The disease involves a progressive disorder of the extrapyramidal system, which controls and adjusts communication between neurons in the brain and muscles in the human body. It also commonly involves depression and disturbances of sensory systems.

Geneticists have, since 1997, found nine different specific genetic defects, each of which causes the disease in one or a few families with extraordinarily high incidences of the disease, but such families are rare.

Symptoms may vary among patients, and additionally may vary greatly over time in a single patient. However, the primary symptoms are:

• tremor (the best known symptom of PD),
• rigidity (increased tone or stiffness in the muscles),
• bradykinesia (slowness of movement) and akinesia (lack of spontaneous movement),
• postural instability (failing balance, walking problems)

These additional signs and symptoms are also commonly associated with Parkinson's Disease:

• depression: occurs in 40-70% of cases; 20% of depression cases are major depressive disorder; severity and persistance of depression is positively associated with executive dysfunction and dementia;
• anxiety or panic attacks
  Note: 70% of individuals with parkinson's disease diagnosed with pre-existing depression go on to develop anxiety; 90% of parkinson's disease patients with pre-existing anxiety subsequently develop depression);
• executive dysfunction, characterized by difficulties in: differential allocation of attention, impulse control, set shifting, prioritizing, evaluating the salience of ambient data, interpeting social cues, and subjective time awareness. This complex is present to some degree in most parkinson's patients; it may progress to:
• dementia: a later development in approximately 20-40% of all patients, typically starting with slowing of thought and progressing to difficulties with abstract thought, memory, and behavioral regulation.
• memory loss; procedural memory is more impaired than declarative memory. Prompting elicits improved recall.
• apathy or abulia: abulia translates from Greek as the absence or negative of will; apathy is an absence of feeling or desire
• altered sexual function: characterized by profound impairment of sexual arousal, behavior, orgasm, and drive is found in mid and late parkinson disease. Current data addresses male sexual function almost exclusively.
• sleep disturbances: including daytime somnolence; initial, intermediate, and terminal insomnia; disturbances in REM sleep, disturbingly vivid dreams, and REM Sleep Disorder, characterized by acting out of dream content;
• slowed reaction time; both voluntary and involuntary motor responses are significantly slowed.

Statistics
Parkinson's disease is widespread, with a prevalence estimated between 100 and 250 cases per 100,000 in North America; globally prevalence estimates range from a low of 15 per 100,000 in China to a high of 657 per 100,000 in Argentina. Because prevalence rates can be affected by socio-economically driven differences in survival, incidence is a more sensitive indicator: rates have ranged from 1.5 per 100,000 in China to a high of 14.8 per 100,000 in Finland. [BC Medical Journal Volume 43, Number 3, April 2001, 133-137 Epidemiology of Parkinson’s disease Benjamin C.L. Lai, MD, MSc, and Joseph K.C. Tsui, MD, FRCP(UK), FRCPC]

About 2% of the population develops the disease some time during life, though the mean age at onset is 58-60. Symptoms usually begin in the upper extremities, and are usually unilateral (one-sided) or asymmetrical at onset.

source: wikipedia.org