American Academy of Ophthalmology Web Site: www.aao.org
Clinical Update: Glaucoma
Imaging the Angle: Anterior Segment Techniques
Angle-closure glaucoma, also known as narrow-angle glaucoma, may be less prevalent than open-angle glaucoma, but it is no less destructive, particularly in its asymptomatic form, when damage can occur unnoticed and unchecked. Thus, “all new patients need to have their angles evaluated,” said Robert N. Weinreb, MD, professor and chairman of ophthalmology and director of the Hamilton Glaucoma Center at the University of California, San Diego. “And those at greater risk—for example, who have a family history of the disease or are farsighted—need to have them reexamined at periodic intervals.”
Angle evaluation is particularly important for certain Asian populations, such as Chinese, Mongolians and Burmese, in whom angle closure is highly prevalent, said Tin Aung, MBBS, PhD, senior consultant and head of the glaucoma service at the Singapore National Eye Centre and professor of ophthalmology at the National University of Singapore. It has been predicted that, by 2020, 87 percent of patients worldwide with primary angle-closure glaucoma will be Asian.1
Ophthalmologists now have a range of options for imaging the angle, from the time-honored practice of gonioscopy through newer, noncontact methods, such as time-domain anterior segment optical coherence tomography (AS-OCT), spectral-domain OCT (SD-OCT) and the EyeCam. Each technique has particular benefits and drawbacks in obtaining reliable, repeatable measurements to assist the clinician in evaluating the angle—here are some considerations for choosing the best approach.
Gonioscopy: Still the Gold Standard
According to Dr. Aung, “Nothing can completely replace gonioscopy by a trained ophthalmologist.” Sanjay G. Asrani, MD, associate professor of ophthalmology at Duke University, said that gonioscopy offers benefits that include a 360-degree view and information about new blood vessels and pigmentation in the angle. In addition, by using the technique of dynamic indentation, the clinician can identify landmarks such as Schwalbe’s line and distinguish peripheral anterior synechiae from appositional closure.2
However, “the reality is that gonioscopy is, unfortunately, not widely performed in routine clinical practice,” said Dr. Weinreb. The reason could be that it has some limitations.
Training, practice required. First, gonioscopy is a skill that requires training and practice to master. For example, recognizing angle landmarks can be difficult when the trabecular meshwork lacks pigmentation. “In such cases, performing gonioscopy with and without indentation as well as comparing the appearance among the different quadrants of the same eye and other eye may help,” said Dr. Asrani.
Subjective assessment. Gonioscopy provides qualitative rather than quantitative information, and the use of different grading schemes and types of gonioscopic lenses leads to variability in angle assessment. Moreover, the definition of occludable angle used by practitioners varies when the trabecular meshwork is not visible.3
Technical confounders. The light of the slit lamp causes pupillary constriction, drawing the iris away from the angle. Along with inadvertent pressure from the goniolens, this can artificially open the angle. “So we can miss intermittent closure or misdiagnose it as another form of glaucoma,” said Dr. Asrani. “It’s not until the disease has become chronic and we start seeing sticky areas [synechiae] in the angle that we say, ‘Aha, there is something going on here.’”
Unwelcome contact. The goniolens is used in contact with the eye, which requires numbing the eye, is uncomfortable to the patient and can cause corneal abrasions.
UBM Penetrates Visual Barriers
First developed in the 1990s, ultrasound biomicroscopy (UBM) offers an objective alternative that has been shown in several studies to have general correlation with gonioscopy. Current high-frequency 50-MHz UBM systems provide axial-by-lateral resolution of about 30 × 60 µm and penetration of approximately 6 mm.3
Because attenuation occurs exponentially with frequency, high-frequency UBM has been more useful for the anterior than the posterior segment, said Ronald H. Silverman, PhD, a developer of high-frequency systems and professor of ophthalmic sciences at Columbia University in New York.
Benefits of UBM. “Keep in mind that ultrasound will be there for you when you need to see through opaque media or structures to make a diagnosis,” said Dr. Silverman, noting that it can be complementary to OCT. The real benefit of UBM is its ability to visualize structures such as the ciliary body, lens and zonules as well as possible causes of angle closure such as iridociliary cysts and masses and plateau iris syndrome.3
“Although OCT provides better resolution, ultrasound gives better penetration,” he said. “For instance, when you’re looking at a choroidal melanoma, all OCT will show is that there is an elevation. But ultrasound can help you to see the tumor in cross section and to assess how thick it is and what is happening in the orbit behind it.”
Dr. Weinreb agreed that ultrasound also is useful for recognizing types of angle closure that might be related to fluid accumulation in the suprachoroidal space or tumors that cause narrowing or closure of the angle. However, he added, in most patients with angle closure, the primary pathology does not originate posterior to the iris.
Downside of UBM. UBM requires fluid media for propagation of the ultrasound waves. This may be accomplished using a saline-filled scleral shell or cup or a water-filled bubble tip over the probe, which is then placed on the eye, with the patient supine. As with gonioscopy, these contact methods can be uncomfortable and may introduce risks such as infection and corneal abrasion as well as inadvertent indentation.3,4 However, some recently developed systems may allow for reduced pressure and increased flexibility of patient positioning.5
OCT: Two Types, Different Features
Largely supplanting UBM, said Dr. Weinreb, OCT provides rapid, noncontact, cross-sectional imaging of biological tissue with higher resolution than that of UBM. It uses low-coherence interferometry to measure the delay and intensity of light reflected from tissue structures.
“OCT allows us to look at the angle with infrared light without shining any white light onto the eye,” said Dr. Asrani. “We can look at the angle and then change the room lighting to see what happens—is the angle really closing when the room lights are off and opening when the lights are on? And, because it’s a noncontact technology, it allows us to study the angle in its natural and dynamic states.”
AS-OCT. The first OCT modified for anterior segment use, the Visante AS-OCT (Carl Zeiss Meditec) can visualize the front part of the eye, including the cornea and anterior chamber. It is a time-domain instrument, in contrast to the more recently developed SD-OCT units. AS-OCT has a number of advantages, said Dr. Weinreb, including ease and speed of operation and better detection of closed angles than with gonioscopy, especially in the superior and inferior quadrants.
“With a wavelength of 1,310 nm, the Visante allows fairly deep penetration of about 6 mm—deeper than that obtained with SD-OCT—and a cross-sectional view of the entire anterior segment, which is about 16 mm.” It also provides lateral resolution of about 60 µm and axial resolution of around 18 µm. Compared with SD-OCT, less light reaches the retina with AS-OCT, resulting in higher power and reduced motion artifact, said Dr. Weinreb. Automated analysis software provides rapid estimation of anterior segment parameters.3
The disadvantages of AS-OCT include high costs and challenges in seeing localized lesions, such as PAS, and structures behind the iris. In addition, in one study, glaucoma specialists using AS-OCT were unable to discern the scleral spur in up to 28 percent of patients.6
SD-OCT. More recently, SD-OCT devices, such as the RTVue (Optovue), have been adapted to look at the anterior segment. Dr. Asrani said, “SD-OCT has an axial resolution of about 5 µm and a typical wavelength of 830 nm. The higher resolution compared with the Visante allows better visualization of the cornea and angle and its substructures. And the higher speed means greater precision.”
Dr. Asrani and colleagues conducted a pilot study with a prototype SD-OCT device that used a 1,310-nm wavelength swept light source. It provided a scan depth greater than 6.3 mm—allowing imaging of the entire anterior chamber depth—while retaining the high resolution characteristic of SD-OCT and reducing an overlap artifact.7
“We were able to see precise details of the Schlemm’s canal and scleral spur, as well as the exact location of the trabecular meshwork in relation to the surface of the iris,” said Dr. Asrani.
Which OCT is better? Each method has its strengths and limitations, said Dr. Aung. “AS-OCT is easy to use and can obtain cross-sectional images of the anterior segment. It has also allowed us to identify novel factors associated with angle closure such as increased iris thickness and area, smaller anterior chamber width, area and volume, and thicker lens vault.
“SD-OCT has higher resolution but cannot easily obtain wide-field views of the anterior segment,” said Dr. Aung, which makes it more difficult to see the angle recess. But its high resolution may be particularly well suited to diagnosing intermittent angle closure, said Dr. Asrani, providing the potential for early treatment and prevention of chronic angle closure and acute angle closure.
Newer OCT devices now in development will combine the advantages of both types: AS-OCT’s enhanced depth and field of image with SD-OCT’s higher resolution.
Originally used for taking wide-field fundus photos—in which application it was called RetCam—EyeCam (Clarity Medical Systems) is a new method for photographing the angles, said Dr. Aung. This technique gives a direct, color image of the angle with excellent optical quality.2
Although it does not provide quantitative measurement of the angle, it can be used to obtain 360-degree visualization of the entire anterior chamber angle, similar to gonioscopy. Unlike gonioscopy, however, the examiner uses a fluid-coupling probe angled against the limbus, avoiding contact with the cornea and reducing compression artifact.3
In a study conducted by Aung and colleagues, EyeCam correlated well with gonioscopy for detecting angle closure, although it detected more closed angles than gonioscopy in all quadrants. The EyeCam captured clear images of angles in 98.8 percent of study participants.2
Disadvantages of EyeCam. The use of EyeCam requires more time, money and space (the patient is supine) than some other imaging techniques. Bright illumination, delivered through a fiberoptic cable, may cause pupillary constriction and alteration of angle configuration. Finally, as with gonioscopy, it may be difficult to discern angle structures if the trabecular meshwork is lightly pigmented.3