Describe the vestibular function tests.

Vestibular Function Tests

• Vestibular tests answer three questions: is the lesion peripheral or central; which side is affected; and which part of the labyrinth. No single test answers all three.
• Caloric test (Fitzgerald-Hallpike) — the only test that examines each labyrinth separately.
• Technique — the patient lies supine with the head raised 30 degrees to bring the lateral canal vertical; each ear is irrigated for 40 seconds with water at 30°C and 44°C (7 degrees either side of body temperature), and the duration of the induced nystagmus is measured
• COWS — Cold Opposite, Warm Same: cold water produces nystagmus beating to the opposite side, warm to the same side
• Canal paresis means reduced response from one ear to both stimuli, and indicates a peripheral lesion on that side
• Directional preponderance means nystagmus in one direction is greater whichever ear is stimulated, and is less localising, occurring with central lesions and with a spontaneous nystagmus
• Contraindicated with a perforation (air or closed-loop irrigation is used instead), and unpleasant enough that patients should be warned
• Electronystagmography (ENG) and videonystagmography (VNG) record eye movements with vision removed, and include saccade, smooth pursuit and optokinetic testing.
• The rotation (rotatory chair) test stimulates both labyrinths simultaneously, so it cannot lateralise; but it tests higher frequencies than calorics and is valuable in bilateral loss and in children
• The head impulse test, clinical or with video (vHIT), tests the vestibulo-ocular reflex at high frequency and can assess each of the six canals individually
• Vestibular evoked myogenic potentials (VEMP) test the otolith organs.
• Posturography measures balance under varying visual and proprioceptive conditions; it does not diagnose but quantifies functional disability and detects a non-organic pattern
• The practical point about all OF them: vestibular tests are confirmatory rather than diagnostic. The diagnosis in this chapter is made from the history and a careful bedside examination; the laboratory quantifies and documents it
• Calorics alone examine each labyrinth separately, which is the unique value that keeps the test in use
• Raise the head thirty degrees to bring the lateral canal vertical, or the stimulus is misdirected and the result meaningless
• Canal paresis localises and directional preponderance does not, which is the distinction to state when interpreting a report
• Avoid water irrigation with a perforation; use air or a closed-loop system instead
• Warn the patient that calorics are unpleasant; an unprepared patient will not tolerate the second ear