SOME EFFECTS OF l‐HYOSCINE HYDROBROMIDE ON POST‐ROTATORY SENSATION AND NYSTAGMUS IN MAN

Abstract

In a balanced double-blind trial six subjects were given, on three successive days, an injection of 0-3 mg. or 0-6 mg. of l-hyoscine hydrobromide or normal saline, 30 min. before rotational tests of vestibular function were performed. Hyoscine caused a significant (P =0-01) depression of the 'slope' of the sensation cupulogram without alteration of the extrapolated 'threshold'. This effect was not significantly greater with the higher dose of the drug. In subjects who had received hyoscine there was an appreciable suppression of the post-rotational nystagmic response, and slow pendular oscillations of the eyes were frequently recorded. These effects were antagonized by mental arithmetic.It was concluded that hyoscine increased adaptation in the vestibular sensory system and released the oculomotor system from the ascending influence of vestibular and reticular projections.THE role of vestibular stimulation in the genesis of motion sickness has been clearly demonstrated both in man and in the experimental animal [Sjoberg, 1931;Johnson, Meek and Graybiel, 1962;Kennedy et al., 1965] though there have been conflicting opinions about which group of specialized receptors of the vestibular apparatus were responsible. It is clear from the experiments of Money and Friedberg [1964] that the functional integrity of the semicircular canals is necessary for the induction of motion sickness in the dog, but in man the evidence is less direct. De Wit [1953] suggested that individual differences in susceptibility to motion sickness were related to the sensitivity of semicircular canal receptors, for he found that a group of subjects who were habitually seasick had longer after-sensations and lower sensory thresholds to angular stimuli than a group who did not suffer from this disability. He also observed that dimenhydrinate 100 mg. (Suprimal, Dramamine) or atropine 0-5 mg., given 11-2 hr. before the cupulometric test of van Egmond, Groen and Jongkees [1948], shortened the duration of the reported after-sensations and produced a cupulogram with a lower slope (Hl/z value) than that obtained in the initial test. Unfortunately, in these experiments as the drug was always administered before the second test, the action of the drug was confounded with the habituation or depression of the cupulogram which normally occurs with iteration of the test procedure [de Wit, 19531. Although dimenhydrinate and perhaps atropine are known to offer protection against motion sickness it would appear that l-hyoscine is a more 296

The Effect of Repetition on Measurements of Post‐rotational Turning Sensation and Nystagmus in Man

“…impulse) indicated that nystagmus could be made to re-appear by the performance of a serial subtraction test. This finding confirms the observations of an earlier experiment [Benson and Brand, 1968].…”

“…Further, although dimenhydrinate is known to be an effective prophylactic against motion sickness there is no good evidence of any such effect for atropine [Brand and Perry, 1966]. However it had been possible to show that l-hyoscine caused a depression of the response in the unhabituated subject even when the effect of habituation due to repetition of the test was taken into account [Benson and Brand, 1968]. The effect of the drug on the responses which might be obtained from a fully habituated subject was quite unknown and this was studied in the experiment described below.…”

“…It may also be partly due to the decrease in mental arousal which develops as the experimental situation becomes in-creasingly familiar to the subject. De Wit [1953] [Benson and Brand, 1968] fig. 6 suggest that a steady increase in the strength of the stimulus would be needed to elicit a similar response when the test is repeated and this may indicate a progressive rise in threshold for the group of subjects as the result of habituation.…”

A comparison of some effects of three antimotion sickness drugs on nystagmic responses to angular accelerations and to optokinetic stimuli.

“…These results and others (1,8,10,13,14,15) suggest that moderate dosages of depressant drugs have effects on vestibular nystagmus that are different depending upon the presence or absence of vision and the mental set of the subject. With respect to the latter, the instructional set assumed by the subject during vestibular testing appears to be one of the most important variables in determining overall slow-phase nystagmic output.…”

“…Among the factors which have a marked influence on vestibular nystagmus are the presence or absence of opportunities for visual fixation (6,8,10,14) and the state of mental alertness of the individual (1,4,5).…”