Instructions for VNG testing

Some medicines, foods and drinkings can affect the outcome of your VNG test by making changes in your body’s performance and thus lead to erroneous and misleading results. Therefore, the following must be observed:
24 hours before VNG test:
• Do not take medications prescribed for dizziness , nausea or motion sickness.
•Do not use sedatives and antidepressants.
• Do not take painkillers.
• Do not take antihistamines.
• Do not take diuretics.
• Alcohol and drug use will also change the results.
On Test Day:
•Don’t eat foods containing caffeine (such as tea, coffee, chocolate, etc.).
• Do not use cosmetics, especially around the eyes.
• Wear comfortable clothing.
2 hours before test:
• Do not smoke.
• Eat nothing except water.
• If you have diabetes, do not change your diet plan and let the audiologist know about it.

The VNG test may cause dizziness that persists shortly after the test. So it is better for someone else to accompany you. Otherwise, plan to stay in the office for about 30 minutes after the test to relieve the remaining vertigo.

Cervical Evoked Myogenic Potential (cVEMP)

In some years ago, independent evaluation of different parts of the inner ear such as semicircular canals, saccule, and utricle was a dream. The introduction of new tests such as cVEMP, oVEMP and vHIT has made it possible to evaluate the performance of these structures. One of the most useful tests to evaluate the cause of vertigo is the cVEMP test, which examines saccule function in the inner ear.
By presenting high-intensity acoustic stimuli to the ear, a set of reflexes are activated. Since both the vestibular organs (such as the saccule) and the auditory components (such as the cochlea) are located near the stapes, acoustic stimuli may trigger acoustic-motor responses by stimulating each of these regions.
cVEMP reflects the activity of the vestibular system, which is triggered by high-intensity sounds and is recognized as a change in neck muscle potential. This response is recorded as a change in the activity of the SCM muscle after stimulation of the vestibular system with acoustic signals of approximately 90 dB HL and greater. cVEMP represents a vestibuloculic reflex, a rapid reflexive change in muscle tone that stabilizes the head after an unexpected stimulation.
cVEMP is a unilateral response that is detected in the SCM muscle on the same side of the stimulated ear. This response indicates a transient inhibition of SCM muscle activity after saccular acoustic stimulation.
The cVEMP waveform is seen as a biphasic response (positive and negative) in the latency range of 10-25 milliseconds. In short, cVEMP is a vestibular response, not an auditory one. That is, it is recorded in people with profound hearing loss as long as the vestibular function is healthy. In addition, this response can be recorded when the perception of the stimulus is masked by BC noise.
Saccule stimulation produces postsynaptic inhibitory potentials in the cervical muscle neurons. That is, after delivering a high-intensity sound, there is a temporary decrease in muscle activity that is recorded as a positive wave in the cVEMP. The upper part of the vestibular nerve innervates the anterior part of the saccular macula, but the lower part innervates the posterior part. Clinical evidence suggests that cVEMP is dependent on normal vestibular function of the inferior auditory nerve. In mammals, saccule fibers enter the inferior vestibular nucleus, along with fibers from the semicircular canals and utricle. From the inferior vestibular nucleus, cVEMP pathways reach the lateral vestibulospinal pathway to motoneurons in the XI cranial nerve that innervates specific muscles in the neck. The SCM muscle is innervated by the XI cranial nerve.
Any device that has the capability to record middle latency potentials can be used for cVEMP recording. Single-channel devices can be used for cVEMP recording, but with two-channel recording, it is possible to better analyze responses. It is best to have a cVEMP test in a quiet room while the patient is sitting or lying down. In various studies, cVEMP registration has been performed from several locations including:
– Recording in Vertex and Inion
– Recording in Trapezius muscle
– Recording in SCM muscle
– Recording in the head splenium muscle
– Recording from hands and feet
– Recording from post-auricular muscle

Videonystagmography (VNG)

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.

  1. Oculomotor tests which include gaze, spontaneous nystagmus, saccadic eye movements, smooth pursuit and optokinetic nystagmus.
  2. Positional and positioning tests in which probable vertigo and nystagmus are evaluated in different positions.
  3. 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[1].

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[2]. 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.

References:

  1. Leigh RJ, Z.D., The saccadic system. The neurology of eye movements, ed. Gilman S and H. WJ. 2006, Oxford: Oxford University Press.
  2. 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.