Parkinson's disease is the textbook example of basal ganglia failure. The pathology is the progressive death of dopaminergic neurons in the substantia nigra. The pathophysiology, as we have just built, is failure of both basal ganglia pathways: the direct pathway loses D1-mediated activation, and the indirect pathway loses D2-mediated inhibition. Both effects suppress movement.
The clinical signature is the cardinal triad: bradykinesia (slowness of movement), rigidity (cogwheel resistance to passive movement), and resting tremor (typically pill-rolling, 4 to 6 Hz, more pronounced at rest than during action). Postural instability emerges later. The classic patient walks with shuffling steps, takes a long time to turn, has a stooped posture, and shows reduced facial expression — the so-called masked face.
The treatment is to replace the missing dopamine. Levodopa, the immediate precursor of dopamine, crosses the blood-brain barrier and is converted to dopamine in surviving nigrostriatal neurons. It is the most effective symptomatic treatment available. Carbidopa is added to prevent peripheral conversion of levodopa to dopamine — peripheral dopamine produces nausea and cardiovascular effects but cannot cross the blood-brain barrier, so blocking peripheral conversion concentrates the active drug where it is needed.
For the first several years of treatment, levodopa works beautifully. Symptoms reduce dramatically. Patients describe feeling like themselves again. But after roughly five to ten years of treatment, the response begins to fluctuate. The system becomes oversensitive — small fluctuations in levodopa blood levels produce large swings in clinical state.
These fluctuations manifest as dyskinesias — involuntary writhing, choreiform, sometimes dystonic movements that emerge at peak-dose times. The patient with end-stage levodopa-treated Parkinson's may toggle through the day between off periods of severe bradykinesia and on periods of dyskinetic involuntary movement, with brief windows of normal function between them. This is one of the most challenging clinical pictures in neurology.
Treatment in advanced Parkinson's becomes about smoothing these fluctuations — adjunct dopamine agonists (pramipexole, ropinirole), MAO-B inhibitors (rasagiline, selegiline), COMT inhibitors (entacapone), and ultimately deep brain stimulation of the subthalamic nucleus or globus pallidus interna for the patients in whom medication alone is no longer adequate.
Hold this trajectory. The early Parkinson's patient is a clear win for medicine. The late Parkinson's patient is a humbling reminder that we treat symptoms, not causes. The substantia nigra continues to die under our pharmacology.