Relation between strength of stimulus and nystagmus frequency in patients with Meniére's disease

Insights on different analysis techniques in the monothermal and bithermal caloric test - which parameter should we use to quantify vestibular function?

OBJECTIVE

To investigate the use of maximum slow component velocity (MSCV) and time-constant duration (TC_d) of elicited nystagmus in quantifying the caloric response and to assess the efficacy of these parameters in the monothermal screening test.

DESIGN

Retrospective analysis of caloric results obtained from 150 patients. Bithermal unilateral weakness (UW), directional preponderance (DP), monothermal caloric asymmetry (MCA) based on warm or cool irrigations only, were calculated using both MSCV and TC_d.

STUDY SAMPLE

66 males and 84 females, aged 16-88 years (μ = 50, σ = 17), who underwent vestibular assessment.

RESULTS

The correlation-coefficient between bithermal caloric unilateral weaknesses calculated using either MSCV or TC_d is 0.155 (p < 0.01). Using MSCV and setting significant MCA at 10%, the sensitivity and specificity were respectively 100% and 34% for monothermal warm and 82% and 53% for monothermal cool tests. Using TC_d and setting significant MCA at 5%, the sensitivity and specificity were respectively 90% and 77% for monothermal warm and 100% and 74% for monothermal cool tests.

CONCLUSIONS

In the monothermal warm caloric test, MSCV can be used more reliably than monothermal cool to predict normal bithermal caloric response. The reverse is true if using TC_d. Unilateral-weakness calculated using TC_d and MSCV were poorly correlated and therefore likely to be demonstrating different aspects of vestibular function.

Which are the parameters of choice from the evaluation of caloric nystagmus?

On hand of caloric tests on 202 healthy subjects, the coefficient of correlation between the currently most frequently used intensity parameters was established separately for each of the four irrigations, i.e. between maximum velocity of the slow phase, maximum amplitude, maximum frequency, total number of beats and duration of reaction. The maximum velocity of the slow phase is so closely correlated to the maximum amplitude, and the maximum frequency is so closely correlated to the total number of beats (r = always greater than 0.75) that it is sufficient to merely determine one of the two well correlated parameters in healthy subjects. However, the parameters maximum velocity of the slow phase and maximum amplitude are less closely correlated to the parameters maximum frequency and total number of beats (r = smaller, in part considerably smaller than 0.75). A better correlation exists between the nystagmus parameters nearly always by warm irrigations, as compared to cold ones. The findings speak for the fact that maximum velocity of the slow phase and maximum amplitude on one hand, and maximum frequency and total number of beats on the other, are an expression of various characteristics of the experimental nystagmus. In a consecutive publication, we are going to report in which way the various nystagmic parameters of patients are being changed by different lesions of the vestibular system.