Degenerative neurological condition, characterized by a progressive loss of mobility, muscular rigidity, tremor, and speech difficulties. The disease destroys a group of cells known as the substantia nigra (‘black substance’) in the upper part of the brainstem, which are concerned with the production of the neurotransmitter dopamine, essential to the control of voluntary movement. There is no cure, but medical treatment can moderate the symptoms.
Types Parkinson's disease can be categorized into age-related groups. Juvenile Parkinson's affects those younger than 20 years old, while early-onset Parkinson's affects those between 20 and 50 years old. However, the majority of cases have late-onset Parkinson's and are over the age of 50.
Parkinsonism is a collective term used to describe all medical conditions that share the symptoms of Parkinson's disease. These include hypokinetic rigid syndrome, vascular Parkinsonism, dementia with Lewy bodies, drug-induced Parkinsonism, multiple system atrophy (MSA), and pure autonomic failure (PFA).
Symptoms Parkinson's disease can also produce a wide range of symptoms unrelated to the standard movement-related effects. The commonest are clinical depression, anxiety, difficulties in swallowing, involuntary muscle movement, loss of balance, and mild cognitive impairment. In around 40% of cases, the last of these may develop into dementia.
Treatment The commonest way of managing symptoms is through the use of drugs that increase the level of dopamine in the brain or stimulate those parts of the brain that use dopamine to be more efficient. However, all drugs tend to become less effective with time. A condition known as an ‘off-episode’ occurs when drugs lose effectiveness and symptoms return suddenly. Consequently, drug treatments have to be regularly managed to keep symptoms under control. Language therapy and physiotherapy are also commonly used.
The introduction of the drug L-dopa in the 1960s was the first effective medicinal treatment for Parkinson's. However, it became evident that long-term use brings considerable problems, and at best it postpones the terminal phase of the disease. Brain grafts with dopamine-producing cells were pioneered in the 1980s, and attempts to graft Parkinson's patients with fetal brain tissue have been made. This experimental surgery has brought considerable improvement to some PAF patients but is ineffective in the MSA form. In 1989, a large US study showed that the drug deprenyl may slow the rate at which disability progresses in patients with early Parkinson's disease.
Genetics Advances in genetics, in particular the emergence of genomics, have allowed genetic risk factors associated with Parkinson's disease to be identified. In 1997, the US National Human Genome Research Institute announced the discovery of a gene that causes the condition. This gene produces a protein called alpha synuclein. When the instructions provided by the gene go wrong, the protein's structure is affected, causing a build-up of deposits on brain cells, seen in most sufferers.
By 2009, 13 genes had been identified as being associated with the condition. These can be grouped into causal genes such as alpha synuclein, where mutations or additional copies of the gene cause the condition to develop, and associated genes, which increase the risk that the condition will develop and are thought to be triggered by environmental conditions, such as exposure to industrial chemicals, insecticides, pesticides, and air pollution from traffic.
Parkinson's disease is very rarely directly inherited within families, although family members can carry genes associated with the condition. It is estimated that there is a genetic cause in around 5% of all cases.
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