By Dr. Rod Nowakowski Director of Genetic and Advanced Diagnostic Evidence for the value of electrophysiological testing in the diagnosis and management of glaucoma continues to evolve and accumulate. I recently lectured on electrophysiology, including applications to glaucoma, at SECO and there was considerable audience interest and interaction that prompted this newsletter. Three electrophysiology tests can add additional evidence helpful in the clinical diagnosis of glaucoma and may detect functional change that significantly precedes structural change. This is important since, up to a point, functional loss from "sick" cells may be slowed or reversed whereas structural change suggests cell death that is not reversible. Given that, a functional measure may present an opportunity for early diagnosis and hence, early intervention. We know early intervention makes a difference from the Early Manifest Glaucoma Trial (EMGT). Pattern Visual Evoked Potential (pVEP or PVEP) The PVEP records signals over the visual cortex generated by a patterned stimulus that looks like a black and white checkerboard. It is convenient to think of the PVEP as a measure of just optic nerve function but the signals measured at the visual cortex are a result of what happens along the entire visual pathway and therefore it is not possible to say an abnormal PVEP is due to abnormal function in a specific part of the pathway. Nevertheless, a sufficiently abnormal optic nerve as seen in glaucoma will give rise to an abnormal PVEP in terms of signal amplitude and/or signal delay. Such abnormalities lend additional evidence to optic nerve dysfunction provided that other parts of the pathway are normal. Pattern ERG (PERG) and steady-state pattern ERG (ssPERG) The typical ERG is used to measure the function of rod and cone photoreceptors by eliciting their electrical response to a bright flash of white light. If instead, the stimulus used is a pattern, like a grating or checkerboard, the signal can be attributable to the retinal ganglion cells (RGCs). A pattern that reverses at a specific rate gives rise to a "steady-state" PERG (ssPERG) that is more sensitive to RGC function. With respect to glaucoma, RGC functional loss precedes the structural change seen by OCT which precedes a measurable loss of visual field; therefore, an abnormal ssPERG may provide an opportunity for early diagnosis and early intervention - much earlier. Photopic Negative Response (PhNR) The PhNR is another test for RGC function but a typical electrophysiology unit may not have that capability or it may be available as an option. If so, it requires a full-field (Ganzfeld) stimulator and these can be expensive. The PhNR is generated by a full-field ERG using a brief flash of light, usually red against a blue background. This is a relatively new test and it may be shown to be more predictive of glaucoma than the ssPERG in some respects but further research is needed. Take Home Message None of these tests conclusively diagnose glaucoma but they all offer important additional measures of electrophysiological function that allow earlier detection of glaucomatous change, primarily related to RGC function, and consequently offer the opportunity for earlier intervention. The ssPERG may do so several years (!) ahead of OCT structural changes which occur ahead of visual field loss. If you are considering adding an electrophysiology unit to your practice, make sure it can perform PVEPs with adjustable contrast and pattern size and can perform an ssPERG. You can wait on the PhNR but it would be nice to have and could serve as a substitute for an ssPERG. If you do not have electrophysiology capability in your office, you should consider referring patients for testing to colleagues who do or to VisionAmerica of Birmingham. We can run a glaucoma test protocol utilizing the tests discussed above and would be happy to provide you with results for your patients. Dr. Nowakowski can be reached via email at [email protected] or via phone at (205) 943-4600.
1 Comment
7/3/2020 09:58:09 am
Glaucoma appears to have a hereditary component. In other words, a family history of glaucoma generally puts one at a higher risk of developing the disease. You usually don’t get its symptoms until later in life.
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