Parkinson Disease

Parkinson disease is a progressive neurodegenerative disorder involving loss of dopaminergic neurons in the substantia nigra pars compacta. These neurons normally project through the nigrostriatal pathway to the striatum, where dopamine helps balance the basal ganglia circuits that facilitate and suppress movement. As dopaminergic input falls, inhibitory output from the basal ganglia increasingly suppresses thalamocortical motor activation. Movement becomes difficult to initiate, smaller, and slower. The resulting motor syndrome is parkinsonism: bradykinesia with rigidity and/or a resting tremor. Bradykinesia is more than moving slowly. Look for a progressive reduction in the amplitude or speed of repetitive movement—for example, finger taps that become smaller and slower with each repetition. Rigidity may feel like sustained resistance during passive movement, sometimes with a ratcheting or “cogwheel” quality. A Parkinson tremor is classically a 4–6 Hz resting tremor that often begins asymmetrically and lessens during purposeful movement. Postural instability and falls can occur, but prominent postural instability early in the illness is not the expected first presentation. It raises concern for an alternative diagnosis. In established disease, impaired postural responses create a major injury...