Motor cortex activation by transcranial magnetic stimulation in ataxia patients depends on the genetic defect

In patients with degenerative ataxia, various abnormalities in motor cortex activation by transcranial magnetic stimulation (TMS) have been observed, including a reduction of intracortical facilitation and a lengthening of the silent period. However, the groups of patients examined in previous studies were heterogeneous, involving patients with autosomal-dominant and idiopathic cerebellar ataxia, and showing different clinical features. The aim of our present study was to investigate whether differences in motor cortex activation by TMS could be observed in genetically defined subtypes of degenerative ataxia. We examined six patients with Friedreich's ataxia, three patients with spinocerebellar ataxia (SCA) type 1, seven patients with SCA2, 12 patients with SCA3, nine patients with SCA6 and 14 healthy controls. In all subjects, motor threshold, central motor conduction time, cortical silent period after TMS, and intracortical inhibition and facilitation (as assessed by TMS using a paired pulses paradigm) were determined. Additionally, F wave amplitudes evoked by electrical peripheral nerve stimulation were measured. We found a significant reduction of intracortical facilitation in SCA2 and SCA3 patients. Furthermore, motor threshold was elevated in SCA1, central motor conduction time was lengthened in patients with Friedreich's ataxia and SCA1, and F wave amplitudes were enlarged in all the genetic subgroups except for SCA6. Silent period and intracortical inhibition did not differ between patients and controls. We conclude that changes of intracortical facilitation induced by TMS and other excitability parameters of the motor system are not a common phenomenon in degenerative ataxia, but are restricted to specific subtypes. This points to differences in the underlying pathophysiological processes in genetic subtypes of ataxia.