The progression of idiopathic Parkinson's disease is not explained by age-related changes. Clinical and pathological comparisons with post-encephalitic parkinsonian syndrome.

In 12 cases of post-encephalitic parkinsonian syndrome (PEPS) starting at a mean age of 25.1 years, the mean disease duration of 35.8 years was twice as long as in 12 young patients with Lewy body-Parkinson's disease (LB-PD), whose disease started at a mean age of 39.9 years (mean duration 16.5 years). In PEPS the rate of progression of disease was extremely slow. The groups were of a similar age at death, but the post-encephalitic cases had 70% fewer cells in the substantia nigra than in idiopathic Parkinson's disease (PD) and 92% fewer than in controls. Nigral cell destruction was not found in PEPS, but active cell breakdown and abundant extraneuronal melanin were present in PD. A second group of 12 LB-PD cases with mean onset at 58.9 years survived for a mean of 10.1 years, but the number of remaining substantia nigra cells was the same as in the young onset cases of PD. This clinical and pathological evidence does not support the notion that progression of disease in PD can be attributed to the superadded effects of normal ageing on top of a primary acute noxious insult sustained in earlier life.