A FEW years ago, visual electrophysiology testing, or electrodiagnostics, was the domain of research facilities and specialized clinics. Today, visually evoked potentials (VEP) and pattern electroretinography (PERG) provide excellent diagnostics, are patient friendly, affordable and integrable in the office setting. What’s more, the technology is incredibly useful for glaucoma suspects and those who show signs of early dysfunction because of AMD and diabetic retinopathy (DR).

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Electrodiagnostic tests evaluate the retina and visual system to aid in the detection of functional issues before irreparable structural damage occurs. Earlier detection of dysfunction leads to earlier intervention and improved long-term outcomes. Additionally, because of the objective nature of VEP and PERG, they are particularly beneficial for non-communicative patients. These tests do not replace standard tests, such as OCT or VF, but instead provide another piece of the diagnostic puzzle and an additional method by which to provide better patient care.

Here, I provide an overview of electrodiagnostics, and four reasons you should consider adding it to your practice.


Electrical activity in the visual cortex is measured using VEP, with the amplitude and latency indicating the presence of healthy visual pathways vs. any dysfunction. Low and high contrasts measure the degradation of the magnocellular and parvocellular pathways, respectively, and results can indicate the presence of neuro-visual disorders, such as optic neuritis and vision problems because of traumatic brain injury (TBI).

In normal visual function, you would expect to see amplitudes between 10 and 20 microvolts, though greater than 5 microvolts is OK, and latencies between 95 and 120 milliseconds. (See photos, on p.23, for normal report results.)

PERG reveals the amplitude and phase of retinal ganglion cell function and aids in the detection of the early signs of diseases, such as glaucoma, DR, AMD and macular toxicity from medications.

PERG produces sinusoidal waves as its raw data. On the interpretation and report printout, the values are expressed as magnitudes, magnitudeD and a ratio of the magnitudeD divided by the magnitude. Typically, normal values are: magnitude ≥ 1.2; magnitudeD should be close in value to the magnitude, ideally it would be exact; MagD/Mag ratio, the most significant number = 0.8 to 1.0. You will follow the ratio through time, with 0.7 to 0.8 representing borderline and <0.7 abnormal.

Examples of normal PERG (top) and VEP (bottom).
Courtesy of Nathan Lighthizer, O.D., F.A.A.O.


Electrodiagnostics aid in evaluating function to help detect abnormalities prior to irreversible damage. Routine exams, fundus photos and OCTs all aid in the assessment of the structure of ganglion cells, the retinal nerve fiber layer, the macula and other aspects of the eye. The condition of that structure can determine whether cells are dead or alive. However, once cells die and structural damage occurs, it is generally too late to fully recover what is lost. Tests, such as VFs, provide vital information, but they are subjective. Electrophysiology, on the other hand, is an objective means of determining cellular dysfunction, providing critical information, enabling you, the O.D., to intervene early enough before irreversible damage occurs.

Specifically, conditions, such as DR and glaucoma, can be detected much earlier than with other diagnostics — years earlier in some cases.1-6 The Academy of Ophthalmology has even added PERG to its definition of a glaucoma suspect.7 These tests also are valuable in tracking disease progression and treatment efficacy and have shown promise in predicting whether ocular hypertensive patients will experience VF loss.8-9

In my experience, patients have been greatly impacted because of these diagnostic tests. Treatments have been confirmed in many patients — and altered in others — based on electrophysiology test results.

In our electrodiagnostics clinic, a patient who had a normal VF and results outside the normal limits on the OCT was able to avoid an erroneous glaucoma diagnosis because of PERG findings, which served as a kind of diagnostic tiebreaker. (See reports, on p.26.)


The electrodes used by these tests are comfortable; set up and testing are straightforward. For VEP, electrodes are attached to the forehead, temple and back of the scalp over the occipital lobe. (See p.27 for pictures on placement.) For PERG, electrodes are attached to the forehead and on the lower eyelids. The patient is then asked to look straight ahead at a computer monitor that displays a checkerboard or grating pattern on the screen. Additionally, testing is objective and reliable, as the patient is required to do very little.


With the ability to diagnose earlier and track treatment efficacy for more accurate care, integrating visual electrophysiology testing has significant practice-building possibilities. These include: Increased rates of positive outcomes, accurate treatment paradigms, because of earlier diagnoses and treatment tracking, and access to cutting-edge technology in the comfort of your practice.

Something else to keep in mind: Both VEP and PERG are reimbursable diagnostic tools under CMS guidelines.


The technology can be logistically fit into most office spaces. Footprints for electrodiagnostic technologies are relatively small. While different systems are available, none are larger than most new OCTs.

In addition, companies provide training. Because this is likely unique technology in your practice, you and your staff may need extra support. Typically, companies send in personnel to train doctors and technicians to run the tests and interpret the results. Training is a valuable part of the investment and one you should inquire about.

OCT, top left, shows significant anomalous results OD, indicating the patient is falling outside the normal ranges for both retinal nerve fiber layer and ganglion cell measurements, compared with normative database. PERG results, top right, show normal magD/mag ratios. At bottom, note normal VF results, without glaucomatous VF defects.
Courtesy of Nathan Lighthizer, O.D., F.A.A.O.


The greatest consideration for incorporating electrophysiology testing into your practice is the significant benefit to the patient. The ability for improved intervention and prevention is undeniable.

Electrodiagnostic testing is no longer the intimidating process of the past. Benefits to patient care outweigh the effort to integrate these systems into a practice. Consider adding both VEP and PERG to your practice. OM

Electrode placement for VEP, forehead, temple and back of the scalp.
Courtesy of Nathan Lighthizer, O.D., F.A.A.O.


  1. Pescosolido N, Barbato A, Stefanucci A, et al. Role of electrophysiology in the early diagnosis and follow-up of diabetic retinopathy. Jour of Diab Res 2015 May; Article ID 319692. doi:10.1155/2015/319692
  2. Tafreshi A, Racette L, Weinreb R, et al. Pattern electroretinogram and psychophysical tests of visual function for discriminating between healthy and glaucoma eyes. Am J Ophthalmol. 2010 Mar; 149(3): 488–495. doi:10.1016/j.ajo.2009.09.027
  3. Bach M, Unsoeld A, Philippin H, et al. Pattern ERG as an early glaucoma indicator in ocular hypertension: a long-term, prospective study. Invest Ophthal & Vis Sci. 2006 Nov;(47)4881-4887. doi:10.1167/iovs.05-0875
  4. Pillai C, Ritch R, Derr P, et al. Sensitivity and specificity of short-duration transient visual evoked potentials (SD-tVEP) in discriminating normal from glaucomatous eyes. Invest Ophthalmol Vis Sci. 2013 Apr.;(54), 2847-2852. doi:10.1167/iovs.12-10097
  5. Karaśkiewicz J, Drobek-Slowik M, Lubinski W. Pattern electroretinogram (PERG) in the early diagnosis of normal-tension preperimetric glaucoma: a case report. Doc Ophthalmol, 2014 Feb.;(128):53–58.
  6. Banitt MR, Ventura LM, Feuer WJ, et al. Progressive loss of retinal ganglion cell function precedes structural loss by several years in glaucoma suspects. Invest Ophthalmol Vis Sci 2013 Mar 28;(54)2346-2352.
  7. American Academy of Ophthalmology. AAO Basic and Clinical Science Course 2015-2016, Section 10, Chapter 4: Open-Angle Glaucoma.
  8. Karaśkiewicz J, Penkala K, Mularczyk M, Lubiński W. Evaluation of retinal ganglion cell function after intraocular pressure reduction measured by pattern electroretinogram in patients with primary open-angle glaucoma. Doc Ophthalmol. 2017 Apr;134(2):89-97. doi: 10.1007/s10633-017-9575-0. Epub 2017 Feb 7.
  9. Pfeiffer N, Tillmon B, Bach M. Predictive value of the pattern electroretinogram in high-risk ocular hypertension. Invest Ophthalmol Vis Sci. 1993 Apr;34(5):1710-5.