To determine the clinical significance of an isolated directional preponderance (DP) on bithermal caloric testing. An isolated caloric DP was defined as a DP, calculated according to the standard Jongkees formula, of ≥40%, with a spontaneous nystagmus (SN) in darkness of ≤2°/s and a canal paresis (unilateral weakness) of ≤25%.
A retrospective analysis of all 15,542 bithermal caloric tests performed in the authors' department in the previous 10 years to identify all tests with an isolated DP of ≥40%. This was followed 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 patients. The study group eventually comprised 114 patients; these were patients in whom a clinical diagnosis could be made at the time the caloric test 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.
A balance disorders clinic in a tertiary referral hospital.
All patients underwent standard bithermal caloric testing. Some of the patients also underwent rotational testing.
A clinical diagnosis for the cause of the isolated DP, made either at the time of the caloric test or on the basis of information supplied at follow-up by the patient or by the referring physician.
Of 114 patients, 39 had benign paroxysmal positioning vertigo, 14 had Ménière's disease, and 5 had migrainous vertigo. Five patients had central 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 the other 54 patients, no definite diagnosis could be made, but 41 of these 54 were either completely well or much better at follow-up.
An isolated DP on caloric testing is usually a transient, benign disorder. About half the patients with an isolated DP have either Ménière'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 ∼5% have a CNS lesion and in almost all this is apparent at the time the caloric test is done. In a relapsing-remitting peripheral vestibular disorder such as benign paroxysmal positioning vertigo or Ménière's disease, the mechanism of an isolated DP could be enhanced dynamic gain of ipsilesional medial vestibular nucleus neurons, perhaps as a result of intermittent 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, perhaps as an adaptive change in response to abnormal input. In an accompanying article, the authors implement a realistic neural network model in which it is possible to simulate an isolated DP by adjusting the dynamic sensitivity of type 1 medial vestibular nucleus neurons on one side or of type 2 medial vestibular nucleus neurons on the other.
*Neurology Department, Royal Prince Alfred Hospital; and †Psychology Department, University of Sydney, Sydney, Australia
Supported in part by the Garnett Passe and Rodney Williams Memorial Foundation, by the RPA Neurology Department Trustees, and by the National Health and Medical Research Council.
Address correspondence and reprint requests to Dr. G. Michael Halmagyi, Neurology Department, RPA Hospital Camperdown, NSW 2050, Sydney, Australia.