Electroretinography is not what it used to be

Optometry Times Journal, November digital edition 2022, Volume 14, Issue 11

The ERG is a game changer for patients with diabetes.

Until recently, if someone had asked me about electroretinography, it would have brought to mind the large equipment stored in the basements of optometry schools, textbooks I’ve never revisited, and content I had skimmed while reviewing complex research studies. In short, I didn’t think electroretinograms (ERG) were practical for the average optometrist, and I wasn’t sure that they added value to patient care. Today, however, my opinion couldn’t be more different.

I now believe the ERG device is one of the easiest-to-use instruments in the optometrist’s arsenal. It is portable, fast, tech-friendly, and—best of all—gives me more confidence when caring for patients who have diabetes and are at-risk for diabetic retinopathy (DR). As a bonus, it’s affordable and reimbursable.

The ABCs of ERG

If you haven’t thought about ERG for a while, allow me to review the basics. An ERG is an electrophysiological test; it is to the retina what the electrocardiogram (ECG) is to the heart. Just as an ECG is crucial to diagnosing illness and monitoring heart function, ERG can play a leading role in the detection of retinal dysfunction.

During an ERG, the patient’s eyes are stimulated by light, and the resulting electrical activity of retinal cells is measured by skin or corneal electrodes. Depending on the type, intensity, and color of the light, information about different areas and retinal cell types can be obtained.

There are 3 kinds of ERGs—full-field flash (ffERG), pattern (PERG), and multifocal (mfERG)—and each has distinct applications and benefits. Different tests allow clinicians and researchers to evaluate patients for retina-related conditions, reliably monitor retinal function over time, and evaluate the efficacy of retinal treatments. In optometry, we are most interested in ffERG because it aids in the diagnosis and management of diseases we see every day.

In ffERG, electrodes record the summed electrical response of all retinal cells to a flash from a Ganzfeld bowl that uniformly illuminates the entire retina with full-field light.1 The summed electrical response is a blend of the results from a range of different cells and regions in the retina, meaning that ffERG shows the status of the retina as a whole.

The major responses measured are the a-wave, the b-wave, and the photopic negative response (PhNR). The first to appear after the flash of light is the negative a-wave, which reflects the electrical activity of the photoreceptors. The second is the positive b-wave, which reflects the electrical activity of the bipolar cells that transmit signals from the photoreceptors to the inner retina in light-adapted conditions and rod response in dark-adapted conditions. After the b-wave comes the photopic negative response (PhNR).

Since each response originates in a different area of the retina, abnormalities can pinpoint the site of retinal dysfunction. The basic measurements are the amplitudes and implicit time values of the response. Amplitudes are how far from the baseline electric potential of the eye dips in the a-wave and climbs in the b-wave. Implicit time is the time that elapses between the flash of light and the trough of the a-wave or the peak of the b-wave. To put it more simply, amplitudes show how strong the retinal response is, and implicit times how quickly the retina reacts to light.

Amplitude results that are lower than reference values may indicate cell death, whereas implicit response times that are slower than reference values indicate cellular stress. Thus, the ffERG is a powerful tool in the diagnosis of retinal pathology, the monitoring of dysfunction severity, and the assessment of treatment effect.

Indeed, ffERGs have proven useful not only in the evaluation of disorders like DR, glaucoma, vein occlusions, and retinitis pigmentosa, but also in the discovery of drug toxicity before severe, vision-threatening damage is done.2

From function to structure

ERG evaluates functional abnormalities of the retina, whereas structural imaging shows its anatomy. Although both assessments are clinically useful, functional changes generally appear well before structural ones. Catching them early, or assessing functional changes over time, is critical for minimizing damage and maximizing vision retention. What’s most appealing to me about ERG is that by detecting functional stress, it can anticipate structural damage.

In studies comparing the ability of ERG and structural imaging to evaluate sight-threatening DR, ERG outperformed traditional imaging at predicting which patients would likely need subsequent medical intervention.3,4

When talking to patients, I often use a weather analogy. Most technology in our practice helps us understand what’s happening now, and thus is akin to looking at the sky to see whether it’s raining. With ERG, we can help predict what tomorrow will be like. That matters as much to my patients as it does to me. It gives them peace of mind, and it gives me confidence in my clinical choices. Whether that means referring patients to a specialist or monitoring them closely, functional tests allow me to make sound clinical decisions.

Power in the palm of your hand!

I use the RETeval Device by LKC Technologies, the only FDA-cleared, portable, battery-operated, nonmydriatic ERG testing instrument on the market in the US. It has skin rather than corneal electrodes, adjusts for pupil size in real time, and doesn’t require dilation.

The clinical value of a handheld device cannot be overstated. The RETeval tests both eyes in less than 5 minutes, and technicians feel comfortable using it. It is easy to interpret due to several simple, yet robust, testing protocols, including 2 that I feel are essential in optometry: the DR and glaucoma-associated protocols.

Confident care

The RETeval offers an objective assessment protocol for measuring likely DR progression risk. By combining a retina cell stress measure and a pupil light response, the device allows you to assess all patients rapidly and noninvasively. It also helps determine if patients are at risk for DR at every visit and determine risk of disease progression.

The RETeval is a reimbursable test (CPT code 92273) that tests both eyes in less than 5 minutes.4 Many practitioners also use the protocol to screen all patients with diabetes, whether or not they’ve been diagnosed with retinopathy, but this application is not currently reimbursable.

The test allows for earlier detection of retinal dysfunction at a lower cost and with less knowledge than is required by traditional ERG imaging.5 The RETeval is also better for evaluating patients with media opacities and small pupils.4 Interpreting results is easy as well; a score of 23.4 or higher indicates an 11-fold risk of requiring medical intervention within 3 years (Figures 1 and 2).4

Conclusion

In optometry school, most of what I learned about ERG had to do with inherited eye diseases, where it continues to play a vital role. But recently its capabilities have expanded dramatically, offering optometrists the power to manage glaucoma and stay on top of DR, which is a benefit that elevates the standard of care we can provide in a particularly profound way.

References
1. Azarmina M. Full-field versus multifocal electroretinography.
J Ophthalmic Vis Res. 2013 Jul;8(3):191-192.
2. Creel DJ. The electroretinogram and electro-oculogram: clinical applications. In: Kolb H, Nelson R, Fernandez E, Jones B, eds. The Organization of the Retina and Visual System. WebVision; 2018: Accessed September 19, 2022. https://webvision.med.utah.edu/book/electrophysiology/the-electroretinogram-clinical-applications/
3. Al-Otaibi H, Al-Otaibi MD, Khandekar R, et al. Validity, usefulness and cost of RETeval system for diabetic retinopathy screening. Transl Vis Sci Technol. 2017;6(3):3. doi:10.1167/tvst.6.3.3
4. Brigell MG, Chiang B, Maa AY, Davis CQ. Enhancing risk assessment in patients with diabetic retinopathy by combining measures of retinal function and structure. Transl Vis Sci Technol. 2020;9(9):40. doi:10.1167/tvst.9.9.40
5. Zeng Y, Cao D, Yu H, et al. Early retinal neurovascular impairment in patients with diabetes without clinically detectable retinopathy. Br. J. Ophthalmol. 103, 1747–1752 (2019). https://bjo.bmj.com/content/103/12/1747