Pathologic studies suggest PD patients may remain symptom free until what percent of the substantia nigra has degenerated?

Pathologic studies suggest PD patients may remain symptom free until what percent of the substantia nigra has degenerated?

• A. 20-40%
• B. 40-60%
• C. 60-80%
• D. 80-100%

Answer: (C)

Motor symptoms in Parkinson disease appear only after substantial loss of the nigrostriatal dopaminergic system. The brain can compensate for early neuron loss, but once a large portion of the substantia nigra pars compacta is degenerated, around 60–80%, those compensatory mechanisms fail and motor features emerge. This threshold explains why patients can be asymptomatic for a long period despite ongoing neurodegeneration—the dopaminergic reserve is simply enough to keep function until that tipping point is reached.

Motor symptoms in Parkinson disease appear only after substantial loss of the nigrostriatal dopaminergic system. The brain can compensate for early neuron loss, but once a large portion of the substantia nigra pars compacta is degenerated, around 60–80%, those compensatory mechanisms fail and motor features emerge. This threshold explains why patients can be asymptomatic for a long period despite ongoing neurodegeneration—the dopaminergic reserve is simply enough to keep function until that tipping point is reached.