Reduced intracortical and interhemispheric inhibitions in corticobasal syndrome

Intracortical inhibitory and facilitatory circuits, interhemispheric inhibitory circuits and corticospinal excitability for both distal and proximal muscles were investigated in seven patients with corticobasal syndrome (CBS) and seven healthy volunteers. Transcranial magnetic stimulation was performed with surface electromyogram recorded from first dorsal interosseous muscle and biceps brachii muscle on both sides in patients and the on right side in controls. Motor thresholds and motor-evoked potential recruitment curves in both muscles at rest and during contraction were not significantly different between CBS and controls, although there was a trend for higher threshold on the more affected side in first dorsal interosseous in CBS. CBS patients had loss of short-interval intracortical inhibition bilaterally in distal and proximal muscles, reduced transcallosal inhibition from both hemispheres measured by ipsilateral silent period and paired pulse method in first dorsal interosseous, and by ipsilateral silent period in biceps brachii. Intracortical facilitation and contralateral silent period duration were not significantly changed. In conclusion, CBS patients had deficits in intracortical and interhemispheric inhibition in the representations for both distal and proximal upper limb muscles bilaterally with relative sparing of motor cortical output pathways.