Rebound caloric nystagmus.

Rebound caloric nystagmus (RCN), a distinctly different phenomenon from secondary phase nystagmus (SPN), was serendipitously discovered during the course of routine neurotological evaluations. RCN occurred when the horizontal semicircular canal was brought into a truly horizontal plane after the primary phase nystagmus (PPN) had ended. The maximum slow-phase velocities (SPVMax) of both PPN and RCN were almost identical. The pathological lesion was an Arnold-Chiari malformation (ACM). Two other patients with posterior fossa lesions also showed RCN. To determine if RCN is an unrecognized normal phenomenon, 11 healthy subjects were tested. The average SPV of RCN in normals was 0.7 degrees per second. Such a small value of RCN is probably of no significance. It therefore appears that RCN is a sign of posterior fossa neuroaxial lesions.