Motor Cortex Disinhibition Does Not Track Parkinson’s Motor Symptoms Longitudinally

Researchers assessed short-interval intracortical inhibition (SICI), a transcranial magnetic stimulation measure of motor cortex inhibition, in three cohorts comprising 95 people with Parkinson’s disease. Assessments included 2-year follow-up with concurrent motor evaluation and, in a subset, 18F-DOPA PET imaging (43 patients; PET data in 24); before and after levodopa (30); and before and after focused ultrasound subthalamotomy (22). Results were compared with a pooled group of 75 healthy controls from two previously published studies. Bayesian statistics were used to evaluate evidence for or against effects.

SICI was reduced in patients relative to controls. The analyses found evidence that this cortical disinhibition did not worsen alongside clinically evident disease progression, including progression confirmed by PET, and did not improve after levodopa or subthalamotomy. Changes in SICI were not associated with changes in motor features across the cohorts.

The findings support a dissociation between this physiological measure and clinical motor features in Parkinson’s disease. SICI therefore did not track motor progression or response to the studied interventions in these cohorts. The study does not establish the measure’s clinical utility; cohort sizes were modest, and the control group was pooled from earlier studies.