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Electrophysiology helps refine localization and diagnosis of visual dysfunction
Electroretinograms and visual evoked potentials are diagnostics within the armamentarium.
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Visual electrophysiology testing provides a powerful set of tools to assist in the clinical investigation of patients with visual dysfunction. Electroretinograms, either flash or multifocal, and visual evoked potentials are particularly beneficial.
“A thorough understanding of the techniques and availability will aid the clinician in proper utilization of the various options,” Eric R. Eggenberger, DO, MSEpi, vice chairman of the Department of Neurology and Ophthalmology at Michigan State University, told Ocular Surgery News. “However, results of such testing should always be interpreted in the context of the entire clinical exam.”
Eggenberger said that electrophysiology can be used as a part of the exam to help refine localization and diagnosis in patients with visual dysfunction.
“[Electrophysiology] has the ability to open new diagnostic avenues in patient evaluation,” he said.
Eggenberger, co-author of a review on electrophysiology in Current Opinion in Ophthalmology, said that visual electrophysiology has changed significantly in recent years. Although ophthalmologists have successfully incorporated electrophysiology into their practices, there is disparity among clinicians in availability of certain techniques, especially the newer testing modalities, he said.
“Not all areas have the ability to provide cutting-edge services,” he said.
Electroretinogram
A flash electroretinogram (FERG) incorporates a widespread retinal response through a mass electrical response of the retina to photic stimulation.
“This is a technique that has been around long enough to be available to most clinicians with access to electrophysiologic services,” Eggenberger said.
Of the three major components of the ERG waveform — a-wave, b-wave and c-wave — the a- and b-waves are most relevant to clinical analysis of human retinal function. Congenital stationary night blindness, congenital achromatopsia, retinitis pigmentosa and cone-rod dystrophies are just some of the retinal conditions for which FERG is particularly useful as a diagnostic adjunct to the fundus examination.
“This technology can provide an overall sense of retinal function, but it may miss subtle retinal abnormalities,” Eggenberger said.
A multifocal electroretinogram (mfERG), on the other hand, provides a more focused look at specific areas of retinal function, Eggenberger said.
“This technique can be quite helpful in sorting out particular retinal causes of visual dysfunction and discerning the location of central visual loss,” he said.
Because mfERG simultaneously records ERG signals from 256 discrete retinal locations, detection of small foci of retinal dysfunction is possible. The technique may also detect glaucomatous visual field defects earlier than automated perimetry would.
Visual evoked potential
Visual evoked potential (VEP) is another time-testing technology that can provide an assessment of optic nerve function, whereby occipital lobe brain wave potential is recorded in response to visual stimulation, starting at the retina and ending at the visual cortex.
“VEP is among the more available visual electrophysiology techniques in most areas of the country,” Eggenberger said.
In early stages of hydroxychloroquine maculopathy, VEP P100 large positive peak latency was found superior to color vision and visual field in one study. VEP results can also be predictive of visual recovery in traumatic optic neuropathy, with lower VEP amplitudes and longer latencies indicative of worse visual acuity. Furthermore, the review noted that VEP can help access visual function in infants and children.
“As my colleague (and co-author of the review) David Kaufman is fond of saying: Electrophysiology needs to be used as an extension of the clinical exam,” Eggenberger said. “This is a useful rule to keep in mind when interpreting electrophysiology test results.”
Eggenberger also said that laboratories often vary widely in their equipment and techniques, so clinicians should be aware of quality controls in the particular lab they use. – by Bob Kronemyer