Parkinson’s Disease
Parkinson’s disease (PD), the second most common neurodegenerative disorder after Alzheimer’s disease (AD), causes a progressive neurologic syndrome characterized by bradykinesia, tremor, rigidity, and, in its later stages, postural instability. The motor signs of PD correspond to loss of dopaminergic neurons in the substantia nigra pars compacta within the ventral midbrain. Neuronal inclusions, called Lewy bodies, are also present in the same region, but they can also be present in limbic and cortical regions and, along with other neurotransmitter deficits, are associated with nonmotor aspects of the disease. PD is to be distinguished from parkinsonism, a general term that refers to clinical conditions with the same motor phenomena, but without reference to a specific etiology. Prevalence rates of PD vary. Epidemiologic studies show ageadjusted prevalence rates (per 100,000 individuals) range from 104.7 in Japan, 114.6 in the United States, 168.8 in Taiwan, and 258.8 in Sicily (Korell and Tanner, 2005). The disease affects about 1 million individuals in North America—approximately 0.5% to 1% of the population older than age 65 years of age. The average age of onset is about 60 years, but 5% to 10% of patients have young-onset PD, beginning before age 40 (Tanner and Ben-Shlomo, 1999). The disease affects all races, and there is a slightly higher prevalence of PD among men. The diagnosis of PD relies on the clinical history and motor examination, which usually distinguish it from other parkinsonian disorders. However, because there is no biological marker that verifies the diagnosis of PD, neuropathologic findings remain the gold standard for confirmation of the clinical diagnosis. Even at specialized movement disorder centers, autopsy studies reveal that 10% to 20% of patients with clinical diagnoses of PD have other neuropathologic diagnoses (Hughes, Daniel, and Lees, 2001). Two of the three cardinal motor signs (tremor, akinesia/bradykinesia, and rigidity) are required to establish the diagnosis of PD, but these motor features overlap with other parkinsonian disorders. However, in patients without an overt tremor, early signs of PD such as decreased arm swing, limb stiffness, and diminished facial expression can be subtle, and the diagnosis of PD maybe delayed for several years.