Videonystagmography (VNG) is one of the commonest battery of tests used to evaluate vertigo. It measures eye movements, including nystagmus. The patient wears goggles with built-in infra-red cameras that record images of the eyes on a computer screen, even when the lenses are closed off with eye shades, thus preventing fixation of the eyes on any object, (as fixation tends to suppress nystagmus). The computer provides tracings of eye movements (as in the older ENG test) but will also show images of eye movements on the computer screen and will provide video recordings which can be replayed and analyzed at a later date.
These tests help to evaluate the balance system of the inner ears and can sometimes provide evidence of more central disorders. The tests listed below take about one and a half hours to perform and are done in the office. The patient, wearing the goggles, watches a light on a screen moving in various ways. Eye movements are then recorded with the patient in various positions. Finally caloric tests are performed in which the ears are gently irrigated, each in turn, with cold and then warm water. This may produce transient dizziness.
On the left the goggles with built-in infra red cameras. On the right the computer screen showing the graphical tracings above and the video of the eyes below.
The folllowing comments are somewhat technical, but may be helpful to physicians who access this site and may be of interest to the lay person who is to have this test.
The first five commonly employed tests listed are performed (in our practice) with an illuminated target projected onto a four feet wide screen, four feet in front of the patient.
Gaze: searching for nystagmus looking at a fixed target to right, left, up and down. Nystagmus may be of central or peripheral origins depending on its characteristics.
Spontaneous: searching for nystagmus looking ahead in the neutral position, with and without fixation. May also be of central or peripheral origin.
Saccades (random): the ability to follow a target that moves suddenly and unpredictably from one position to another. The velocity of the eye movements, their accuracy and latency are measured.
Smooth pursuit: the ability accurately to follow a target that moves smoothly to and fro across the screen.
Optokinetic nystagmus: the ability of the eyes to follow successive columns of targets moving across the visual field in both directions.
Saccade, smooth pursuit and optokinetic abnormalities are usually of central origin.
Dix-Hallpike positional test: the subject is moved back quickly from sitting to supine head hanging position, with each ear down, in turn. Benign paroxysmal positional vertigo (BPPV) of the posterior canal (the canal most often affected) will be indicated by a brief latent period followed by torsional or rotatory nystagmus which settles after some seconds. The horizontal or anterior canals may be less commonly affected and the nystagmus in these cases will have different characteristics. VNG allows accurate recording of the nystagmus (both horizontal and vertical elements) with eye shades closed to abolish fixation.
Positional tests: to search for nystagmus in the supine position with head turned right, center, and left and also whole body turned onto left and right sides.
Caloric tests of vestibular function: each ear is irrigated in turn with water at 7 ° C below and 7 °C above body temperature for 30 seconds each with suitable intervals between irrigations. Nystagmus is induced and analyzed by the computer for slow phase velocity of each beat. The two sides are compared.
Caloric weakness on one side (unilateral weakness or reduced vestibular response) is often described as a “peripheral” finding and is usually vestibular in origin, but may occasionally result from pathology of the vestibular nerve or its root entry zone to the brainstem and may therefore be encountered in “central” pathologies.
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