Altered somatosensory processing in Parkinson's disease and modulation by dopaminergic medications

Background: Somatosensory abnormalities contribute to the pathophysiology of Parkinson's disease (PD). The goal of this study was to identify abnormalities in the tactile-evoked activation of the somatosensory and motor cortices in PD, and in a sensorimotor circuit that traverses both of these cortical loci. The second goal was to investigate the impact of dopaminergic medication on these measures. Methods: Individuals with PD (n = 10, age 61 +/- 8 years) and aged-matched controls (n = 11, age 52.3 +/- 10.4 years) were studied. PD subjects were studied on and off dopaminergic medications. Using high-resolution functional magnetic resonance, imaging data was acquired over the primary somatosensory and motor cortices during passively delivered, computer-automated tactile stimulation of digits 2 and 5 of the more affected hand in PD and the analogous hand in controls. Short and long-latency afferent inhibition (SAL LAI) were assessed via median nerve stimulation followed by transcranial magnetic stimulation over the motor cortical representation of the first dorsal interosseous muscle. Results: Compared to controls, PD subjects demonstrated diminished activation within the somatosensory cortex, reduced LAI and normal SAL of which all were insensitive to dopaminergic medications. In addition to improving motor symptoms, dopaminergic medications reduced the hyperactivity observed within primary motor cortex in PD. Conclusions: Somatosensory processing is deficient in PD. Reduction in tactile-evoked activation within primary motor cortex may contribute to improvement in motor symptoms with dopaminergic medications.