Analysis of single-joint rapid movements in patients with sporadic olivopontocerebellar atrophy

Patients with pure cerebellar cortical atrophy (CCA) present isolated cerebellar signs, whereas patients with sporadic olivopontocerebellar atrophy (sOPCA) present various combinations of cerebellar and extracerebellar signs. However, the differential diagnosis between these two forms of cerebellar degeneration is often a challenge for the clinician. Therefore, any test helping in this differential diagnosis might have a potential clinical interest. In this study, our goal was to investigate the adaptation to increased inertia in patients with sOPCA exhibiting combined cerebellar and pyramidal signs, during the performance of fast wrist flexions. We found that these patients exhibited a hypermetria which remained unchanged after addition of inertia, because they were unable to increase neither their agonist activity (launching force), nor their antagonist activity (braking force). This contrasts with our previous findings in patients with CCA. In these latter, the hypermetria worsened when the inertial load of the hand increased because those patients were able to increase their agonist activity, but not their antagonist activity. The adaptation to inertia might thus help to differentiate CCA and sOPCA.