Transcranial magnetic stimulation follow-up study in early Parkinson's disease: A decline in compensation with disease progression?

BACKGROUND

A number of neurophysiological abnormalities have been described in patients with Parkinson's disease, but very few longitudinal studies of how these change with disease progression have been reported. We describe measures of motor cortex inhibition and plasticity at 6 and 12 mo in 12 patients that we previously reported at initial diagnosis. Given the well-known interindividual variation in these measures, we were particularly concerned with the within-subject changes over time.

METHODS

Patients were assessed clinically, and transcranial magnetic stimulation (TMS) was used to measure motor cortical excitability, inhibition (short interval intracortical inhibition, cortical silent period), and plasticity (response to excitatory paired associative stimulation protocol) in both hemispheres. All measurements were performed 6 mo and 12 mo after the baseline experiments.

RESULTS

Asymmetry in clinical motor symptoms was reflected in asymmetry of plasticity and inhibition. In the group as a whole, little change was seen in any of the parameters over 12 mo. However, analysis of within-individual data showed clear correlations between changes in clinical asymmetry and asymmetry of response to paired associative stimulation protocol and cortical silent period.

CONCLUSIONS

Longitudinal changes in cortical silent period and response to paired associative stimulation protocol in Parkinson's disease reflect dynamic effects on motor cortex that are related to progression of motor signs. They are useful objective markers of early disease progression that could be used to detect effects of disease-modifying therapies. The decline in heightened plasticity that was present at disease onset may reflect failure of compensatory mechanisms that maintained function in the preclinical state.