What Is Electrophysiology?
Electrophysiology is used to diagnose and follow progression of disorders affecting vision similar to the manner electrocardiograms (ECGs) are used to monitor heart disease. The retina of the eye, optic pathways in the brain and visual cortex create electrical signals that can be recorded directly from the eye or extracted by computer from brain electrical signals recorded from the scalp, similar to recording electroencephalograms (EEG).
Moran Eye Center electrophysiologists are leaders in the field and are actively involved in research to improve electrophysiological testing.
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Learn more about electrophysiological testing:
- Electroretinograms
- Multifocal electroretinograms
- Visually evoked potentials
- Multifocal visually evoked potentials
- Electro-oculograms
- Auditory brainstem responses
- Dark adaptometry
- Saccadic velocities
Electroretinograms
The electroretinogram (ERG) is an electrical response of the retina to photic stimulation. The ERG is similar to an electrocardiogram in that it shows the electrical health of the retina, similar to the ECG reflecting the health of the heart muscles. The eye does not beat, so the retina must be stimulated to start its electrical response.
A flash of light or bright appearance of a pattern sparks a biphasic negative/positive waveform. Chemical changes in the retina start in 18 nanoseconds. The a-wave is the initial electrochemical discharge of the rods (night receptors) and cones (day and color receptors) and is fully formed in 10–15 milliseconds.
The b-wave is generated by the mid-retina where editing begins before electrochemical signals are sent into the brain via the optic nerves. There are several methods used to record ERGs depending on whether one is most interested in the global health of the retina or whether one wishes to map the retina using multifocal patterns to determine areas producing blind spots.
Multifocal Electroretinograms
Multifocal electroretinograms are used to map the retina-detecting blind spots and are usually best to detect drug toxicity. Abbreviated recording methods are used with infants and in the operating room as part of exams under anesthesia. None of these procedures are painful. The eye is numbed with drops, so contact lenses are not uncomfortable.
Visually Evoked Potentials
The terms visually evoked response (VER) and visually evoked potential (VEP) are synonymous. They refer to electrical potentials recorded from scalp overlying visual cortex at the back of the head, which are extracted from the electroencephalogram (EEG) by signal averaging performed by computer. The most common stimuli are checkerboard patterns displayed on a television monitor or flashes of light. Usually, the recording electrode is placed on the midline overlying the central pole of the occipital cortex at the back of the head.
Visually evoked potentials are used primarily to measure the functional integrity of the optic nerves and pathways to the visual cortex of the brain. Any abnormality that affects the visual pathways or cortex in the brain can affect the VEP. A few examples are cortical blindness, due to meningitis; demyelination, associated with multiple sclerosis; optic neuritis; optic atrophy; stroke; compression of the optic pathways, such as by tumors; oxygen deprivation; visual-field defects; neurofibromatosis; and several dozen others.
In general, slowing neuronal transmission such as produced by myelin plaques, common in multiple sclerosis or gliomas on optic nerves in neurofibromatosis, slow the speed of the VEP wave peaks. Compression of the optic pathways, such as from hydrocephalus or from a tumor, reduces the amplitude of wave peaks.
There are several methods of recording VEPs. In patients over about three years of age, VEPs are usually recorded using a television monitor to present patterned stimuli. In sedated patients and infants, flashes of light from a strobe flash or an array of LEDs are used to stimulate the eye.
Multifocal VEPs can be recorded to map the projection over the occipital cortex. Visually evoked potentials are also recorded in the operating room as part of an exam under anesthesia or during a surgical procedure. Learn more about the various methods of recording VEPs, along with a number of examples of VEPs from patients:
Electro-Oculogram
The electro-oculogram (EOG) measures the potential between the cornea and the back of the eye. The potential produces a dipole field with the cornea electrically positive compared to the back of the eye. With the cornea constantly positive, movement of the eye produces a shift of this electrical potential. By attaching skin electrodes near the inner and outer corners of the eye, the potential can be measured by having the patient move their eyes horizontally.
Procedure
The procedure is simply that the patient keeps their head still while moving the eyes back and forth, alternating between the two red lights. The movement of the eyes produces a voltage swing of approximately 5 millivolts between the electrodes on each side of the eye, which is charted on graph paper or stored in the memory of a computer.
Typically the voltage becomes smaller in the dark, reaching its lowest potential after about 8–12 minutes, often referred to as the dark trough. When the lights are turned on, the potential rises reaching its peak in about 10 minutes. When the size of the light peak is compared to the dark trough, the relative size should be at least near 2:1 or twice as big in light compared to low voltage in dark. A ratio of less than about 1.7 is considered abnormal.
The origin of electro-oculographic potentials is the pigment epithelium of the retina. Although, the light rise of the potential requires both a normal pigment epithelium and normal mid-retinal function. A number of retinal diseases produce an abnormal electro-oculogram (EOG), which can also be demonstrated with the electroretinogram (ERG).
Most Common Use of ERG
The most common use of the electro-oculogram is to confirm Best's disease. Best's dystrophy is usually identified by the appearance of the fundus and can be confirmed by recording both the electroretinogram (ERG) and electro-oculogram (EOG): the ERG will be normal, and the EOG will be abnormal.
Auditory Brainstem Responses
Brainstem auditory evoked potential (BAEP), brainstem auditory evoked response (BAER), and auditory brainstem response (ABR) are synonymous terms. They all refer to a test that is little used in ophthalmology. For decades, this test has been commonly used as a clinical tool in neurology and audiology to detect pathology in the brainstem.
Auditory brainstem responses are recorded by placing an electrode at the top of the head in reference to an electrode on the ear lobe or mastoid behind ear. Click stimuli of several intensities, including about 70 dB above minimum hearing threshold, are presented to one ear through earphones at a rate of 10 or more per second. A computer averages brain potentials evoked by about 2000 clicks.
These small-amplitude potentials are very reliable, and usually five peaks can be identified in the first 10 milliseconds. The best feature of this test is that each peak has been associated with the level of the brainstem generating each component. Alterations in the brainstem, which are adjacent to the auditory pathways, will also alter the brainstem auditory evoked potential (BAEP). The auditory relay center in the mid-pontine region of brainstem lies adjacent to the cranial nerve nuclei IV, V, VI, and VII, so there is a high probability that anomalies associated with these nerves will produce abnormal ABRs.
It has been shown that brainstem auditory evoked potentials reflect abnormal brainstem pathways in visual centers controlling eye movement such as in Duane's retraction syndrome. We have found that half of patients with Marcus-Gunn ptosis demonstrate abnormal brainstem auditory evoked potentials as do about one-third of patients with blepharospasm.
Although this test does not have the broad-base application to ophthalmology, it is of diagnostic use in patients with eye movement disorders that may be of central origin. The brainstem auditory pathways are also the second most common sites of demyelination seen in patients with multiple sclerosis. Neuro-ophthalmologists and neurologists often request the ABR.
Dark Adaptometry
This test measures how quickly and how well people adapt to the dark. When passing from the light into dark, the light sensitivity of the eye improves up to one-thousand-fold or more. This is due to increase in the sensitivity of night receptor cells in the retina (rods) and is called dark adaptation. A person views a black and white three-stripe circle the size of a tennis ball that blinks on about once per second. Over a period of about 30 minutes, a dark-adaptation curve, which reflects level of dark adaptation is created.
Saccadic Velocities
To quantitate eye movement disorders, some doctors measure the relative strength and speed of eye muscles. The patient has small discs placed near the inner and outer corners of the eye, which measures electrical potentials when the patient moves their eyes horizontally a set distance.
The procedure is similar to an electro-oculogram without the dark adaptation. The patient keeps their head still while moving the eyes back and forth, alternating between the two small red lights. The movement of the eyes produces a voltage swing between the electrodes on each side of the eye, which is charted on graph paper or stored in the memory of a computer and reflects relative speed and strength of eye muscles.