Vestibular Function Tests

This is intended as a simple overview of vestibular function tests and audiometry. It can be used to help interpret test results and help to decide when they should be ordered.

The inner ear

Who should have vestibular function tests?

These tests are clinically useful in any patient with a suspected vestibular disorder. This includes patients with rotatory vertigo, patients with dizziness influenced by head position or movement and dizzy patients without a clear diagnosis.

An overview of vestibular function tests

A range of tests are performed that assess various aspects of the vestibular system and indeed the full battery of tests is able to examine all 3 canals and both otolith organs and also auditory function from the ear to the brainstem. Reflecting the importance of the vestibulo-ocular reflex (VOR), most of the tests rely on the recording of eye movements using an infrared camera. The traces you will see in the body of the report represent eye position in horizontal (and sometimes vertical) planes.

Nystagmus

We look for spontaneous nystagmus in a range of eye and head positions. Nystagmus gives a “saw-tooth” appearance on eye position traces. Nystagmus can occur in patterns suggestive of either central or peripheral vestibular disease.

Visual fixation

Nystagmus due to peripheral vestibular disorders is suppressed by visual fixation, so we always record with and without visual fixation. Less than 50% reduction in nystagmus with visual fixation can indicate a cerebellar disorder.

Dix-Hallpike test

This involves quickly placing the patient in a head hanging position, to one side and then the other. A positive test is often diagnostic of Benign Paroxysmal Positioning Vertigo (BPPV) – the commonest cause of vertigo in the community. This type of vertigo is treatable by a simple repositioning or rolling manoeuvre (Epley manoeuvre) on the examination couch.

Head-shaking nystagmus

Passive side to side head-shaking is performed for approximately 20 seconds. The central nervous system stores this information in a system called “velocity storage”. If there is any asymmetry or imbalance within the vestibular system (peripheral or central) then a burst of nystagmus can follow the stimulus. The most common abnormality is nystagmus away from the affected ear in a unilateral peripheral vestibular lesion.

Caloric test

Caloric stimulation is probably still the most important of all vestibular tests, as it is the best established means of demonstrating a unilateral peripheral vestibular lesion. However, it is important to note that it only tests horizontal semicircular canal function. We perform bilateral bithermal caloric stimulation, resulting in two sets of responses from each ear. Warm water irrigation is excitatory and cool water is inhibitory to horizontal semicircular canal function, so producing nystagmus of opposite direction. The maximum velocity of the slow phase of the nystagmus is measured for each test. Two important values are derived from these four measures, “canal paresis” and “directional preponderance”.

a) Canal paresis (or unilateral weakness)

This is a measure of unilateral peripheral vestibular loss and represents the difference between the sum of the hot and cold responses in one ear and those in the other.

Greater than 25% canal paresis is considered abnormal and indicates a peripheral vestibular lesion on the weaker side.

b) Directional preponderance

This is a measure of the difference between nystagmus beating in one direction (i.e. warm stimulation to one ear and cool stimulation to the opposite ear) and those beating in the other direction and, while useful, it is generally a non-specific marker of vestibular disease.

Greater than 30% directional preponderance is considered abnormal.

Video Head impulse test (vHIT)

This is another newer test of the vestibulo-ocular reflex (VOR) that is also able to demonstrate loss of semicircular canal function and is particularly useful in conjunction with caloric testing. It relies on a fast head rotation (impulse) in one direction chiefly stimulating the labyrinth on that side. This is a relatively quick and generally well-tolerated test that doesn’t trigger any vertigo or nausea but can be problematic where there is neck stiffness or pain. All 3 canals on each side can be examined but the results are most reliable for the horizontal canals and occasionally the vertical canals can’t be adequately studied. The VOR for each canal is expressed as proportion of the head rotation. A ratio of 1 reflects a perfect VOR (compensatory eye movements are equal and opposite to the induced head impulse). A ration of 0 reflects total loss of VOR , although in practice a value of 0 is almost never seen.

For the horizontal canal vHIT a ration of less than 0.8 is generally considered abnormal, while for the vertical (anterior and posterior) canals the cut-off is usually 0.6.

Vestibular evoked myogenic potentials (VEMPs)

Vestibular evoked myogenic potentials (VEMPs) are a recently developed test of vestibular function that depend on vestibular projections to the neck and eyes and the ability of loud sound to excite the vestibular system (specifically the otoliths). It complements the vHIT caloric test well, as together they can test all of the inner ear organs. While this is not a test of hearing or cochlear function, it is important to be aware that conductive hearing loss can influence the result. Thus any abnormality of this test must be interpreted in conjunction with an audiogram.

The most commonly used version of the VEMP is recorded from neck muscles in response to loud clicks delivered through headphones – cervical or cVEMP. This is a test of the sacculus. It is also possible to record cVEMPs in response to a forehead tap and this bypasses conductive hearing problems but is probably less specific to the sacculus. Ocular or oVEMPs record from eye muscles and are more sensitive to problems with the utricle. Different combinations of VEMPs can be used, depending on the clinical problem, but the cVEMP is the most established and almost always performed.

The most common abnormality of the cVEMP is a small or absent response on one side (difference of greater than 2.5 times), but occasionally a large or delayed response can be abnormal.

Tests of auditory function in dizzy patients.

Brainstem auditory evoked responses (BAER)

BAER measure auditory conduction through the cochlear nerve and brainstem. Delay or absence of waves on one side can alert us to the possibility of an acoustic neuroma (or other retrocochlear lesion).

Audiometry

Audiometry is often used in conjunction with vestibular function tests, even in patients without obvious hearing problems. Subtle hearing abnormalities often provide important clues to the cause of dizziness. Our audiologist will usually perform pure tone audiometry to air and bone conducted sound. Sensorineural hearing loss is indicated if air and bone conduction thresholds are similar. Conductive hearing loss is indicated if bone conduction is significantly better than air conduction. Tympanometry provides information about the middle ear. Speech recognition provides important information regarding functional impact of the hearing loss and unexpectedly poor speech recognition may suggest a retrocochlear lesion.

The interpretation of tests in specific clinical situations.

The patient with a single severe attack of prolonged rotatory vertigo.

The commonest diagnosis here is acute loss of peripheral vestibular function on one side, often referred to as vestibular neuritis, labyrinthitis or vestibular neurolabyrinthitis – the pathophysiology is usually neuritis. There shouldn’t be any hearing loss or specific brainstem symptoms or signs. The differential diagnosis includes a number of conditions, including a first attack of Meniere’s disease. The most important differential however is a small cerebellar stroke. Clinical features and imaging can be very helpful, but vestibular function tests are often definitive. The commonest abnormality seen in vestibular neuritis is unilateral reduction in canal function (canal paresis and small vHIT), less often with small VEMP on the same side. If this abnormality is seen then the diagnosis is highly likely. If the tests are normal or there are other abnormalities then scanning (CT may not be sufficient) or specialist referral may be required.

The patient with recurrent attacks of prolonged rotatory vertigo.

The commonest diagnoses here are Meniere’s disease and vestibular migraine. Meniere’s disease will usually show unilateral canal paresis on caloric testing, often with small VEMP on the same side. Often the vHIT is normal in Meniere’s disease and a dissociation between caloric and vHIT responses is increasingly recognised as a marker of this condition. Audiometry will usually be abnormal, the typical pattern being unilateral sensorineural loss at low frequency. Tests will usually be normal in vestibular migraine, but occasionally abnormalities are seen. Most patients with this pattern of vertigo should have specialist referral, but performing the tests first is often appropriate and speeds up the assessment process.

The patient with recurrent attacks of very brief rotatory vertigo.

The commonest cause of vertigo in the community is Benign Paroxysmal Positioning Vertigo (BPPV) and this usually presents with brief attacks triggered by specific head movements. Common triggers are lying or turning in bed and looking up or stooping. It is important to make an accurate diagnosis as it can be very frightening and disabling and simple treatment is often effective. Vestibular function tests are often very useful in these patients. The Dix-Hallpike test is usually diagnostic of BPPV and shows a highly specific pattern of nystagmus. There should be no other abnormalities on vestibular or auditory tests and if there are referral is usually appropriate. Our testing is performed by a registered nurse (Imelda Hannigan) who has huge experience in the diagnosis and treatment of BPPV. She will perform the appropriate simple treatment (Epley or particle repositioning manoeuvre) on patients diagnosed with BPPV. If there are ongoing problems then specialist referral might be required.

Patients with non-specific dizziness

It is sometimes hard to know whether dizziness is vestibular or due to one of the myriad other causes. Finding an abnormality on vestibular (or hearing) tests can provide an important clinical clue in these patients and lead to appropriate management or referral. It should be noted however that normal results on vestibular and hearing tests do not rule out a vestibular disorder.

What is involved for your patients?

Vestibular function tests take between around 60 minutes to perform. Audiometry is often required and takes 15-30 minutes and sometimes needs to be done at a separate visit. The tests are simple and safe. Caloric testing will make patients dizzy for a few minutes. This is usually minor, but occasionally severe and associated with vomiting and this kind of response can be an indicator of vestibular migraine. vHIT, VEMPs and head-shaking occasionally causes neck discomfort and can’t be completed in some elderly patients or patients with neck injury or arthritis.

Specific instructions are included on the back of our request form. Please note that the ears must be free of significant wax and that vertigo, nausea and sedative medications are best avoided. If you consider that it is important for the patient to continue taking these medications or they might be hazardous to cease (e.g. long term benzodiazapine usage), we can still perform the tests but certain abnormalities may be harder to interpret. They should continue all other regular medications.

We remain happy to see any dizzy patients in consultation, but in some cases it might prove simpler and more professionally satisfying to order the tests directly. In some patients with vestibular neuritis or BPPV (for example) no further action will be required. Please feel free to call and speak to any of us, if guidance is required.

Written by Dr Shaun Watson, 2020.