Imaging's Role in the Diagnosis and Management of Glaucoma

Two primary modalities are used for imaging glaucoma patients: retinal photographs and optical coherence tomography (OCT). Each has a role, and both can be complementary to the examination findings and the management plan. Both photography and OCT have important strengths and limitations that are relevant to how they are used at different stages of glaucoma.

An advantage of retinal photography is that it is a stable platform. It has been around for many years and photographs taken 25 years ago are still relevant. That cannot be said for imaging devices purchased 25 years ago. With advances in the technology used for image capture, the optic nerve, retinal nerve fiber layer, and the macula can be visualized better than ever before.

OCT provides extremely useful clinical information, and its versatility has allowed it to eclipse several other technologies used in glaucoma imaging. Scanning laser polarimetry for imaging the retinal nerve fiber layer (RNFL), for example, is less commonly used. Confocal scanning laser ophthalmoscopy, as with the Heidelberg Retinal Tomograph (Heidelberg Instruments), is an excellent tool for tracking progression over time, but its single functionality makes it less desirable for the average clinic than the more flexible OCT.

There is some thought that OCT imaging is so useful that retinal photography is no longer indicated. I find retinal photographs to be invaluable, and I utilize both photography and OCT as I monitor individuals with glaucoma. Certain findings such as disc hemorrhages are best seen with photography. Also, some clinicians have taken to performing OCT in lieu of performing visual field examinations, under the mistaken notion that this advanced technology can be a substitute for other parts of the examination and can by itself drive clinical decision-making. However, there is no rationale for replacing a functional test with one that provides information only on structure. Moreover, any and all imaging is still subject to interpretation by the investigator. Even if said imaging provides a perfect representation of the retina, fundus, and optic nerve, at its best, imaging is additive to the physician’s clinical impression and findings.

OCT IN DIAGNOSIS AND FOLLOW-UP

Spectral-domain OCT (SD-OCT) offers granular detail about the health of the RNFL and ganglion cell complex. The technology can image the macula to demonstrate whether there has been tissue loss that may be attributable to glaucoma. The normative databases used by SD-OCT platforms greatly facilitate the clinician’s ability to classify disease and severity. Most SD-OCT devices also contain some form of tracking software, which functions both to correct for ocular micromovements (eliminating noise and image artifacts) and to ensure that serial imaging is performed based on a consistent reference point. The former provides greater image clarity, and the latter improves the ability to track progression over time.

OCT is valuable as a diagnostic tool. For diagnostic purposes, OCT provides insights on rim thickness, rim area, and cup volume. The technology is also useful for tracking progression over time and fine-tuning therapy. If serial imaging confirms no change from one visit to the next, there may be no reason to alter a patient’s established therapeutic approach. I may adjust therapy in response to OCT findings even when functional correlates (ie, visual field testing) do not demonstrate change.

However, OCT is useful only in the early stages of glaucomatous disease. It starts to lose applicability in more advanced stages. In a healthy eye, the RNFL is about 100 to 110 µm thick, but glaucoma results in irreversible thinning of the layer. The RNFL is detectable on OCT down to about 50 to 55 µm thickness, which corresponds in most eyes with moderate glaucoma. Subsequent optic neuropathy in more advanced stages of glaucoma does not result in any further detectable thinning of the retinal nerve on OCT because the imaging depicts remnant glial tissue.

This so-called floor effect limits the applicability of OCT for imaging advanced glaucoma. OCT can demonstrate progression only to moderate disease, when patients typically still have a lot of visual field. At that point, visual field testing becomes more relevant for monitoring disease progression.

Of course, for visual field testing to be relevant in advanced disease, it is helpful to have a baseline to compare against. Therefore, visual field testing should be initiated early in the disease course. No single imaging or testing modality functions in isolation in the management of patients with glaucoma.

FUTURE INNOVATIONS

There are two innovations in OCT that may add to its clinical utility in glaucoma. Swept-source OCT (SS-OCT), now available or in development by a number of manufacturers, uses a faster image capture mechanism via a moving reference lens. (SD-OCT uses a fixed reference lens to capture backscattered light, and images are mathematically reconstructed using Fourier calculations that determine how wavelengths of projected light behaved in reference to spatial objects in the field of reference.) Faster image capture means less image noise and the ability to perform more scans per second to allow more accurate reconstruction of images. In most models, SS-OCT has been paired with a 1080-nm laser light source, a wavelength that penetrates deep into retinal structures, including the choroid. It appears that SS-OCT may also penetrate media opacities such as cataracts.

Faster image capture also allows OCT devices capture hundreds of thousands of sequential images of the retinal vasculature. With this information, the series of static images can be reconstructed to provide insight into the function of the retinal vasculature over a period of time (ie, the duration of the scan). This makes OCT angiography (OCT-A) possible. The modality is somewhat akin to fluorescein angiography, in that it provides information about blood flow at the back of the eye. Unlike fluorescein angiography, OCT-A does not require use of an intravenous injection; however, it also does not provide dynamic moving images with which to assess vascular function. There appears to be rationale for OCT-A in a number of retinal pathologies, although it is uncertain whether and how OCT-A might be applicable to glaucoma.

CONCLUSION

SS-OCT, OCT-A, and other proposed novel modalities may change the paradigm of how patients with glaucoma are followed using imaging. However, as much as greater clarity in imaging and the ability to gain deeper understanding of ocular structures will enhance clinical impressions, it is likely that imaging will always play a complementary role to the physician’s interpretive skill in the management of patients.

Murray Fingeret, OD
• chief of the Optometry Section, Department of Veterans Affairs,
•ew York Harbor Healthcare System, Brooklyn, New York
• clinical professor, SUNY College of Optometry
• murrayf@optonline.net
• financial disclosure: consultant to and a member of the advisry board for Carl Zeiss Meditec, Heidelberg Engineering, and Topcon