Optic disc morphology--rethinking shape

Optical Coherence Tomography-Derived Measurements of the Optic Nerve Head Structure of Aboriginal and Torres Strait Islander Children

PRCIS

This study demonstrated significant differences in optic nerve head characteristics in Aboriginal and Torres Strait Islander children compared with non-Indigenous children, which has implications for glaucoma risk and diagnosis in Aboriginal and Torres Strait Islander populations.

PURPOSE

The purpose of this study was to examine the optic nerve head (ONH) characteristics of visually normal Aboriginal and Torres Strait Islander children and non-Indigenous Australian children.

MATERIALS AND METHODS

Spectral domain optical coherence tomography imaging was performed on the right eye of 95 Aboriginal and Torres Strait Islander children and 149 non-Indigenous Australian children (5-18 years). Horizontal and vertical line scans, centered on the ONH, were analyzed to determine the dimensions of the ONH (Bruch membrane opening diameter), optic cup diameter, Bruch membrane opening minimum rim width, and the peripapillary retinal nerve fiber layer thickness.

RESULTS

The vertical but not horizontal Bruch membrane opening diameter of Aboriginal and Torres Strait Islander children was significantly larger than non-Indigenous children (mean difference: 0.09 mm, P = 0.001). The horizontal (mean difference: 0.12 mm, P = 0.003) and vertical cup diameter (mean difference: 0.16 mm, P < 0.001) were also significantly larger in Aboriginal and Torres Strait Islander children, as were the horizontal and vertical cup-to-disc ratios (both P < 0.01). Aboriginal and Torres Strait Islander children also had a significantly thinner Bruch membrane opening minimum rim width in the superior, nasal, and temporal meridians (all P < 0.001). Peripapillary retinal nerve fiber layer thickness did not differ between groups.

CONCLUSIONS

Differences exist in the ONH structure between Aboriginal and Torres Strait Islander children and non-Indigenous children, which may have implications for the detection and monitoring of ocular disease in this population and highlights the need to extend this research to the adult population.

Myopia: Histology, clinical features, and potential implications for the etiology of axial elongation

Myopic axial elongation is associated with various non-pathological changes. These include a decrease in photoreceptor cell and retinal pigment epithelium (RPE) cell density and retinal layer thickness, mainly in the retro-equatorial to equatorial regions; choroidal and scleral thinning pronounced at the posterior pole and least marked at the ora serrata; and a shift in Bruch's membrane opening (BMO) occurring in moderately myopic eyes and typically in the temporal/inferior direction. The BMO shift leads to an overhang of Bruch's membrane (BM) into the nasal intrapapillary compartment and BM absence in the temporal region (i.e., parapapillary gamma zone), optic disc ovalization due to shortening of the ophthalmoscopically visible horizontal disc diameter, fovea-optic disc distance elongation, reduction in angle kappa, and straightening/stretching of the papillomacular retinal blood vessels and retinal nerve fibers. Highly myopic eyes additionally show an enlargement of all layers of the optic nerve canal, elongation and thinning of the lamina cribrosa, peripapillary scleral flange (i.e., parapapillary delta zone) and peripapillary choroidal border tissue, and development of circular parapapillary beta, gamma, and delta zone. Pathological features of high myopia include development of macular linear RPE defects (lacquer cracks), which widen to round RPE defects (patchy atrophies) with central BM defects, macular neovascularization, myopic macular retinoschisis, and glaucomatous/glaucoma-like and non-glaucomatous optic neuropathy. BM thickness is unrelated to axial length. Including the change in eye shape from a sphere in emmetropia to a prolate (rotational) ellipsoid in myopia, the features may be explained by a primary BM enlargement in the retro-equatorial/equatorial region leading to axial elongation.

Geometric Morphometrics Can Predict Postoperative Visual Acuity Changes in Patients With Epiretinal Membrane: A Retrospective Study

Purpose

To investigate the efficacy of the geometric morphometrics method for the evaluation of retinal deformation in patients with epiretinal membrane (ERM) and determine whether the degree of deformation can serve as a predictive factor for postoperative visual outcome.

Methods

We retrospectively evaluated data from 29 eyes of 29 patients with primary ERM. Preoperative optical coherence tomography images were compared with images of their normal fellow eyes using the geometric morphometrics thin-plate spline technique. Conventional parameters such as retinal layer thickness and previously reported indices were also measured. The correlation between the preoperative parameters and visual acuity was evaluated. Statistical comparisons were performed using a paired t-test, and associations between the optical coherence tomography image parameters and visual acuity were determined using Spearman's rank correlation coefficient.

Results

Bending energy, which was calculated using geometric morphometrics, was significantly associated with visual acuity as well as conventional optical coherence tomography parameters and previously reported indices. Multiple regression analysis showed that bending energy was an independent predictive factor for postoperative visual acuity changes.

Conclusions

The geometric morphometrics method is an effective approach for evaluating the severity of ERM and predicting the efficacy of surgery.

Translational Relevance

Geometric morphometrics can effectively evaluate retinal deformation in eyes with epiretinal membrane.

Optic disc area frequency distribution in a large sample of retinographic images

Objective

To describe a new method to estimate the frequency distribution of optic nerve disc area, using digital retinographic images.

Methods and analysis

We analysed 492 023 fundus images obtained with seven fundus cameras, mainly in Caucasian subjects. They were grouped by resolution and zoom. They were automatically segmented by identifying the inner edge of the Elschnig scleral ring. For this purpose, a neural network trained by deep learning previously described was used. The number of pixels contained within the segmentation and their frequency distribution were calculated. The results of each camera, using different number of images, were compared with the global results using the Kolmogorov-Smirnov test to confront frequency distributions.

Results

The frequency distribution was non-Gaussian, more limited in small sizes than in large ones. If the median is assigned a theoretical value of 1.95 mm2, the 1th, 5th, 25th, 50th, 75th, 95th and 99th percentiles would correspond to 1.29, 1.46, 1.73, 1.95, 2.20, 2.64 and 3.03 mm2 in all the dataset. The overall differences were significant for the smaller series, but for each percentile their mean value was only 0.01 mm2 and the maximum 0.10 mm2, so they can be considered similar for practical purposes in all cameras.

Conclusion

By automatically segmenting the edges of the optic nerve and observing the frequency distribution of the number of pixels it delimits, it is possible to estimate the frequency distribution of the disc area in the population as a whole and that of each individual case.

Optic nerve head anatomy in myopia and glaucoma, including parapapillary zones alpha, beta, gamma and delta: Histology and clinical features

The optic nerve head can morphologically be differentiated into the optic disc with the lamina cribrosa as its basis, and the parapapillary region with zones alpha (irregular pigmentation due to irregularities of the retinal pigment epithelium (RPE) and peripheral location), beta zone (complete RPE loss while Bruch's membrane (BM) is present), gamma zone (absence of BM), and delta zone (elongated and thinned peripapillary scleral flange) within gamma zone and located at the peripapillary ring. Alpha zone is present in almost all eyes. Beta zone is associated with glaucoma and may develop due to a IOP rise-dependent parapapillary up-piling of RPE. Gamma zone may develop due to a shift of the non-enlarged BM opening (BMO) in moderate myopia, while in highly myopic eyes, the BMO enlarges and a circular gamma zone and delta zone develop. The ophthalmoscopic shape and size of the optic disc is markedly influenced by a myopic shift of BMO, usually into the temporal direction, leading to a BM overhanging into the intrapapillary compartment at the nasal disc border, a secondary lack of BM in the temporal parapapillary region (leading to gamma zone in non-highly myopic eyes), and an ocular optic nerve canal running obliquely from centrally posteriorly to nasally anteriorly. In highly myopic eyes (cut-off for high myopia at approximately -8 diopters or an axial length of 26.5 mm), the optic disc area enlarges, the lamina cribrosa thus enlarges in area and decreases in thickness, and the BMO increases, leading to a circular gamma zone and delta zone in highly myopic eyes.

Mechanical Deformation of Human Optic Nerve Head and Peripapillary Tissue in Response to Acute IOP Elevation

Purpose

To measure the deformation of the human optic nerve head (ONH) and peripapillary tissue (PPT) in response to acute intraocular pressure (IOP) elevation.

Methods

The ONH and PPT of 14 human donor globes were imaged with high-frequency ultrasonography during inflation testing from 5 to 30 mm Hg. A correlation-based speckle tracking algorithm was used to compute tissue displacements, and the through-thickness, in-plane, and shear strains were calculated by using least-squares strain estimation methods. The ONH and PPT were segmented along the anterior-posterior direction and the nasal-temporal direction. Regional displacements and strains were analyzed and compared.

Results

The ONH displaced more posteriorly than the PPT in response to an acute IOP increase. Scleral canal expansion was minimal but correlated with ONH posterior displacement at all IOP levels. Through-thickness compression was concentrated in the anterior of both the ONH and the PPT. Shear was concentrated in the vicinity of the canal with higher shear in the peripheral ONH than the central ONH and higher shear in the PPT near the scleral canal than that further away from the canal.

Conclusions

High-resolution ultrasound speckle tracking showed a displacement mismatch between the ONH and the PPT, larger compressive strains in the direction of IOP loading in the anterior ONH and PPT, and higher shear strains in the periphery of ONH in response to acute IOP elevation in the human eye. These findings delineate the deformation patterns within and around the ONH and may help understand IOP-associated optic nerve damage.

Gaze-Evoked Deformations in Optic Nerve Head Drusen: Repetitive Shearing as a Potential Factor in the Visual and Vascular Complications

PURPOSE

To determine if ocular ductions deform intrapapillary and peripapillary tissues in optic nerve head drusen (ONHD) and to compare these deformations with healthy eyes and eyes with other optic neuropathies.

DESIGN

Observational case series.

PARTICIPANTS

Twenty patients with ONHD.

METHODS

Axial rasters of the optic nerve from a spectral-domain OCT device (Cirrus 5000; Carl Zeiss Meditec, Inc, Dublin, CA) were used to analyze the shape of the peripapillary basement membrane (ppBM) layer in 20 confirmed cases of ONHD. We compared registered images obtained from 2 eye positions: 10° to 15° in adduction and 30° to 40° in abduction. Geometric morphometrics was used to analyze the shape of the ppBM layer defined by placing 10 equidistant landmarks extending 2500 μm on both sides of the basement membrane opening. We also adapted an image strain tracking technique to measure regional intrapapillary strains in 6 patients. Using manually placed nodes on the reference image (in adduction), an iterative, block-matching algorithm is used to determine local displacements between the reference and its paired image in abduction. Displacement vectors were used to calculate the mean shear and effective strain (percent change).

MAIN OUTCOME MEASURES

Peripapillary shape deformations, intrapapillary shear strains, and effective strains.

RESULTS

We found a statistically significant difference in the shape of the ppBM layer between abduction and adduction (P < 0.01). The deformation was characterized by a relative posterior displacement temporally in adduction that reversed in abduction. Strain tracking in all 6 patients showed substantial gaze-induced shearing and effective strains. Mean effective strains were 7.5% outside the drusen. Shear and effective strains were significantly larger outside versus within the drusen (P < 0.003 and P < 0.01, respectively).

CONCLUSIONS

This study demonstrates that horizontal ocular ductions induce significant shearing deformations of the peripapillary retina and prelaminar intrapapillary tissues. We also found that the deformations in healthy persons are similar in magnitude to ONHD. Based on these findings, we speculate that patients with intrapapillary calcifications exposed to the long-term effects of repetitive shearing (induced by ocular ductions) may contribute to the progressive axonal loss and vascular complications associated with ONHD.

Heritability of the morphology of optic nerve head and surrounding structures: The Healthy Twin Study

Background

Optic nerve head (ONH) and surrounding structures such as β-zone peripapillary atrophy (PPA) are important structures in glaucomatous pathogenesis. Thus, for understanding genetic components in glaucoma pathogenesis, it is necessary to evaluate the heritability of ONH and surrounding structures. The present study investigated the genetic influences on ONH and surrounding structures such as β-zone PPA and retinal vessels.

Methods

A total of 1,205 adult twins and their family members (362 monozygotic (MZ) twin subjects (181 pairs), 64 dizygotic (DZ) twin subjects (32 pairs), and 779 singletons from 261 families), were part of the Korean Healthy Twin Study. ONH parameters including the vertical cup-to-disc ratio, the presence, the area and the location of β-zone PPA and the angular location of retinal vein were measured. The genetic influences on the structures were evaluated using variance-component methods.

Results

The intraclass correlation coefficient (ICC) values of axial length were highest among the parameters. The ICCs of the area and location of PPA were similar to those of vertical cup-to-disc ratio. However, retinal vessel angular locations showed low ICC values even in MZ twins. After age and sex adjustment, for axial length, vertical cup-to-disc ratio, the presence, area and location of PPA, the estimated narrow-sense heritability was 0.85, 0.48, 0.76, 0.50 and 0.65 in the right eye and 0.84, 0.47, 0.72, 0.46 and 0.72 in the left eye, respectively. The estimated narrow-sense heritability of angular location of the superior and inferior vein was 0.17 and 0.12 in the right eye and 0.13 and 0.05 in the left eye, respectively.

Conclusions

ONH and surrounding structures such as vertical cup-to-disc ratio and the presence, the area and the location of β-zone PPA seemed to be determined by the substantial genetic influence, whereas the venous angular location did not.

Morphometric parameters of the optic disc in normal and glaucomatous eyes based on time-domain optical coherence tomography image analysis

BACKGROUND AND OBJECTIVE

Assessment of optic disc morphology is essential in diagnosis and management of visual impairment. The aim of this study was to evaluate associations between optic disc morphometric parameters, i.e., size and shape, and age, gender, and ocular axial length in normal and glaucomatous eyes based on time-domain optical coherence tomography image analysis.

MATERIALS AND METHODS

It was a case-control study of 998 normal and 394 eyes with primary open angle glaucoma that underwent an ophthalmological examination and time-domain optical coherence topography scanning. Areas and shapes of the disc, cup, and neuroretinal rim were analyzed.

RESULTS

The shape of the optic disc did not differ between the study groups, i.e., normal and glaucomatous case groups, but the disc area of the primary open angle glaucoma group was significantly larger. The shape of the small disc was significantly different, but the shape of the medium and the large disc did not differ between the study groups. The central area of the disc, i.e., cup area was significantly larger in the case group and its shape was significantly different between the study groups. No significant differences in the area of the cup and its shape, nerve fibers on the edge of the disc, i.e., neuroretinal rim area, were found between the study groups of the small discs. There were significant associations between age, gender, and ocular axial length and morphometric parameters of the optic disc.

CONCLUSIONS

Informative results with regard to the size and shape due to various ocular characteristics between the healthy control group and patients suffering with primary open angle glaucoma were obtained. Both study groups were significant in size, which makes the findings interesting and important contribution in the field.

Stereo Photo Measured ONH Shape Predicts Development of POAG in Subjects With Ocular Hypertension

PURPOSE

To identify objective, quantitative optic nerve head (ONH) structural features and model the contributions of glaucoma.

METHODS

Baseline stereoscopic optic disc images of 1635 glaucoma-free participants at risk for developing primary open-angle glaucoma (POAG) were collected as part of the Ocular Hypertension Treatment Study. A stereo correspondence algorithm designed for fundus images was applied to extract the three-dimensional (3D) information about the ONH. Principal component analysis was used to identify ONH 3D structural features and the contributions of demographic features, clinical variables, and disease were modeled using linear regression and linear component analysis. The computationally identified features were evaluated based on associations with glaucoma and ability to predict which participants would develop POAG.

RESULTS

The computationally identified features were significantly associated with future POAG, POAG-related demographics (age, ethnicity), and clinical measurements (horizontal and vertical cup-to-disc ratio, central corneal thickness, and refraction). Models predicting future POAG development using the OHTS baseline data and STEP features achieved an AUC of 0.722 in cross-validation testing. This was a significant improvement over using only demographics (age, sex, and ethnicity), which had an AUC of 0.599.

CONCLUSIONS

Methods for identifying objective, quantitative measurements of 3D ONH structure were developed using a large dataset. The identified features were significantly associated with POAG and POAG-related variables. Further, these features increased predictive model accuracy in predicting future POAG. The results indicate that the computationally identified features might be useful in POAG early screening programs or as endophenotypes to investigate POAG genetics.

Optical coherence tomography shape analysis of the peripapillary retinal pigment epithelium layer in presumed optic nerve sheath meningiomas

BACKGROUND

Geometric morphometrics (GM) was used to compare the shape of the peripapillary retinal pigment epithelium-Bruch's membrane (ppRPE) layer imaged on spectral domain optical coherence tomography (SD-OCT) of patients with presumed optic nerve sheath meningiomas (pONSM) and normal subjects.

METHODS

We compared 2 groups: 30 normals to 10 patients (11 eyes) with pONSM. We digitized 20 equidistant semi-landmarks on OCT images of the ppRPE-layer, spanning 2500 μm on each side of the neural canal opening (NCO). Data were analyzed using standard GM techniques including a generalized least squares Procrustes superimposition, principal component analysis (PCA), thin-plate spline, and permutation statistical analysis to evaluate differences in shape. We also analyzed other variables with respect to shape including tumor size-proximity to the globe, age, retinal nerve fiber layer, and optic disc height.

RESULTS

All pONSM patients were female (age 37-66 years); 10 had unilateral and 1 had bilateral optic nerve involvement. Ten of the eyes had optic disc edema at presentation, 4 went on to develop shunt vessels, and 4 had optic atrophy. The ppRPE-layer bordering the NCO in normals is V-shaped pointing away from the vitreous; the ppRPE-layer in pONSM is indented causing an inverted-U shaped deformation skewed nasally toward the vitreous. PCA showed a significant difference between normals and pONSM (permutation, n = 10,000, P = 0.001). The size and proximity of the tumor to the globe correlates with the shape of the ppRPE-layer (r = 0.75, P = 0.04). Correlation between shape variables and RNFL thickening (r = 0.51), optic disc height (r = 0.67), and age (r = 0.67) were not statistically significant.

CONCLUSION

The shape of the RPE layer in pONSM is characterized by an inverted-U shape or indentation that differs significantly from normals. It is indistinguishable from the shape we previously reported in papilledema and is not caused by disc edema. The mechanism in pONSM is unknown but may involve a change in the compliance of the nerve and/or localized sequestration of cerebrospinal fluid in the distal optic nerve sheath.

Effects of lowering cerebrospinal fluid pressure on the shape of the peripapillary retina in intracranial hypertension

PURPOSE

To analyze the deformations of the peripapillary retinal pigment epithelium-basement membrane (ppRPE/BM) layer in response to procedures that lower intracranial pressure (ICP). Second, to demonstrate how shape changes may complement the mean retinal nerve fiber layer (RNFL) thickness as a measure of intracranial hypertension (ICH) and papilledema.

METHODS

We used geometric morphometrics on spectral-domain optical coherence tomography images to analyze shape change of the ppRPE/BM layer after several interventions that lower cerebrospinal fluid (CSF) pressure. We also evaluated the effects of pressure-lowering interventions on both the anterior-posterior displacement of ppRPE/BM and the mean RNFL thickness. Forty-one patients with ICH and papilledema were studied before and after lumbar puncture (20), CSF shunt (9), and medical treatment of idiopathic ICH (23). We also compared the shape of 30 normal subjects to 23 patients whose papilledema resolved after medical treatment.

RESULTS

The ppRPE/BM-layer in ICH and papilledema is characterized by an asymmetric anterior deformation that moves posteriorly and becomes more V-shaped after each pressure-lowering intervention. The differences were statistically significant for all three groups. These shape changes also occur in patients with ongoing ICH who have secondary optic atrophy (without papilledema). Posterior displacement at the margin of the ppRPE/BM layer correlated strongly with overall shape changes.

CONCLUSIONS

The subsurface contour of the ppRPE/BM layer is a dynamic property that changes with CSF pressure-lowering interventions. It can supplement the RNFL thickness as an indirect gauge of ICP and is particularly helpful in patients with secondary optic atrophy. Direct measurements of displacement at the basement membrane opening may serve as a more convenient office-based surrogate for shape analysis.

Application of Elliptic Fourier analysis to describe the lamina cribrosa shape with age and intraocular pressure

The lamina cribrosa (LC) plays an important biomechanical role in the optic nerve head (ONH). We developed a statistical shape model of the LC and tested if the shape varies with age or IOP. The ONHs of 18 donor eyes (47-91 years, mean 76 years) fixed at either 5 or 50 mmHg of IOP were sectioned, stained, and imaged under a microscope. A 3D model of each ONH was reconstructed and the outline of the vertical sagittal section closest to the geometric center of the LC extracted. The outline shape was described using Elliptic Fourier analysis, and principal components analysis (PCA) employed to identify the primary modes of LC shape variation. Linear mixed effect models were used to determine if the shape measurements were associated with age or IOP. The analysis revealed several modes of shape variation: thickness and depth directly (PC 1), or inversely (PC 2) related, and superior-inferior asymmetry (PC 3). Only PC 3 was associated with IOP, with higher IOP correlating with greater curvature of the LC superiorly compared to inferiorly. Our analysis enabled a concise and complete characterization of LC shape, revealing variations without defining them a priori. No association between LC shape and age was found for the relatively old population studied. Superior-inferior asymmetry of LC shape was associated with IOP, with more asymmetry at higher IOP. Increased IOP was not associated with LC thickness or depth.

Detecting abnormality in optic nerve head images using a feature extraction analysis

Imaging and evaluation of the optic nerve head (ONH) plays an essential part in the detection and clinical management of glaucoma. The morphological characteristics of ONHs vary greatly from person to person and this variability means it is difficult to quantify them in a standardized way. We developed and evaluated a feature extraction approach using shift-invariant wavelet packet and kernel principal component analysis to quantify the shape features in ONH images acquired by scanning laser ophthalmoscopy (Heidelberg Retina Tomograph [HRT]). The methods were developed and tested on 1996 eyes from three different clinical centers. A shape abnormality score (SAS) was developed from extracted features using a Gaussian process to identify glaucomatous abnormality. SAS can be used as a diagnostic index to quantify the overall likelihood of ONH abnormality. Maps showing areas of likely abnormality within the ONH were also derived. Diagnostic performance of the technique, as estimated by ROC analysis, was significantly better than the classification tools currently used in the HRT software - the technique offers the additional advantage of working with all images and is fully automated.

Morphometric analysis of pelvic sexual dimorphism in a contemporary Western Australian population

Requisite to routine casework involving unidentified skeletal remains is the formulation of an accurate biological profile, including sex estimation. Choice of method(s) is invariably related to preservation and by association, available bones. It is vital that the method applied affords statistical quantification of accuracy rates and predictive confidence so that evidentiary requirements for legal submission are satisfied. Achieving the latter necessitates the application of contemporary population-specific standards. This study examines skeletal pelvic dimorphism in contemporary Western Australian individuals to quantify the accuracy of using pelvic measurements to estimate sex and to formulate a series of morphometric standards. The sample comprises pelvic multi-slice computer tomography (MSCT) scans from 200 male and 200 female adults. Following 3D rendering, the 3D coordinates of 24 landmarks are acquired using OsiriX® (v.4.1.1) with 12 inter-landmark linear measurements and two angles acquired using MorphDb. Measurements are analysed using basic descriptive statistics and discriminant functions analyses employing jackknife validation of classification results. All except two linear measurements are dimorphic with sex differences explaining up to 65 % of sample variance. Transverse pelvic outlet and subpubic angle contribute most significantly to sex discrimination with accuracy rates between 100 % (complete pelvis-10 variables) and 81.2 % (ischial length). This study represents the initial forensic research into pelvic sexual dimorphism in a Western Australian population. Given these methods, we conclude that this highly dimorphic bone can be used to classify sex with a high degree of expected accuracy.

Glaucomatous optic neuropathy evaluation (GONE) project: the effect of monoscopic versus stereoscopic viewing conditions on optic nerve evaluation

PURPOSE

To determine whether monoscopic vs stereoscopic viewing impacts evaluation of optic disc photographs for glaucoma diagnosis in an expert population.

DESIGN

Prospective observational study.

METHODS

Twenty pairs of high-quality monoscopic and stereoscopic photographs of similar size and magnification (ie, 40 images), were selected to demonstrate a range of optic disc features from a total of 197 eyes of 197 patients with glaucoma and normal subjects recruited from a tertiary clinic. These were presented in randomized order via an interactive platform (http://stereo.gone-project.com/). Participants assessed 9 topographic features and estimated glaucoma likelihood for each photograph. Main outcome measures were intra- and inter-observer agreement.

RESULTS

There was good intra-observer agreement between monoscopic and stereoscopic assessments of glaucoma likelihood (κw = 0.56). There was also good to substantial agreement for peripapillary atrophy (κw = 0.65), cup shape (κw = 0.65), retinal nerve fiber layer loss (κw = 0.69), vertical cup-to-disc ratio (κw = 0.58), and disc shape (κw = 0.57). However, intra-observer agreement was only fair to moderate for disc tilt, cup depth, and disc size (κw = 0.46-0.49). Inter-observer agreement for glaucoma likelihood in monoscopic photographs (κw = 0.61, 95% confidence interval [CI] = 0.55-0.67) was substantial and not lower than in stereoscopic photographs (κw = 0.59, CI = 0.54-0.65). Monoscopic photographs did not lead to lower levels of inter-observer agreement compared to stereoscopic photographs in the assessment of any optic disc characteristics, for example disc size (mono κw = 0.65, stereo κw = 0.52) and cup-to-disc ratio (mono κw = 0.72, stereo κw = 0.62).

CONCLUSIONS

For expert observers in the evaluation of optic disc photographs for glaucoma likelihood, monoscopic optic disc photographs did not appear to represent a significant disadvantage compared to stereoscopic photographs.

A Geometric Morphometric Assessment of Hand Shape and Comparison to the 2D:4D Digit Ratio as a Marker of Sexual Dimorphism

Twin studies are extremely useful for investigating hypotheses of genetic influence on a range of behavioral and physical traits in humans. Studies of physical traits, however, are usually limited to size-related biological characteristics because it is inherently difficult to quantify the morphological counterpart – shape. In recent years, the development of geometry-preserving analytical techniques built upon multivariate statistical methodologies has produced a new discipline in biological shape analysis known as geometric morphometrics. In this study of hand shape analysis, we introduce the reader already familiar with the field of twin research to the potential utility of geometric morphometrics and demonstrate the cross-discipline applicability of methods. We also investigate and compare the efficacy of the 2D:4D ratio, a commonly used marker of sexual dimorphism, to the fully multivariate approach of shape analysis in discriminating between male and female sex. Studies of biological shape variation utilizing geometric morphometric techniques may be completed with software freely available on the Internet and time invested to master the small learning curve in concepts and theory.

Eye

The Eye chapter of the 3rd edition of Haschek and Rousseaux’s Handbook of Toxicologic Pathology brings a comprehensive description of pathological processes affecting the ocular tissues in the most commonly used laboratory animals and their correlations with human diseases of interest in toxicology. Also presented are detailed descriptions of the structure and function of the different ocular tissues, the most advanced techniques applied in the toxicological evaluation of the eye, useful animal models of human disease, and known mechanisms of ocular toxicity. The introductory sections of the chapter also feature such essential topics as ocular embryology, an overview of clinical ophthalmic evaluation, and eye-specific techniques of tissue processing.

Shape analysis of the peripapillary RPE layer in papilledema and ischemic optic neuropathy

PURPOSE

Geometric morphometrics (GM) was used to analyze the shape of the peripapillary retinal pigment epithelium-Bruch's membrane (RPE/BM) layer imaged on the SD-OCT 5-line raster in normal subjects and in patients with papilledema and ischemic optic neuropathy.

METHODS

Three groups of subjects were compared: 30 normals, 20 with anterior ischemic optic neuropathy (AION), and 25 with papilledema and intracranial hypertension. Twenty equidistant semilandmarks were digitized on OCT images of the RPE/BM layer spanning 2500 μm on each side of the neural canal opening (NCO). The data were analyzed using standard GM techniques, including a generalized least-squares Procrustes superimposition, principal component analysis, thin-plate spline (to visualize deformations), and permutation statistical analysis to evaluate differences in shape variables.

RESULTS

The RPE/BM layer in normals and AION have a characteristic V shape pointing away from the vitreous; the RPE/BM layer in papilledema has an inverted U shape, skewed nasally inward toward the vitreous. The differences were statistically significant. There was no significant difference in shapes between normals and AION. Pre- and posttreatment OCTs, in select cases of papilledema, showed that the inverted U-shaped RPE/BM moved posteriorly into a normal V shape as the papilledema resolved with weight loss or shunting.

CONCLUSIONS

The shape difference in papilledema, absent in AION, cannot be explained by disc edema alone. The difference is a consequence of both the translaminar pressure gradient and the material properties of the peripapillary sclera. GM offers a novel way of statistically assessing shape differences of the peripapillary optic nerve head.

Leaf morphology, taxonomy and geometric morphometrics: a simplified protocol for beginners

Taxonomy relies greatly on morphology to discriminate groups. Computerized geometric morphometric methods for quantitative shape analysis measure, test and visualize differences in form in a highly effective, reproducible, accurate and statistically powerful way. Plant leaves are commonly used in taxonomic analyses and are particularly suitable to landmark based geometric morphometrics. However, botanists do not yet seem to have taken advantage of this set of methods in their studies as much as zoologists have done. Using free software and an example dataset from two geographical populations of sessile oak leaves, we describe in detailed but simple terms how to: a) compute size and shape variables using Procrustes methods; b) test measurement error and the main levels of variation (population and trees) using a hierachical design; c) estimate the accuracy of group discrimination; d) repeat this estimate after controlling for the effect of size differences on shape (i.e., allometry). Measurement error was completely negligible; individual variation in leaf morphology was large and differences between trees were generally bigger than within trees; differences between the two geographic populations were small in both size and shape; despite a weak allometric trend, controlling for the effect of size on shape slighly increased discrimination accuracy. Procrustes based methods for the analysis of landmarks were highly efficient in measuring the hierarchical structure of differences in leaves and in revealing very small-scale variation. In taxonomy and many other fields of botany and biology, the application of geometric morphometrics contributes to increase scientific rigour in the description of important aspects of the phenotypic dimension of biodiversity. Easy to follow but detailed step by step example studies can promote a more extensive use of these numerical methods, as they provide an introduction to the discipline which, for many biologists, is less intimidating than the often inaccessible specialistic literature.

Retinal nerve fiber layer assessment: area versus thickness measurements from elliptical scans centered on the optic nerve

PURPOSE

An evaluation of the retinal nerve fiber layer (RNFL) provides important information on the health of the optic nerve. Standard measurements of the RNFL consider only thickness, but an accurate assessment should also consider axial length, size of the optic nerve head (ONH), blood vessel contribution, and distance of the scan from the ONH margin. In addition, although most primate ONHs are elliptical, the circular scan centered on the ONH is the mainstay in both clinical and research analyses. The purpose of this study was to evaluate thickness and area measures of RNFL cross sections when axial length and ONH shape are included.

METHODS

Circular, raster, and radial scans of left eye optic nerves were acquired from 40 normal rhesus monkeys (Macaca mulatta) using spectral domain optical coherence tomography. The disc margin was identified by manually selecting the RPE/Bruch's membrane opening and ONH border tissue. With a pixel-to-micrometer conversion computed from a three-surface schematic eye, RNFL scans were interpolated at 300 to 600 μm (50-μm increments) from the edge of the ONH. The thickness and area of the RNFL at each distance were obtained by custom programs. Blood vessels in the RNFL were selected and removed from the overall RNFL measures.

RESULTS

The average RNFL thickness decreased systematically from 149 ± 12.0 μm for scans 300 μm from the disc margin to 113 ± 7.2 μm at an eccentricity of 600 μm (P < 0.05). In contrast, the cross-sectional areas of the RNFL did not vary with scan location from the disc margin (0.85 ± 0.07 mm(2) at 300 μm compared with 0.86 ± 0.06 mm(2) at 600 μm). Blood vessels accounted for 9.3% of total RNFL thickness or area, but varied with retinal location. On average, 17.6% of the superior and 14.2% of the inferior RNFL was vascular, whereas blood vessels accounted for only 2.3% of areas of the temporal and nasal RNFL regions.

CONCLUSIONS

In nonhuman primates, with appropriate transverse scaling and ONH shape analysis, the cross-sectional area of the RNFL is independent of scan distance, up to 600 μm from the rim margin, indicating that the axonal composition changes little over this range. The results suggest that, with incorporation of transverse scaling, the RNFL cross-sectional area, rather than RNFL thickness, provides an accurate assessment of the retinal ganglion cell axonal content within the eye.

Glaucomatous optic neuropathy evaluation project: a standardized internet system for assessing skills in optic disc examination

BACKGROUND

Development of a standardized internet-based system to self-assess skills in optic disc examination for glaucoma risk assessment.

DESIGN

Prospective internet-based observational study.

PARTICIPANTS

Total of 197 participants (glaucoma subspecialists, general ophthalmologists and trainees) from 22 countries.

METHODS

Forty-two optic disc images demonstrating a range of features were selected from 2500 monoscopic disc photographs of normal and glaucomatous eyes. Images were presented to clinicians via website (http://www.gone-project.com). Participants were asked to assess nine topographic features and make a subjective assessment of glaucoma likelihood.

MAIN OUTCOME MEASURES

Inter-observer agreement using kappa (κ) or weighted kappa (κ(w) ).

RESULTS

There was substantial level of inter-observer agreement between glaucoma subspecialists for assessment of glaucoma likelihood (κ(w) = 0.63). Inter-observer agreement was high for haemorrhage (κ= 0.83) and substantial for disc size, disc shape, cup:disc ratio, peripapillary atrophy and cup shape (κ(w) = 0.59-0.68). Subspecialists had stronger inter-observer agreement for glaucoma likelihood and for most disc characteristics than did trainees: the greatest difference being the assessment for retinal nerve fibre layer loss. Analysis of individual disc answers from ophthalmology trainees showed that discs leading to lower agreement of glaucoma likelihood tend to produce lower agreement for the assessment of cup:disc ratio, cup shape, cup depth and retinal nerve fibre layer. Discs with features of moderate to deep cup or cup:disc ratio between 0.6 and 0.8 also lead to lower agreement in glaucoma likelihood.

CONCLUSIONS

This internet-based system is a readily accessible and standardized tool, for clinicians globally, that permits self-assessment and benchmarking of skills in optic disc examination.

Automated quantification of inherited phenotypes from color images: a twin study of the variability of optic nerve head shape

PURPOSE

Discovery and description of heritable optic nerve head (ONH) phenotypes have been haphazard. In this preliminary study, the authors test the hypothesis that inheritable phenotypes can be discovered and quantified computationally by estimating three-dimensional ONH shape parameters from stereo color photographs from the Twins Eye Study in Tasmania and determining how much of the variability in ONH shape is accounted for by genetic influence.

METHODS

Three-dimensional ONH shape was estimated by an automated algorithm from stereoscopic optic disc color photographs of a random sample of 172 subjects (344 eyes, 45 pairs of monozygotic [MZ] and 41 dizygotic [DZ] twins). Shape resemblances between eyes were quantified with a distance metric. The heritability of the shape resemblance was determined both through the distribution of the discongruence indices and through structural equation modeling techniques (ACE model).

RESULTS

Significantly different discongruence indices were found for MZ (1.0286; 95% CI, 0.9872-1.0701) and DZ twins (1.4218; 95% CI, 1.2631-1.5804); larger indices for DZ twins indicated that variability was substantially determined by genetic factors. The standardized variances of the A(dditive genetic), C(ommon environmental), and (nonshared) E(nvironmental) components were 0.80, 2.00 × 10(-15) and 0.20, respectively, for all OD, and 0.79, 3.24 × 10(-14), and 0.21 for all OS.

CONCLUSIONS

This preliminary study shows that quantitative phenotyping of the ONH shape from color images leads to phenotypes that can be measured and are largely under genetic control. The association of these inherited phenotypes with genotypes deserves confirmation and further study.