Although PD is more common in older people, it is not exclusive to them. The peak onset of PD is 60 years, which is hardly considered old these days. Moreover, 15% of PD patients are younger than 50, and 10% are younger than 40. In the United States, for every million people about 3,500 have PD; altogether there are about 1.2 million people with PD in the United States.
Given that the development of symptoms is slow, the time between onset and diagnosis may be from 2 to 10 years. Due to the long period between the onset of the “process of PD” (which is usually silent) and diagnosis, it’s estimated that for every person who’s diagnosed with PD there are at least two or more who have PD but have not yet been diagnosed.
The reason age plays a role in the development of PD is that the slowness of movement and the rigidity of PD results from a loss of neurons (cells) in a region of the brain called the substantia nigra. As we age, we lose approximately 2,000 neurons a year. The “process of PD” results in an increased loss of neurons: 5,000 to 20,000 neurons per year.
The older you are, the fewer neurons you have from the normal age-related loss of neurons, the more vulnerable you are to the accelerated loss of neurons in PD, and therefore the more likely you are to develop PD. The incidence of PD may increase as the overall population ages unless we find a way of stopping or slowing it down. Although easily diagnosed when advanced, mild or early PD can be difficult to diagnose, especially in young people who, it’s thought, are not as likely to get PD.
Approximately 5.0% of people with PD have Young Onset Parkinson Disease (YOPD). YOPD is defined as onset at 40 years or younger. In the United States, there are approximately 50,000 people with YOPD. YOPD and PD are similar but different. Except for age of onset, YOPD in large part resembles adult onset PD. YOPD is thought to represent the lower end of the age spectrum for idiopathic, adult onset PD. In YOPD, as in adult PD, symptoms start insidiously and progress slowly over several years. YOPD is more likely to be genetic than older onset PD.
Tremor is often the most common initial symptom in YOPD, occurring in 70% of patients. In some patients it’s present all the time, in others only during times of stress or fatigue. Tremor usually starts in the thumb or the wrist on one side of the body.
Occasionally the tremor in YOPD may start in or involve the jaw. Dystonia (involuntary muscle spasms) is a common initial symp-tom in YOPD, but not in adult onset PD. Dystonia may appear as an aching pain in the shoulder or upper arm (where it’s often mistaken for arthritis) or a cramping pain in a calf or foot (where it’s often mistaken for a muscle ache related to sports).