Although optic nerve damage and a progressive loss of the visual field are the final common pathways of the glaucomas, the configuration of the anterior segment is one of the most important things the clinician must assess. A determination of anterior segment anatomy (such as the angle grade, iris configuration, and lens position) provides essential guidance to proper clinical therapeutic decisions. The various ways in which physicians evaluate the anterior segment, aside from a detailed examination and gonioscopy, include technologies such as Scheimpflug photography, scanning slit-lamp systems, ultrasound biomicroscopy, and anterior segment optical coherence tomography (AS-OCT). Although Huang et al1 first described optical coherence tomography of the eye in 1991, Izatt et al2 described its use in the anterior segment in 1994.
AS-OCT imaging allows clinicians to document progression or changes in anterior segment anatomy, and it provides a cross-sectional view. The technology also precisely documents parameters such as the angle's width, the iris' thickness, and anterior chamber depth. In addition, it permits physicians to view dynamic images of the angle's configuration under different lighting conditions. AS-OCT is therefore a useful adjunct in the diagnosis and treatment of glaucoma.
IMAGING THE IRIS, LENS ,
AND ANTERIOR CHAMBER
Two common flaws in the performance of clinical
gonioscopy include the placement of pressure on the
cornea and the use of excessive amounts of light. Both
typically produce the illusion of an open angle in a
patient who otherwise may have narrow or even appositionally
closed angles. AS-OCT provides little light artifact.
It can actually dynamically show how an angle
imaged in bright light may appear open but look narrow
when assessed in the dark (Figure 1). The noncontact test
also eliminates the problem of corneal compression.
AS-OCT can quantitatively measure the exact angle recess (Figure 2) as well as provide insight into the possible mechanism of the angle's narrowing or closure, including a primarily lens-related mechanism, plateau iris, a mixed mechanism of combined lens rise and plateau iris, and even malignant glaucoma. In the case of a lens-related narrow angle, any contributing component of pupillary block may be alleviated by a peripheral laser iridotomy. The angle may still be at risk for closure, however, due to a large lens or high lens rise pushing the iris leaflets forward (Figure 3). Although not always as dramatic, one striking example of primary phacomorphic angle closure is microspherophakia. The definitive treatment in such cases is lensectomy; any other treatment modalities will ultimately fail to prevent intermittent, chronic, or even acute angle closure (Figure 4).
Classically in plateau iris, the central anterior chamber is deep, but the peripheral anterior chamber is shallow, leading to the illusion of an open angle on cursory examination. On gonioscopy, however, irido-trabecular proximity can be noted along with the so-called double-hump sign upon light compression. This finding occurs where it is thought the ciliary body is more anteriorly positioned or relatively enlarged, propping up the peripheral iris and resulting in the narrow angle (Figure 5). Although AS-OCT provides important information and various clues, clinicians must remember that plateau iris, or any ciliary bodymediated posterior pushing of the peripheral iris (such as a cyst or tumor), cannot be diagnosed with this technology alone. AS-OCT is unable to image any tissues posterior to the iris due to its pigmentation. Physicians therefore should use ultrasound biomicroscopy for definitive documentation and evaluation of the ciliary processes.
Another relatively rare but diagnostic and therapeutically challenging entity is that of malignant glaucoma. Although sometimes obvious with complete obliteration of the anterior chamber and an elevated IOP (Figure 6), the manifestation of this disease can be subtle. AS-OCT is able to show an obliterated sulcus space and angle closure in a patient with a myopic shift after routine cataract surgery and elevated IOP (Figure 7). Although the technology is unable to image the ciliary processes or through the pigmented iris, it can show the suprachoroidal space and effusions, another potential cause of angle closure (Figure 8).
In addition to its utility in the imaging of narrow angles and selection of appropriate therapeutics, AS-OCT can provide unique views of pigment dispersion syndrome (Figure 9) and tilted IOLs. The technology can reveal some characteristics of the crystalline lens (Figure 10) and possibly provide a means by which to monitor and assess abnormal structures in the anterior chamber (Figure 11).
IMAGING IN GLAUCOMA SURGERY
As new glaucoma surgical technologies are developed,
the role of AS-OCT continues to expand. The technology
helps surgeons obtain supplemental information about the
mechanism by which a surgical procedure lowers IOP in a
given patient. It also aids in preoperative planning in some
cases. Examples of the utility of AS-OCT in glaucoma surgery
include the imaging of subconjunctival blebs in trabeculectomy;
not only can the internal ostium be visualized
but also the internal bleb structure itself (Figure 12).
Likewise, in tube shunt surgery, the position of the device in
the anterior chamber, its proximity to the corneal endothelium,
and obstructions in the lumen or irregularities in the
course of the tube can be readily visualized (Figure 13).
The surgical objective of canaloplasty is to produce suture-mediated re-expansion of a collapsed canal in an eye with open-angle glaucoma. Studies have shown that higher suture tension in the canal results in a greater IOP-lowering effect.3 AS-OCT allows visualization of the distended Schlemm's canal postoperatively, confirming that suture tension has been achieved (Figure 14). Further, distal intrascleral aqueous veins are sometimes visible on AS-OCT (Figure 15). As mentioned previously, AS-OCT technology can image the suprachoroidal space, thus permitting the evaluation of such devices' success as the gold suprachoroidal microshunt (Solx Gold Shunt; Solx, Inc., Waltham, MA). This device is designed to provide a conduit by which aqueous humor can exit the anterior chamber into the suprachoroidal space and thereby lower the IOP (Figure 16).
Although the definitive surgical treatment of phacomorphic angle closure consists of lens extraction, this procedure alone does not alleviate plateau iris-related narrowing of the angle in eyes with a mixed mechanism of angle narrowing.4 In these cases, endoscopic cycloablation in a pattern Ike Ahmed, MD, terms endocycloplasty may be of use. With the technique, the surgeon treats the posterior tails of the ciliary processes to induce shrinkage and posterior curling, allowing the peripheral iris to fall away from the angle (Figure 17). The goal of this treatment is different from that of traditional endoscopic cyclophotocoagulation for refractory glaucoma, where the surgeon attempts to treat the entire pigmented epithelium. In endocycloplasty, the ophthalmologist merely shrinks the ciliary processes posteriorly to eliminate any propping up of the peripheral iris into the angle, a process that alleviates any ciliary body-mediated angleclosure mechanism ( for more information, see page 29).
CONCLUSION
No technology can replace a careful clinical examination
and the diagnostic and therapeutic decision making of an
experienced ophthalmologist. Moreover, AS-OCT has limitations,
most notably its inability to image through pigmented
tissue. It is therefore inadequate for the assessment of the ciliary
body, zonules, posterior chamber, or anterior vitreous.
Nevertheless, AS-OCT yields unique cross-sectional views of the anterior segment, and it has the ability to precisely measure various spaces and angles between tissues. This technology is a powerful tool not only for documenting disease and providing diagnostic clues but also for monitoring change and progression of tissue's position over time. ASOCT's noncontact nature, excellent high-resolution real-time images, and rapid scanning rate make it an excellent adjunctive tool for both the physician and the patient. With developments in three-dimensional AS-OCT likely5 and increasingly rapid scanning speeds,6,7 the future applications of this technology in glaucoma are promising and exciting.
Diamond Y. Tam, MD, is a clinical assistant professor in the Department of Ophthalmology at Stanford University School of Medicine in California. He is a consultant to iScience Interventional. Dr. Tam may be reached at (650) 723-6995; diamondtam@gmail.com.
