Electrophysiology
Visual Evoked potential (VEP)
This test measures how long takes the signal to go from the optic nerve to the visual center of the brain (located in the back of the head).
VEP is an objective test of cortical visual activity. An electrode is placed on the inion, which is located at the back of the head near the cerebral cortex. This records a mass electrical signal from cells in the visual cortex. A positive result from this test depends upon the adequate functioning of the afferent visual pathway from the retina to the brain.
There are several types of VEP tests:
- Pattern VEP – The standard VEP test
- Flash VEP – Typically for lower-vision patients
- 3-Lead VEP – Typically looking for ocular albinism
During this test electrodes are placed on the head and right ear with electrode paste with conductive paste. The patient will see flashing lights while they look straight ahead. The electrodes read the retinal response of eye eye. Blinking is allowed but the patient must sit as still as possible to get the best results.
Electroretinogram (ERG)
This test checks the electrical activity of the retina. Electroretinogram (ERG) is an objective test of the electrical function of the retina. Dim lights test the rod responses and the brighter lights test the cone responses.
During this test electrodes will be placed on the head and ears and a tiny thread will be placed in the tear-film of the numbed eye. The patient will see a fixation light straight ahead and will look at that light throughout the test as they are exposed to different types of flashing lights to check their retina’s response. The patient may blink but should sit as still as possible to get the best results.
This test lasts approximately 45 minutes to one hour. This is due to the time is takes for the placement of the electrodes and also the 30 minute dark adaptation period in the middle of the test. The patient is not tested during the dark adaptation period but must sit in the dark for a half hour to allow their eyes to adjust to the dark and check their response to lights in a darker environment.
Multifocal ELECTRORETINOGRAM (MfERG)
This test stimulates all different parts of the retina. MFERG records complex electrical response from individual areas of the retina to standardized stimuli. This provides objective information about retinal function of the cone systems. Individual ERG waveforms are mapped out in the report which tests 108 parts of the retina.
During the test a contact lens electrode is placed on the cornea, which will be numbed, allowing the recording of a complex electrical response from the entire retina to light stimulation. The patient will look straight ahead the entire time at fixation point while seeing a flashing pattern of light. The test itself lasts only 8 minutes per eye once everything is in place.
Electrooculogram (EOG)
The EOG is typically used to diagnose or rule out Best Disease.
During this test electrodes will be placed on the head and near the eyes. The patient will see a fixation light which they will follow as it moves back and forth. There will be parts in the dark and parts in a very bright light. The patient will get short breaks throughout the test. The patient may blink but should sit as still as possible to get the best results.
This test lasts approximately 45 minutes to one hour. This is due to the time is takes for the placement of the electrodes and the length of the test itself.
The EOG is used to assess the function of the pigment epithelium. The eye acts as a dipole in which the anterior pole is positive and the posterior pole is negative. The patient is asked to switch the eye position repeatedly between two points (usually to the left and right of the center). Since these positions are constant, a change in the recorded potential originates from a change in the resting potential.