Isolated directional preponderance of caloric nystagmus: I. Clinical significance

Objectives: To determine the clinical significance of an isolated direction al preponderance (DP) on bithermal caloric testing. An isolated caloric DP was defined as a DP, calculated according to the standard Jongkees formula, of greater than or equal to 40%, with a spontaneous nystagmus (SN) in dark ness of less than or equal to2 degrees /s and a canal paresis (unilateral w eakness) of less than or equal to 25%. Study Design: A retrospective analysis of all 15,542 bithermal caloric test s performed in the authors' department in the previous 10 years to identify all tests with an isolated DP of greater than or equal to 40%. This was fo llowed by a review of the clinical data on the 144 patients identified with such a result and then by a telephone or postal follow-up study of these p atients. The study group eventually comprised 114 patients; these were pati ents in whom a clinical diagnosis could be made at the time the caloric tes t was done, or who responded to requests for follow-up information. The 34 patients in whom a clinical diagnosis could not be made at the time of the caloric test, and who did not respond to requests for follow-up information , were excluded. Study Setting: A balance disorders clinic in a tertiary referral hospital. Intervention: All patients underwent standard bithermal caloric testing. So me of the patients also underwent rotational testing. Outcome Measures: A clinical diagnosis for the cause of the isolated DP, ma de either at the time of the caloric test or on the basis of information su pplied at follow-up by the patient or by the referring physician. Results: Of 114 patients, 39 had benign paroxysmal positioning vertigo, 14 had Meniere's disease, and 5 had migrainous vertigo. Five patients had cent ral nervous system (CNS) disorders, and this was clinically apparent at the time of the caloric test in 4, so that only 1 patient with an isolated DP developed evidence of a CNS disorder after the caloric test was done. In th e other 54 patients, no definite diagnosis could be made, but 41 of these 5 4 were either completely well or much better at follow-up. Conclusions: An isolated DP on caloric testing is usually a transient, beni gn disorder. About half the patients with an isolated DP have either Menier e's disease or benign paroxysmal positioning vertigo; in most of the other half, no definite diagnosis is made but most of these patients will do well . Only similar to5% have a CNS lesion and in almost all this is apparent at the time the caloric test is done. In a relapsing-remitting peripheral ves tibular disorder such as benign paroxysmal positioning vertigo or Menieres disease, the mechanism of an isolated DP could be enhanced dynamic gain of ipsilesional medial vestibular nucleus neurons, perhaps as a result of inte rmittent hyperfunction of primary semicircular canal vestibular afferents. The authors postulate that an isolated DP reflects a gain asymmetry between neurons in the medial vestibular nucleus on either side, caused either by increased sensitivity on one side or by reduced sensitivity on the other, p erhaps as an adaptive change in response to abnormal input. In an accompany ing article, the authors implement a realistic neural network model in whic h it is possible to simulate an isolated DP by adjusting the dynamic sensit ivity of type 1 medial vestibular nucleus neurons on one side or of type 2 medial vestibular nucleus neurons on the other.