Anterior Segment Optical Coherence Tomography and its Clinical Applications in Glaucoma

The role of anterior segment optical coherence tomography in post-cataract surgery Descemet membrane detachment

This review seeks to evaluate anterior segment optical coherence tomography (AS-OCT) in the diagnostic procedure and management of Descemet's membrane detachment (DMD) in cataract surgery. DMD may present diagnostic challenges, particularly in pronounced corneal edema where traditional methods such as slit lamp biomicroscopy may be inadequate in evaluating the corneal layers. The role of AS-OCT in providing high-resolution images in the preoperative, intraoperative, and postoperative phases of cataract surgery is analyzed with a focus on its role in the early diagnosis of DMD and in evaluating the extent, morphology, and topographic localization of DMD allowing for immediate intervention during surgery and precise pneumodescemetopexy procedures where conservative treatment has failed. This review explores the integration of AS-OCT into the standard perioperative diagnostic workflow, highlighting its potential role in the prevention, accurate diagnosis, and prompt management of DMD, a complication of cataract surgery that, while low in incidence, can be highly disruptive when it occurs. The emerging role of artificial intelligence (AI) in AS-OCT analysis of anterior segment conditions and surgical procedures is discussed, though refinement of AI algorithms is warranted.

In Vivo Quantification of Anterior and Posterior Chamber Volumes in Mice: Implications for Aqueous Humor Dynamics

Purpose

Aqueous humor inflow rate, a key parameter influencing aqueous humor dynamics, is typically measured by fluorophotometry. Analyzing fluorophotometric data depends, inter alia, on the volume of aqueous humor in the anterior chamber but not the posterior chamber. Previous fluorophotometric studies of the aqueous inflow rate in mice have assumed the ratio of anterior:posterior volumes in mice to be similar to those in humans. Our goal was to measure anterior and posterior chamber volumes in mice to facilitate better estimates of aqueous inflow rates.

Methods

We used standard near-infrared (NIR) optical coherence tomography (OCT) and robotic visible-light OCT (vis-OCT) to visualize, reconstruct, and quantify the volumes of the anterior and posterior chambers of the mouse eye in vivo. We used histology and micro-computed tomography (CT) scans to validate relevant landmarks from ex vivo tissues and facilitate in vivo measurement.

Results

Posterior chamber volume is 1.1 times the anterior chamber volume in BALB/cAnNCrl mice, that is, the anterior chamber constitutes about 47% of the total aqueous humor volume, which is very dissimilar to the situation in humans. Anterior chamber volumes in 2-month-old BALB/cAnNCrl and C57BL6/J mice were 1.55 ± 0.36 µL (n = 10) and 2.05 ± 0.25 µL (n = 10), respectively. This implies that previous studies likely overestimated the aqueous inflow rate by approximately twofold.

Conclusions

It is necessary to reassess previously reported estimates of aqueous inflow rates and, thus, aqueous humor dynamics in the mouse. For example, we now estimate that only 0% to 15% of aqueous humor drains via the pressure-independent (unconventional) route, similar to that seen in humans and monkeys.

In vivo quantification of anterior and posterior chamber volumes in mice: implications for aqueous humor dynamics

Purpose

Aqueous humor inflow rate, a key parameter influencing aqueous humor dynamics, is typically measured by fluorophotometery. Analyzing fluorophotometric data depends, inter alia, on the volume of aqueous humor in the anterior, but not the posterior, chamber. Previous fluorophotometric studies of aqueous inflow rate in mice have assumed the ratio of anterior:posterior volumes in mice to be similar to those in humans. Our goal was to measure anterior and posterior chamber volumes in mice to facilitate better estimates of aqueous inflow rates.

Methods

We used standard near-infrared optical coherence tomography (OCT) and robotic visible-light OCT (vis-OCT) to visualize, reconstruct and quantify the volumes of the anterior and posterior chambers of the mouse eye in vivo. We used histology and micro-CT scans to validate relevant landmarks from ex vivo tissues to facilitate in vivo measurement.

Results

Posterior chamber volume is 1.1 times the anterior chamber volume in BALB/cAnNCrl mice, i.e. the anterior chamber constitutes about 47% of the total aqueous humor volume, which is very dissimilar to the situation in humans. Anterior chamber volumes in 2-month-old BALB/cAnNCrl and 7-month-old C57BL6/J mice were 1.55 ± 0.36 μL (n=10) and 2.41 ± 0.29 μL (n=8), respectively. This implies that previous studies likely over-estimated aqueous inflow rate by approximately two-fold.

Conclusions

It is necessary to reassess previously reported estimates of aqueous inflow rates, and thus aqueous humor dynamics in the mouse. For example, we now estimate that only 0-15% of aqueous humor drains via the pressure-independent (unconventional) route, similar to that seen in humans and monkeys.

Repeatability and agreement of central corneal thickness measurements with a new handheld non-contact pachymeter

PURPOSE

To compare the repeatability of central corneal thickness (CCT) measurements taken with a new handheld pachymeter (Occuity PM1 pachymeter) and to assess its agreement with ultrasound biometry and two commercially available optical biometers in participants with normal eyes.

METHODS

Three consecutive CCT measurements of the right eye of 105 participants with normal corneas were acquired by the PM1 pachymeter, Lenstar LS 900 and Oculus Pentacam HR in a random order. This was followed by three measurements with a handheld ultrasound pachymeter (UP) (Pachmate 2). Repeatability and the repeatability limit were calculated for each device and Bland-Altman limits of agreement (LoA) were determined for the PM1 pachymeter compared to the other devices.

RESULTS

The mean CCT (±SD) was 551.04 ± 33.43, 558.62 ± 31.46, 549.41 ± 31.00 and 539.73 ± 29.50 μm for the PM1 pachymeter, UP, Lenstar and Pentacam, respectively. The repeatability limits (expressed as the within subject SD for repeat measurements) were 14.02, 13.68, 4.99 and 9.90 μm, respectively. The closest agreement was between the PM1 and Lenstar (mean difference = -1.63 μm with LoA 10.72 μm below and 13.97 μm above the readings obtained with the Lenstar). The PM1 underestimated CCT compared to UP (mean difference = 7.58 μm, LoA 24.63 μm below and 9.47 μm above UP). The agreement was lowest between the PM1 and Pentacam (mean difference = -11.30 μm, LoA between 4.29 and 26.89 μm).

CONCLUSIONS

The PM1 pachymeter shows excellent precision for CCT measurements across a range of corneal thicknesses in normal eyes and provides a safe and easy-to-use alternative to ultrasound pachymetry.

[The effect of phacoemulsification of age-related cataract on intraocular pressure and iridocorneal angle parameters in primary open-angle glaucoma]

Cataract phacoemulsification with intraocular lens implantation significantly affects the morphology of the anterior chamber angle (ACA) and contributes to a reduction of intraocular pressure (IOP).

PURPOSE

This study investigates the changes in anterior chamber angle configuration and IOP level before and after cataract surgery in primary open-angle glaucoma (POAG) eyes, and assesses its possible relationship with the axial length of the eye.

MATERIAL AND METHODS

The study included 38 patients with immature senile cataract and compensated unoperated stage I-II POAG. Patients underwent anterior segment optical coherence tomography (AS-OCT) with measurement of the angle opening distance (AOD750) and trabecular-iris space area (TISA750) before and six weeks after the surgery, as well as axial length and corneal-compensated IOP (IOPcc).

RESULTS

After the surgery 92.11% of patients showed a decrease in IOPcc an average of 1.84±1.95 mm Hg (by 12.33±10.21% of the baseline); the maximum decrease in the IOPcc level was 8.35 mm Hg (by 40.97% of the baseline). AS-OCT data indicates that cataract phacoemulsification causes an increase in anterior chamber angle width: AOD750 increased from 0.510±0.175 to 0.771±0.156 mm, and TISA750 increased from 0.266±0.090 to 0.494±0.096 mm2. The strongest correlations were found between the axial length, postoperative TISA750 increase (Δ%TISA750) and postoperative IOPcc reduction (Δ% IOPcc). Postoperatively, specific changes could be observed in the trabecular meshwork in 28.95% of cases in the form of elongation and strain of the membrane.

CONCLUSIONS

Cataract extraction contributes to a significant reduction in IOP due to anterior chamber angle expansion, trabecular meshwork stretching, and improved aqueous outflow.

Two-Photon Imaging for Non-Invasive Corneal Examination

Two-photon imaging (TPI) microscopy, namely, two-photon excited fluorescence (TPEF), fluorescence lifetime imaging (FLIM), and second-harmonic generation (SHG) modalities, has emerged in the past years as a powerful tool for the examination of biological tissues. These modalities rely on different contrast mechanisms and are often used simultaneously to provide complementary information on morphology, metabolism, and structural properties of the imaged tissue. The cornea, being a transparent tissue, rich in collagen and with several cellular layers, is well-suited to be imaged by TPI microscopy. In this review, we discuss the physical principles behind TPI as well as its instrumentation. We also provide an overview of the current advances in TPI instrumentation and image analysis. We describe how TPI can be leveraged to retrieve unique information on the cornea and to complement the information provided by current clinical devices. The present state of corneal TPI is outlined. Finally, we discuss the obstacles that must be overcome and offer perspectives and outlooks to make clinical TPI of the human cornea a reality.

Evaluation of anterior segment parameters with two anterior segment optical coherence tomography systems: Visante and Casia, in primary angle closure disease

Purpose:

To determine the comparability of anterior chamber biometric measurements in primary angle closure disease (PACD) patients using two commercially available anterior segment optical coherence tomography machines (ASOCT): Visante and Casia.

Methods:

This was a cross-sectional observational study, which included clinically, diagnosed cases of PACD. Anterior segment biometric measurements were done using Casia and Visante ASOCT. Parameters studied were central corneal thickness (CCT), anterior chamber depth (ACD), nasal (N) and temporal (T) angle opening distance at 500 μm (AOD500) and 750 μm (AOD750), and N and T trabecular iris space area at 500 μm (TISA500) and 750 μm (TISA750).

Results:

Total 36 PACD patients (72 eyes) with average age of 59.48 ± 7.95 years were recruited, out of which 25 were females (69.44%) and 11 males (30.56%). The mean measurements of CCT, ACD, AOD500, and TISA on Casia and Visante machines were 522.5 ± 34.75 μm and 539.55 ± 29.56 μm (P = 0.00); ACD- 2.144 ± 0.38 mm and 2.133 ± 0.39 mm (P = 0.487); AOD500-0.27 ± 0.16 μm and 0.21 ± 0.10 μm (P = 0.04); and TISA500-0.100 ± 0.07 μm and 0.063 ± 0.03 μm (P = 0.00), respectively. A statistically significant difference was noted in CCT, N and T AOD, and TISA. A good corelation for ACD and CCT (ACD = 0.9816 and CCT = 0.772) only were noted between the two machines. The Bland-Altman plot analysis of different parameters between two machines has revealed good agreement of measurement of ACD and CCT but poor agreement for rest of the parameters.

Conclusion:

It is advisable not use the two machines interchangeably because of the wide limits of agreement and poor correlation of angle measurement values of Casia and Visante ASOCT.

Anterior Segment Optical Coherence Tomography: Applications for Clinical Care and Scientific Research

Anterior segment optical coherence tomography (AS-OCT) is a non-contact imaging technique that produces high-resolution images and quantitative measurements of the anterior segment and its anatomical structures. There has been rapid development of OCT technology over the past 2 decades, with the transition from time-domain to Fourier-domain OCT devices. By integrating these advancements in OCT technology, AS-OCT devices have evolved into versatile clinical and research tools for studies of the anterior segment and ocular surface. The primary purpose of this article was to review OCT technology and AS-OCT devices as well as applications of AS-OCT for clinical practice and scientific research. We first describe the different types of OCT technology, how they have been adapted for AS-OCT imaging, and differences between various AS-OCT devices. We then review the applications of AS-OCT for characterizing the anatomical structures of the anterior segment and aqueous outflow pathways, including the anterior chamber angle, trabecular meshwork, and Schlemm canal. We also describe glaucoma-related applications of AS-OCT imaging, which include evaluating patients for static and dynamic biometric risk factors of primary angle closure disease and assessing the efficacy of glaucoma interventions, such as laser peripheral iridotomy and glaucoma surgery. Finally, we review other clinical applications of AS-OCT imaging for detection and management of diseases of the ocular surface, cornea, and lens.

Cellular resolution corneal imaging with extended imaging range

Current optical coherence tomography (OCT) technology, which is used for imaging the eye's anterior segment, has been established as a clinical gold standard for the diagnosis of corneal diseases. However, the cellular resolution level information that is critical for many clinical applications is still not available. The major technical challenges toward cellular resolution OCT imaging are the limited ranging depth and depth of focus (DOF). In this work, we present a novel ultrahigh resolution OCT system that achieves an isotropic spatial resolution of <2 µm in tissue. The proposed system could approximately double the ranging depth and extend the DOF using the dual-spectrometer design and the forward-model based digital refocusing method, respectively. We demonstrate that the novel system is capable of visualizing the full thickness of the pig cornea over the ranging depth of 3.5 mm and the border of the corneal endothelial cells 8 times Rayleigh range away from the focal plane. This technology has the potential to realize cellular resolution corneal imaging in vivo.

To Study and Determine the Role of Anterior Segment Optical Coherence Tomography and Ultrasound Biomicroscopy in Corneal and Conjunctival Tumors

Purpose. To analyze and describe corneal and conjunctival tumor thickness and internal characteristics and extension in depth and size and shape measured by two noninvasive techniques, anterior segment optical coherence tomography (AS-OCT) and ultrasound biomicroscopy (UBM). Design. Systematic review. Methods. This systematic review is based on a comprehensive search of 4 databases (Medline, Embase, Web of Science, and Cochrane Library). Articles published between January 1, 1999, and December 31, 2015, were included. We searched for articles using the following search terms in various combinations: “optical coherence tomography”, “ultrasound biomicroscopy”, “corneal neoplasm”, “conjunctival neoplasm”, “eye”, “tumor” and “anterior segment tumors”. Inclusion criteria were as follows: UBM and/or AS-OCT was used; the study included corneal or conjunctival tumors; and the article was published in English, French, Dutch, or German. Results. There were 14 sources selected. Discussion. Several studies on the quality of AS-OCT and UBM show that these imaging techniques provide useful information about the internal features, extension, size, and shape of tumors. Yet there is no enough evidence on the advantages and disadvantages of UBM and AS-OCT in certain tumor types. Conclusion. More comparative studies are needed to investigate which imaging technique is most suitable for a certain tumor type.

A simple and non-contact optical imaging probe for evaluation of corneal diseases

Non-contact imaging techniques are preferred in ophthalmology. Corneal disease is one of the leading causes of blindness worldwide, and a possible way of detection is by analyzing the shape and optical quality of the cornea. Here, a simple and cost-effective, non-contact optical probe system is proposed and illustrated. The probe possesses high spatial resolutions and is non-dependent on coupling medium, which are significant for a clinician and patient friendly investigation. These parameters are crucial, when considering an imaging system for the objective diagnosis and management of corneal diseases. The imaging of the cornea is performed on ex vivo porcine samples and subsequently on small laboratory animals, in vivo. The clinical significance of the proposed study is validated by performing imaging of the New Zealand white rabbit's cornea infected with Pseudomonas.

Bulbar Conjunctival and Tenon's Layer Thickness Measurement using Optical Coherence Tomography

Purpose: Observations made during glaucoma filtering surgery (trabeculectomy) suggest variability in the thickness of the bulbar conjunctiva and Tenon's layers between individuals. We propose that this could infuence the final bleb morphology and function.

We designed a pilot study to assess this using optical coherence tomography (OCT) to measure bulbar conjunctival and Tenon's layer thickness.

Materials and methods: A total of 67 eyes of 48 individuals were scanned using an optovue Mode RT100 version 2.0 OCT machine. Cross-line CAM-L scans were taken and the com bined bulbar conjunctival and Tenon's layer thickness was measured 3 mm above the superior limbus. Conjunctival and Tenon's layers appeared as a hyper-refective section as opposed to the hypo refective underlying sclera. Measurements were taken using the inbuilt review software.

Results: The age ranged from 23 to 91 years. There were 20 mal e s and 28 females. The mean conjunctival and Tenon's layer thick ness was 393 ± 67 microns (mean ± SD) ranging from 194 to 573 microns.

Conclusion: Optical coherence tomography conjunctival and Tenon's layer thickness measurements appear to vary significantly between individuals. We postulate that this could infuence the final bleb morphology and may predict the risk of bleb encapsulation and failure or thin avascular blebs. Further assessment could establish cut-offs on which patients should receive intraoperative antimetabolites and/or Tenon's layer excision.

How to cite this article: Howlett J, Vahdani K, Rossiter J. Bulbar Conjunctival and Tenon's Layer Thickness Measurement using Optical Coherence Tomography. J Curr Glaucoma Pract 2014;8(2):63-66.