Sensitivity and specificity of a portable system measuring postural tremor.

A portable, accelerometric system measuring tremor was evaluated. That is, the validity and consistency of measurements as well as its ability to discriminate pathologic from physiological tremor were investigated. Control subjects and patients with Parkinson's disease were tested with this portable system and with an independent system that gave precise displacement data using lasers. It was found that amplitude of postural tremor as measured by the two systems differed significantly, but further investigation revealed that this difference was due 1) to the difference between amplitude of acceleration and amplitude of displacement, and 2) to changes in tremor over the time between tests, rather than to any inaccuracy or unreliability in the portable system. The other characteristics of tremor reported by the portable system were also valid and reasonably reliable in test-retest experiments, with the exception of the "harmonic index," which proved less stable. Most of the reported characteristics were distributed differently for the control group and for the patients with Parkinson's disease, but the large overlaps between distributions would make diagnosis difficult when tremor is not very pronounced. These results suggest that until better discriminating measures of tremor are available, tremor tests should be repeated and combined with other tests of motor function.