Gonioscopy and Anterior Segment Imaging

The high prevalence of angle-closure glaucoma in Asia^1-3 coupled with its potentially blinding course have prompted the need for identifying people with early stages of disease. Assessment of anterior chamber angles remains the most effective approach for screening eyes at risk.⁴ Advances in imaging technology make it possible to visualize and analyze the anterior segment structures objectively and study their role in screening of angle closure.^5-7

Gonioscopy

Gonioscopy is widely accepted as the reference standard for assessing anterior chamber angle structures and their configuration. However, this highly subjective technique can lead to varying results depending on the type of lens used, the illuminating conditions and the skill of the examiner, and only moderate agreement is reported among observers.^2,3,8 Mechanical compression of the cornea or excessive illumination during the procedure can result in an inadvertent widening of the angles. The varying annotation of angle findings across different grading schemes^9,10 can also lead to significant variability in assessment. Definition of what constitutes an occludable angle varies, ranging from 180˚ to 270˚ of angle, in which the trabecular meshwork is not visible.¹¹ Despite its many limitations, gonioscopy is still an indispensable tool in the assessment of anterior chamber angle. Some of the advantages of gonioscopy are low cost and the abilities to view a large extent of the angle simultaneously and to perform dynamic indentation to assess for peripheral anterior synechiae.

Ultrasound Biomicroscopy

Ultrasound biomicroscopy (UBM) uses high-frequency ultrasound to image the angle of the eye, generating high-resolution images amenable to quantitative analysis.^12,13 UBM and gonioscopy exhibit good agreement in anterior chamber assessment when each is performed in a completely dark room.¹⁴ While gonioscopy sometimes overestimates angle width in subjects with occludable angles,¹⁵ the angle dimensions estimated by UBM correlate significantly with gonioscopy.¹⁶ UBM can image structures located behind the iris, such as the ciliary body, making the technique useful in identifying posterior causes of angle closure, such as plateau iris, iridociliary masses causing secondary angle closure, or choroidal effusion.^17,18 (Figure 1) UBM is also helpful in the evaluation of certain types of secondary glaucoma, such as pigment dispersion,¹⁹ and assessing for tilted or subluxed lenses in exfoliation syndrome.²⁰ The other advantage of UBM is the ability to acquire real-time images of the eye.²¹ The major disadvantage of UBM is that the procedure requires contact, either in the form of a scleral cup or a corneal probe, which can be uncomfortable for the patient, can lead to corneal abrasion or infection, and can distort the angles due to inadvertent indentation.²²

Figure 1. Ultrasound Biomicroscopy

Pigment epithelial cyst causing secondary angle closure.
Source: Monisha E. Nongpiur, MD, and Tin Aung, FRCOpth, PhD

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Pentacam

The Pentacam (Oculus) is a noncontact optical system that uses a rotating Scheimpflug camera to image the anterior segment of the eye. In 2 seconds, the device captures up to 50 images, which are then reconstructed into a 3-dimensional image, enabling a rapid assessment of the anterior chamber. The Pentacam does not allow visualization of the angle recess—hence angle assessment cannot be evaluated in detail—but the system is more useful for measurement of corneal pachymetry, anterior chamber depth and lens thickness.²³

Anterior Segment OCT

Anterior segment optical coherence tomography (AS-OCT) uses low-coherence interferometry to measure the delay and intensity of light reflected from tissue structures and compares these values with light that has traversed a known reference path length by using a Michelson-type interferometer (Figure 2).²⁴ The technology was initially employed for retinal OCT and was later adapted to image the anterior segment by altering the light to a longer wavelength of 1,310 nm.

Figure 2. Anterior Segment OTC

Visante anterior segment optical coherence tomography image showing closed angles.
Source: Monisha E. Nongpiur, MD, and Tin Aung, FRCOphth, PhD

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Two AS-OCT devices are commercially available. The Visante AS-OCT (Carl Zeiss Meditec) obtains scans at a rate of 2,000 A-scans per second, with an axial and transverse resolution of 18 μm and 60 μm, respectively. The slit-lamp OCT (SL-OCT; Heidelberg Engineering) has a slower image-acquisition speed and a lower axial and transverse resolution of < 25 μm and 20 to 100 μm, respectively. Additionally, the SL-OCT requires manual rotation of the scanning beam. In addition to angle parameters such as angle-opening distance (AOD) and trabecular-iris space area (TISA),²⁵ quantitative measurements of the structures such as iris,^5,6 lens,⁷ anterior chamber width (ACW),²⁶ area (ACA) and volume (ACV)²⁷ are also possible with AS-OCT images by using customized image analysis software (Figure 3).

Figure 3. Anterior Segment OCT

Visante anterior segment optical coherence tomography image illustrating the lens vault and anterior chamber width (ACW). Bold arrows indicate the position of the scleral spur.
Source: Monisha E. Nongpiur, MD, and Tin Aung, FRCOpth, PhD

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Studies found that AS-OCT detected more closed angles, particularly in the superior and inferior quadrants,^28,29 than gonioscopy, probably due to inadvertent pressure on the globe and exposure of the pupil to light, artificially widening the angle, during gonioscopy. Another reason could be the differences in definitions used to define angle closure. On gonioscopy, angle closure was defined as the apposition between the iris and the posterior trabecular meshwork, whereas on the AS-OCT, it was defined as any contact between the iris and the angle structures anterior to the scleral spur. The better agreement between SL-OCT and gonioscopy was attributed to the use of visible light during both examinations.³⁰

Recent studies evaluating AS-OCT–derived parameters have highlighted their potential role as objective markers for angle-closure screening, including thicker irides with greater iris area and curvature,^5,6 smaller ACW,²⁶ ACA and ACV,²⁷ and greater lens vault,⁷ which is the increased thickness and bulk of the lens anterior to the plane of the scleral spur.

Spectral Domain OCT

Fourier or Spectral domain OCT (SD-OCT) allows for the acquisition of high-resolution cross-sectional images at an axial resolution of 5 μm and a transverse resolution of 15 μm.³¹ By utilizing light of shorter wavelength (830 nm), the depth of penetration lessens, making it less useful for imaging the iris or more posterior structures. However, the high-resolution images show better structural delineation and views of anterior chamber angle landmarks, such as Schwalbe’s line, trabecular meshwork and Schlemm’s canal (Figure 4).^32,33

Figure 4. Spectral Domain OCT

High-resolution spectral domain optical coherence tomography image showing angle structures: trabecular meshwork (TM), Schwalbe’s line (SL) and scleral spur (SS).
Source: Monisha E. Nongpiur, MD, and Tin Aung, FRCOpth, PhD

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The major advantages of the OCT devices for angle imaging include noncontact method of examination, ease of operation and rapidity of image acquisition. Quantitative estimation of the various anterior segment parameters can be obtained by using either customized or the device-incorporated image analysis software. The main disadvantage of these devices is the inability to distinctly detect and measure structures posterior to the iris or to detect peripheral anterior synechiae. OCT devices also allow only cross-sectional imaging of the angles, unlike gonioscopy, which allows dynamic visualization of the entire angle quadrant. Moreover, high cost of these devices may be a limiting factor.

EyeCam

The EyeCam (Clarity Medical Systems) is an imaging system originally designed to obtain wide-field photographs of the pediatric fundus.³⁴However, with modifications in the optical technique, the device can also be used to view angle structures in a manner similar to direct gonioscopy (Figure 5). The images produced are similar to what is seen during gonioscopy, making it easy for clinicians to interpret. The objective documentation of angle findings allows for monitoring of angle changes over time, tracking of angle changes with disease progression and treatment effects.³⁵

Figure 5. EyeCam

A: Image of an open angle obtained using EyeCam, clearly detailing clearly Schwalbe’s line, pigmented trabecular meshwork, scleral spur and iris processes. B: Image of a closed angle obtained using EyeCam, showing Schwalbe’s line. The pigmented trabecular meshwork is not visualized.
Source: Monisha E. Nongpiur, MD, and Tin Aung, FRCOphth, PhD

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A preliminary study comparing EyeCam “gonio-graphy” with conventional gonioscopy in 60 eyes demonstrated that results from the EyeCam have good agreement with gonioscopy findings.³⁶ Another comparison between the two modalities found good agreement between them (0.72 to 0.76), and the EyeCam had a sensitivity of 76% and specificity of 81% for detecting eyes with angle closure using the two-quadrant definition of angle closure.³⁷

Although the EyeCam is unable to provide quantitative measurements of anterior chamber angle (similar to AS-OCT or UBM), the device provides a 360˚ visualization of the entire anterior chamber angle compared with the ASOCT and UBM, which provide only cross-sectional views. Limitations of the EyeCam compared with gonioscopy are that the EyeCam takes longer to perform (about 5 to 10 minutes per eye), is more expensive, and requires additional space since imaging is performed with the patient supine. In addition, unlike with dynamic gonioscopy, discerning the presence of PAS with the EyeCam is difficult due to the inability to indent the angle. Finally, the fiber-optic light source of the EyeCam may cause inadvertent pupillary constriction that can alter the angle configuration.

Conclusions

New anterior-segment imaging devices offer advantages of easy and rapid image acquisition and storage, qualitative and quantitative image assessment, and imaging in the presence of corneal or anterior chamber opacities. None of these new devices can completely replace conventional slit-lamp biomicroscopy and gonioscopy, but they can complement clinical evaluation and diagnosis by providing more objective assessment of anterior segment structures and angles.

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