Effects of a startle on heart rate in patients with multiple system atrophy

The patient cooperation usually required for neurophysiological assessment of autonomic cardioregulatory function is difficult to obtain from patients with bradykinesia. A particularly interesting condition occurs in multiple system atrophy (MSA), which features both bradykinesia and autonomic dysfunction. Another characteristic of patients with MSA is their normal motor reaction to a startling stimulus. We used startle as a stimulus for testing autonomic cardioregulatory function in patients with MSA, thus avoiding the need for patient cooperation. In 10 healthy volunteers and 8 MSA patients, we recorded the electrocardiographic QRS complex with surface electrodes attached over the chest and delivered an acoustic startle stimulus after 8 seconds of baseline recording. We calculated the ratio between the pre-stimulus and the post-stimulus heart beat intervals (R-R ratio) by dividing the mean prestimulus R-R interval by the shortest R-R interval obtained within 10 seconds poststimulus. Healthy volunteers had a significant shortening of the R-R interval. The peak of the effect occurred after 2 to 5 seconds, with a mean R-R ratio of 1.14 (S.D. = 0.09). In contrast, R-R shortening was markedly reduced in patients, even though they had a normal motor response. The mean R-R ratio in patients was 1.03 (S.D. = 0.03), significantly lower than in healthy volunteers (P < 0.01). Our results demonstrate an abnormally reduced modulation of the heart beat frequency in patients with MSA, compatible with a dysfunction on pathways responsible for autonomic regulation. The method described here may be useful in the assessment of cardioregulatory function in poorly cooperative patients with normal startle responses.