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Electrophysiology testing includes a battery of tests which can be used to provide information about the visual system beyond the standard clinical examination of the eye. The primary objective of the electrophysiologic examination is to assess the function of the visual pathway from the photoreceptors of the retina to the visual cortex of the brain. Information obtained from these diagnostic tests helps establish the correct diagnosis or may rule out related ophthalmic diseases.
Electrophysiologic testing is useful in diagnosing a variety of inherited retinal and optic nerve diseases, toxic drug exposure, inflammatory conditions, intraocular foreign bodies, and retinal vascular occlusions. Electrophysiologic testing is performed most often in large referral centers which have expertise in obtaining and interpreting these data. The data are used in conjunction with the clinical examination and other tests (perimetry, dark adaptometry) to establish a correct diagnosis.
Arlene V. Drack, MD, Medical Director
Wanda Pfeifer, OC(C),COMT, Coordinator, (319) 356-1985, Wanda-Pfeifer@uiowa.edu
Andy Kemerley, Intern, firstname.lastname@example.org
For more detailed information or consultation call 319-356-1985.
Full Field Electroretinography (ERG) is a test used to detect abnormal function in the retina (the light-detecting portion of the eye). Specifically, this test examines the function of the light-sensitive cells of the eye (the photoreceptors), and several other cells that process these light signals before they are sent to the brain. Photoreceptors are specialized nerve cells in the eye that are sensitive to light. There are two types of photoreceptors in the human eye: rods and cones. Cones provide central high-acuity vision used for reading and are also color vision. There are 6 to 7 million cones in the retina, of which about 650,000 are concentrated in the foveola for central vision. Rods provide vision in dim light. There are about 120 million rods in the retina, of which none are located in the foveola.
During the ERG test, the photoreceptors produce tiny amounts of electricity in response to brief flashes of light. If we know exactly how much light enters the eye, and how much electrical response is generated, we can figure out how well the rods and cones are working. The differences in responses are analyzed to differentiate diseases which affect the rods from those which affect the cones or other cells in the retina.
To detect the electrical response of both the rods and the cones, special electrodes are placed on the surface of the eye. There are several types of electrodes; the type of electrode used will depend on the type of test being performed, the age of the patient, and the suspected condition.
The patient assumes a comfortable position (lying down or sitting up). Usually the patient's eyes are dilated beforehand with standard dilating eye drops.
The patient will be asked to sit in the dark for 20 minutes. The patient may have someone sit with them or they may listen to music, but they will not be able to use any device that will emit a light. This allows the eyes to adjust to the darkness so that the function of the rod photoreceptors can be examined. After sitting in the dark, the technician will place numbing drops in the eyes. The patient is asked to look up, and an electrode is gently placed on each eye. 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 ERG performed in the operating room is very similar to the ERG preformed in the clinic. 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 a technique for assessing the local ERG from different regions of the retina. 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.
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. The pattern will seem to flicker randomly, but each element follows a fixed, predetermined sequence called an ‘m sequence’. 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. Usually the patient's eyes are dilated beforehand with standard dilating eye drops. 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. An additional electrode is placed on the forehead. The eye not being tested is covered with a black eye patch.
The patient will be asked to continuously look at an ‘X’ located in the center of a monitor. 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.
Dark adaptometry (DA) measures the eye’s ability to detect a dim light, a measure of rod photoreceptor function. 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. Dark adaptometry may be used in concert with other tests such and the ERG or mfERG.
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. The light will continue to be presented at regular intervals until it is seen. Once seen, the light will be dimmed more and the test will continue until the dimmest light is no longer seen. The time at which each light intensity is seen is documented and recorded on a graph. This test will take approximately 30 to 45 minutes to perform.
This test is performed in a dark room, after the patient is asked to sit in the dark for 30 to 45 minutes. This allows the eyes to be their most sensitive for the light. A light is presented to the patient and its brightness is changed until the dimmest light that can be seen has been determined. This process usually takes 3-5 minutes.
Burian-Allen Electrodes are bipolar, reusable contact lens electrode with built-in lid speculum and lead wire. They are available in a number of sizes. We use these electrodes mainly on sedated patients.
DTL Plus Electrodes are disposable, mono-polar Silver-Nylon thread electrode with adhesive fasteners. This is our main electrode.