Electrooculography (EOG) is a non-invasive diagnostic test that measures the electrical potential between the front and back of the human eye. This technique tracks eye movements and assesses retinal health. The resulting electrical signal is known as an electrooculogram. It evaluates eye function by detecting changes in electrical potential as the eye moves.
The Science of Electrooculography
The fundamental principle behind electrooculography lies in the eye’s inherent electrical property, often described as a small battery or an electric dipole. A standing potential, known as the corneoretinal potential, exists between the cornea (the front, which is positively charged) and the retina (the back, which is negatively charged). This potential difference is approximately 6 mV, with the cornea being positive relative to the retina.
This consistent electrical potential is primarily generated by the retinal pigment epithelium (RPE), a layer of cells located at the back of the eye. As the eye rotates within its orbit, this electrical dipole changes its orientation relative to stationary electrodes placed on the surrounding skin. This change in orientation leads to measurable shifts in the electrical field around the eyes.
When the eye moves from a central position, the positive cornea approaches one electrode, while the negative retina moves closer to the opposing electrode. This creates a potential difference recorded by the electrodes, with the magnitude of the recorded potential directly corresponding to the eye’s position.
The Electrooculography Procedure
The patient sits comfortably in an examination chair. Small, adhesive electrodes are placed on the skin around the eyes. Two electrodes are usually positioned near the outer and inner corners of each eye for horizontal movements, and sometimes electrodes are placed above and below one eye for vertical movements. An additional ground electrode is often placed on the forehead or behind the ear to complete the circuit.
The procedure involves two main phases: dark adaptation and light adaptation. During the dark adaptation phase, the patient sits in a dimly lit or dark room for about 15 minutes, allowing their eyes to adjust to the low light conditions. In this period, the resting potential of the eye slightly decreases, reaching a minimum point often referred to as the “dark trough”.
Following dark adaptation, the light adaptation phase begins. A calibrated light source is introduced, and the patient is asked to move their eyes back and forth between two fixed points, typically alternating between central gaze and looking left or right. During this light exposure, the electrical potential of the eye increases, reaching a peak known as the “light peak” after several minutes.
Medical and Research Applications
Electrooculography serves various purposes in both medical diagnosis and scientific research. In ophthalmology, EOG is valuable for assessing the function of the retinal pigment epithelium (RPE), which plays a significant role in maintaining retinal health. The test helps in diagnosing specific inherited retinal diseases, especially those affecting the RPE’s electrical activity.
A prominent medical application is the diagnosis of Best’s disease, also known as vitelliform macular dystrophy, a genetic disorder characterized by abnormal RPE function. While other conditions like retinitis pigmentosa or Stargardt disease might involve the RPE, EOG is most directly indicative of disorders where the RPE’s light-dependent potential changes are compromised.
Beyond clinical diagnosis, EOG finds diverse applications in research settings. In sleep studies, it is routinely used to track rapid eye movement (REM) sleep, a distinct phase of sleep characterized by rapid, involuntary eye movements. Psychologists utilize EOG to study reading patterns, attention allocation, and cognitive states, observing how eye movements correlate with mental processes. Furthermore, EOG signals are explored in human-computer interface (HCI) research for developing eye-gaze control systems, allowing individuals, including those with disabilities, to interact with technology using only their eye movements.
Interpreting Electrooculography Results
The data collected during an EOG test is analyzed to derive a specific metric that reflects retinal pigment epithelium function. The key measurement obtained from an EOG is the “Arden ratio”. This ratio quantifies the change in the corneoretinal potential between dark and light conditions.
To calculate the Arden ratio, the highest electrical potential recorded during the light adaptation phase, known as the “light peak,” is divided by the lowest electrical potential measured during the dark adaptation phase, referred to as the “dark trough”. A normal Arden ratio typically falls around 2:1 or higher, indicating healthy RPE function.
An Arden ratio below a certain threshold, often cited as less than 1.7, suggests an abnormal RPE response. A low Arden ratio signifies a dysfunction of the retinal pigment epithelium, as seen in conditions such as Best’s disease. This measurement provides a direct indicator of RPE health, helping clinicians pinpoint the nature of certain retinal disorders.