Ophthalmic Electrophysiology Service
Electrodiagnostic consultation for review of findings
obtained in other laboratories is available. Please contact 319-384-9381
for details.
Ophthalmic Electrophysiology
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 electophysiologic
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 diseases, toxic drug exposure, 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 the correct diagnosis.
The Electrophysiology Service specializes in standardized electroretinography
(ERG), electrooculography (EOG), focal macular electroretinography (FERG)
and Goldmann-Weekers dark adaptometry.
Electroretinography (ERG)
Photoreceptors are nerves in the eye which are sensitive to light. There
are two types of photoreceptors in the human eye: rods and cones.
Cones provide central reading vision, and are responsible for 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 night vision
and detect motion. There are about 120 million rods in the retina of which
none are located in the foveola.
Minute electrical voltages are produced by the eye. Small electrodes
placed about the patient's eye record these micro-voltages as different
lighting conditions are presented. The differences in voltages are analyzed
to differentiate diseases which affect the rods from those which affect
the cones. As an example, patients with retinitis pigmentosa
initially have problems with their rods, causing them to complain of
poor peripheral vision and reduced night vision. Patients with rarer
cone dystrophies complain of poor visual acuity and color vision.
 
Normal ERG (left), ERG of patient with retinitis pigmentosa (right).
The ERG is performed in a specially shielded room called a Faraday's
cage. It is commonly known as a copper room because the walls,
ceiling, and floor are covered with copper sheeting to reduce electrical
interference from generators and electric motors.
Inside the copper room, special contact lenses are placed on the patient.
The lenses have a thin metal ring which detects the small electrical
signal produced by the eye. A single flash of light is used to stimulate
the retina. The light is so dim that only the rods respond. The response
is sensed by the lens, amplified, and displayed on a computer screen.
The intensity of the testing light is progressively increased Cones
require more light to be stimulated than rods and will eventually contribute
to the overall waveform. Data is also collected by presenting a uniform
white background continuously for 10 minutes, suppressing the rods and
enhancing cone function. The cone function is recorded using the standard
flash. 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 being
selectively tested.
Electro-oculography(EOG)
Electro-oculography (EOG) measures the difference in the electrical potential
between the front and the back of the eye in response to periods of dark
and light. Although the EOG is performed in the darkened "copper room"
it differs from ERG.
Skin electrodes are placed on the face at each side of each eye. The
patient is positioned in front of a bowl-shaped illuminator called a
Ganzfeld bowl. Three small lights positioned across the back of the
bowl are used to direct the patients direction of gaze from left to
right.
EOG electrodes in place.
The test begins with the white background light inside the bowl on.
After six minutes, or when it appears that the baseline voltage is stable,
the white light is turned off. The patient is now in total darkness,
but still following the three small target lights. At the end of 16
minutes of darkness, the white background light comes back on for 14
minutes.
In normal patients, the EOG tracing demonstrates a dark trough
where there is low voltage during darkness, and a light peak.
where there is higher voltage during illumination. The ratio of dark
to light responses is called the Arden ratio. Analysis of this
pattern aids in diagnosis. As an example, patients with Best's vitelliform
macular dystrophy have little or no increase in voltage in the light
compared to the voltage in darkness.
 
Normal EOG (left), EOG of patient with Best's macular dystrophy
(right).
Standardized Testing
There is currently an effort to standardize electrophysiology testing
throughout the world so that data collected in one laboratory can be used
by others. In the past, each laboratory had its own specific method of
testing. Data from one laboratory could not be accurately compared with
data from another. With the growing acceptance of standardized electrophysiologic
testing, collaborative studies on patients with rare diseases as seen
by different investigators can be performed.
The electrodiagnostic evaluation plays a crucial role in advancing
our understanding of inherited vitreoretinal diseases. Without a proper
clinical diagnosis, molecular diagnostics would be impossible. Accurate
clinical classification in "affected" and "unaffected" disease status
is essential.
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