REBOUND CALORIC NYSTAGMUS

Rebound caloric nystagmus (RCN), a distinctly different phenomenon fro m secondary phase nystagmus (SPN), was serendipitously discovered duri ng 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 (SPV(Max)) of both PPN and RCN were almost ident ical. The pathological lesion was an Arnold-Chiari malformation (ACM). Two other patients with posterior fossa lesions also showed RCN. To d etermine if RCN is an unrecognized normal phenomenon, 11 healthy subje cts 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 les ions.