Vestibular (labyrinthine) stimulation can also be produced by rotating the patient in a Bárány chair or any type of swivel chair. The patient is asked not to fixate or is defocused with Frenzel lenses during rotation to avoid the effects of optokinetic nystagmus. The normal response is nystagmus in the direction opposite to rotation. In contrast, if the patient is asked to focus on his own thumb in an outstretched arm, there should be no nystagmus if the rotational velocity is slow; the ability to suppress this vestibuloocular response reflects the integrity of the vestibular organ and nerve on the side toward the direction of rotation. Electronystagmography (ENG) provides a more refined method of detecting disordered labyrinthine function because it permits the accurate recording of eye movements without visual fixation. ENG is usually coupled with caloric stimulation or with modern devices for rotational testing that allow precise control of the velocity, acceleration, and extent of rotation beyond what can be done with a traditional chair.
Irrigation of the ear canal alternately with cold and warm water (caloric testing) may be used to disclose a reduction in labyrinthine function in the form of an impairment or loss of thermally induced nystagmus on the involved side. Caloric testing is accomplished by having the patient lay supine on the examining table with the head tilted forward 30 degrees to bring the horizontal semicircular canal into a vertical plane, the position of maximal sensitivity of this canal to thermal stimuli. Each external auditory canal is irrigated for 30 s, first with water at 30°C (86°F) and then at 44°C (111.2°F), with a pause of at least 5 min between each irrigation. In normal persons, cold water induces a slight tonic deviation of the eyes to the side being irrigated, followed, after a latent period of about 20 s, by nystagmus to the opposite side (direction of the fast phase). Warm water induces nystagmus to the irrigated side. (As noted in Chap. 16, this is the basis for the mnemonic COWS: cold opposite, warm same, referring to the direction of fast phase of the nystagmus.) In normal subjects, the nystagmus usually persists for 90 to 120 s, although the range is considerably larger. Nausea and symptoms of excessive reflex vagal activity may occur in sensitive individuals.
Simultaneous irrigation of both canals with cold water causes a tonic downward deviation of the eyes with nystagmus (quick component) upward. Bilateral irrigation with warm water yields a tonic upward movement and nystagmus in the opposite direction (“cold upward, warm down, referring again to the fast phase of nystagmus; “CUWD”). Caloric testing will reliably answer whether the vestibular end organs react, and comparison of the responses from the two ears will indicate which one is paretic. Recording of eye movements during the test allows quantification of these responses. Galvanic stimulation of the labyrinths is effective but offers no particular advantage over caloric stimulation.
Ménière Disease and Other Forms of Labyrinthine Vertigo