The dysrhythmia and the exponential constants of the postrotatory nystagmus qualities

Dysrhythmia of caloric nystagmus

OBJECTIVES

Objectives of the study were 1) to determine the prevalence and characteristics of dysrhythmia, 2) to identify associations between dysrhythmia and other patient variables, and 3) to determine whether dysrhythmia is more a characteristic of individuals than simply a correlate of alertness or response intensity.

STUDY DESIGN

Cross-sectional.

METHODS

Review of records from 150 patients undergoing the caloric test.

RESULTS

Dysrhythmia severe enough to interfere with accurate analysis of the caloric test was present in 40% of the cases. Individuals were entered into an "orderly" group (n = 67) or a "dysrhythmic" group (n = 83) and were 5.8 times more likely to remain in the same group on a second visit than to change groups. Stronger response scores were associated with less dysrhythmia. Higher caloric-induced vertigo scores were associated with both stronger response scores and lower dysrhythmia scores. Cases in the dysrhythmic group were 3.8 times more likely to have abnormal caloric test results than individuals in the orderly group.

CONCLUSIONS

Dysrhythmia frequently interferes with valid and accurate interpretation of the caloric test. Current methods of "mental alerting" frequently fail to eliminate dysrhythmia. The presence of dysrhythmia appears to be a strong predictor of an abnormal caloric test result, and there is a moderate correlation between dysrhythmia and response intensity. Nevertheless, our results suggest that dysrhythmia is specific to individuals, rather than simply a manifestation of peripheral vestibular disease or a correlate of alertness or response intensity.

Clinical evaluation of dysrhythmia of postrotatory nystagmus

Postrotatory responses of nystagmus were analysed in an exponential model by utilizing linear regression analysis. Four nystagmus qualities (velocity and duration of slow and fast phases) were studied in 10 patients with vestibular peripheral lesions, 10 patients with frontal lobe lesions and 10 patients with brain-stem lesions, together with 10 control subjects. In addition, pauses during the responses were quantified. Patients with frontal lobe lesions differed from other groups by scoring higher values of slow phase velocity and by exhibiting more pauses. The time constant was significantly shorter in patients with brain-stem lesion than in any other group. As regards other qualities, e.g. slow phase duration and fast phase velocity, or duration, no differences were observed. The pathological dysrhythmia may therefore be presented as changes in the gain and time constant of slow phase velocity as well as in pauses during nystagmus. Since all these changes may be encountered in normal subjects, one should be cautious in interpreting these changes as being pathological in each individual case.