Over time, we have evolved to incorporate the use of optical coherence tomography (OCT) into our day-to-day management of patients with glaucoma and other ocular diseases. As the software has evolved, it has become even more important to be aware of pitfalls that can affect our interpretation of OCT. Here are a few quick tips to make the most of OCT.
1. Know the OCT signal strength. The strength of the OCT signal does not tell you the quality of a scan, but it is true that a scan with a low signal strength will not be of high quality. Signal strength can be decreased by problems with the ocular surface, cataracts, posterior capsular opacification, or axial myopia. It is possible to use this information to aid in the diagnosis of the above problems. Problems related to signal strength arise when the signal strength changes from one examination to the next. For example, in dry eye patients, signal strength changes can affect values, such as average retinal nerve fiber layer thickness, and these apparent changes can mask or mimic progression. Before concluding that the exam has changed, make sure the signal strength is it is similar between the two exams. If it fluctuates, consider taking a closer look at the ocular surface.
2. Beware of myopia. Keep in mind that almost all of the OCT normative data bases exclude patients with moderate to high myopia. Myopes will have thinner ocular tissues in part because of anatomy, and in part because of an OCT measurement artifact (signal attenuation). Signal strength will be lower, often diminishing values and occasionally causing segmentation errors. In summary, use OCT in myopes to compare changes between exams, and to diagnose gross pathology that doesn’t require normative data bases, but use caution otherwise.
3. Edema in all of its forms. I recently had a patient whose glaucoma appeared to have gotten better. His visual field defects went away, and his retinal nerve fiber layer became thicker. Of course, what had actually happened was that I had removed his cataract. The visual field had apparently improved without the cataract; the OCT had improved because, several months after the cataract surgery, there was some subclinical edema that had not affected his vision, but had modestly elevated his retinal nerve fiber layer thickness measurements. These cases of subtle edema masking retinal nerve fiber layer thinning can be present in uveitis, post-operative conditions, and in diabetic patients with any form of retinal edema.
4. Macula and nerve together, forever... When we scan the optic nerve or macula, it is to everyone’s benefit to scan the nerve and the macula at the same time. Why? Because optic nerve problems can affect macular measurements, and vice versa. CME will often elevate retinal nerve fiber layer measurements, and optic nerve edema, or thinning, can affect macular measurements. Furthermore, OCT provides a fantastic opportunity to evaluate for a variety of optic nerve and macular diseases, and it serves our patients well to be on the lookout for both — even if our focus for one particular visit is just the optic nerve, or the macula.
In summary, OCT technology has evolved wonderfully over the past several decades, and we will do well to continuously refine our strategies and interpretations.
Dr. Nathan M. Radcliffe is the director of the glaucoma service and a clinical assistant professor at New York Univeristy Langone Ophthalmology Associates and is a cataract and glaucoma surgeon at the New York Eye Surgery Center.