Visual Pathways: Electrophysiology and Pathology

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Some electrodes are contact lenses with small wires attached; others are fine threads that lay across the surface of the eye. An additional electrode is placed on the forehead. The test is painless. However, the electrode that rests on the eye may feel a little like an eyelash is in the eye. This sensation may persist up to several hours following completion of the ERG.

One should not rub the eyes for at least an hour after an ERG or any test in which the cornea has been numbed , to avoid scratching the cornea. Flashes of light are used to stimulate the retina. The first light is so dim that only the rods respond. The response is sensed by the electrode, amplified, and displayed on a computer screen. The intensity of the testing light is progressively increased.

We refer to this as the dark adapted phase and this part of the test may take 10 to 15 minutes. We also record responses from the eye in the light adapted state. This is done by having the patient view a medium — bright light for 10 minutes. This light suppresses the rods and isolates cone function. Cone function is recorded in response to another series of flashes. In another test of cone function, a light flickering at 30 flashes per second is used as the stimulus. Rods are unable to respond to light flickering at such a fast frequency, so cone photoreceptors are selectively tested. We refer to this portion of the ERG testing as the light adapted phase, and this part of the test may take 10 to 15 minutes.

No special preparation is necessary for this test. We ask that you do not wear any eye make-up the day of testing. A normal ERG contains an a-wave initial negative deflection followed by a b-wave positive deflection. The leading edge of the a-wave is produced by the photoreceptors, while the remainder of the wave is produced by a mixture of cells. An abnormal ERG result suggests abnormal function of the retina which may be caused by a number of diseases of the retina. The results of the ERG will be discussed with your doctor. An electroretinogram ERG can be performed in the operating room under general anesthesia in children who are too young to understand and perform the test in the clinic.

Children will first be evaluated in the clinic by Dr. Arlene Drack. If it is determined that an ERG under anesthesia is necessary, a pre-operative elevation will be conducted which will determine the need for a further anesthesia work up. The dark adaptation is done under anesthesia, and then both the dark adapted and light adapted responses are recorded.

The ERG under anesthesia takes approximately 90 minutes to perform. Multifocal techniques, in particular the multifocal ERG, are used in the diagnosis of ophthalmological diseases. The mfERG is particularly valuable in cases in which the retina appears normal and it is difficult to distinguish between diseases of the retina and diseases of the optic nerve.

Visual Electrophysiology Diagnostic Lab | Children's Hospital Pittsburgh

Electrical responses from the eye are recorded with an electrode just as in conventional ERG recording, but the special nature of the stimulus and analysis produces a more detailed map of where in the central retina ERG responses are normal or abnormal. This can help distinguish among different diseases. For the routine mfERG, the retina is stimulated with a pattern of hexagons. By correlating the continuous ERG signal with the on or off phases of each stimulus element, the focal ERG signal associated with each element is calculated. As in full field ERG the patient assumes a comfortable seated position.

The technician will place a numbing drop in the eye. The patient is asked to look up, and an electrode is gently placed on the eye. The eye not being tested is covered with a black eye patch. The monitor will display a flashing hexagon pattern for 30 seconds, and then stop. This will be repeated 8 times for each eye. The test will take approximately one hour to complete. Data can be displayed in various ways, such as waveform traces or colored graphs.

The results of the mfERG will be given to your doctor for analysis and may be discussed with you at a later date. Two values are measured: the dimmest light the eye can detect the dark adaptation threshold and the time it takes to detect a dim light after going from condition of bright light to darkness, dark adaptation curve.

The test is performed in a dark room. The patient will hold a special light to their eye and look at a bright light for 1 minute. After 1 minute, the patient will be asked if they can see another dimmer light. 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. 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. 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 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. 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.

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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. 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.

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  5. 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.

    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.

    Department of Ophthalmology and Visual Sciences

    Tell us what you think about Healio. Login Register My Saved. Clinicians may detect glaucoma, diabetic retinopathy and optic neuritis sooner than with traditional testing. Please provide your email address to receive an email when new articles are posted on this topic. Receive an email when new articles are posted on this topic. You have already added this topic to your email alerts.

    Visual Pathways: Electrophysiology and Pathology Visual Pathways: Electrophysiology and Pathology
    Visual Pathways: Electrophysiology and Pathology Visual Pathways: Electrophysiology and Pathology
    Visual Pathways: Electrophysiology and Pathology Visual Pathways: Electrophysiology and Pathology
    Visual Pathways: Electrophysiology and Pathology Visual Pathways: Electrophysiology and Pathology
    Visual Pathways: Electrophysiology and Pathology Visual Pathways: Electrophysiology and Pathology
    Visual Pathways: Electrophysiology and Pathology Visual Pathways: Electrophysiology and Pathology

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