Parkinson's Disease

Epidemiology

Misdiagnosis of PD is common, and therefore figures for its prevalence and incidence must be taken cautiously. In addition, there is a growing recognition that PD encompasses a number of etiologically distinct disorders, including some purely genetic and some purely environmental forms. The term "parkinsonian syndrome" (PS) or "parkinsonism" refers to idiopathic (typical) PD, genetic forms, cases with known environmental causes (such as toxins or medications), and the other atypical forms of parkinsonism, including progressive supranuclear palsy, multiple systems atrophy, and corticobasal degeneration. The latter three are often referred to as "parkinson-plus" syndromes, since each includes features not found in idiopathic PD.

A classification of parkinsonian syndromes is given in the Table below. Idiopathic PD is by far the most common, and includes the genetic forms caused by mutations in the genes for LRRK2, alpha-synuclein, parkin, and others. Known environmental causes include poisoning by MPTP (extremely rare), carbon monoxide, and manganese, as well as recurrent head trauma. Neuroleptic exposure, on the other hand, is a relatively common cause of drug-induced parkinsonism (and is reversible). No new cases of postencephalitic parkinsonism have been reported since the 1960s.

Distribution of Parkinson Syndromes (PS)
Syndrome	% of PS Cases
Idiopathic PD	85%
Neuroleptic-induced	7%-9%
MSA	~2.5%
PSP	~1.5%
Vascular	~3%
Known toxins	Very rare
Recurrent head trauma	Very rare

From Hughes et al. Neurology. 2001 Nov;57(10 Suppl 3):S34-8.

The incidence of PS increases with age. The median age of onset for all forms of parkinsonian syndrome is 61.6 years, with median idiopathic PD onset at 62.4 years. Onset before age 30 is rare, but up to 10% of cases of idiopathic PD begin by age 40. In a study in the United States, the incidence of Parkinson's disease was 10.9 cases per 100,000 person years in the general population, and 49.7 per 100,000 person-years for those over age 50. An important caveat is that perhaps 40% of cases may be undiagnosed at any given time.

Incidence and prevalence of PD are growing as the population ages. The number of US cases was estimated at 340,000 in 2005, and is predicted to rise to 610,000 by 2030.