The vestibular system is an integral part of the labyrinth in the temporal bone. The vestibular system has two labyrinths: the membranous labyrinth within the bony labyrinth. Both the labyrinths are fluid-filled: endolymph in the membranous labyrinth and perilymph in the bony labyrinth.
What Does The Vestibular System Do?
The vestibular system is responsible for balance and consists of 5 distinct end organs, they are,
1. The utricle and saccule which is sensitive to linear motion (movements in the straight-line), they are two sack-like structures.
2. The three semicircular canals that are joined together in the same fluid environment which is sensitive to angular motions (circular in motion ex. head rotations).
Medical history and findings from a physical examination are used as a basis for ordering diagnostic tests to assess the function of the vestibular system and to rule out alternative causes of symptoms.
Types of Vestibular Tests?
The most common type of vestibular tests are
Vestibular Evoked Myogenic Potential (VEMP)
Gaze Fixation Test
Smooth Pursuit Tracking
Positioning & Positional Tests
Dix-Hall Pike Maneuver
Computerized Dynamic Posturography (CDP)
Videonystagmography (VNG) Test
1. Vestibular Evoked Myogenic Potential (VEMP) are further classified as C VEMP and O VEMP.
C VEMP (Cervical Vestibular Evoked Myogenic Potential)
Sound stimulates the saccule which activates inferior vestibular nerve, lateral vestibular nucleus, 11th nerve nucleus, ipsilateral sternocleidomastoid muscle. The VEMP will be recorded using an evoked response computer, sound generator and surface electrodes. Electrodes are placed accordingly where as positive electrode on sternum, negative on the sternocleidomastoid muscle and ground on the forehead.
O VEMP (Ocular Vestibular Evoked Myogenic Potential)
Ocular VEMPs are obtained from contralateral inferior oblique muscle after stimulation of the ipsilateral utricle by placing electrodes in their respective placements. Electrodes are placed in such a way that positive electrode is below to the non-testing eye, negative to below the testing eye and Ground electrode to the forehead.
2. Electronystagmography - ENG is a test used most commonly for evaluation of patients with dizziness, vertigo, disequilibrium and various types of recordable eye movements.
• Occulomotor evaluation - the patient is presented with various visual stimuli and asked to follow them while his or her eye movement is recorded.
• Saccatle (Reflection) Test – The test is assessed by redirecting the eye from one position to another in shortest period of time. This is done by correcting the error of retinal position by bringing the object to fovea of the eye as fast as possible and thus serves a re-fixation function.
3. Gaze fixation test - Gaze test is conducted to evaluate for the presence of nystagmus in the absence of vestibular stimulation. This is also to maintain visual fixation of an object on the fovea of the eye.
• Fixation Suppression - Nystagmus which is evident with eyes closed or in dark room should be suppressed with visual fixation. If visual fixation does not suppress nystagmus, CNS pathology is possible.
4. Smooth Pursuit Tracking - The test is to stabiles the slowly moving object such as pendulum on the fovea of eye which matches the angular velocity of eye with that of moving object.
5. Dix-Hallpike Maneuver
• This is mainly to assess the presence or absence of nystagmus associated with Benign Paroxysmal Positioning Vertigo (BPPV).
• The static positional tests are conducted to determine if changes in head position cause or modify nystagmus.
Caloric Tests - Caloric Stimulation
The bithermal caloric test procedure is used to induce endolymph flow in the semicircular canals by creating a temperature gradient from the lateral to medial port of the canal.
Rotational Tests – The rotational test is done by manually driven rotatory chair that produced observable nystagmic eye movements and the test was performed in clockwise and anti-clockwise rotational movements.
Computerized Dynamic Posturography (CDP) - uses a computer controlled moveable platform that track the anterior posterior (AP) sway of the patient when they stands on the platform surface.
Videonystagmography (VNG) Test - The videonystagmography (VNG) test is the electronic recording of nystagmus (eye jerks) which are recorded by performing certain tasks using the pair of goggles, outfitted with an infrared camera over one eye.