Vestibular Tests
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Direct answer
Vestibular testing provokes each half of the balance end-organ in a controlled way and watches the eyes. The bithermal caloric test irrigates each ear alternately with water at 30 and 44 degrees celsius (30 and 50 by air), and the resulting nystagmus — remembered as COWS, cold opposite, warm same, for the direction of the fast phase — is charted to reveal canal paresis (a weak ear) or directional preponderance (a biased brainstem). The ice water caloric tests the vestibular reserve and belongs to brainstem death protocols. Beyond calories, the video head impulse test examines all six canals at high physiological frequencies, rotational chair testing assesses both labyrinths together, and VEMP (vestibular-evoked myogenic potentials) tests the saccule and utricle with their inferior and superior vestibular nerve connections — the modern panel that maps peripheral versus central loss.
What you must remember
- Caloric stimulus specifics: water at 30 and 44 degrees celsius (or air at about 24 and 50) irrigated for roughly 40 seconds with the patient supine and head flexed 30 degrees to put the horizontal canal vertical; nystagmus is recorded from its peak slow-phase velocity.
- COWS: Cold — fast phase to the Opposite side; Warm — fast phase to the Same side.
- Canal paresis (unilateral weakness): the summed responses of one ear are significantly smaller than the other — commonly a difference exceeding about 20-25 per cent — indicating reduced peripheral function on the weak side (neuritis, Meniere disease, schwannoma).
- Directional preponderance: responses beating toward one side are larger across both ears — a central or peripherally generated bias, classically seen in unilateral peripheral lesions too.
- Ice water caloric: 5-10 millilitres of ice water should still provoke nystagmus in a normal ear; an absent response bilaterally suggests bilateral vestibular paresis, and its total absence forms part of brainstem death testing.
- Video head impulse test (vHIT): passive rapid head turns assess each canal's high-frequency function; reduced gain with catch-up (covert) saccades indicates peripheral loss, while a normal vHIT despite strong clinical suspicion shifts attention toward a central lesion.
- VEMP: cVEMP from the sternocleidomastoid tests saccule/inferior vestibular nerve; oVEMP from beneath the eyes tests utricle/superior vestibular nerve — separating the nerve divisions in superior canal dehiscence and vestibular neuritis.
- Fixation suppression: failure of visual fixation to suppress caloric nystagmus is a central sign — a bedside discriminator between peripheral and central lesions.
Worked example: interpreting one caloric chart
A 45-year-old with spinning episodes and right aural fullness undergoes bithermal calorics. Charted peak slow-phase velocities: right-warm 8, right-cold 6, left-warm 24, left-cold 22 degrees per second. The right ear contributes 14 against the left's 46 — unilateral weakness well beyond the usual 20-25 per cent threshold, so right canal paresis; leftward-beating responses dominate, giving a left directional preponderance. Read together: an underactive right labyrinth with the brainstem biased away from it — the signature of a right peripheral disorder such as Meniere disease or burnt-out neuritis, next sorted by audiometry and, if retrocochlear doubt arises, magnetic resonance imaging.
The same chart with a normal ear but persistent nystagmus despite visual fixation changes everything: failure of fixation suppression points to a cerebellar or brainstem lesion, not the ear. Peripheral weakness on the one hand, central failure of suppression on the other — examiners pair a number with a sign, and interpretation, not technique, earns the marks.
Where students slip
The mnemonic gets reversed under pressure — COWS refers to the fast phase, while the slow phase is the actual vestibular response being charted. Candidates also treat absent calorics as total vestibular death: calorics test only the horizontal canal at low frequency, so the video head impulse test and calorics can disagree, and bilateral failure needs rotational chair confirmation. Do not forget the 30-degree head flexion that aligns the horizontal canal, and remember that directional preponderance often accompanies an acute peripheral lesion rather than always signalling central disease.
Frequently asked questions
At what temperatures is the bithermal caloric test performed?
Water at 30 and 44 degrees celsius, or air at approximately 24 and 50 degrees, irrigated for about 40 seconds with the head flexed 30 degrees to place the horizontal canal in the vertical plane.
What does COWS stand for in caloric testing?
Cold Opposite, Warm Same — the fast phase of caloric nystagmus beats away from the irrigated ear with cold water and toward it with warm water.
What is canal paresis on caloric testing?
Significant unilateral reduction of the summed responses of one ear, commonly exceeding about 20-25 per cent asymmetry, indicating peripheral vestibular hypofunction on the weaker side.
What is the clinical use of the ice water caloric test?
It tests maximal vestibular stimulability: preserved ice water response indicates residual function, bilateral absence supports bilateral vestibular paresis, and absent responses to ice water form part of the brainstem death evaluation.
Which vestibular test evaluates the saccule and inferior vestibular nerve?
The cervical vestibular-evoked myogenic potential recorded from the sternocleidomastoid, whereas the ocular VEMP tests the utricle and superior vestibular nerve.