Electrophysiology facilitates earlier diagnosis for a range of ocular conditions

Clinicians may detect glaucoma, diabetic retinopathy and optic neuritis sooner than with traditional testing.

Issue: March 2017

ByAlberto González García, MD

ByNate Lighthizer, OD, FAAO, FAAOMS

For most optometrists, visual electrophysiology testing has been considered the last test to turn to when a patient presents with unexplained vision loss.

However, the increased accessibility of electroretinogram (ERG) and visual-evoked potential (VEP) testing and our improved understanding of how to apply it in our clinics shows us that these tests are valuable for diagnosing, evaluating and managing a large variety of ocular disease states.

Electrophysiology can objectively illustrate how the retina and visual system are working and identify functional abnormalities in the cells prior to cell death. Clinicians are then able to track disease progression and tailor treatment to improve that function.

Pattern electroretinography

Pattern electroretinography (PERG) is an objective test of retinal ganglion cell function. While optical coherence tomography looks at the structure of the eye, PERG looks at how well the cells in the eye are functioning.

This test is particularly valuable in the early detection of glaucoma. It has been shown that functional abnormalities stemming from glaucoma can be detected up to 8 years earlier with PERG than with traditional structural tests (Banitt et al.).

Diopsys VEP test result of a patient with multiple sclerosis with optic neuritis showing visual pathway dysfunction. Click here for larger image.

Depending on the health of the retina, a stronger or weaker signal is measured from electrically active retinal ganglion cells. Dysfunctional cells producing a weaker signal can improve with therapy, and the increase in function can be seen on subsequent PERG test results. This, in turn, leads to the ability to preserve healthy cells through earlier treatment, which can slow the progression of the disease.

Clinical diagnosis of glaucoma has typically focused on elevated intraocular pressure, optic nerve cupping, and visual field and retinal nerve fiber layer defects. However, these are often insufficient due to the nature of the disease. IOP is not always elevated enough to raise concern, allowing the disease to progress unchecked. Moreover, once the disease has reached the stage where the optic nerve has cupped or a defect is seen on a visual field or OCT, it is already advanced. PERG enables cell dysfunction to be identified in time to instigate early intervention and treatment.

Diopsys ffERG test result of a patient with dense cataracts in both eyes and no retinal dysfunction. Click here for larger image.

As with glaucoma, PERG testing is beneficial for diabetic retinopathy (DR) suspects, as it is often possible to detect electrophysiological abnormalities of the retina prior to the development of any obvious clinical retinopathy.

The fact that dysfunction in the retina and visual pathways occurs well before structural changes are typically identified has been extensively documented. Other tests for DR, such as OCT or intravenous fluorescein angiography, can also be informative; however, PERG is not only more efficient in detecting abnormalities earlier (Caputo et al. and Parisi et al.), test results can also demonstrate treatment efficacy (Ozkiri and Ozkiri et al.).

Plaquenil (hydroxychloroquine sulfate, Sanofi-Aventis) is used to treat malaria and inflammatory disorders such as rheumatoid arthritis and lupus. This drug has also been known to damage the structure of the outer retina and retinal pigment epithelium. According to the American Academy of Ophthalmology, the preferred tests to evaluate patients with potential macular toxicity are OCT, visual field and multifocal electroretinography. Early detection is the key to minimizing this damage, and PERG has proven to be a valuable tool in identifying toxic retinopathy (Neubauer et al.). Testing can be repeated after the agent has been discontinued to track further progression.

Full field electroretinography

Full field electroretinography (ffERG), sometimes referred to as flash, is helpful when assessing retinal dysfunction such as in retinal vascular occlusions and DR.

This test measures generalized dysfunction in response to stimulus of the entire retina, with flicker ffERG particularly inciting a response from the cone cells. As the flash stimulus is intense, ffERG is especially helpful in monitoring disease progression and treatment efficacy in moderate to severe retinopathies.

This test is also beneficial for patients with media opacities, as the full field light stimulus is able to penetrate through to the retina. Therefore, ffERG is a useful tool for optometrists managing patients with dense cataract. The technology can be used prior to surgery and after the procedure to monitor healing and vision improvement.

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Visual-evoked potential

Visual evoked potential (VEP) measures the electrical activity in the entire vision system, from the patient’s retina to the visual cortex. Latency (peak time) is the first important measure, which indicates the response time along the visual pathway. Amplitude is the second important measure and indicates the strength of the signal.

Possible ocular damage, optic nerve impairment, amblyopia or neurological diseases such as optic neuritis due to multiple sclerosis can be signified by delayed response time or reduced signal strength. A delayed latency confirms that the visual pathway is dysfunctional. Early identification of these abnormalities allows for earlier intervention.

As an objective test, it is also ideal for nonverbal patients such as children or those with communication issues, as it does not necessitate a response from the patient.

Patient set-up for Diopsys ffERG vision testing using the handheld mini-Ganzfeld.

Optic neuritis is often associated with multiple sclerosis. Useful clinical and subclinical information can be provided by OCT; however, VEP has a higher sensitivity, especially in identifying subtle past optic neuritis (Naismith et al.). VEP is also more efficient than visual acuity or optic nerve appearance in determining optic pathway damage (Kelly et al.).

Treatment of any pathology is significantly improved with early detection. This is particularly true with amblyopes, as the underlying potential of vision improvement is an essential element in establishing a treatment arc for this population.

VEP is especially beneficial in diagnosing and projecting treatment plans for these patients because it identifies abnormal changes along the entire visual pathway. Amplitude increases on pattern VEP test results reflect vision improvement over the course of treatment, and efficacy can also be followed.

Traumatic brain injury (TBI) can cause ophthalmic issues such as blurry vision, motion hypersensitivity, reading impairment and posttraumatic vision syndrome, which optometrists can address with vision therapy. VEP is especially beneficial in not only diagnosing and projecting treatment courses for TBI patients, but also in tracking the efficacy of vision therapy. As the test results are obtained objectively, patients cannot inadvertently skew results, allowing for accurate readings of ocular and visual dysfunctions.

Patient taking a Diopsys ERG (pattern) vision test.

ERG and VEP tests allow optometrists to treat a wide variety of patients, many of whom may have been difficult to examine in the past.

References:
Arden GB, et al. Br J Ophthalmol. 1974;58:183–192.
Banitt MR, et al. Invest Ophthalmol Vis Sci. 2013;54(3):2346–2352. doi: 10.1167/iovs.12-11026.
Caputo S, et al. Diabetes Care. 1990;13(4):412–418.
Ciuffreda KJ, et al. Optometry. 2011;82(6):337-339. doi: 10.1016/j.optm.2011.03.006.
Kelly JP, et al. Ophthalmology. June 2012; 119(6). doi: 10.1016/j.ophtha.2011.12.035.
Naismith RT, et al. Neurology. 2009;73(1):46–52. doi: 10.1212/WNL.0b013e3181aaea32.
Neubauer AS, et al. Ophthalmic Res. 2004;36(2):106-113. doi: 10.1159/000076890.
Oner A, et al. Doc Ophthalmol.2004;109:223–227.
Ozkiris A, et al. Doc Ophthalmol. 2004;109(2):139-145.
Ozkiri A. Doc Ophthalmol. 2010;120(3):243–250.
Parisi V, et al. Diabetes Metab Res Rev. 2001;17(1):12-18. 10.1007/s10633-010-9219-0.
Sokol S. Br J Ophthalmol. 1983;67:310–314.
Sokol S. Int Ophthalmol Clin. 1980;20:251–268.
Yadav NK, et al. Brain Inj. 2015;29(3):352-365. doi: 10.3109/02699052.2014.979229.
Yin ZQ, et al. Chin J Ophthalmol. 1989;5:312–315.
Yu M, et al. Invest Ophthalmol Vis Sci. 1998;39:2033–2040.

For more information:
Alberto González García, MD, a neuro-ophthalmologist, is the research director at Diopsys and chairman of the Diopsys Scientific Advisory Board. He may be reached at: agonzalez@diopsys.com.
Nathan Lighthizer, OD, is an associate professor at the Northeastern State University Oklahoma’s College of Optometry, assistant dean of Clinical Care Services, chief of Specialty Care Clinics and chief of Electrodiagnostics Clinic. He may be reached at lighthiz@nsuok.edu.

Disclosures: González García is employed by Diopsys. Lighthizer reported he is a consultant for Aerie Pharmaceuticals, Alcon, BioTissue, Diopsys, Optovue and Shire.