Primary angle closure glaucoma (PACG) is a significant cause of visual disability worldwide. It predominantly affects the Eastern and South Asian population of the world. Early detection of anatomically narrow angles is important, and the subsequent prevention of visual loss from PACG depends on an accurate assessment of the anterior chamber angle (ACA). Gonioscopy has given way to modern day imaging technologies such as ultrasound biomicroscopy (UBM) and more recently, anterior segment optical coherence tomography (AS-OCT). Ultrasound biomicroscopy provides objective, high-resolution images of anterior segment anatomy, including the cornea, iris, anterior chamber, anterior chamber angle, and ciliary body. Optical coherence tomography (OCT) is a noncontact optical signal acquisition and processing device that provides magnified, high-resolution cross-sectional images of ocular tissues. Recent technological advances towards three-dimensional visualization broadened the scope of AS-OCT in ophthalmologic evaluation. Optical coherence tomography systems use low-coherence, near-infrared light to provide detailed images of anterior segment structures at resolutions exceeding that of UBM. This paper summarizes the clinical application of UBM and OCT for assessment of anterior segment in glaucoma.
Primary angle closure glaucoma (PACG) is a leading cause of blindness worldwide [
Angle-closure glaucoma is three times as likely as open angle glaucoma (OAG) to cause blindness. Angle-closure glaucoma studies conducted in Asian countries estimate 4.3 million blind from ACG and 3.3 million from OAG and that bilateral blindness affects fewer than 10% of those with OAG but 25–30% of ACG sufferers [
Anterior segment pathologies, resulting from anatomic, structural, or mechanical abnormalities which cause apposition of the iris to the trabecular meshwork can contribute to angle-closure and thus the risk of progressive trabecular damage, elevated intraocular pressure (IOP), peripheral anterior synechiae (PAS), and acute angle-closure. Primary angle-closure glaucoma can, like other types of glaucoma, ultimately result in blindness if not diagnosed and treated timely.
The pathophysiology of angle closure can be divided into primary and secondary causes. Primary angle closure is known as pupillary block, and it accounts for more than 90% of cases. In pupillary block, the flow of aqueous from the posterior chamber, where it is produced by nonpigmented ciliary epithelium, to the anterior chamber is limited because of resistance to aqueous flow through the pupil in the region of iridolenticular contact. This limitation of flow creates an increased pressure gradient between the anterior and posterior chambers, which in turn forces the iris anteriorly and causes anterior iris bowing, narrowing of the angle, and acute/chronic or acute on chronic iridotrabecular apposition or angle closure glaucoma. In relative pupillary block, all the anatomical structures are usually normal. Secondary causes of angle closure, by contrast, can result from structural or anatomic abnormalities in the anterior or posterior segments, such as plateau iris configuration, lens subluxation, or malignant glaucoma (ciliary block or aqueous misdirection).
Clinically, the gold standard for diagnosis of narrow angles is dark-room gonioscopy, in which the iridocorneal angle and aqueous outflow through the trabecular meshwork can be assessed; however, this technique is subjective, and there are currently no standards related to gonioscopy to determine which angles require treatment.
Qualitative studies of the anterior segment structures can provide only limited information for diagnosis and subsequently, fail to provide a single, worldwide standard of care for narrow angles. Ideally, quantitative studies of the anterior chamber and the relationships of structures therein could provide objective measurements, which will standardize the anterior chamber (AC) parameters requiring interventions and treatments. The best way to achieve these quantitative measurements is through anterior chamber imaging devices. Ultrasound Biomicroscopy (UBM) has been used for this very purpose for more than 15 years [
Ultrasound biomicroscopy image showing normal angle structures. S: sclera; CB: ciliary body, PC: posterior chamber, AC: anterior chamber, L: lens, C: cornea. Dark arrows delineate the trabecular meshwork from the scleral spur towards schwalbe’s line while the white arrow signals points towards an open angle.
UBM image showing relative pupillary block with bowing of the iris anteriorly prior to laser iridotomy.
Ultrasound biomicroscopy image showing plateau iris with the classic double-hump sign. Contrary to angle closure on the basis of relative pupillary block, where indentation gonioscopy results in deepening of the peripheral anterior chamber, in plateau iris the iris contour follows the lens, dips posteriorly, then rises anteriorly before reaching the angle recess. The iris root remains angulated forward with a deepening of the anterior chamber confined to the region of the central iris. In this figure iridotrabecular contact (ITC) can be appreciated.
Ultrasound biomicroscopy image showing peripheral anterior synechia (PAS). S: sclera; CB: ciliary body, AC: anterior chamber, I: iris, C: cornea. Dark arrows delineate the PAS.
Ultrasound biomicroscopy image of an eye with plateau iris configuration status post-argon laser peripheral iridoplasty (PICP). An open angle and no ITC can be appreciated.
Ultrasound biomicroscopy image showing angle closure consequent of phacomorphic causes. A large lens is pushing iris anteriorly causing angle closure.
Ultrasound biomicroscopy image showing anteriorly rotated ciliary body in aqueous misdirection/ciliaryblock/malignant glaucoma.
Imaging technologies have proven extremely useful for explaining the nature of various pathologies and in determining a rationale for treatment in patients who may be confused concerning open-angle and angle-closure glaucoma and the laser treatment modality which best suits their condition.
In a UBM image, the scleral spur can be seen as the innermost point of the line separating the ciliary body and the sclera at its point of contact with the anterior chamber (please see Figure
Despite the advantages of UBM, there are some disadvantages as well [
The risks of performing UBM, the requirement for a skilled operator, and the incomplete or nonideal information provided by the supine position leave room for improvement in anterior segment imaging. The next generation of imaging would provide images taken in the seated position, preferably without any contact, and, with less invasiveness, less risk and less discomfort for the patient (please see Table
Presents the details of UBM as well as ASOCT devices available in the market.
UBM (50 MHz) | Stratus-OCT | Visante-OCT (TD-OCT) | RTVue (FD-OCT) | Cirrus high-definition OCT (HD-OCT) | |
---|---|---|---|---|---|
Light source | Ultrasound | Super luminescent Diode 820 nm | Super luminescent Diode 1310 nm | Super luminescent Diode 840 nm | Super luminescent Diode 840 nm |
Scan size | Up to 7 mm tissue depth | 6 mm (width) × 2 mm (depth) | 16 mm × 6 mm | 2 mm × 2 mm (CAM-S) OR 6 mm × 2 mm (CAM-L) | 3 mm × 1 mm |
Scans rate (A-scans/second) | 1000 | 400 | 2,000 | 26,000 | 27,000 |
Axial resolution | 30 | 10 | 18 | 5 | 5 |
With new anterior segment optical coherence tomography (ASOCT) imaging techniques, detailed spatial relationships of the anterior segment structures can be visualized and objective anterior chamber angle (ACA) measurements can be performed in a noncontact manner (please see Figure
Anterior segment optical coherence tomography image showing relative pupillary block. AC: anterior chamber; ITC: iridotrabecular contact.
Anterior segment optical coherence tomography image showing plateau iris configuration. CB: ciliary body, AC: anterior chamber, ITC: iridotrabecular contact.
Anterior segment optical coherence tomography image showing angle closure consequent of phacomorphic causes.
Anterior segment optical coherence tomography image showing iridotrabecular and iridocorneal contact in a case of aqueous misdirection/ciliaryblock/malignant glaucoma.
Enhancing the clinical applicability of ASOCT, Prata et al., described a novel dynamic technique to differentiate appositional from synechial angle closure and to understand the underlying mechanisms of angle closure using indentation ASOCT [
Asrani et al. recently reported the successful visualization of details of anterior chamber drainage angle (Schlemm’s canal, trabecular meshwork and configuration details of the iris with respect to the angle) using a swept source Fourier-domain OCT system [
The scleral spur is a protrusion of sclera anchoring the trabecular meshwork anteriorly and the longitudinal muscle of the ciliary body posteriorly. It represents an anatomical landmark for the trabecular meshwork which is located approximately 250 to 500
The differences in anterior chamber angle measurements in different lighting conditions have been investigated with UBM and ASOCT [
Significant correlations were found among ACA measurements by ASOCT, gonioscopy, and UBM [
Furthermore, UBM offers a better view of the ciliary body, which is rarely visible during ASOCT, since attenuated light from the overlying sclera obscures the view of the ciliary body. However, recent studies have confirmed the ability of ASOCT to evaluate and confirm a clinical suspicion of plateau iris configuration and syndrome [
Finally, different factors can influence the appearance of the angle, including background illumination, blinking, patient posture, contact with the eye, and image processing software.
In most cases the software to analyze OCT images is based on manual labeling of the scleral spur, cornea, and iris which is not only a tedious process but sometimes not possible to accomplish as in about 20–30% of the cases; these landmarks cannot be identified. Sakata et al. found that the sclera spur could not be detected in approximately 30% of the ACA quadrants, this problem being worse in the superior and inferior quadrants [
Clinically, ASOCT has been applied to the observation of ACA change after glaucoma surgeries, such as laser peripheral iridotomy (LPI), argon laser peripheral iridoplasty (ALPI), trabeculectomy combined with cataract extraction, and intraocular lens implantation.
In a retrospective study involving 71 Caucasian eyes, Ang and Wells compared AS-OCT parameters before and after laser iridotomy [
In most of the priory reported studies where LPI was done, the authors found a proportion of patients in whom the angle did not widen but did not specify the possible etiologies for it, and this is consistent with the limitation of current OCT technologies to evaluate the structures behind the iris like an anterior insertion of the ciliary body causing plateau iris configuration/syndrome. Despite this limitation in a small case series some authors have suggested the presence of signs that are suspicious for plateau iris syndrome in AS-OCT [
It is envisioned that the new anterior segment imaging devices would have as significant impact as the new posterior segment imaging devices. The new imaging devices do not aim to replace conventional slit-lamp biomicroscopy. They would act to supplement and augment clinical practice and become invaluable tools for ophthalmic research. The major advantages of the newer devices are the noncontact nature of examination, high scan speed, good repeatability and reproducibility for quantitative and qualitative measurements, and cross-sectional visualization of anterior segment structures. Since ASOCT can visualize the entire anterior chamber, all the essential parameters for detection of angle closure/narrow angle can be examined in a single scan. The ASOCT would become an essential tool for screening PAC, making screening programs for PACG more feasible and less doctor dependent. The application of ASOCT has led to a better understanding of anterior segment diseases. It can now be readily quantified making longitudinal followups and assessments possible. The outcome of treatment can be monitored without discomfort or risks of inflammation. The new devices may improve our understanding of current limitations of surgery. The potential clinical applications of these methods are only starting to be explored and the range of information they may yield has yet to be determined. Therefore, the use of the newer anterior segment imaging devices could well be the start of a new era for ophthalmic diagnosis.
The authors acknowledge Ocular Imaging Laboratory at the Shelley and Steven Einhorn Clinical Research Center at New York Eye and Ear Infirmary for providing images. The authors also thank Vishal Jhanji, MD from Department of Ophthalmology and Visual sciences Chinese University of Hong Kong, for helping with critical review of the paper. T. M. Grippo is supported by a departmental challenge grant from research to prevent blindness, Inc.