The electrooculogram is the electrical pulse created by the difference between the cornea of the eye and the retina. The difference is caused by a concentration of electrically live nerves in the retina compared to the frontal eye area. The EOG test measures the resting electrical potential that exists between the retina and the cornea of the eye. The eyes are always in motion during daily activities to maintain one's line of vision pointed in the object of interest. There are three opposing pairs of muscles attached to the eyeball that control the eye movement along any direction. The muscle pairs work to move the eye vertically, torsionally and horizontally.
Positive or negative electric signals are produced when the eyes move along any axis. The amplitude of the signal increases with the increment of the movement angle and the width of the signal is proportional to the time it takes to make the eye movement. There is no change in the amplitude and potential of the signal when eyes are looking straight ahead or stationary.
The EOG test enables the doctor to:
- Track the pupil
- Diagnose retinal disorders
- Examine vestibular function
- Identify focal points on images
- Calculate eye movement velocity
- Determine eye travel and position
- Study jerky or pendular nystagmus
- Record horizontal and vertical movement
- Examine the saccadic and micro saccadic movement
Also Known As
The EOG analyses the function of the pigment epithelium of the retina and the interaction between the retinal pigment epithelium and photoreceptors. The test is conducted by:
The measure of retinal function depends on the cohesion of the retinal pigment epithelial layer, which is the outermost layer of the retina. The EOG investigates disorders of the retinal pigment epithelium which allows the early diagnosis of certain progressive retinal diseases, including:
- Best's disease
- Stationary night blindness
- Inherited macular diseases
- Pigment epithelial dystrophies
Preparation & Expectation
The patient's pupils should be dilated during the EOG test. So the doctor will apply eye drops medication in the patient's eyes before the exam. Dilating the patient's pupils allows more light to pass through for better results. The patient will also be required to stay in a room with stable lighting for a minimum of 30 minutes before the exam. The patient cannot undergo eye examinations that involve a robust retinal illumination and retinal imaging, such as fundus photography, indirect ophthalmoscopy, fluorescein angiogram, etc., during this period.
It is important to note that the effects of the eye drops can last for up to 15 hours and so it is advisable for the patient to bring sunglasses for use after the test. Furthermore, the patient should come with someone or arrange for transportation back home because they may experience blurry vision for some time after the test.
The patient will lie down on a couch and remain still. Four recording skin electrodes will be placed at the middle and corners of the patient's eyes, while the grounding conductor will be placed on the forehead. When the patient's eyes move from a center position toward the sides of the eyes or one of the wires, the electrode detects the positive retinal side while the opposite one gets the negative side. A potential difference will happen between the electrodes. If the resting potential is assumed to be constant, then the recorded potential is a measure of the patient's eye's position.
Where the pigment epithelium layer is healthy, the standing potential is determined between the back and front of the eye that matches changes in background illumination. During the dark adjustment, the resting potential declines marginally and reaches a minimum after a few minutes. However, there is a substantial rise in the resting potential when the light is switched on. It drops off after several minutes as the retina adjusts to the light and returns to a stable equilibrium.
The ratio of the light peak divided by the dark trough, i.e., the maximum amplitude in the light modified environment divided by the minimum amplitude in dark adjusted conditions, is the Arden ratio. It is the standard diagnostic assessment of the EOG and is comparable to the eye movement recordings in practice. The patient will be asked to change eye position frequently alternating from center to the right and center to the left. A change in the recorded potential derives from a difference in the resting potential because these positions are constant.
Some cases where EOG is abnormal include:
- Progressive high shortsightedness
- Diffuse chronic chorioretinal swelling
- Chloroquine and hydroxychloroquine toxicity
- Didanosine use which is reversible after stopping therapy
- Siderosis bulbi which are retained intraocular iron particles
- Exposure to silicone oil in retinal detachment repair surgery
- Retinal detachment, the retina has separated from the underlying layer
- Membranoproliferative glomerulonephritis with electron-rich matter removal in the choriocapillaris
- Oguchi disease- a rare inherited form of stationary night blindness linked to very slow dark adaptation and discoloration of the fundus
- Diabetes- the disease may be getting worse or progressing with time and damaging the optic nerve. It is the nerve cord that transmits light and pictures from the eye to the brain. Optic nerve damage affects the patient’s vision and can lead to blindness.
Risks & Complications
The electrooculogram is safe and there are no risks to the patient. However, the patient should tell the doctor if one is allergic to eye drops and if they are taking any medications, including dietary supplements and over the counter medicines. Patients suffering from glaucoma or with a family member who has the disease should inform the doctor because, in sporadic cases, the pupil dilating medication may have side effects, such as dry mouth, dizziness, narrow-angle glaucoma, nausea and vomiting.
A patient should inform the doctor at once or seek immediate medical assistance if they get sudden and severe eye pain or eye vision problems after the test.