6The Vestibular Audiogram I: Canal Tests Across Frequency

CThe multifrequency concept

Caloric irrigations roughly emulate one cycle of 0.005-0.01 Hz sinusoidal rotation at ~50°/s — the lowest-frequency probe Clinical rotary chairs typically test ~0.01-1 Hz at 50-100°/s; head-shake examines ~1-2 Hz; head impulse probes ~1-15 Hz Together these span vestibular frequency space like an audiogram spans the cochlea, so hypofunction on one test is NOT a dead labyrinth.[2004]

TCaloric and rotary chair

A side-to-side difference >20% in summed 30°C+44°C responses suggests weaker-canal hypofunction; summed responses <10°/s indicate horizontal-canal hypofunction regardless of asymmetry An absent ice-water caloric response suggests profound sensory loss in that horizontal canal Rotary chair is partially side-specific and useful when calorics are unavailable, intolerable or uninterpretable, and is often preferred for young children.[2004][2002]

CHead impulse testing

Quantitative head-impulse testing with scleral search coils can independently measure each of the six canals and detects hypofunction even when corrective saccades occur before the head stops, making it more accurate than the bedside head thrust Semicircular-canal-plane impulses can detect absent function of individual canals The video head impulse test (vHIT) is cheap, well tolerated and usable from as young as ~3 months, mapping the horizontal, superior and posterior canals.[1998][1988]

TCaloric and vHIT disagree — and that teaches something

Caloric and vHIT frequently disagree, especially for mild dysfunction, because they probe different frequencies Caloric testing detects vestibular impairment in candidates more often than vHIT, with roughly a 1.8:1 ratio of abnormal caloric to abnormal vHIT results vHIT may be LESS sensitive to post-surgical change than caloric testing, so a normal vHIT alone does not exclude post-op vestibular loss.[2004]