Neuropathology of parkinsonism

The common denominator of virtually all disorders associated with clinical parkinsonism is neuronal loss in the substantia nigra, particularly the dopaminergic neurons in the pars compacta that project to the striatum (Fig. 12.1). The ventrolateral group of neurons (A9) appears to be the most vulnerable in many parkinsonian disorders (1), and these tend to project heavily to the putamen (2). The more medial group of neurons send projections to limbic forebrain and medial temporal lobe and are less affected, but may be preferentially affected in individuals with cognitive impairment (3). The dorsal tier of neurons may be vulnerable to neuronal loss associated with aging (2).