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Vestibular physiology and testing: the VOR, calorics, vHIT and VEMP

Each inner ear holds three semicircular canals for angular acceleration and two otolith organs for linear acceleration. Each vestibular test checks a different part: calorics — the horizontal canal at low frequency; vHIT — all six canals at high frequency; cVEMP — the saccule and inferior nerve; oVEMP — the utricle and superior nerve.

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Why it matters in the exam

Anatomy and physiology: the canal hair cells sit on the crista in the ampulla, under the cupula; linear acceleration and gravity are picked up by the otoconia-laden utricle (largely horizontal) and saccule (largely vertical). The hair cells fire at rest: bending the stereocilia toward the kinocilium raises firing, and bending them away lowers it. The two horizontal canals act in opposition: turning the head right excites the right canal and inhibits the left, and the vestibulo-ocular reflex (VOR) moves the eyes left at the same speed. The superior vestibular nerve carries the anterior canal, the horizontal canal and the bulk of the utricle; the inferior nerve carries the posterior canal and the saccule.

Calorics: by COWS (cold — opposite, warm — same), warm water (44°C) in the left ear produces nystagmus beating to the left. It is quantified by slow-phase velocity (degrees per second). Unilateral weakness = [(RW + RC) − (LW + LC)] ÷ the total × 100: for example (58 − 22) ÷ 80 × 100 = 45% — abnormal (usually above about 20–25%). vHIT: VOR gain = eye velocity ÷ head velocity; normal for the horizontal canal is close to 1, and 120 ÷ 200 = 0.60 is low. Covert saccades occur during the turn, so the bedside test misses them — vHIT catches them. In the bedside head impulse test, a corrective saccade after a turn to one side points to weakness on that side. Peripheral nystagmus weakens with visual fixation and grows in darkness; by Alexander's law it increases when looking toward the fast phase. An air-conducted VEMP needs a normal middle ear — even a moderate conductive loss abolishes it; with a third window (dehiscence, enlarged vestibular aqueduct) the amplitude grows and the threshold drops.

Key points

• Canals — angular; otoliths — linear.
• Toward the kinocilium — more firing.
• Superior: horizontal, anterior, utricle; inferior: posterior, saccule.
• COWS; unilateral weakness above about 20–25%; vHIT: gain close to 1.
• The VEMP needs a normal middle ear; third window — a large VEMP.

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Sources

• Jacobson, Shepard, Barin, Janky & McCaslin (eds.), Balance Function Assessment and Management, 3rd edition (2021) — Vestibular physiology; caloric testing; vHIT; VEMP
• Halmagyi et al., Frontiers in Neurology, 2017 — The video head impulse test
• Rosengren, Colebatch, Young, Govender & Welgampola, Clinical Neurophysiology Practice, 2019 — Vestibular evoked myogenic potentials in practice

Sources are given for further reading; the text is written afresh and not copied from them.

Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.