Paired transcranial magnetic stimulation for the early diagnosis of corticobasal degeneration

Objective : To investigate cortical excitability in patients with corticobasal degeneration (CBD) and to find a reliable diagnostic technique for differentiating CBD from Parkinson's disease (PD). Methods : Using a paired transcranial magnetic stimulation technique, we studied motor cortex excitability at rest in 6 patients with clinically probable CBD, 10 patients with PD, and 10 normal subjects. The recovery cycle of the motor evoked potentials was tested by delivering paired magnetic stimulation over the hand area of the motor cortex at interstimulus intervals (ISIs) from 1 to 17ms. Results : In patients with CBD, paired magnetic stimuli delivered at short ISIs invariably elicited enlarged test MEPs. At ISIs of 1–10ms, the conditioned test MEPs were significantly larger in patients with CBD than in control subjects; and at ISIs of 1, 2, 4, and 6ms ,they were also larger in patients with CBD than in patients with PD. At the other ISIs tested, patients and control subjects had similar amplitude conditioned test responses. Conclusions : Our findings suggest that the unusual clinical manifestations of CBD might arise partly from motor cortex disinhibition. Paired magnetic stimulation could be a useful diagnostic test particularly in the early stages of the disease.