Pathophysiology of pain and fatigue in Parkinson's disease

In Parkinson's disease (PD), nigral degeneration determines an altered neuronal ouput from the subthalamic nucleus and globus pallidus, and as a consequence functional changes in the motor circuits linking basal ganglia to the motor cortical areas. Movement slowness, rigidity and tremor are among the principal motor symptoms of PD. Studies of movement execution have shown that PD patients have difficulty in performing simultaneous and sequential movements. In executing sequential movements the abnormalities of PD patients worsen as the sequence progresses. This phenomenon, called sequential effect, may be one of the mechanisms underlying the fatigue of PD patients. Cortical deafferentation is thought to be responsible for the motor disturbances of PD and studies using transcranial magnetic stimulation showed that in PD patients there are abnormalities in cortical plasticity and in cortical connectivity. Sensorimotor integration refers to the processes that link sensory input to motor output to produce appropriate voluntary movements. Sensory information is important for motor preparation and execution in parkinsonian patients, and PD patients have greater difficulty in performing movements when no external cues are provided. Investigating the role of sensory information, several studies provided evidence that PD patients have numerous somatosensory deficits, including tactile temporal discrimination threshold. Neurophysiological testing in PD has also found altered central somatosensory processing. Finally PD patients may experience painful sensations after the onset of the disease and various evidence suggests an abnormal nociceptive input processing in the central nervous system that might predispose PD patients to developing pain.