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Standard testing, electrophysiology can work together for early glaucoma detection
Objective electrophysiological tests can detect abnormalities sooner than subjective functional tests.
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In recent years, the approach to glaucoma care has begun to change. Earlier detection is enabling earlier treatment, resulting in greatly improved outcomes. There are many medical and surgical options available to treat glaucoma, but catching the disease as early as possible is what truly alters the whole disease course.
PERG and VEP aid in earlier detection
Structural tests such as Humphrey visual field (HVF) and OCT provide valuable information, but electrophysiological tests of the retina and neurovisual pathways are objective tests that detect functional abnormalities. Studies have determined that pattern electroretinography (PERG) can detect abnormalities in patients up to 8 years earlier than structural testing. Additionally, because electrophysiological tests are objective, as opposed to subjective functional testing such as visual fields, the results are particularly reliable.
Visual evoked potentials (VEP) can help determine field loss by measuring the amplitude and latency of electrical activity from the retina to the visual cortex. Amplitude indicates the strength of the signal. Latency measures the length of time it takes for the signal to travel to the visual cortex from the retina. Increased latencies can indicate issues that affect the myelin sheath, such as optic neuritis or multiple sclerosis.
PERG measures function specific to the retinal ganglion cells. This test is useful in detecting diseases that have an expected pattern of macular degeneration. Electrically active cells in the retina elicit a stronger or weaker signal depending on the cell types that are stimulated.
Using PERG in conjunction with VEP and other tests is useful in distinguishing between retinal and optic nerve disorders as well as improving detection of neuropathies and maculopathies.
While in the past these tests were more difficult to conduct, VEP and PERG can now easily be performed in the office. For patients who are suspects for diseases such as glaucoma or for tracking disease progression, these tests give us the ability to detect issues much earlier and can help alter the patient’s therapeutic regimen.
A huge spectrum of patients would be well served with both PERG and VEP testing, including those with diagnosed or suspected glaucoma, diabetic retinopathy, amblyopia, multiple sclerosis and other macular diseases. For those with diabetic retinopathy, the sensitive nature of the testing allows for a more comprehensive analysis of the visual function and the effect the disease has on it. This sensitivity is also helpful in confirming a diagnosis of multiple sclerosis because VEP can detect even the slightest slowing of electrical conduction earlier than might be found with a neurologic examination. Additionally, as electrophysiological testing is particularly useful when testing children, it can be a significant aid in the early diagnosis of amblyopia, especially in cases in which no structural abnormalities are detected. Because these tests are objective, they give the eye care provider a lot of confidence in the results.
Furthermore, these tests can be useful for patients with toxic retinopathies that may be related to medications such as hydroxychloroquine. Often used to treat conditions such as rheumatoid arthritis and lupus, hydroxychloroquine can cause visual loss due to toxicity in the outer retina and retinal pigment epithelium. However, while this drug has been shown to cause the gradual decline of the density of multifocal electroretinography responses, this damage can be improved upon termination of the medication. VEP testing is especially useful in the early detection of this damage, allowing time to halt the formation of irreparable abnormalities.
Working in tandem for best results
Combining standard testing with electrophysiological testing can provide a more complete diagnosis and lead to necessary treatment far sooner than standard testing alone. In order to catch any possible issues before they become a true problem, I take advantage of the full suite of testing options currently available, including HVF, OCT/HRT, VEP and PERG. Electrophysiological tests do not detract from or replace more traditional testing — they only add to it.
Case study
A female patient in her mid-50s presented with a presumed diagnosis of ocular hypertension and a strong family history of glaucoma. She had been told by another physician that she may have glaucoma. When given a full exam, including anterior segment slit lamp examination, ophthalmoscopy, fundus photos, gonioscopy, visual field and OCT, she was found to have ocular hypertension with a pressure of 27 mm Hg and a cup-to-disc ratio of 0.45 in both eyes with no apparent cupping. Because there was no structural damage on OCT and HVF, she was placed on one medication with the goal of reducing the pressure to around 19 mm Hg or 20 mm Hg, thereby lowering her chances of developing optic nerve damage. Her central corneal thickness was 554 µm in the right eye and 547 µm in the left eye.
With electrophysiology testing utilizing the Diopsys NOVA vison testing system, functional damage was detected to the extent that if we were not more progressive with treatment, the patient may have developed eventual structural damage. A second medication was added to help lower pressure to a target of 13 mm Hg or 14 mm Hg.
This case illustrates how the various diagnostics now available can work in tandem to provide the best care for patients. Many will be diagnosed and successfully treated for ocular hypertension through structural tests, but electrophysiology can detect functional damage that can lead to structural issues far in advance, giving physicians the ability to head off more serious issues.
- References:
- Banitt MR, et al. Invest Ophthalmol Vis Sci. 2013;doi:10.1167/iovs.12-11026.
- Lai TY, et al. Am J Ophthalmol. 2005;doi:10.1016/j.ajo.2005.05.046.
- Maturi RK, et al. Arch Ophthalmol. 2004;doi:10.1001/archopht.122.7.973.
- Tsang AC, et al. Ophthalmology. 2015;doi:10.1016/j.ophtha.2015.02.011.
- For more information:
- Jai G. Parekh, MD, MBA, managing partner at Brar-Parekh Eye Associates and a clinical associate professor of ophthalmology at New York Eye and Ear Infirmary of Mt. Sinai/The Icahn School of Medicine, can be reached at 1031 McBride Ave., Woodland Park, NJ 07424; email: kerajai@gmail.com.
Disclosure: Parekh reports no relevant financial disclosures.