New Bio-marker for Photoreceptor Integrity
Posted on Tue, Jan 05, 2010
A recently discovered OCT artifact proves enormously predictive. By Jerry Sherman, OD
 | "We now have a simple test which
in seconds can differentiate between
maculopathy and optic neuropathy!" |
The ophthalmoscope has served vision care well for 150 years, but in light of recent advancements, its primacy has become outdated. We wrongly assume that if a retina looks normal via ophthalmoscopy it is healthy. In fact, we now know the earliest stages of virtually all posterior ocular diseases are invisible to ophthalmoscopy. Further, many such conditions can be diagnosed using new technologies, such as ocular coherence tomography (OCT).
Ocular coherence tomography has revolutionized eye care by imaging detailed cross-sections of the retina, revealing what previously was hidden (in vivo, at least); while at the same time OCT software technology allows us to measure quantitative parameters that otherwise couldn't be measured-for example, the exact thickness of the macula and the retinal nerve fiber layer (RNFL).
There are many remarkable applications for this technology, but I'll focus here on one so recent it hasn't received much attention in the literature.
We were taught that the retina consists of 10 layers, the lower nine of which are invisible to ophthalmoscopy. Much of this area contains photoreceptors (rods and cones). The advent of OCT has revealed these structures, and better still, we've found a bio-marker of photoreceptor health unique to Spectral Domain OCT images that appears never to fail.
Located at the border between the outer and inner segments of the photoreceptors, this bio-marker is a line which looks like the retina's "11th layer." It does not appear on histologies, but only on OCT images, probably due to the intense change in light refraction between the inner and outer segments. Astoundingly, this line is present in every single normal patient we have studied (a total of about 1,500) and is broken or abnormal in every single patient with outer retinal disease. [[art suggestion: image comparing histology to OCT image, 13:50 on presentation, illustrating PIL]]
Additionally, the nature of the line-which we've named the photoreceptor integrity line (PIL)-corresponds to the type of disease. Once basics like media opacity are ruled out, patients with reduced acuity and a PIL that is healthy under the fovea, where most cones are located, are probably suffering from rod degeneration around the disk. Conversely, patients with normal PILs near the RNFL likely have cone degeneration.
(Although the photoreceptor integrity line, or the PIL (defined as the junction between the inner and outer segments) is barely visible in most histological sections, it is highly prominent in normal SD OCTs. The PIL, as shown above, should be continuous throughout the entire scan in normal eyes. The PIL is considered by some as a mere artifact that is due to the difference in the index of refraction of the inner and outer segments but this artifact is remarkably useful in SD OCT interpretation.)In essence, we now have a simple test which in seconds can differentiate between maculopathy and optic neuropathy!
The clinical value of the PIL is enormous-just one factor which leads me to believe the retina represents the new frontier for optometry. For more information about how OCT can help provide optimal patient care, visit www.retinarevealed.com, where I present and discuss new case studies every week.
Dr. Sherman is President of the Optometric Retina Society.
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