The eyes are a window to the vestibular system. They provide information about peripheral vestibular system function and the ability to produce high-performance voluntary eye movements that are necessary to maintain eye contact with the environment. The videonystagmography test, formerly known as electronystagmography (with a different recording method), consists of three subtests that are listed below.
- Oculomotor tests which include gaze, spontaneous nystagmus, saccadic eye movements, smooth pursuit and optokinetic nystagmus.
- Positional and positioning tests in which probable vertigo and nystagmus are evaluated in different positions.
- Caloric test.
VNG test usually begins by calibrating the patient’s eye movements relative to the targets. Usually an illuminated dot in a certain distance from the person is placed as a target and the patient should look at them. The assessment method was to put the electrodes around the patient’s eyes and to ask the patient to look at the targets. With eye movements, eye potentials were altered and recorded by electrodes. With the progress in technology and the introduction of videonystagmography, cameras that are located inside a google detect the pupils of the eye and fix on them. Each time the patient’s eye moves, these cameras record the motion of the patient’s eyes and show them in a graph. These cameras can record the fastest eye movements of the individual, and their level of error is very low. Before performing videonystagmography test, some issues should be considered. Small eyes, drooping eyelids and eye makeup cause the camera to fail to detect the pupil. The duration of the videonystagmography test is usually about 45 minutes. The Audiologist will provide the patient with information about the things that must be followed before the test is performed several days before performing the test.
The first part of videonystagmography tests is oculomotor assessment in which gaze, saccadic eye movements, smooth pursuit and optokinetic nystagmus are evaluated. These tests measure eye movements that result from the coordination and health of the central nervous system. Observed abnormalities in these tests are signs of neurologic lesions. In addition to central nervous system disorders, in cases that person is tired, or has taken some medications or his mental level has altered, the outcome of these tests will be affected.
In practice, the first eye movement which is evaluated in oculomotor tests is saccade. Saccades are fast eye movements that are used to place an image at the center of the visual line. In fact, saccades are the fastest eye movements in humans. There is no sight during the saccades. These eye movements could be done voluntarily and non-voluntarily. The most important parameters in evaluation of saccadic eye movements include accuracy, velocity and latency. Saccade latency is about 150-250ms when target is unpredictable or random, and when it is predictable, latency is about 76ms. Normal people undershoot target about 10-15% which is known as “hypometry”. In cases that saccade amplitude is less than 79% of target amplitude, it is considered abnormal. This problem may be more seen in patients with cerebellar dysfunctions. Overshooting the target is defined as “hypermetry”. If the hypermetry and amplitude of saccades is 15-20% more than target amplitude, it will be considered as abnormal. Saccade velocity is measured as the maximum speed of the eye, when the gaze moves from one target to another. Velocities less than 430deg/sec are considered abnormal. In addition to cortical activity, pontine reticular formation and vestibulocerebellum are involved in controlling saccade velocity, latency and accuracy. This test is useful in diagnosis of neurologic disorders especially neurodegenerative lesions.
Smooth pursuit eye movements maintain gaze on objects which are moving in visual field. Smooth pursuit needs a real or imagined target, and eye velocity should match target velocity. In smooth pursuit, gain and phase are assessed. Smooth pursuit abnormalities show abnormalities of central nervous system especially cerebellum.
Optokinetic nystagmus assessment is performed with different stimuli. Initially, OKN was done using a cylinder with black and white lines on it. It is currently being implemented by delivering targets in the form of LEDs or images on the display.
In gaze test, the evaluated person should be able to look at a number of luminous dots that are usually presented at the center of the person’s vision, right and left. Presence of nystagmus in gaze test shows abnormalities of central nervous system.
Another part of videonystagmography tests is spontaneous nystagmus. In order to assess spontaneous nystagmus, goggle is placed on patients eyes. In this test, patient should perform intellectual task which could be caounting, telling name of flowers, etc. different central and peripheral vestibular disorders lead to horizontal and vertical nystagmus, and based on nystagmus type and its characteristics, it is possible to determine the location of the lesion. It is recommended to perform spontaneous nystamus before other VNG tests because it could affect the results of other tests.
Positioning or Dix-Hallpike test is conducted to diagnose BPPV. In this test, the patient’s head is rotated to the right or left. Then, examiner helps patient to lye on the bed so that patients head would be hanging from the bed. BPPV patients and some patients with some central disorders would feel nustagmus and vertigo. After 30 seconds, examiner helps patient to return to the sitting position. This process should be performed for the other side. With this test, diagnosis of many peripheral and central causes of vertigo is possible.
Positional test is performed in supine position, head right and head left. In this test, intellectual task is necessary. Central and peripheral vestibular lesions could cause abnormalities in positional test such as geotropic and ageotropic nystagmus.
The last videonystagmographic test which would be mentioned here is caloric test. This test was introduced in 1942. This test shows the reaction of human vestibular system to temperature stimulation. It is possible to conduct caloric test with cold or warm water and air. Different methods have used for this test. Patient should lie down on the bed while his head is 30 degree higher. After about 60s air stimulation, the presence of nystagmi and their degree is evaluated. In vestibular neuritis and some central vestibular dysfunction as well as in Meniere’s disease, caloric test results are abnormal. In caloric test intellectual task is necessary, too. Caloric test is considered as the most important part of videonystagmography tests.
Videonystagmography test should be requested by a specialist physician and performed by an audiologist.
- Leigh RJ, Z.D., The saccadic system. The neurology of eye movements, ed. Gilman S and H. WJ. 2006, Oxford: Oxford University Press.
- Fitzgerald G, H.C., Studies in human vestibular function I: observations on the directional preponderance of caloric nystagmus resulting from cerebral lesions. Brain Behav, 1942. 62: p. 22.