Using glaucoma diagnostic imaging

Over the last decade, there has been rapid growth in the management of glaucoma. Innovative imaging technologies that aid in the diagnosis of this vision-threatening disease have also been on the rise. Instead of relying on a single diagnostic tool, clinicians may consider leveraging the benefits of several modalities available today. Ultrasound biomicroscopy (UBM), optical coherence tomography (OCT), and gonioscopy are three imaging technologies I use to diagnose glaucoma and assess its progression, and ultimately, to make treatment decisions.

Ultrasound biomicroscopy

The first effective advantage of UBM is it offers a real-time, accurate assessment of the anatomy of the anterior chamber angle. And because of the rapidly changing demographics of North America, a substantial portion of patients are potentially narrow-angle glaucoma suspects or glaucoma patients and mixed mechanism patients who have a combination of open-angle, elevated intraocular pressure (IOP), and phacomorphic lens-induced angle narrowing.¹

The second advantage of UBM is its ability to assess structures posterior to the iris. There are no commercially available OCT units that can image behind the iris plane because OCTs cannot penetrate the tissue. For conditions in which the patient’s intraocular lens may be imparting a clinical symptomatology or if there’s an iris cyst, those factors need to be differentiated.

UBM enhances ophthalmic imaging applications

Finally, UBM is easy to use, and that is a major reason why I have incorporated it into my practice. In the past, UBM was conducted by sending a patient to a laboratory where an ophthalmic water bath system was utilized, and an ultrasonic probe was lowered into the setup to increase image sensitivity and penetration. The entire procedure took 30 to 40 minutes. Now, I need to only apply topical anesthetic and artificial tears to the patient’s eye and prep the tip of the probe. Ease of use has made the UBM a more useful and productive instrument for the busy primary-care practice. The best way to produce high-quality imaging with UBM is to use a tear supplement to create contact between the surface of the tip and the patient’s cornea. A tear supplement typically amplifies the quality and distinctiveness of the image, and the anatomy is more clearly defined. My UBM features a video loop that records imaging, so I can assess the vertical, nasal, horizontal, and inferior position of the angle. I can stop and review a loop at any time in order to examine the anatomy in detail or explore an area of concern.

Gonioscopy

In the world of multi-diagnostic assessment of glaucoma patients, gonioscopy is complimentary to UBM because UBM assesses the natural anatomy of the angle-specifically, the relationship among the anterior iris plane, the trabecular meshwork, and the posterior cornea. Gonioscopy should be done to differentiate whether there is pigment in the angle, trauma from angular recession, or neovascularization. Together, UBM and gonioscopy provide different streams of complimentary information, each with a unique ability to differentiate a component of the angle, but neither of them can provide the entire picture. 

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Some clinicians, however, are not as comfortable with gonioscopy as they need to be to perform it well, and many patients are not comfortable having it done. To improve the outcome, a few simple tips are helpful. While most clinicians use a drop or two of anesthesia, I use tetravisc  before the procedure, which provides a deeper anesthesia and a more comfortable and cooperative patient. Once the eye is anesthetized, it can tolerate the maneuvers of placing the lens. I always anesthetize both eyes, even if I am going to look at only one eye, because it reduces blinking and decreases the patient’s squeezing effect that occurs when the lens is placed. Finally, many clinicians often overuse the light, and the brighter the light, the more likely it will change the anatomy. The room light should be off, and the slit-lamp beam should be bright enough that the clinician can see the anatomy without washing out the ability to get a sense of the real angle.

OCT

An OCT helps in the diagnosis of glaucoma because it provides information about the optic nerve. Its primary roles in glaucoma are the imaging of the nerve and the diagnostic algorithms that are used to calculate endpoint assessment. The subsequent primary use of OCT is its tracking of the patient’s progression over time. OCT has incredible horizontal capacity to assist the clinician in diagnosis of a variety of diseases, including retina and optic nerve disease. Gonioscopy and UBM provide an assessment of the anterior anatomy, which is essential to understanding the relative importance of IOP. OCT, then, effectively provides an assessment of the anatomy of the optic nerve that can be integrated into the anterior assessment and, together with gonioscopy and UBM, provides a comprehensive picture of the patient’s glaucoma presentation. 

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The key to a successful OCT is to perform the image with a pupil size that’s big enough to have a full capture and, if possible, take the image before dilation. If an anesthetic is applied to the eye to dilate the pupil, it can cause a keratitis that makes the OCT imaging less clear. Capturing the image without having to put drops in the eye or pre-instillation is a better alternative. It is important to note that the software in the OCT does not address patients with hypoplasia or megalopapilla.. It is the clinician’s job to understand the natural range of the algorithm and what happens when the nerve is too small or too large. By understanding the inherent limits of the OCT, the clinician can master its use in diagnosing and treating glaucoma. 

Capturing images

There is great variability in operating imaging devices that is relative to the healthcare professional who is performing the test. A well-trained technician or clinician can create exquisitely clear, useful imaging on a regular basis if they have  protocols to assist in image capture. Additionally they need to understand the necessity for an accurate image in order to properly interpret the outcome. Without a clear image as a source for analysis, the purpose of using an imaging instrument diminishes to the clinician and, eventually, to the patient. 

Reference

1. Francis BA, Varma R, Chopra V, et al. Los Angeles Latino Eye Study Group. Intraocular pressure, central corneal thickness, and prevalence of open-angle glaucoma: the Los Angeles Latino Eye Study. Am J Ophthalmol. 2008 Nov;146(5):741-6.