Magnetic Resonance Imaging of Optic Nerve Traction During Adduction in Primary Open-Angle Glaucoma With Normal Intraocular Pressure

Postmortem Digital Image Correlation and Finite Element Modeling Demonstrate Posterior Scleral Deformations during Optic Nerve Adduction Tethering

Postmortem human eyes were subjected to optic nerve (ON) traction in adduction and elevated intraocular pressure (IOP) to investigate scleral surface deformations. We incrementally adducted 11 eyes (age 74.1 ± 9.3 years, standard deviation) from 26° to 32° under normal IOP, during imaging of the posterior globe, for analysis by three-dimensional digital image correlation (3D-DIC). In the same eyes, we performed uniaxial tensile testing in multiple regions of the sclera, ON, and ON sheath. Based on individual measurements, we analyzed eye-specific finite element models (FEMs) simulating adduction and IOP loading. Analysis of 3D-DIC showed that the nasal sclera up to 1 mm from the sheath border was significantly compressed during adduction. IOP elevation from 15 to 30 mmHg induced strains less than did adduction. Tensile testing demonstrated ON sheath stiffening above 3.4% strain, which was incorporated in FEMs of adduction tethering that was quantitatively consistent with changes in scleral deformation from 3D-DIC. Simulated IOP elevation to 30 mmHg did not induce scleral surface strains outside the ON sheath. ON tethering in incremental adduction from 26° to 32° compressed the nasal and stretched the temporal sclera adjacent to the ON sheath, more so than IOP elevation. The effect of ON tethering is influenced by strain stiffening of the ON sheath.

Finite element model of ocular adduction with unconstrained globe translation

Details of the anatomy and behavior of the structures responsible for human eye movements have been extensively elaborated since the first modern biomechanical models were introduced. Based on these findings, a finite element model of human ocular adduction is developed based on connective anatomy and measured optic nerve (ON) properties, as well as active contractility of bilaminar extraocular muscles (EOMs), but incorporating the novel feature that globe translation is not otherwise constrained so that realistic kinematics can be simulated. Anatomy of the hemisymmetric model is defined by magnetic resonance imaging. The globe is modeled as suspended by anatomically realistic connective tissues, orbital fat, and contiguous ON. The model incorporates a material subroutine that implements active EOM contraction based on fiber twitch characteristics. Starting from the initial condition of 26° adduction, the medial rectus (MR) muscle was commanded to contract as the lateral rectus (LR) relaxed. We alternatively modeled absence or presence of orbital fat. During pursuit-like adduction from 26 to 32°, the globe translated 0.52 mm posteriorly and 0.1 mm medially with orbital fat present, but 1.2 mm posteriorly and 0.1 mm medially without fat. Maximum principal strains in the optic disk and peripapillary reached 0.05-0.06, and von-Mises stress 96 kPa. Tension in the MR orbital layer was ~ 24 g-force after 6° adduction, but only ~ 3 gm-f in the whole LR. This physiologically plausible simulation of EOM activation in an anatomically realistic globe suspensory system demonstrates that orbital connective tissues and fat are integral to the biomechanics of adduction, including loading by the ON.

Scanning Laser Ophthalmoscopy Demonstrates Pediatric Optic Disc and Peripapillary Strain During Horizontal Eye Rotation

Purpose: We employed automated analysis of scanning laser ophthalmoscopy (SLO) to determine if mechanical strains imposed on disc, and retinal and choroidal vessels during horizontal duction in children differ from those of adults.Methods: Thirty-one children aged 11.3 ± 2.7 (standard deviation) years underwent SLO in central gaze, and 35° ab- and adduction. Automated registration with deep learning-based optical flow analysis quantified vessel deformations as horizontal, vertical, shear, and equivalent strains. Choroidal vessel displacements in lightly pigmented fundi, and central disc vessel displacements, were also observed.Results: As in adults, strain in vessels during horizontal duction was greatest at the disc and decreased with distance from it. Strain in the pediatric disc was similar to published values in young adults,1 but in the peripapillary region was greater and propagated significantly more peripherally to at least three disc radii from it. During adduction in children, the nasal disc was compressed and disc vessels distorted, but the temporal half experienced tensile strain, while peripapillary tissues were compressed. The pattern was similar but strains were less in abduction (p < .001). Choroidal vessels were visualized in 24 of the 62 eyes and shifted directionally opposite overlying retinal vessels.Conclusions: Horizontal duction deforms the normal pediatric optic disc, central retinal vessels, peripapillary retina, and choroid, shearing the inner retina over the choroid. These mechanical effects occur at the sites of remodeling of the disc, sclera, and choroid associated with typical adult features that later emerge later, including optic cup enlargement, temporal disc tilting, and peripapillary atrophy.

Adduction induces large optic nerve head deformations in subjects with normal-tension glaucoma

PURPOSE

To assess intraocular pressure (IOP)-induced and gaze-induced optic nerve head (ONH) strains in subjects with high-tension glaucoma (HTG) and normal-tension glaucoma (NTG).

DESIGN

Clinic-based cross-sectional study.

METHODS

The ONH from one eye of 228 subjects (114 subjects with HTG (pre-treatment IOP≥21 mm Hg) and 114 with NTG (pre-treatment IOP<21 mm Hg)) was imaged with optical coherence tomography (OCT) under the following conditions: (1) OCT primary gaze, (2) 20° adduction from OCT primary gaze, (3) 20° abduction from OCT primary gaze and (4) OCT primary gaze with acute IOP elevation (to approximately 33 mm Hg). We then performed digital volume correlation analysis to quantify IOP-induced and gaze-induced ONH tissue deformations and strains.

RESULTS

Across all subjects, adduction generated high effective strain (4.4%±2.3%) in the LC tissue with no significant difference (p>0.05) with those induced by IOP elevation (4.5%±2.4%); while abduction generated significantly lower (p=0.01) effective strain (3.1%±1.9%). The lamina cribrosa (LC) of HTG subjects exhibited significantly higher effective strain than those of NTG subjects under IOP elevation (HTG: 4.6%±1.7% vs NTG: 4.1%±1.5%, p<0.05). Conversely, the LC of NTG subjects exhibited significantly higher effective strain than those of HTG subjects under adduction (NTG: 4.9%±1.9% vs HTG: 4.0%±1.4%, p<0.05).

CONCLUSION

We found that NTG subjects experienced higher strains due to adduction than HTG subjects, while HTG subjects experienced higher strain due to IOP elevation than NTG subjects-and that these differences were most pronounced in the LC tissue.

Changes in Optic Nerve Head Blood Flow During Horizontal Ocular Duction

Purpose

In this study, we aimed to compare blood flow changes in the optic nerve head (ONH) during horizontal ocular duction among normal, primary open-angle glaucoma (POAG), and normal-tension glaucoma (NTG) eyes.

Methods

In this cross-sectional study, we included 90 eyes from 90 participants (30 control eyes, 30 POAG eyes, and 30 NTG eyes). ONH blood flow was measured with laser speckle flowgraphy using an external fixation light to induce central gaze, abduction, and adduction at 30 degrees for each eye. The mean blur rate (MBR) of the entire ONH area (MA), vascular region (MV), and tissue region (MT), and the change ratio were analyzed. The change ratio was defined as abduction or adduction value/central gaze value.

Results

In the control group, MA significantly decreased during adduction (22.9 ± 3.7) compared with that during central gaze (23.6 ± 3.9, P < 0.05). In the POAG group, MA (adduction = 17.4 ± 3.8 and abduction = 17.3 ± 3.6) and MV (adduction = 37.9 ± 5.6 and abduction = 38.0 ± 5.6) significantly decreased during adduction and abduction compared with those during central gaze (18.0 ± 4.1 and 39.5 ± 6.3, respectively, P < 0.05). In the NTG group, MA significantly decreased during adduction (17.4 ± 4.2) compared with that during central gaze (18.1 ± 4.6) and abduction (18.1 ± 4.8, P < 0.05). The change ratio did not differ between the glaucoma and control groups.

Conclusions

ONH blood flow decreased during horizontal ocular duction regardless of normal or glaucoma states; however, the change ratio was comparable between the normal and glaucoma groups.

Intraocular pressure changes at different gaze positions after superior rectus muscle-lateral rectus muscle loop myopexy in highly myopic strabismus

PURPOSE

To evaluate changes in intraocular pressure (IOP) at different gaze positions before and after superior rectus muscle-lateral rectus muscle (SR-LR) loop myopexy in highly myopic strabismus (HMS).

STUDY DESIGN

Nonrandomized clinical, prospective, interventional trial.

METHODS

Fourteen patients with HMS (18 eyes) who underwent SR-LR loop myopexy were divided into 3 groups: < 100 prism diopters (PD) (mild esotropia [ET] group), > 100 PD (large ET group), and > 100 PD, and simultaneous recession of the medial rectus (MR) muscle was performed (large ET + MR group). Intraocular pressure was measured preoperatively and postoperatively at the primary, abduction, and adduction positions in each group.

RESULTS

Intraocular pressure did not change after surgery in the mild ET group. Intraocular pressure significantly decreased in the abduction position (from 20.0 ± 2.1 to 16.0 ± 1.9 mmHg, P = 0.043) in the large ET group and in the abduction (from 22.2 ± 5.9 to 15.6 ± 4.3 mmHg, P = 0.048) and primary positions (from 15.8 ± 5.0 to 10.2 ± 2.8 mmHg, P = 0.043) in the large ET + MR group. The preoperative significant differences in IOP between the abduction and adduction positions in the large ET group (7.4 ± 3.4 mmHg) and the large ET + MR group (10.0 ± 5.5 mmHg) disappeared postoperatively (3.2 ± 2.8 mmHg and 3.6 ± 1.7 mmHg, respectively). The differences in IOP between abduction and adduction were similar in all the groups.

CONCLUSION

SR-LR loop myopexy decreased IOP in patients with HMS in the abduction and primary positions.

Scanning Laser Ophthalmoscopy Demonstrates Disc and Peripapillary Strain During Horizontal Eye Rotation in Adults

PURPOSE

We used automated image analysis of scanning laser ophthalmoscopy (SLO) to investigate mechanical strains imposed on disc, and retinal and choroidal vessels during horizontal duction in adults.

DESIGN

Deep learning analysis of optical images.

METHODS

The peripapillary region was imaged by SLO in central gaze, and 35° abduction and adduction, in younger and older healthy adults. Automated image registration was followed by deep learning-based optical flow analysis to track determine local tissue deformations quantified as horizontal, vertical, and shear strain maps relative to central gaze. Choroidal vessel displacements were observed when fundus pigment was light.

RESULTS

Strains in the retina and disc could be quantified in 22 younger (mean ± SEM, 26 ± 5 years) and 19 older (64 ± 10 years) healthy volunteers. Strains were predominantly horizontal and greater for adduction than for abduction. During adduction, maximum horizontal strain was tensile in the nasal hemi-disc, and declined progressively with distance from it. Strain in the temporal hemi-retina during adduction was minimal, except for compressive strain on the disc of older subjects. In abduction, horizontal strains were less and largely confined to the disc, greater in older subjects, and generally tensile. Vertical and shear strains were small. Nasal to the disc, choroidal vessels shifted nasally relative to overlying peripapillary retinal vessels.

CONCLUSIONS

Strain analysis during horizontal duction suggests that the optic nerve displaces the optic canal, choroid, and peripapillary sclera relative to the overlying disc and retina. This peripapillary shearing of the optic nerve relative to the choroid and sclera may be a driver of disc tilting and peripapillary atrophy.

Prognostic value of extraocular muscles for intraocular pressure in thyroid-associated ophthalmopathy patients

Background

Persistent increase in intraocular pressure (IOP) is often observed in eyes with thyroid-associated ophthalmopathy (TAO), which has irreversible effects on the visual function of patients. This retrospective cross-sectional study evaluated the value of magnetic resonance imaging (MRI) measurements of extraocular muscle (EOM) volume in prognosing IOP in patients with TAO.

Methods

This single-center study was conducted in Beijing Friendship Hospital (Beijing, China), a tertiary hospital. From 35 participants, 70 eyes (normal IOP group: 50 eyes; high IOP group: 20 eyes; random samples) were enrolled in this study. Basic data from patients were collected and compared using 2-sample t-tests and chi-squared tests. The volume of the EOM, orbit fat, and whole orbit were measured by orbital MRI. Moreover, proptosis, optic nerve (ON) sheath diameter, optic nerve angle (ONA), and gaze angle were additionally measured with MRI. These parameters were compared between the two groups using 2-sample t-tests and chi-squared tests. Before and after post-methylprednisolone therapy, the MRI data of 20 eyes were obtained and compared with a paired t-test. Peripapillary retinal nerve fiber layer (RNFL) thickness was obtained using an optical coherence tomography (OCT) scan. Spearman rank correlation analysis, logistic regression analysis, and receiver operating characteristic (ROC) curves were performed to evaluate the role of these factors in IOP changes.

Results

The EOM volume and axial and sagittal ONAs in the high-IOP group were significantly increased compared to the normal-IOP group (P<0.001, P=0.001, P=0.02, respectively). Logistic regression analysis indicate that the cutoff value for EOM volume for the diagnosis of high IOP was significantly larger than that of the other parameters except orbit volume (P=0.03, P=0.004, P=0.08, respectively). Spearman correlation analysis revealed a significant correlation between EOM volume and ON sheath diameter and average RNFL thickness (P=0.01, P=0.02, respectively). A paired t-test indicated a significant decrease of EOM volume and ON sheath diameter as well as a significant enlargement of the axial ONAs after methylprednisolone therapy (P=0.002, P=0.02, P=0.05, respectively).

Conclusions

EOM volume was an effective diagnostic factor for tracking IOP changes and ON patterns in patients with TAO. Methylprednisolone therapy is recommended for patients with TAO with secondary glaucoma to quickly reduce the EOM volume.

Pharmaceutical Approaches to Normal Tension Glaucoma

Normal tension glaucoma (NTG) is defined as a subtype of primary open-angle glaucoma (POAG) in which the intraocular pressure (IOP) values are constantly within the statistically normal range without treatment and represents approximately the 30-40% of all glaucomatous cases. The pathophysiology of this condition is multifactorial and is still not completely well known. Several theories have been proposed to explain the onset and progression of this disease, which can be divided into IOP-dependent and IOP-independent factors, suggesting different therapeutic strategies. The current literature strongly supports the fundamental role of IOP in NTG. The gold standard treatment for NTG tends to be based on the lowering IOP even if "statistically normal". Numerous studies have shown, however, that the IOP reduction alone is not enough to slow down or stop the disease progression in all cases, suggesting that other IOP-independent risk factors may contribute to the NTG pathogenesis. In addition to IOP-lowering strategies, several different therapeutic approaches for NTG have been proposed, based on vaso-active, antioxidant, anti-inflammatory and/or neuroprotective substances. To date, unfortunately, there are no standardized or proven treatment alternatives for NTG when compared to traditional IOP reduction treatment regimes. The efficacy of the IOP-independent strategies in decreasing the risk or treating NTG still remains inconclusive. The aim of this review is to highlight strategies reported in the current literature to treat NTG. The paper also describes the challenges in finding appropriate and pertinent treatments for this potentially vision-threatening disease. Further comprehension of NTG pathophysiology can help clinicians determine when to use IOP-lowering treatments alone and when to consider additional or alternatively individualized therapies focused on particular risk factors, on a case-by-case basis.

Empirical Quantification of Optic Nerve Strain Due to Horizontal Duction

Magnetic resonance imaging (MRI) was used to measure in vivo local strains in the optic nerve (ON) associated with horizontal duction in humans. Axial and coronal MRI were collected in target-controlled gazes in 24 eyes of 12 normal adults (six males and six females, 59 ± 16 years) during large (~28°) and moderate (~24°) ductions. The ON, globe, and extraocular muscles were manually identified, and the pixels were converted to point-sets that were registered across different imaging planes and eye positions. Shape of the ON was parameterized based on point-sets. Displacements and strains were computed by comparing deformed with initial ON configurations. Displacements were the largest in the most anterior region. However, strains from adduction were uniform along the length of the ON, while those during abduction increased with distance from the globe and were maximal near the orbital apex. For large gaze angles, ON strain during abduction was primarily due to bending near the orbital apex that is less transmitted to the eye, but during adduction the ON undergoes uniform stretching that transmits much greater loading to the posterior eye, implied by greater strain on the ON.

Optical Coherence Tomography Angiography Demonstrates Strain and Volume Effects on Optic Disk and Peripapillary Vasculature Caused by Horizontal Duction

PURPOSE

The optic nerve mechanically loads the eye during ocular rotation, thus altering the configuration of the disk and peripapillary tissues. We used optical coherence tomography (OCT) angiography (OCTA) to investigate mechanical strains and volume changes in disk and peripapillary blood vessels during horizontal duction.

METHODS

Structural OCT and OCTA were performed centered on optic disks; imaging was repeated in central gaze, and in 30° ab- and adduction. By an algorithm employing point-set registration of 3 D features, we developed a novel approach for measuring disk strains, and strains and volumes of the blood vessels associated with horizontal duction. Repeatability was demonstrated in each gaze position.

RESULTS

19 eyes of 10 healthy adults of average age 37 ± 15 (standard deviation, SD) years were imaged. The method was validated by demonstrating numerically consistent vascular volumes and strains for repeated imaging under identical conditions. Compared with central gaze, vascular volume increased by 5.2 ± 4.1% in adduction. Adduction and abduction caused strains of 3.0 ± 1.6% and 2.6 ± 1.8% in the optic disk, whereas blood vessels showed greater strains of 8.1 ± 1.3% and 8.2 ± 1.7%. Decomposition of strain components depending on directionality and regions demonstrated that adduction induces significant net tensile strains, suggesting traction exerted by the optic nerve. The decomposition also showed that nasotemporal compressive strains are larger in temporal hemidisks than nasal hemidisks. The Bruch's membrane opening was significantly compressed horizontally in adduction by 1.1% (p = .009).

CONCLUSION

This novel analysis combining structural OCT and OCTA demonstrates that optic disk compression during adduction is associated with disk and vascular strains much larger than reported for intraocular pressure elevation and pulsatile perfusion, as well as compressing the disk and increasing peripapillary vascular volume. These changes may be relevant to the pathogenesis of optic nerve and retinal vascular disorders.

Linear viscoelasticity of human sclera and posterior ocular tissues during tensile creep

PURPOSE

Despite presumed relevance to ocular diseases, the viscoelastic properties of the posterior human eye have not been evaluated in detail. We performed creep testing to characterize the viscoelastic properties of ocular regions, including the sclera, optic nerve (ON) and ON sheath.

METHODS

We tested 10 pairs of postmortem human eyes of average age 77 ± 17 years, consisting of 5 males and 5 females. Except for the ON that was tested in native shape, tissues were trimmed into rectangles. With physiologic temperature and constant wetting, tissues were rapidly loaded to tensile stress that was maintained by servo feedback as length was monitored for 1,500 sec. Relaxation modulus was computed using Prony series, and Deborah numbers estimated for times scales of physiological eye movements.

RESULTS

Correlation between creep rate and applied stress level was negligible for all tissues, permitting description as linear viscoelastic materials characterized by lumped parameter compliance equations for limiting behaviors. The ON was the most compliant, and anterior sclera least compliant, with similar intermediate values for posterior sclera and ON sheath. Sensitivity analysis demonstrated that linear behavior eventually become dominant after long time. For the range of typical pursuit tracking, all tissues exhibit Debora numbers less than 75, and should be regarded as viscoelastic. With a 6.7 Deborah number, this is especially so for the ON during pursuit and convergence.

CONCLUSIONS

Posterior ocular tissues exhibit creep consistent with linear viscoelasticity necessary for describing biomechanical behavior of the ON, its sheath, and sclera during physiological eye movements and eccentric ocular fixations. Running Head: Tensile Creep of Human Ocular Tissues.

Differing Associations between Optic Nerve Head Strains and Visual Field Loss in Patients with Normal- and High-Tension Glaucoma

PURPOSE

To study the associations between optic nerve head (ONH) strains under intraocular pressure (IOP) elevation with retinal sensitivity in patients with glaucoma.

DESIGN

Clinic-based cross-sectional study.

PARTICIPANTS

Two hundred twenty-nine patients with primary open-angle glaucoma (subdivided into 115 patients with high-tension glaucoma [HTG] and 114 patients with normal-tension glaucoma [NTG]).

METHODS

For 1 eye of each patient, we imaged the ONH using spectral-domain OCT under the following conditions: (1) primary gaze and (2) primary gaze with acute IOP elevation (to approximately 35 mmHg) achieved through ophthalmodynamometry. A 3-dimensional strain-mapping algorithm was applied to quantify IOP-induced ONH tissue strain (i.e., deformation) in each ONH. Strains in the prelaminar tissue (PLT), the retina, the choroid, the sclera, and the lamina cribrosa (LC) were associated (using linear regression) with measures of retinal sensitivity from the 24-2 Humphrey visual field test (Carl Zeiss Meditec). This was performed globally, then locally according to a previously published regionalization scheme.

MAIN OUTCOME MEASURES

Associations between ONH strains and values of retinal sensitivity from visual field testing.

RESULTS

For patients with HTG, we found (1) significant negative linear associations between ONH strains and retinal sensitivity (P < 0.001; on average, a 1% increase in ONH strains corresponded to a decrease in retinal sensitivity of 1.1 decibels [dB]), (2) that high-strain regions colocalized with anatomically mapped regions of high visual field loss, and (3) that the strongest negative associations were observed in the superior region and in the PLT. In contrast, for patients with NTG, no significant associations between strains and retinal sensitivity were observed except in the superotemporal region of the LC.

CONCLUSIONS

We found significant negative associations between IOP-induced ONH strains and retinal sensitivity in a relatively large glaucoma cohort. Specifically, patients with HTG who experienced higher ONH strains were more likely to exhibit lower retinal sensitivities. Interestingly, this trend in general was less pronounced in patients with NTG, which could suggest a distinct pathophysiologic relationship between the two glaucoma subtypes.

Finite element modeling of effects of tissue property variation on human optic nerve tethering during adduction

Tractional tethering by the optic nerve (ON) on the eye as it rotates towards the midline in adduction is a significant ocular mechanical load and has been suggested as a cause of ON damage induced by repetitive eye movements. We designed an ocular finite element model (FEM) simulating 6° incremental adduction beyond the initial configuration of 26° adduction that is the observed threshold for ON tethering. This FEM permitted sensitivity analysis of ON tethering using observed material property variations in measured hyperelasticity of the anterior, equatorial, posterior, and peripapillary sclera; and the ON and its sheath. The FEM predicted that adduction beyond the initiation of ON tethering concentrates stress and strain on the temporal side of the optic disc and peripapillary sclera, the ON sheath junction with the sclera, and retrolaminar ON neural tissue. However, some unfavorable combinations of tissue properties within the published ranges imposed higher stresses in these regions. With the least favorable combinations of tissue properties, adduction tethering was predicted to stress the ON junction and peripapillary sclera more than extreme conditions of intraocular and intracranial pressure. These simulations support the concept that ON tethering in adduction could induce mechanical stresses that might contribute to ON damage.

Analysis of Peripapillary Intrachoroidal Cavitation and Myopic Peripapillary Distortions in Polar Regions by Optical Coherence Tomography

Purpose

To compare the peripapillary polar characteristics in eyes combining peripapillary staphyloma and gamma peripapillary atrophy according to whether peripapillary intrachoroidal cavitation (PICC) was present or absent (combination-group).

Patients and methods

This prospective non-interventional cross-sectional study included 667 eyes of 334 subjects. From the polar peripapillary regions to the opening of Bruch’s membrane, the following elements and their topographic relationships were analyzed using optical coherence tomography sections: configuration of the posterior curvature of the choroid, visibility of the subarachnoid space (SAS), and suprachoroidal detachment (SCD). Chi-squared and Fisher exact tests were used for statistical analysis.

Results

The protrusion of the posterior choroidal wall, with anterior elevation on either side, observed in both groups progressed and transformed into a wedge-shaped deformity on the side of gamma peripapillary atrophy. This wedge configuration was significantly more frequent in PICC-group than in combination-group (p = 0.004 and p < 0.001) for the upper and lower poles, respectively. SAS was more frequently observed in PICC-group than in combination-group (p = 0.002 and p < 0.001) for the upper and lower poles, respectively. SCD was detected exclusively in PICC-group (p < 0.001, both poles). The wedge-shaped configuration and the SCD were aligned antero-posteriorly with the SAS.

Conclusion

We confirmed that PICC is an SCD. We observed its constant alignment with the SAS. We suggest that the tensile forces of the optic nerve sheaths during adduction promote the collapse of the scleral flange onto the SAS, leading to PICC. Further studies are warranted to confirm this hypothesis.

Advanced Diffusion MRI of the Visual System in Glaucoma: From Experimental Animal Models to Humans

Simple Summary

This review summarizes current applications of advanced diffusion magnetic resonance imaging (MRI) throughout the glaucomatous visual system, focusing on the eye, optic nerve, optic tract, subcortical visual brain nuclei, optic radiations, and visual cortex. Glaucoma continues to be the leading cause of irreversible blindness worldwide and often remains undetected until later disease stages. The development of non-invasive methods for early detection of visual pathway integrity could pave the way for timely intervention and targeted treatment strategies. Principles of diffusion have been integrated with MRI protocols to produce a diffusion-weighted imaging modality for studying changes to tissue microstructures by quantifying the movement of water molecules in vivo. The development and applications of diffusion MRI in ophthalmology have allowed a better understanding of neural pathway changes in glaucoma. The feasibility of translating diffusion MRI techniques to assess both humans and experimental animal models of glaucoma and other optic neuropathies or neurodegenerative diseases is discussed. Recent research focuses on overcoming limitations in imaging quality, acquisition times, and biological interpretation suggest that diffusion MRI can provide an important tool for the non-invasive evaluation of glaucomatous changes in the visual system.

Abstract

Glaucoma is a group of ophthalmologic conditions characterized by progressive retinal ganglion cell death, optic nerve degeneration, and irreversible vision loss. While intraocular pressure is the only clinically modifiable risk factor, glaucoma may continue to progress at controlled intraocular pressure, indicating other major factors in contributing to the disease mechanisms. Recent studies demonstrated the feasibility of advanced diffusion magnetic resonance imaging (dMRI) in visualizing the microstructural integrity of the visual system, opening new possibilities for non-invasive characterization of glaucomatous brain changes for guiding earlier and targeted intervention besides intraocular pressure lowering. In this review, we discuss dMRI methods currently used in visual system investigations, focusing on the eye, optic nerve, optic tract, subcortical visual brain nuclei, optic radiations, and visual cortex. We evaluate how conventional diffusion tensor imaging, higher-order diffusion kurtosis imaging, and other extended dMRI techniques can assess the neuronal and glial integrity of the visual system in both humans and experimental animal models of glaucoma, among other optic neuropathies or neurodegenerative diseases. We also compare the pros and cons of these methods against other imaging modalities. A growing body of dMRI research indicates that this modality holds promise in characterizing early glaucomatous changes in the visual system, determining the disease severity, and identifying potential neurotherapeutic targets, offering more options to slow glaucoma progression and to reduce the prevalence of this world’s leading cause of irreversible but preventable blindness.

Statics of plant mechanics

David A. Robinson took all that was known in his time about ocular anatomy, extraocular muscle force generation, and neural control of extraocular muscles, and integrated this information into a quantitative model of the static behavior of the ocular motor plant suitable for application to strabismus, the pathological misalignment of the eyes. Robinson's comprehensive mathematical descriptions of the essential details he knew to be the properties of the ocular motor plant highlighted the critical gaps in the state of knowledge that he very explicitly bridged by quantitative assumptions that later motivated focused research that ultimately revealed many missing pieces of the puzzle. Robinson suggested that it should be possible, in principle, to account (in a computational model) for all the mechanical factors and neural drives that regulate binocular alignment and strabismus, and that such modeling could assist in diagnosis and treatment of this common ophthalmic disorder.

A Review of Potential Novel Glaucoma Therapeutic Options Independent of Intraocular Pressure

Glaucoma, a progressive optic neuropathy characterized by retinal ganglion cell degeneration and visual field loss, is the leading cause of irreversible blindness worldwide. Intraocular pressure (IOP) is presently the only modifiable risk factor demonstrated to slow or halt disease progression; however, glaucomatous damage persists in almost 50% of patients despite significant IOP reduction. Many studies have investigated the non-IOP-related risk factors that contribute to glaucoma progression as well as interventions that can prevent or delay glaucomatous neurodegeneration and preserve vision throughout life, independently of IOP. A vast number of experimental studies have reported effective neuroprotection in glaucoma, and clinical studies are ongoing attempting to provide strong evidence of effectiveness of these interventions. In this review, we look into the current understanding of the pathophysiology of glaucoma and explore the recent advances in non-IOP related strategies for neuroprotection and neuroregeneration in glaucoma.

Intraocular Pressure According to Eye Gaze by iCare Rebound Tonometry in Normal Participants and Glaucoma Patients

PRECIS

We investigated changes of intraocular pressure (IOP) according to eye gaze. IOP was significantly elevated in adduction, abduction, and supraduction. However, there was no significant difference between glaucoma and control groups.

PURPOSE

We assessed changes in IOP according to eye gaze and identified their correlations with various risk factors of glaucoma.

PATIENTS AND METHODS

In this prospective observational study that included 56 glaucoma patients and 34 healthy participants, we measured IOP in the primary position with a Goldmann applanation tonometry and rebound tonometer. Then, this IOP was measured in abduction, adduction, supraduction using a rebound tonometer. IOP changes according to eye gaze were measured based on the baseline IOP, and IOP changes between glaucoma and the control groups were compared. Correlations between IOP changes and risk factors of glaucoma were evaluated.

RESULTS

The baseline IOP was not significantly different between glaucoma and the control groups. Compared with the IOP in the primary position, a significant increase in IOP was 2.3±2.7 mm Hg during abduction (P<0.0001), 0.7±2.7 mm Hg during adduction (P<0.0001), and 1.2±2.8 mm Hg during supraduction (P<0.0001). However, there was no significant difference in the amount of IOP elevation or the ratio of IOP change between glaucoma and the control groups in all gazes. The baseline IOP measured by Goldmann applanation tonometry and IOP changes according to eye gaze showed a significant negative correlation in all gazes.

CONCLUSIONS

IOP was significantly elevated in adduction, abduction, and supraduction than in the primary position in both the normal and glaucoma groups. However, there was no significant difference of IOP changes between glaucoma and normal groups.

Optic nerve tortuosity and displacements during horizontal eye movements in healthy and highly myopic subjects

AIMS

(1) To assess the morphology and 3-dimensional (3D) displacements of the eye globe and optic nerve (ON) in adduction/abduction using MRI. (2) To assess differences between healthy emmetropic and highly myopic (HM) subjects.

METHODS

MRI volumes of both eyes from 18 controls and 20 HM subjects in primary gaze, abduction and adduction (15°) were postprocessed. All ONs were manually segmented and fitted to a 3D curve to assess ON tortuosity. ON displacements were evaluated in four quasicoronal planes which were perpendicular to the ON in primary gaze and were 3 mm apart.

RESULTS

Axial length was higher in the HM group (28.62±2.60 vs 22.84±0.89 mm; p<0.0001). Adjusted ON tortuosities (ie, ON tortuosities estimated before myopia onset) were lower in HM eyes (0.9063±0.0591) versus controls (1.0152±0.02981) in primary gaze, adduction (0.9023±0.05538 vs 1.0137±0.0299) and abduction (0.9100±0.0594 vs 1.0182±0.0316); p<0.0001 for all cases. In all eyes, ON displacements in adduction were significantly different from those in abduction in the naso-temporal direction (p<0.0001 in all planes) but not in the supero-inferior direction. ON displacements in the posterior segments of the ON were smaller in the HM group in both gaze directions and were larger in the anterior-most ON segment in adduction only.

CONCLUSION

The adjusted tortuosity of the ON was significantly lower in HM eyes, suggesting that eyes destined towards HM exhibited higher ON traction forces during eye movements before the onset of myopia. Our ON metrics may be valuable to explore a potential link between eye movements and axial elongation.

Material properties and effect of preconditioning of human sclera, optic nerve, and optic nerve sheath

The optic nerve (ON) is a recently recognized tractional load on the eye during larger horizontal eye rotations. In order to understand the mechanical behavior of the eye during adduction, it is necessary to characterize material properties of the sclera, ON, and in particular its sheath. We performed tensile loading of specimens taken from fresh postmortem human eyes to characterize the range of variation in their biomechanical properties and determine the effect of preconditioning. We fitted reduced polynomial hyperelastic models to represent the nonlinear tensile behavior of the anterior, equatorial, posterior, and peripapillary sclera, as well as the ON and its sheath. For comparison, we analyzed tangent moduli in low and high strain regions to represent stiffness. Scleral stiffness generally decreased from anterior to posterior ocular regions. The ON had the lowest tangent modulus, but was surrounded by a much stiffer sheath. The low-strain hyperelastic behaviors of adjacent anatomical regions of the ON, ON sheath, and posterior sclera were similar as appropriate to avoid discontinuities at their boundaries. Regional stiffnesses within individual eyes were moderately correlated, implying that mechanical properties in one region of an eye do not reliably reflect properties of another region of that eye, and that potentially pathological combinations could occur in an eye if regional properties are discrepant. Preconditioning modestly stiffened ocular tissues, except peripapillary sclera that softened. The nonlinear mechanical behavior of posterior ocular tissues permits their stresses to match closely at low strains, although progressively increasing strain causes particularly great stress in the peripapillary region.

Finite Element Model of Ocular Adduction by Active Extraocular Muscle Contraction

Purpose

In order to clarify the role of the optic nerve (ON) as a load on ocular rotation, we developed a finite element model (FEM) of incremental adduction induced by active contractility of extraocular muscles (EOMs), with and without tethering by the ON.

Methods

Three-dimensional (3-D) horizontal rectus EOM geometries were obtained from magnetic resonance imaging of five healthy adults, and measured constitutive tissue properties were used. Active and passive strain energies of EOMs were defined using ABAQUS (Dassault Systemes) software. All deformations were assumed to be caused by EOM twitch activation that rotated the eye about a fixed center. The medial rectus (MR) muscle was commanded to additionally contract starting from 26 degrees adducted position, and the lateral rectus (LR) to relax, further adducting the eye either with or without loading by the ON. Tridimensional heat maps were generated to represent the stress and strain distributions.

Results

Tensions in the EOMs were physiologically plausible during incremental adduction. Force in the MR increased from 10 gm at 26 degrees adduction to approximately 28 gm at 32 degrees adduction. Under identical MR contraction, adduction with ON loading reached 32 degrees but 36 degrees without it. Maximum and minimum principal strains within the MR were 16% and 22%, respectively, but when ON loading was included, resulting stress and strain were concentrated at the optic disc.

Conclusions

This physiologically plausible method of simulating EOM activation can provide realistic input to model biomechanical behavior of active and passive tissues in the orbit to clarify biomechanical consequences of ON traction during adduction.

Age-dependent changes in visual sensitivity induced by moving fixation points in adduction and abduction using imo perimetry

Visual field (VF) testing has usually been performed with the central gaze as a fixed point. Recent publications indicated optic nerve head deformations induced by optic nerve traction force can promote the progression of optic neuropathies, including glaucoma. We generated a new static test protocol that adds 6° adduction and abduction to gaze position (fixation points) movement. The aim of this study was to investigate both whether quantifying VF sensitivities at lateral horizontal gaze positions is feasible and whether horizontal gaze positions change sensitivities differently in subjects of different ages. Healthy adult eyes from 29 younger (≤ 45 years) and 28 elderly (> 45 years) eyes were examined in this cross-sectional study. After VF testing with central gaze as a fixation point using 24 plus (1) imo static perimetry, subjects underwent VF testing with 6° adduction and 6° abduction as fixation points. The average mean sensitivities with central gaze, adduction, and abduction were 29.9 ± 1.0, 29.9 ± 1.3, and 30.0 ± 1.2 decibels (dB) in younger subjects and 27.7 ± 1.2, 27.5 ± 1.7, and 28.1 ± 1.3 dB in elderly subjects, respectively. Visual sensitivity in young healthy subjects was similar among the three fixation points, whereas visual sensitivity in elderly healthy subjects was significantly better with abduction as a fixation point than with central gaze and adduction (both p < 0.05). We expect this test protocol to contribute to our understanding of visual function during horizontal eye gaze movement in various eye diseases.

Association between esodeviation and primary open-angle glaucoma: the 2010-2011 Korea National Health and Nutrition Examination Survey

BACKGROUND/AIMS

To evaluate the association between strabismus and primary open-angle glaucoma (POAG) in a representative Korean population.

METHODS

A total of 11 114 participants aged 20 years or older in the Korea National Health and Nutrition Examination Survey database for the years 2010 through 2011 were reviewed. A standardised protocol was used to interview every participant and to perform comprehensive ophthalmic examinations. Glaucoma diagnosis was based on fundus photography and frequency-doubling technology perimetry results, according to the International Society of Geographical and Epidemiological Ophthalmology criteria. Ocular alignment was evaluated using the alternate prism and cover test, and clinically significant horizontal strabismus was defined as exodeviation of ≥15 prism dioptres (PD) and esodeviation of ≥10 PD. Univariate and multivariate regression analyses were used to evaluate the potential risk factors for POAG.

RESULTS

In the Korean population, subjects with clinically significant esodeviation had a much higher prevalence of POAG (12.32%) than those without clinically significant esodeviation (3.14%, p=0.016). After adjusting for age and intraocular pressure, clinically significant esodeviation was independently associated with POAG (OR 7.61, p=0.002).

CONCLUSION

Esodeviation was independently associated with POAG in the Korean population. This could be the result of, at least in part, ocular-adduction-induced greater strain on the temporal optic nerve head and peripapillary tissues, which makes eyes with esodeviation more vulnerable to POAG.

Orbital Fat Volume After Treatment with Topical Prostaglandin Agonists

Purpose

Topical prostaglandin analogs (PGAs) are common treatment for primary open-angle glaucoma (POAG) but reportedly may cause adnexal fat atrophy. We asked if patients with POAG treated with PGAs have abnormalities in orbital fat volume (OFV).

Methods

We studied 23 subjects with POAG who had never experienced intraocular pressure (IOP) exceeding 21 mm Hg and were treated long term with PGAs, in comparison with 21 age-matched controls. Orbital volume, non-fat orbital tissue volume, and OFV were measured using high-resolution magnetic resonance imaging.

Results

Subjects with POAG had been treated with PGAs for 39 ± 19 months (SD) and were all treated within the 4 months preceding study. In the region from trochlea to orbital apex, OFV in POAG was significantly less at 9.8 ± 1.9 mL than in the control subjects at 11.1 ± 1.3 mL (P = 0.019). However, between the globe-optic nerve junction (GONJ) and trochlea, OFV was similar in both groups. Width and cross sectional area of the bony orbit were significantly smaller in POAG than in controls (P < 0.0001). Posterior to the GONJ, the average orbital cross-sectional area was 68.2 mm² smaller, and the orbital width averaged 1.5 mm smaller throughout the orbit, in patients with POAG than in controls.

Conclusions

Patients with POAG who have been treated with PGAs have lower overall OFV than controls, but OFV in the anterior orbit is similar in both groups. Lower overall OFV in POAG may be a primary association of this disorder with a horizontally narrower bony orbit, which may be a risk factor for POAG at nonelevated IOPs.

Adduction-Induced Strain on the Optic Nerve in Primary Open Angle Glaucoma at Normal Intraocular Pressure

PURPOSE/AIM

The optic nerve (ON) becomes taut during adduction beyond ~26° in healthy people and patients with primary open angle glaucoma (POAG), but only retracts the globe in POAG. We used magnetic resonance imaging (MRI) to investigate this difference.

MATERIALS AND METHODS

MRI was obtained in 2-mm quasi-coronal planes in central gaze, and smaller (~23-25°) and larger (~30-31°) adduction and abduction in 21 controls and 12 POAG subjects whose intraocular pressure never exceeded 21 mmHg. ON cross-sections were analyzed from the globe to 10 mm posteriorly. Area centroids were used to calculate ON path lengths and changes in cross-sections to calculate elongation assuming volume conservation.

RESULTS

For both groups, ON path was nearly straight (<102.5% of minimum path) in smaller adduction, with minimal further straightening in larger adduction. ON length was redundant in abduction, exceeding 103% of minimum path for both groups. For normals, the ON elongated 0.4 ± 0.5 mm from central gaze to smaller adduction, and 0.4 ± 0.5 mm further from smaller to larger adduction. For POAG subjects, the ON did not elongate on average from central gaze to smaller adduction and only 0.2 ± 0.4 mm from smaller to larger adduction (P = .045 vs normals). Both groups demonstrated minimal ON elongation not exceeding 0.25 mm from central gaze to smaller and larger abduction. The globe retracted significantly more during large adduction in POAG subjects than normals (0.6 ± 0.7 mm vs 0.2 ± 0.5 mm, P = .027), without appreciable retraction in abduction. For each mm increase in globe axial length, ON elongation in large adduction similarly increased by 0.2 mm in each group.

CONCLUSIONS

The normal ON stretches to absorb force and avert globe retraction in adduction. In POAG with mild to severe visual field loss, the relatively inelastic ON tethers and retracts the globe during adduction beyond ~26°, transfering stress to the optic disc that could contribute to progressive neuropathy during repeated eye movements.

Glaucoma conversion of the contralateral eye in unilateral normal-tension glaucoma patients: a 5-year follow-up study

BACKGROUND/AIMS

To investigate clinical characteristics and risk factors for glaucoma conversion of the contralateral eye in unilateral normal-tension glaucoma (NTG) patients.

METHODS

A retrospective observational cohort study was conducted on a total of 76 subjects who had been diagnosed with unilateral NTG at the baseline and followed up for more than 5 years. Glaucoma conversion in the contralateral eye was defined as increased thinning of neuro-retinal rim, development of retinal nerve fibre layer defect and/or development of glaucomatous visual field defect.

RESULTS

During the mean follow-up period of 7.3±2.4 years, 21 of 76 (27.6%) subjects were confirmed to have developed glaucoma in the non-glaucomatous contralateral eye. The 5-year rate of glaucoma conversion in contralateral eyes was 19.7%. The maximum width of β-zone parapapillary atrophy (MWβPPA)-disc diameter (DD) ratio at the baseline and the presence rate of disc haemorrhage during follow-up period were significantly greater in the contralateral eyes of the conversion group than in those of the non-conversion group (p=0.011, <0.001, respectively). A multivariate Cox-proportional hazard model revealed intraocular pressure (IOP) over 17 mm Hg (HR 5.05, p=0.031), central corneal thickness (CCT) under 491 μm (HR 4.25, p=0.025) and MWβPPA-DD ratio over 0.32 (HR 6.25, p=0.003) in contralateral eye at the baseline as the independent risk factors for glaucoma conversion.

CONCLUSIONS

Among unilateral NTG patients, those with low CCT and high MWβPPA-DD ratio as well as high IOP in the contralateral eye are more likely to develop glaucoma in that eye during long-term follow-up.

The Effect of Axial Length on Extraocular Muscle Leverage

PURPOSE

Magnetic resonance imaging was used to determine the effect of axial length (AL) on globe rotational axis and horizontal extraocular muscle leverage during horizontal duction.

DESIGN

Prospective observational case series.

METHODS

At a single academic center, 36 orthophoric adults with a wide range of ALs underwent high-resolution axial orbital magnetic resonance imaging in target-controlled adduction and abduction. ALs were measured in planes containing maximum globe cross-sections. Area centroids were calculated to determine globe centers. Rotational axes in orbital coordinates were calculated from displacements of lens centers and globe-optic nerve attachments. Lever arms were calculated as distances between published extraocular muscle insertions and rotational axes.

RESULTS

ALs averaged 26.3 ± 0.3 mm (standard error [range 21.5-33.4 mm]). Rotational axes from adduction to abduction averaged 1.1 ± 0.2 mm medial and 1.1 ± 0.2 mm anterior to the globe's geometric center in adduction. Linear regression demonstrated no significant correlation between AL and rotational axis horizontal (R² = 0.06) or anteroposterior (R² = 0.07) position. Medial rectus (MR) lever arms averaged 12.0 ± 0.2 mm and lateral rectus (LR) lever arms averaged 12.8 ± 0.2 mm. Both MR (R² = 0.24, P < .001) and LR (R² = 0.32, P < .001) lever arms significantly increased by about 0.3 mm per 1.0-mm of increased AL, with a corresponding reduction in predicted per-millimeter effect of surgical repositioning of their insertions.

CONCLUSIONS

Regardless of AL, the globe rotates about a point nasal and anterior to its geometric center, giving the LR more leverage than the MR. This eccentricity may diminish the effect of tendon repositioning in moderate to highly myopic patients, with reductions in per-mill imeter dose/response predicted with longer AL.

Unraveling the Enigma of Nonarteritic Anterior Ischemic Optic Neuropathy

Non-arteritic anterior ischemic optic neuropathy (NAON) is the second most common optic neuropathy in adults. Despite extensive study, the etiology of NAION is not definitively known. The best evidence suggests that NAION is caused by an infarction in the region of the optic nerve head (ONH), which is perfused by paraoptic short posterior ciliary arteries (sPCAs) and their branches. To examine the gaps in knowledge that defies our understanding of NAION, a historical review was performed both of anatomical investigations of the ONH and its relevant blood vessels and the evolution of clinical understanding of NAION. Notably, almost all of the in vitro vascular research was performed prior our current understanding of NAION, which has largely precluded a hypothesis-based laboratory approach to study the etiological conundrum of NAION. More recent investigative techniques, like fluorescein angiography, have provided valuable insight into vascular physiology, but such light-based techniques have not been able to image blood vessels located within or behind the dense connective tissue of the sclera and laminar cribrosa, sites that are likely culpable in NAION. The lingering gaps in knowledge clarify investigative paths that might be taken to uncover the pathogenesis of NAION and possibly glaucoma, the most common optic neuropathy for which evidence of a vascular pathology also exists.

Bilaminar Mechanics of the Human Optic Nerve Sheath

PURPOSE/AIM

The adult human optic nerve (ON) sheath has recently been recognized to be bilaminar, consisting of inner layer (IL) and outer layer (OL). Since the ON and sheath exert tension on the globe in large angle adduction as these structures transmit reaction force of the medial rectus muscle to the globe, this study investigated the laminar biomechanics of the human ON sheath.

MATERIALS AND METHODS

Biomechanical characterization was performed in ON sheath specimens from 12 pairs of fresh, post-mortem adult eyes. Some ON sheath specimens were tested completely, while others were separated into IL and OL. Uniaxial tensile loading under physiological temperature and humidity was used to characterize a linear approximation as Young's modulus, and hyperelastic non-linear behavior using the formulation of Ogden. Micro-indentation was performed by imposing small compressive deformations with small, hard spheres. Specimens of the same sheaths were paraffin embedded, sectioned at 10 micron thickness, and stained with van Gieson's stain for anatomical correlation.

RESULTS

Mean (± standard error of the mean, SEM) tensile Young's modulus of the inner sheath at 19.8 ± 1.6 MPa significantly exceeded that for OL at 9.7 ± 1.2 MPa; the whole sheath showed intermediate modulus of 15.4 ± 1.1 MPa. Under compression, the inner sheath was stiffer (7.9 ± 0.5 vs 5.2 ± 0.5 kPa) and more viscous (150.8 ± 10.6 vs 75.6 ± 6 kPa s) than outer sheath. The inner sheath had denser elastin fibers than outer sheath, correlating with greater stiffness.

CONCLUSIONS

We conclude that maximum tensile stiffness occurs in the elastin-rich ON sheath IL that inserts near the lamina cribrosa where tension in the sheath exerted during adduction tethering may be concentrated adjacent the ON head.

Bilaminar Structure of the Human Optic Nerve Sheath

PURPOSE/AIM

We aimed to characterize the connective tissue microanatomy, elastin abundance, and fiber orientation in the human optic nerve sheath, also known as the optic nerve dura mater, for correlation with its biomechanical properties.

MATERIALS AND METHODS

Seven whole human orbits aged 4-93 years, and five isolated human optic nerve sheaths aged 26-75 years were formalin fixed, paraffin embedded, coronally sectioned, stained by Masson trichrome and van Gieson's elastin methods, and analyzed quantitatively for elastin fiber abundance and orientation. Elastin area fraction was defined as area stained for elastin divided by total area.

RESULTS

While unilaminar in children, the adult ON sheath exhibited distinct inner and outer layers. Collagen was denser and more compact in the inner layer. Elastin area fraction was significantly greater at 6.0 ± 0.4% (standard error of mean) in the inner than outer layer at 3.6 ± 0.4% (P < 10^-5). Elastin fibers had three predominant orientations: longitudinal, diagonal, and circumferential. Of circumferential fibers, 63 ± 4.7% were in the inner and 37 ± 4.7% in the outer layer (P < 10^-4). Longitudinal and diagonal fibers were uniformly distributed in both layers. Elastin density and sheath thickness increased significantly with age (P < .01).

CONCLUSIONS

The adult human optic nerve sheath is bilaminar, with each layer containing elastin fibers oriented in multiple directions consistent with isotropic properties. Differences in laminar elastin density and orientation may reflect greater tensile loading in the inner than in the outer layer.

Optic Nerve Traction During Adduction in Open Angle Glaucoma with Normal versus Elevated Intraocular Pressure

Purpose/Aim: We used magnetic resonance imaging (MRI) to investigate effects of intraocular pressure (IOP), race, and other factors on optic nerve (ON) traction in adduction, a phenomenon proposed as neuropathic in open angle glaucoma (OAG).Materials and Methods: Thirty-five patients with OAG (26 with maximal untreated IOP ≤21 mmHg, 9 with IOP >21mmHg) and 48 controls underwent axial and quasi-coronal MRI in central gaze and large (27-33°) abduction and adduction. Some underwent MRI at smaller ductions (21-28°). Effects of presence vs. absence of OAG; within OAG whether maximum IOP level was ≤21 mmHg vs. >21 mmHg; adduction angle; race; age; and gender on ON path length and globe translation were analyzed using generalized estimating equations to account for possible intereye correlations of individual subjects.Results: Average visual field mean deviation (±standard error of mean, SEM) was -8.2 ± 1.2 dB in OAG with normal IOP, and -6.1 ± 1.4 in high IOP. In central gaze, ON path in OAG was significantly more redundant than in controls but in both groups the ON became significantly and almost equally straighter in small (~21°) or large (~27°) adduction than in central gaze. With progressive adduction only, globes retracted in OAG (P < 0.005) but not in controls; this was only weakly related to globe size and not to IOP elevation. Globe retraction in adduction was significant only in OAG, and in that group was significantly greater in Asian than white patients (P < 0.02).Conclusions: Although ON tethering in adduction is normal, progressive adduction is associated with abnormal globe retraction in OAG regardless of IOP level. This phenomenon is more prominent in Asians who have OAG. Traction in adduction may cause repetitive strain injury to the ON and peripapillary sclera, thus contributing to the optic neuropathy of glaucoma independent of IOP.

Compression of the Choroid by Horizontal Duction

Purpose

The optic nerve becomes tethered in adduction in most people, which deforms the disc. We investigated the effect of horizontal ocular duction and subject age on choroidal volume at the macular side of the optic disc.

Methods

In 25 younger (18-33 years) and 15 older (50-73 years) normal subjects, the disc and the peripapillary choroid were imaged with optical coherence tomography (OCT) in central gaze and 35° adduction and abduction. The choroid temporal to the optic disc underlying the region between the Bruch's membrane opening and fovea was segmented into regions that were multiples of the disc radius for determination of local choroidal thickness. Regional volume changes from central gaze were determined in adduction and abduction.

Results

In adduction, regional choroidal volume decreased by 42.4 ± 3.4 nanoliters (nL) (standard error of the mean) in younger (P < 0.0001) and 6.2 ± 2.6 nL in older (P < 0.02) subjects. Relative volume reduction in adduction was 7.5% ± 0.6% in younger (P < 0.001) and 1.3% ± 0.6% in older (P < 0.02) subjects. Volume reduction was greatest near the disc and significant up to three disc radii from it in younger and 1 radius in older subjects but was insignificant in abduction.

Conclusions

Horizontal duction compresses the temporal peripapillary choroid, more in adduction than in abduction and more in younger than older subjects. This reflects duction-related peripapillary tissue deformation probably related, at least in part, to optic nerve tethering in adduction.

Age-dependent Deformation of the Optic Nerve Head and Peripapillary Retina by Horizontal Duction

PURPOSE

To study effects of age and horizontal duction on deformation of the optic nerve head (ONH) and peripapillary retina (PPR), as reflected by displacement of vascular landmarks, to explore the influence of adduction tethering.

DESIGN

Cross-sectional study.

METHODS

Setting: University.

STUDY POPULATION

Single eyes of 20 healthy young adults (average age 23.9 ± 3.9 [SD] years) were compared to 20 older subjects (average age 61.4 ± 9.3 years). Observational Procedure: The disc and PPR were imaged by scanning laser ophthalmoscopy in central gaze and at 35 degrees abduction and adduction.

MAIN OUTCOME MEASURE

Deformations of the disc and adjacent PPR were measured by comparing positions of epipapillary and epiretinal blood vessels.

RESULTS

Vessels within the ONH of younger subjects shifted temporally during adduction and nasally during abduction. Displacement of the nasal hemi-disc in adduction was greater at 38.5 ± 1.7 μm (standard error of mean) than the temporal half at 4.1 ± 2.1 μm (P < .001). PPR within 1 radius of the disc margin underwent 7.6 ± 1.6 μm average temporal displacement in adduction in young subjects. In abduction, the young temporal hemi-disc shifted 4.4 ± 0.6 μm nasally without significant displacement in the nasal half. Older subjects' ONH showed less temporal shift and less displacement in the PPR within 1 disc radius (P < .0001) in adduction; the nasal hemi-disc shifted 24.5 ± 1.3 μm compared with 4.4 ± 2.1 μm in the temporal half. There were no significant deformations of the disc during abduction by older subjects.

CONCLUSION

Large horizontal duction, particularly adduction, deforms the disc and peripapillary vasculature. This deformation, which is larger in younger than older subjects, may be due to optic nerve tethering in adduction.

Non-commutative, nonlinear, and non-analytic aspects of the ocular motor plant

The ocular motor plant, consisting of the globe, extraocular muscles (EOMs), and connective tissue suspension, constitutes an intricate and non-linear actuator of eye movements. The pulley system of the rectus EOMs constitutes a non-linear inner gimbal actuated by the orbital layers of these EOMs that renders the sequence of ocular rotations effectively commutative to the central controller, and can be rotated by the outer gimbal driven by the oblique EOMs. Optic nerve (ON) length is insufficient to permit large angle adduction without tethering by the ON and sheath, creating at and beyond this threshold a large additional load on the medial rectus muscle. Finite element modeling suggests that adduction may eventually cause repetitive strain injury to the ON and glaucomatous optic nerve damage.

Functional anatomy of extraocular muscles during human vergence compensation of horizontal heterophoria

We employed magnetic resonance imaging to quantify human extraocular muscle (EOM) contractility during intermittent convergent and divergent strabismus with each eye viewing monocularly at 20 cm compared with centered target fusion. Contractility, indicated by posterior partial volume change, was analyzed in transverse rectus and in medial and lateral superior oblique (SO) muscle compartments. In five subjects with intermittent esotropia, abduction of the deviated eye to monocular target fixation was associated with significant whole lateral rectus (LR) contraction, but with medial rectus (MR) relaxation that was significantly greater in the superior than inferior compartment. Esotropic eye abduction to binocular fusion was associated with similar relaxation in the two MR compartments, but with greater contraction in the LR's superior than inferior compartment. The whole diverging eye SO muscle relaxed. In three subjects with intermittent exotropia, converging eye fusional adduction was associated with significant whole LR relaxation and with MR contraction attributable to significantly greater contraction in the superior than inferior compartment. In adduction of the exotropic eye to monocular target fixation but not fusional adduction, the whole SO exhibited significant relaxation. Rectus pulley positions were not significantly altered by fusion of either form of intermittent strabismus. Globe rotational axis was eccentric in intermittent strabismus, rolling the eye so that rectus EOM lever arms facilitated vergence. These results confirm, and extend to fusion of intermittent horizontal strabismus, differential compartmental function in horizontal rectus EOMs and suggest a novel role for the SO in compensation of both intermittent esotropia and exotropia. NEW & NOTEWORTHY Disjunctive eye movements normally permit binocular fixation in near visual space but also compensate for mechanical imbalances in binocular alignment developing over the life span. Magnetic resonance imaging of the extraocular muscles demonstrates important differential function in muscle compartments during compensation of large-angle intermittent convergent and divergent strabismus in humans. Eye translation during rotation also enhances vergence compensation of intermittent strabismus.

Optic Nerve Sheath Tethering in Adduction Occurs in Esotropia and Hypertropia, But Not in Exotropia

Purpose

Repetitive strain to the optic nerve (ON) due to tethering in adduction has been recently proposed as an intraocular pressure-independent mechanism of optic neuropathy in primary open-angle glaucoma. Since strabismus may alter adduction, we investigated whether gaze-related ON straightening and associated globe translation differ in horizontal and vertical strabismus.

Methods

High-resolution orbital magnetic resonance imaging was obtained in 2-mm thick quasi-coronal planes using surface coils in 25 subjects (49 orbits) with esotropia (ET, 19 ± 3.6Δ SEM), 11 (15 orbits) with exotropia (XT, 33.7 ± 7.3Δ), 7 (12 orbits) with hypertropia (HT, 14.6 ± 3.2Δ), and 31 normal controls (62 orbits) in target-controlled central gaze, and in maximum attainable abduction and adduction. Area centroids were used to determine ON path sinuosity and globe positions.

Results

Adduction angles achieved in ET (30.6° ± 0.9°) and HT (27.2° ± 2.3°) did not significantly differ from normal (28.3° ± 0.7°), but significantly less adduction was achieved in XT (19.0° ± 2.5°, P = 0.005). ON sheath tethering in adduction occurred in ET and HT similarly to normal, but did not in XT. The globe translated significantly less than normal, nasally in adduction in XT and temporally in abduction in ET and HT (P < 0.02, for all). Globe retraction did not occur during abduction or adduction in any group.

Conclusions

Similar to normal subjects, the ON and sheath become tethered without globe retraction in ET and HT. In XT, adduction tethering does not occur, possibly due to limited adduction angle. Thus, therapeutic limitation of adduction could be considered as a possible treatment for ON sheath tethering.

Lycium barbarum polysaccharides alleviate hydrogen peroxide-induced injury by up-regulation of miR-4295 in human trabecular meshwork cells

Glaucoma is a chronic neurodegenerative disease which produces damage to the optic nerve and causes sightlessness. Current remains lack of effective method for glaucoma. Lycium barbarum polysaccharides (LBPs) have pleiotropic effects on various diseases. However, the effect of LBPs on glaucoma remains unclear. The study aimed to clarify the protective effect of LBPs against hydrogen peroxide (H₂O₂)-induced oxidative damage in human trabecular meshwork (HTM) cells. HTM cells were exposed to H₂O₂ (0-400 μM) for 24 h to construct an oxidative damage model. Then, the different concentrations of LBPs (0-500 μg mL^-1) were used to pre-treated HTM cells, and cell viability, apoptosis, protein levels of pro-/cleaved-caspase-3 and pro-/cleaved-caspase-9, and reactive oxygen species (ROS) generations were detected. MicroRNA (miR)-4295 inhibitor and its control were transfected into HTM cells, and the biological functions of miR-4295 were assessed in H₂O₂ and LBPs treated cells. Phosphatidylinositol 3-kinase (PI3K)/protein Kinase B (AKT) and extracellular regulated protein kinases (ERK) pathways were determined by western blot assay. LBPs significantly promoted cell viability, reduced apoptosis, declined cleaved-caspase-3/-9 and ROS level in HTM cells after H₂O₂ administration. MiR-4295 expression was up-regulated in H₂O₂ and LBPs treated cells. The protective effect of LBPs on H₂O₂-injured HTM cells was obviously reversed by miR-4295 inhibition. LBPs activated PI3K/AKT and ERK signaling pathways through up-regulation of miR-4295 in H₂O₂-injured HTM cells. These data demonstrated that LBPs alleviated H₂O₂-induced injury by up-regulation of miR-4295 in HTM cells, indicating the protective effect of LBPs on HTM cells against oxidative damage.

Pluripotent Stem Cell-Based Approaches to Explore and Treat Optic Neuropathies

Sight is a major sense for human and visual impairment profoundly affects quality of life, especially retinal degenerative diseases which are the leading cause of irreversible blindness worldwide. As for other neurodegenerative disorders, almost all retinal dystrophies are characterized by the specific loss of one or two cell types, such as retinal ganglion cells, photoreceptor cells, or retinal pigmented epithelial cells. This feature is a critical point when dealing with cell replacement strategies considering that the preservation of other cell types and retinal circuitry is a prerequisite. Retinal ganglion cells are particularly vulnerable to degenerative process and glaucoma, the most common optic neuropathy, is a frequent retinal dystrophy. Cell replacement has been proposed as a potential approach to take on the challenge of visual restoration, but its application to optic neuropathies is particularly challenging. Many obstacles need to be overcome before any clinical application. Beyond their survival and differentiation, engrafted cells have to reconnect with both upstream synaptic retinal cell partners and specific targets in the brain. To date, reconnection of retinal ganglion cells with distal central targets appears unrealistic since central nervous system is refractory to regenerative processes. Significant progress on the understanding of molecular mechanisms that prevent central nervous system regeneration offer hope to overcome this obstacle in the future. At the same time, emergence of reprogramming of human somatic cells into pluripotent stem cells has facilitated both the generation of new source of cells with therapeutic potential and the development of innovative methods for the generation of transplantable cells. In this review, we discuss the feasibility of stem cell-based strategies applied to retinal ganglion cells and optic nerve impairment. We present the different strategies for the generation, characterization and the delivery of transplantable retinal ganglion cells derived from pluripotent stem cells. The relevance of pluripotent stem cell-derived retinal organoid and retinal ganglion cells for disease modeling or drug screening will be also introduced in the context of optic neuropathies.

The Globe's Eccentric Rotational Axis: Why Medial Rectus Surgery Is More Potent than Lateral Rectus Surgery

PURPOSE

Tables typically recommend greater lateral rectus (LR) than medial rectus (MR) surgical doses for horizontal strabismus of any given magnitude, a difference unexplained by mechanical models that assume globe rotation about its center. We tested this assumption during horizontal ductions.

DESIGN

Prospective observational study.

PARTICIPANTS

Eighteen adult subjects with normal binocular vision.

METHODS

Surface coil magnetic resonance imaging at 390 or 430 μm resolution was obtained using 2-mm-thick contiguous axial planes while subjects fixated targets in central, right, and left gaze. Angular displacements of lines connecting the corneal apex through the minor lens axis to the retina were measured to approximate clinical ductions. Globe centers were calculated from their area centroids. Apparent lens and globe-optic nerve (ON) junction rotations around the globe center were then compared with clinical ductions.

MAIN OUTCOME MEASURES

Apparent angular rotations of lenses and globe-ON junctions during horizontal ductions.

RESULTS

Globe-ON junctions appeared to rotate significantly less around globe centers than did lenses for abduction (20.6°±4.7° vs. 27.4°±7.4°, ± standard deviation (SD), P < 0.001) and adduction (25.3°±6.7° vs. 31.9°±8.3°, P < 0.001). Both rotations differed significantly from clinical adduction (27.9°±8.3°, P < 0.007), but only in abduction was globe-ON junction rotation significantly less than clinical abduction (28.6°±9.4°, P < 0.001). The true geometric globe rotational center was 2.2±0.5 mm medial and 0.8±1.0 mm posterior to the geometric globe center and was displaced farther medially and posteriorly during adduction. This eccentricity imbues each millimeter of MR recession with approximately 30% more trigonometric rotational effect than equivalent LR recession.

CONCLUSIONS

The medial and posterior eccentricities of the normal ocular rotational axis profoundly influence horizontal rectus action. The proximity of the globe's rotational axis to the MR shortens its lever arm relative to the LR, explaining why mechanical effects of smaller MR recessions are equivalent to larger LR recessions.

Knobby Eye Syndrome

INTRODUCTION

A spherical globe is traditionally assumed, but this study employed magnetic resonance imaging (MRI) to demonstrate frequent occurrence of non-spherical staphylomata in strabismic patients.

METHODS

High-resolution, surface coil MRI was obtained in multiple image planes in 21 highly myopic subjects (36 eyes) and compared with 17 normal controls (33 eyes). Images were analyzed for axial length, aspect ratio of eye shape, and deflection of muscle paths.

RESULTS

All but two high myopes had strabismus. While myopic globes were generally spherical in 10 myopic eyes including both orthotropic subjects, 15 globes exhibited diffuse posterior staphylomata, 16 equatorial staphylomata, and 4 both posterior and equatorial staphylomata. Equatorial scleral ectasias were positioned to contact and elongate paths of horizontal rectus muscles in some gaze positions. Axial length in myopes averaged 28.8 ± 3.8 (SD) mm and did not differ significantly between regular vs. irregular staphylomata. Globe aspect ratios in the coronal, axial, and sagittal planes were significantly greater than normal in myopes (P < 0.005), but correlated significantly with axial length only in the axial and sagittal planes (P < 0.03). While myopes with irregular staphylomata were older at 57 ± 11 years than subjects with spherical globes at 24 ± 8 years (P < 0.0005), other clinical features were similar.

CONCLUSION

Irregular equatorial or posterior staphylomata are common in strabismic axial high myopes, acting, like "cams" affixed to the normally spherical globe so that they may have no mechanical effect until rotating eccentrically against muscles. After rotational contact, staphylomata would nonlinearly increase muscle tension with further duction. Imaging may be clinically informative about this "knobby eye syndrome."

Progressive Deformation of the Optic Nerve Head and Peripapillary Structures by Graded Horizontal Duction

Purpose

We investigated the effect of graded range of horizontal duction on the shape of the peripapillary Bruch's membrane (ppBM) and optic nerve head (ONH).

Methods

In 50 eyes of 25 normal subjects, the ONH and peripapillary retina were imaged by optical coherence tomography (OCT) in central gaze and incremental angles of add- and abduction. Displacements of the Bruch's membrane opening (BMO), optic cup (OC), and change in ONH angle in eccentric gazes were compared to those of central gaze, in add- and abduction.

Results

With increasing duction, the nasal edge of the BMO (nBMO) shifted progressively anteriorly in adduction and posteriorly in abduction, while the temporal edge of the BMO (tBMO) shifted posteriorly in adduction and anteriorly in abduction. The summed absolute nBMO and tBMO displacements in 30° and 35° adduction significantly exceeded those in comparable abduction angles (P < 0.005 for both). The ONH progressively tilted temporally in adduction and nasally in abduction; absolute ONH tilt in adduction was significantly greater than that in abduction for 30° and 35° ductions (P < 0.005 for both). BMO displacement and ONH tilt in adduction exhibited bilinear behavior, with greater effects for both at angles exceeding 26°. The OC shifted significantly farther anteriorly in abduction than adduction at every angle from 10° to 35°.

Conclusions

Horizontal duction deforms the ONH and ppBM, but more in adduction than in abduction, and increasingly so for angles greater than 26°. This behavior is consistent with optic nerve sheath tethering for adduction exceeding 26°.