Electronystagmography

Overview
Electronystagmography (ENG) is a diagnostic test to record involuntary movements of the eye caused by a condition known as nystagmus. It can also be used to diagnose the cause of vertigo, dizziness or balance dysfunction by testing the vestibular system.

Technique and Results
The test is performed by attaching electrodes around the eye and measuring the movements of the eye in relation to the ground electrode. The vestibular system monitors the position and movements of the head to stabilize retinal images. This information is integrated with the visual system and spinal afferents in the brain stem to produce the vestibulo-ocular reflex (VOR). ENG provides an objective assessment of the oculomotor and vestibular systems. A newer standard for the recording is the use of infrared video systems which allow for a more detailed observation and analysis of these eye movements, called Video Nystagmography. A similar test is performed for testing vertigo by using caloric reflex test, which can be induced by air or water of specific temperatures, typically +/- 7 degrees Celsius from body temperature.

The standard ENG test battery consists of 3 parts:
 * oculomotor evaluation
 * positioning/positional testing
 * caloric stimulation of the vestibular system

The comparison of results obtained from various subtests of ENG assists in determining whether a disorder is central or peripheral. In peripheral vestibular disorders, the side of lesion can be inferred from the results of caloric stimulation and, to some degree, from positional findings.

ENG or VNG can be used to record nystagmus during oculomotor tests such as saccades, pursuit and gaze testing, optokinetics and also calorics (bithermal or monothermal). Abnormal oculomotor test results may indicate either systemic or central pathology as opposed to peripheral (vestibular) pathology.

Optokinetics generally are used as a cross check on abnormal responses to oculomotor tests. Both of these tests use a "light bar" involving a moving light (usually red) which the patient will track with their eyes.

The caloric irrigation is the only vestibular test which allow the clinician to test the vestibular organs individually, however, it only tests one of the three semi circular canals - the horizontal canal.

While ENG is the most widely used clinical laboratory test to assess vestibular function, normal ENG test results do not necessarily mean that a patient has typical vestibular function. ENG abnormalities can be useful in the diagnosis and localization of site of lesion; however, many abnormalities are nonlocalizing; therefore, the clinical history and otologic examination of the patient are vital in formulating a diagnosis and treatment plan for a patient presenting with dizziness or vertigo.