Causes

The most striking pathological change noted in Parkinson’s disease is a loss
of nerve cells in a region of the brain known as the substantia nigra, a layer
of deeply pigmented gray matter located in the midbrain. The region contains
nerve cells that produce dopamine, a neurotransmitter associated with
the control of movement. The levels of dopamine are normally in balance
with another neurotransmitter, acetylcholine. In Parkinson’s disease, the
loss of dopamine-producing cells causes a decrease in the levels of dopamine,
with a consequent imbalance with acetylcholine. This leads to the
symptoms of Parkinson’s disease.
The factors that lead to an upset of the dopaminergic system in the disease
are complex. The disease is found throughout the world and occurs in
nearly equal frequency in men and women, with slightly more men being affected
than women. Parkinson’s disease is found in all ethnic groups, although
there are some striking ethnic differences. The disease is relatively
high among whites and relatively low among African blacks and Asians. Ethnic
differences may reflect genetic and environmental differences. American
blacks have a higher incidence than African blacks, indicating a likely
role of local environmental factors.
The role of genetics in Parkinson’s disease has been difficult to establish.
A family history of Parkinson’s disease appears to be a strong indicator of an
increased risk of the disease. As part of its comprehensive genetic profiling
of its entire population, Iceland has gathered an immense amount of data
on genetic diseases, including Parkinson’s disease. In the study of late-onset
Parkinson’s disease, the risk ratio increased with degree of relatedness, with
a 2.7 greater probability of developing the disease for nephews and nieces of
patients, 3.2 for children of patients, and 6.7 for brothers and sisters of patients.
Much research remains to be done to determine whether single genes are playing a major causative role or whether the disorder is multifactorial,
involving genetic and environmental factors. 587