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Parkinson disease is a progressive neurodegenerative disorder characterized by motor symptoms including tremor, rigidity, bradykinesia, and postural instability. It results from the loss of dopamine-producing neurons in the substantia nigra, leading to disrupted motor control and eventual functional decline.
The disease involves selective degeneration of dopaminergic neurons in the substantia nigra pars compacta, with accumulation of alpha-synuclein protein aggregates forming Lewy bodies. This neuronal loss disrupts the basal ganglia circuitry, particularly the direct and indirect pathways that regulate voluntary movement and motor planning.
Diagnosis relies on recognizing the cardinal motor features and observing response to dopaminergic therapy, as there are no definitive biomarkers. Early symptoms may be subtle and unilateral, progressing to bilateral involvement with increasing functional impairment, requiring comprehensive assessment of both motor and non-motor manifestations for optimal management.