MEASUREMENT OF RIGIDITY IN PARKINSONS-DISEASE

Clinical assessment of rigidity in parkinsonian patients is largely qu alitative. The reliability and validity of the assessments are sometim es in doubt. Several ''engineering'' methods of quantifying rigidity h ave been described, but none has been adopted into general clinical pr actice. A possible reason is that these methods differ in crucial aspe cts from the clinical exam. We therefore tackled the problem by monito ring the clinical exam itself, using small sensors to measure the forc es and displacements applied. Limb impedance (Z) was computed using pa rameter identification methods and compared to raters' verbalized rati ngs of rigidity based on a 5-point scale: the Unified Parkinsons Disea se Rating System. The qualitative and quantitative estimates of impeda nce covaried over a fourfold range, depending on the fortes imposed an d the subject's motor set. Raters differed by up to 1 full point in th eir mean qualitative ratings and sometimes disagreed on whether levodo pa reduced rigidity, This was not due to any significant differences i n the overall range of rigidity they evoked, but rather to the way the y scored this range [the ratio of mean rating to mean impedance (R/Z) varied between raters and subjects]. On the other hand. the R/Z ratio was reproducible over separate sets of ratings and may therefore serve to convert measured impedance into a standardized rating. Our results indicate that the current clinical exam may be too abbreviated to det ect the sometimes quite small reductions in rigidity after levodopa. W e conclude that a device that conveniently quantifies the clinical ass essment of rigidity is now available and will lead to more standardize d protocols for rating rigidity in the near future.