PERTURBATION OF PRECISION GRIP IN FRIEDREICHS ATAXIA AND LATE-ONSET CEREBELLAR-ATAXIA

Perturbations of precision grip were tested in 7 patients with Friedre ich's ataxia (FA) and 11 patients with late-onset cerebellar ataxia (C A). Subjects were instructed to hold a small compressible manipulandum between thumb and index finger and to resist any perturbation of main tained finger position. A sudden increase of load induced a displaceme nt of fingers until this was stopped by subjects' active intervention. The amount of initial displacement emerged as a highly sensitive para meter to differentiate the clinical subgroups: Responses in FA patient s were missing or massively delayed, whereas displacements in CA patie nts were normal or only moderately abnormal. This discrimination of im paired hand function in FA and CA patients has not been possible by us ing only tasks of isometric grip force control. We concluded that our task relies more on intact sensory afferents, which are known to be im paired in FA, than on cerebellar function. In a second task the stiffn ess of the maintained grip was determined. On the average, preresponse stiffness was lower in FA patients as compared with CA patients and n ormal controls. However, stiffness appeared to be an independent param eter that did not influence the amount of displacement in the perturba tion task.