Lunt, W.; Moore, J. A.; Cottard, E.; Murphy, A. E.; Shah, M.; Sang, J.; Choi, J.; Dash, H.; Dawson, S.; Green, N.; Nagaeva, E.; Burke, S.; Higgins, J. P. T.; Skene, N. G.

Parkinson's disease is defined clinically by motor dysfunction, but its pathology is not confined to nigral dopaminergic neurons. Prominent non-motor features including cognitive impairment, autonomic failure and sleep disturbance indicate widespread neurodegeneration that remains incompletely characterised. We pre-registered and conducted a multilevel meta-analysis of 166 case-control post-mortem studies published between 1963 and 2025, mapping neuronal loss across 85 brain regions, 38 cell types and 145 region-cell populations. The evidence base behind this map is thin. Only 4 of 145 populations are adequately powered, and 82% of Allen Brain Atlas regions have never been quantified in Parkinson's disease. A further 18 populations would reach adequate power with five or fewer additional studies, identifying an e[ff]icient route to closing current gaps. Noradrenergic neurons of the locus coeruleus degenerate to a similar extent as substantia nigra dopaminergic neurons, with both populations losing more than 60% of neurons. Cholinergic neurons of the basal nucleus and pedunculopontine tegmental nucleus and dopaminergic neurons of the ventral tegmental area show significant but less severe loss. These findings establish Parkinson's disease as a multi-system neurodegenerative disorder and expose key gaps and biases in the existing literature.