Abnormal somatotopic arrangement of sensorimotor interactions in dystonic patients

The aim of the study was to detect abnormalities of sensorimotor interactions and their topographic distribution in the hand muscles of dystonic patients. We investigated the effect 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 (TMS) and transcranial electrical stimulation (TES) in the relaxed first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles on both sides of eight patients with unilateral hand dystonia (HD) and in four patients with cervical dystonia (CD). Six Parkinson's disease patients were used as the disease control group and 10 healthy subjects served as normal controls. For each muscle, the digital stimulation was applied to a contiguous finger (CF) and to a non-contiguous finger (NCF). The digital stimulation was set at three times the sensory threshold and preceded TMS or TES at intervals ranging from 10 to 100 ms. In normal subjects, a somatotopic inhibitory effect was detected, since the CF stimulation was significantly more powerful in determining the reduction of MEPs in response to TMS at intervals ranging from 20 to 50 ms. In dystonic patients, on the contrary, the somatopic effect was not present, because both CF and NCF stimulation evoked a consistent MEP inhibition and no significant difference was detected between the conditioning effect of CF and NCF stimulation. These abnormalities were present in the muscles of both the affected and unaffected hands of HD patients, as well as in CD patients. TES conditioning provoked MEP inhibition only at interstimulus intervals (ISIs) <40 ms. Significant MEP potentiation was found at ISIs of 20-40 ms to CF stimulation in Parkinson's disease patients, while there was no effect after NCF stimulation. These data suggest that MEP suppression in response to digital stimulation is preserved in dystonia, but the somatotopically distributed input-output organization of the sensorimotor interactions is lost in dystonic patients' hands. The comparison between TMS and TES experiments indicates that abnormalities may be present at both the spinal and the cortical level, at least in some patients. These findings suggest that a mechanism that normally operates in order to focus the effect of somatosensory afferences on the motor system may be impaired in dystonia. This abnormality seems specific to dystonia.