Objective: To examine the sensorimotor interactions in cerebellar patients. Methods: We investigated the effects of electrical stimulation of the second (D2) and fifth (D5) fingers on the amplitude of motor evoked potentials (MEPs) in response to transcranial magnetic stimulation and transcranial electrical stimulation (TES) in the relaxed right abductor digiti minimi muscles of 7 patients with cerebellar syndromes and of 14 age-matched controls. The digital stimulation was set at 3 times the sensory threshold and preceded brain stimulation at interstimulus intervals (ISIs) ranging from 10 to 100 ms. Results: D5 stimulation produced significant MEP inhibition in normal subjects at ISIs of 20-50 ms, while D2 stimulation resulted in a non-significant inhibitory trend with the same intervals. In contrast, digital stimulation had no effect on MEP amplitude in cerebellar patients. A significant difference was found between patients and controls at ISIs of 20-50 ms with D5 stimulation. The difference in amplitude of MEPs conditioned by D5 and D2 stimulation was statistically significant between patients and controls at ISIs of 30 and 50 ms. TES conditioning induced MEP inhibition only at ISIs <40 ms. Conclusions: Digital stimulation would appear to modulate motor system excitability less effectively in cerebellar patients. MEP inhibition by cutaneous afferences is reduced in response to stimulation of contiguous, as well as non-contiguous fingers. The difference between the conditioning effects of the two fingers is also decreased, and therefore the somatotopic distribution of cutaneomotor inhibition is absent in patients. These abnormalities may contribute to the genesis of cerebellar motor symptoms and their time course suggests involvement of subcortical and cortical sites.
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