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Visual electrophysiology enables early diagnosis of confounding conditions
Unexplained vision loss, unusual vision and inconsistent test results can be investigated with objective testing.
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While visual fields, OCT and other diagnostic tests can supply important data regarding visual health and performance, they may be insufficient in identifying the functional changes consistent with early disease states. The results of subjective tests can be unreliable, and some tests are difficult to administer to patients who are young, cognitively or physically impaired, or in some other way unable to participate.
Office-based electroretinography (ERG) and visual evoked potential (VEP) tests can supplement existing testing measures for earlier, more accurate diagnosis.
Diagnostic capabilities
ERG and VEP tests are a useful addition to a clinician’s arsenal. Visual electrophysiology testing can aid in the diagnosis of patients who exhibit:
- unexplained vision loss;
- unusual complaints regarding visual symptomatology;
- visual field loss inconsistent with clinical appearance;
- potential for developing glaucoma or inheritable diseases; and
- marginal or borderline results with other tests.
The technology can also be useful in helping differentiate visual pathway disease in children and special needs patients who either cannot be examined with traditional testing or who are uncooperative for procedures such as visual field testing.
Primary uses of ERG, VEP
Visual fields are useful but subjective. VEP is able to test visual function without requiring a manual response and generates objective results that can guide diagnosis and be used to document change over time. VEP Multi-Contrast looks at optic nerve function from the retina to the visual cortex relative to a reference range database and may be effective in diagnosing traumatic optic neuropathy, infectious optic neuropathy, autoimmune optic neuropathy, amblyopia and glaucoma.
ERG, which focuses on ganglion cells in the retina, can enable doctors to examine specific maculopathies that may be at play in hereditary diseases, diabetic retinopathies, glaucoma and macular degeneration. By running both ERG and VEP tests on patients, a clinician can achieve the maximum understanding of the visual process and visual processing system.
Early diagnosis, treatment
ERG demonstrating pre-perimetric glaucoma changes in a patient who had been followed as a glaucoma suspect.
Image: Diopsys
Because it can detect retinal change up to 8 years prior to visual symptomatology or structural damage visible via OCT, ERG tests can be effective tools in identifying patients for early intervention. VEP can show early functional changes in latency or amplitude that are undetectable by other means. The technology can confirm a diagnosis that was uncertain and allow the practitioner to begin treatment much earlier.
VEP tests can also be useful in witnessing asymmetry, an early indicator of a number of conditions. In the normative population, a high concordance of symmetry between a patient’s two eyes exists, so any asymmetry immediately leads to a concern regarding etiology. It is especially instructive for diagnosing glaucoma, which is rarely symmetrical. In addition, if corneal hysteresis is low in one eye, a full pattern analysis with VEP may identify changes in latency and amplitude that clarify the picture.
Training, implementation in practice
The world of diagnostic systems has seen a number of technologies that never caught on, but visual electrophysiology has been used effectively as a research tool for more than 50 years. Over the past decade the technology has also proven its worth as a valuable primary care tool and has reached the point where it is cost effective for both doctors and patients. In the same way that General Electric has made portable ultrasound technology accessible for private practitioners, Diopsys has created a system that is efficient, effective and economical enough for clinical use.
The system no longer requires that sensors be placed directly on the eyes, a practice that was effective but placed the patient at risk for irritation, structural damage and infection. This updated version utilizes a new lid sensor technology that has proven just as indicative while maintaining patient safety and comfort.
Like any advancing technology, clinicians have to invest time and energy into learning to read the patterns that are typical of an abnormal vs. normal outcome. However, the Diopsys NOVA (cart-based) and Diopsys ARGOS (tabletop) devices are much easier to use than previous systems, and technicians can be trained in a number of days.
The technician first uses a sanitary wipe to remove any lotion or oils on the skin. For VEP testing, sensory pads are then placed on the forehead, near the temple, and on the back of the head at the visual cortex. The patient will be given an occluder to cover one eye at a time and will be asked to focus on the computer screen with the uncovered eye. The patient simply has to look at the patterns that appear to “flip” quickly for the duration of the test, and the sensors capture the patient’s response. The system then creates a report for the doctor’s interpretation of the results.
ERG testing is similar, except advanced lid sensors are placed under each eye and a separate sensory pad goes on the forehead. Both tests should be completed before any drops or dilating agents have been administered.
Ideally a technician assigned to electrophysiology testing will possess a high sense of structure and attention to detail. This, together with training and gained experience, will enable them to complete multiple tests per hour. A competent technician can administer testing without the assistance of the clinician, acquiring the data for a doctor to analyze at a later time.
Expanding clinical practice
Many clinical practices are large enough to justify the acquisition of a testing unit for their own patient base. In our clinic we see more than 150 patients each day, and we were sending patients out for electrophysiology testing, an average of several a week just for glaucoma evaluation. We realized that the Diopsys NOVA ERG and VEP Vision Testing System would easily fit into our own clinical flow and that it made more sense to do the testing in-house.
Clinicians may also maximize the use of the device and expand the scope of their practice by approaching other professionals in the community they serve, including pediatricians, neurologists, general practitioners and other primary care optometrists, and making them aware that the technology is available and that the office is available to accept patient referrals. Outside referrals can maximize the system’s use while strengthening ties with other members of the medical community.
Office-based visual electrophysiology can be a valuable tool in creating an objective, multi-faceted picture of a patient’s visual pathway and performance. It can be effective in identifying early changes that indicate disease and examining patients who are difficult to assess with other forms of testing. As the technology has advanced, it has become efficient and cost-effective to incorporate into clinical practice.
- References:
- Banitt MR, et al. Invest Ophthalmol Vis Sci. 2013;doi:10.1167/iovs.12-11026.
- Shengelia A, et al. Evaluation of pattern ERG responses using various electrodes. Presented at: Association for Research in Vision and Ophthalmology meeting; May 1-5, 2016; Seattle.
- For more information:
- J. James Thimons, OD, is a board-certified optometrist at the Ophthalmic Consultants of Connecticut who has served as the chief of optometry at the Veterans Administration Medical Center and the director of the Glaucoma Institute at the State University of New York. In addition to his clinical practice, he is also a clinical professor at seven schools and colleges of optometry in the U.S. and Australia. He can be reached at jimthimons@gmail.com.
Disclosure: Thimons reports he is a consultant with Diopsys.