Caloric tests examine the vestibulo-ocular reflexes (VOR). They are mainly used in two circumstances: to identify vestibular pathology in the assessment of dizziness/vertigo when clinical tests of VOR are unhelpful and to assess brainstem integrity in coma. Each labyrinth may be separately assessed by irrigating each outer ear. Head flexion to 30? above the horizontal allows maximum stimulation of the horizontal semicircular canals, whereas 60? below horizontal maximally stimulates the lateral semicircular canals. Water 7?C above and below body temperature (i.e.,30?C and 44?C) is used, applied for 30-40 seconds. Induced nystagmus is then timed both with and without visual fixation (in the dark, Frenzel glasses). This method is cheap but has poor patient acceptability.
Normally, the eyes show conjugate deviation toward the ear irrigated with cold water, with corrective nystagmus in the opposite direction; with warm water the opposite pattern is seen. (The direction of nystagmus may thus be recalled by the mnemonic COWS: cold opposite, warm same.) Dysconjugate responses suggest brainstem damage or depression. A reduced duration of induced nystagmus is seen with canal Paresis; enhancement of the nystagmus with removal of visual fixation suggests this is peripheral in origin (labyrinthine, vestibulocochlear nerve), whereas no enhancement suggests a central lesion.
In coma the deviation may be present but without corrective saccades, even at a time when the oculocephalic responses elicited by the doll’s head maneuver are lost. As coma deepens even the caloric reflexes are lost as brainstem involvement progresses.
Rudge P, Bronstein AM. Investigations of disorders of balance. In: Hughes RAC (ed.). Neurological Investigations. London: BMJ Publishing, 1997: 283-314