Effects of glycerol intake and body tilt on otoacoustic emissions reflect labyrinthine pressure changes in Menière's disease.

It is known that by influencing stapes stiffness thus the ear's impedance, changes in intracranial and intralabyrinthine pressure induce a characteristic phase shift in otoacoustic emissions (OAE) around 1 kHz in human ears. Thus, if the regulation of pressure in intralabyrinthine compartments were abnormal in Menière patients, OAEs might help detect it. Body tilt, which acts on intracranial pressure, and administration of an osmotically active substance provide two simple ways of manipulating intralabyrinthine pressure. Here, 14 patients with typical signs of an attack of unilateral endolymphatic hydrops were submitted to postural changes and a glycerol test. Their OAEs initially collected in upright position served as references, then OAEs were measured in supine position, and back to the upright posture one and 3h after glycerol intake. Twenty control subjects were also tested for body tilt. The main effect of body tilt and glycerol was a phase rotation of OAEs peaking around 1 kHz. Its frequency dependence matched the one due to a pressure-related change in stapes or basilar membrane stiffness predicted by the ear model of Zwislocki (1962). The average glycerol-induced phase shifts were similar in size in Menière vs. asymptomatic ear and audiometric thresholds were stable after glycerol intake in line with the model predicting little change in the magnitude of the transfer function. These data support a simple conductive pressure-related mechanism explaining the action of glycerol on inner ear responses. The fact that the mean postural shift was three times larger in Menière than asymptomatic and control ears suggests an additional effect in allegedly hydropic ears.