Postcaloric nystagmus by positional change. 1. Study in normal subjects.

We investigated the postcaloric nystagmus related to positional change. Twenty-two healthy subjects were examined (experiment 1). A caloric test was carried out in the supine position. Following the cessation of the primary phase, the subject was reoriented to the upright position. An additional 10 subjects were also examined with other positional changes (experiment 2): following the cessation of the primary phase, the subject was reoriented to the lateral positions, irrigated-ear-up and irrigated-ear-down positions. As a control study, 6 subjects were examined without positional change (experiment 3). In 37 (84%) ears, postcaloric nystagmus was observed in the upright position. Furthermore, in 12 of 20 (60%) ears in the irrigated-ear-up position and in 13 of 20 (65%) ears in the irrigated-ear-down position, the horizontal component of postcaloric nystagmus was observed. Without positional change, no postcaloric nystagmus was observed. These findings indicate that caloric nystagmus is modified by an adaptive process and that postcaloric nystagmus as it relates to positional change might be attributed mainly to the vestibular adaptation.