Action-induced clonus mimicking tremor.

Action tremor has been described in cerebellar, task-specific, dystonic, or Holmes tremor. We report 2 patients who developed unilateral kinetic or isometric action tremor of the upper extremity, following cervical spondylotic myelopathy and capsular ischemic stroke. Slight motor weakness and spasticity with exaggerated tendon jerks and passive stretch-induced clonus were present on the same limb. The central motor pathways lesions might have been responsible for a hyperexcitability of the stretch-reflex arc and an enhancement of the coactivation of skeletal muscles through a loss of the descending or segmental control of the spinal reflexes. The unusual topography of the symptoms, their occurrence during motion, and the similar frequency of the passive clonus and the action tremor, led us to hypothesize that both patients had prolonged action-induced clonus, mimicking action tremor. Lesions of the central motor pathways lesions might be responsible for action tremor under certain conditions.