Take-home message: Recent technologic advances have helped move visual electrophysiological testing into the clinical setting.
By Steven Silverstein, MD, Special to Ophthalmology Times
Kansas City, MO
—Though electrophysiological testing has been around for decades, until recently, its use has been more common in research centers than in clinical settings.
I had the privilege of being involved in some of early-generation diagnostics in the 1980s. To be able to benefit from these technologies decades later is exciting. Becoming an early adapter was an easy decision.
Our practice consists of three physicians in two locations, each equipped with a visual-evoked potential (VEP) and electroretinography (ERG) vision testing system (Diopsys NOVA VEP and ERG Vision Testing System, Diosys Inc.).
The high-volume, comprehensive practice deals with many different pathologies but has a corneal specialty. The practice also has a high volume of glaucoma patients and sees a significant volume of patients with maculopathy, including age-related macular degeneration (AMD) and diabetic retinopathy.
Electrophysiological testing has fit seamlessly into the practice. A separate schedule is maintained for ancillary testing, with technicians conducting visual fields and optical coherence tomography (OCT), as well as pattern electroretinography (PERG) and VEP with the system. While scheduling depends on the clinic’s volume on each particular day, the attempt is to bring patients in for testing when there is less stress on the clinic.
This typically means the patient will be coming in for a separate testing appointment than when he or she is scheduled to see the physician, either earlier that same day or on a prior day. This ensures the physician will have all necessary data when the patient is seen. However, there is no second co-payment for this added visit so the patient need not worry about anything other than fitting the appointment into his or her schedule.
VEP and PERG
VEP and PERG are easily performed in an office setting. As objective tests, they do not rely on responses from the patients. As such, they are considerably useful with non-communicative patients, stroke patients, non-English speaking patients, and children.
In addition, they are excellent for rare cases of malingerers and workman’s compensation claims issues, as the testing provides documented evidence of damage.
VEP measures the electrical activity of the visual cortex. The amplitude of the signal will be greater or lesser depending on the presence of healthy retinal cells versus the amount of disease present. Latency indicates the amount of time it takes for the signal to reach the visual cortex from the retina.
Patients are tested with an 85% high contrast and 15% low contrast stimulus. The former is weighted more toward the function of the parvocellular pathway. Abnormal results from high contrast testing can be an indicator of central vision loss. Low-contrast testing is weighted more toward the function of the magnocellular pathway, where abnormal results may be an early indicator of glaucoma.
PERG focuses on the electrical activity of the retinal ganglion cells rather than the full visual system. ERG concentric stimulus fields protocol is valuable in detecting diseases that have an expected pattern of macular degeneration, whereas the contrast sensitivity protocol is valuable in detecting diseases that affect the retina in a diffuse pattern.
Appropriate patient population
The vision testing system is used for a range of purposes, most notably with glaucoma patients and those at risk for worsening maculopathy, such as significant AMD. When it comes to initializing treatment, these tests are helpful in defining stability or progression of glaucomatous disease, AMD, and optic neuropathy.
If the visual field is equivocal, but the PERG is normal, I may wait to begin treatment. This applies as well in diabetic retinopathy, especially in cases of borderline patients when considering whether to do focal laser treatment. This objective, physiological adjunctive technology to visual fields and OCT can illustrate stability or progression and either speed or delay the need for focal laser or other treatments.
In situations when there is a questionable defect in one eye but OCT and ERG are normal and the other eye is normal, I might wait and retest in 6 months rather than initiating treatment. But if the field and ERG are both abnormal, the likelihood of initiating or adding to treatment is greater.
OCT can be highly variable, whereas PERG seems to be more reliable. The same can be true of visual fields, as there tends to be a learning curve associated with its use. When there is a patient with suspected glaucoma and there is a deficit during visual field testing that is suspicious for glaucoma, follow up with that patient is typically a month later. About 35% of the time, the visual field defect that was present during the patient’s initial testing disappears.
VEP and PERG have greatest utility in borderline cases, especially those in which it is difficult to obtain reliable visual fields, and in non-verbal patients with suspected central nervous system or organic disease. While the visual field test is still the most definitive study I trust—provided it is performed with accurate testing parameters and the patient is practiced taking the test—studies have shown that PERG testing can detect glaucoma in patients up to 8 years earlier than visual fields or OCT.1
Visual electrophysiological testing provides reliable objective data that can impact clinical decision making. The ability to detect disease earlier presents the opportunity to treat much earlier, affording patients a chance for a better prognosis.
Steven Silverstein, MD
E: [email protected]
Dr. Silverstein is a board-certified ophthalmologist and fellow of the American College of Surgeons. He also serves as assistant professor of ophthalmology, University of Missouri Kansas City Medical School, and clinical professor of ophthalmology, Kansas City University of Medicine and Biosciences. Dr. Silverstein receives payment for lectures.