Myoclonus or tremor in orthostatism: an under-recognized cause of unsteadiness in Parkinson's disease.

Patients with Parkinson's disease (PD) often com- plain of unsteadiness. This can occur as the result of various neurological dysfunctions, including changes in postural ad- justments, loss of postural reflexes, axial akinesia and rigidity, freezing and/or postural hypotension. In some cases these symptoms remain unexplained, and rare cases of unsteadiness have been attributed to tremor on standing. To delineate this condition, we investigated 11 consecutive PD patients with unexplained unsteadiness because of tremor on standing, seen in our department over a 6-year period. All the patients had detailed clinical and electrophysiological investigations based on surface polygraphic electromyographic recordings. Four patients had fast orthostatic tremor (13-18 Hz), one had inter- mediate orthostatic tremor (8 -9 Hz), and three had slow ortho- static tremor (4 - 6 Hz). The remaining 3 patients had ortho- static myoclonus, a condition that has not previously been reported in PD. Patients with fast tremor improved on clonaz- epam. Patients with slow tremor and myoclonus improved on levodopa and sometimes benefited further when clonazepam was added. These observations show the usefulness of neuro- physiological investigations for diagnosing and treating unex- plained unsteadiness in Parkinson's disease. © 2007 Movement Disorder Society