Parkinson's disease: description and incidence
Description
Parkinson's disease (PD) is a progressive (degenerative) condition involving a disturbance in the co-ordination of movement. It was first described in 1815 by Dr James Parkinson. His Essay on the shaking palsy was so detailed that his description of the characteristic tremor and slowness of movement is still used as a basis for diagnosis.
He described 6 patients with 'involuntary tremulous motion with lessened muscular power in parts not in action even when supported, with a propensity to bend forward, and to pass from a walking to a running pace' [Parkinson 1817].
The term 'Parkinson's disease' (or primary parkinsonism) should be reserved for those cases that are idiopathic (i.e. causative factor unknown).
PD is characterised by 4 major and several minor symptoms. The major symptoms are tremor (shaking), rigidity (stiffness), bradykinesia (slowness of movement), and postural instability. The disease usually begins slowly, with tremor in the hands, arm or leg. The muscles begin to stiffen. In most cases, one side of the body is affected before the other, but generally both sides are affected over time. A person may develop a shuffle, and have difficulty with postural correction and with fine movement such as writing.
Facial expression also may become rigid and inflexible. Despite the gradual progression of symptoms, the disease itself does not shorten life.
PD has no known cure at this time. It is now the subject of study and debate as to whether the cause is an externally generated toxin, an internally produced poison or a defect in the cell itself resulting in premature self destruction (apoptosis).
It is known that there is a loss of nerve cell bodies in the substantia nigra (a pigmented area in the base of the brain) which is associated with diminishing levels of dopamine — a neurotransmitter (chemical carrier) which transmits impulses from one nerve cell to another — but there is no substantial evidence pointing to the cause of the disease.
Dopamine is produced by the nerve cells (neurones) in the substantia nigra. The degeneration of these neurones reduces the amount of dopamine produced and leads to an interference with the normal function of the basal ganglia — a linked area of the brain involved in the integration of muscle action. Cell death in PD is not unlike the biologically related processes of oxidation and rancification; researchers are currently studying oxidative stress in PD.
Studies have shown that the reduction of dopamine is already well advanced in the brains of patients with only mild symptoms, which indicates that the underlying disease process has been progressing over a number of years (at least 4) before clinical deficits present. Even when symptoms become noticeable, it is sometimes up to 2 years before a definite diagnosis is made.
The nerve cells of the substantia nigra have their cell bodies within the substantia nigra, but their terminals are in a related area of the brain called the corpus striatum. The cell terminals release dopamine into the cells of the corpus striatum, which modulates neurotransmitter function. It is commonly agreed that the function of the corpus striatum depends upon a balance being maintained between all its neurotransmitters, including dopamine and acetylcholine.
With the deterioration of the substantia nigra, there is a loss of dopamine input into the corpus striatum, causing an effective increase in acetylcholine and a resulting imbalance of neurotransmitters. This imbalance in the basal ganglia brings about postural abnormalities and difficulties with smooth execution of movement.
The striatal dopamine deficiency leads to excitation of the medial segment of the globus pallidus and this is responsible for the bradykinesia (abnormal slowness of movement and mental responses).
Recent magnetic resonance imaging and positron-emission tomography (PET) studies have allowed for surgical intervention (such as pallidotomy) to be redeveloped to counteract this.
Although PD is progressive, it can be alleviated with appropriate medication to enhance the level of dopamine in the brain and effectively reduce the symptoms for a period of time. Medication does not alter the progression of the disease, and drug therapy becomes less predictable in its effect and more difficult to manage later in the course of the disease.
Recent research has shown that neurones are capable of regenerating themselves. This can occur under the influence of trophic (growth) factors, each one specific for one or 2 neurones. This area of research gives hope for an eventual cure for PD.
Another pathological change noted in PD is the loss of pigmentation of the nerve cells in the substantia nigra. This has been regularly observed in autopsies on patients who have had PD. It has also been noted that there are Lewy bodies (discovered by F.H. Lewy in 1913) present within the degenerating pigmented cells. These invade the cells after they begin to degenerate and some researchers have suggested that idiopathic PD be renamed Lewy Body Disease.
Incidence and prevalence
PD is present in all countries. It can affect young people as well as old, but the most common age of onset is 50-75; 10 per cent of people are less than 50 years old at the time of onset. Slightly more men than women are affected (in the ratio 5:4) but there appears to be no preponderance in particular races. A few families show an inherited pattern for the disease, typically transmitted via an autosomal dominant trait, but twin studies show that hereditary factors are minimal.
There are virtually no accurate figures of incidence and prevalence of PD, and worldwide estimates vary from 1:1000 to 2:1000, with this figure increasing for those over 65 to 1:100 and 4:100 in the over-70 population. A survey carried out in South Australia in 1985 resulted in a figure of 2300 people in South Australia, which would translate to an incidence of 1.6:1000. This could mean that about 25,000 people in Australia have PD.
There have been many famous people who have suffered from PD, including Harry Truman, Mao Tse Tung, General MacArthur and actor Terry Thomas.
Other Parkinsonian syndromes
- Multiple system atrophy is unlike PD in that more than one system degenerates.
- Olivopontocerebellar atrophy or OPCA: the main characteristic is a wide gait due to a loss of balance.
- Nigrostriatal degeneration: there is no shortage of dopamine, but the dopamine receptors do not connect to dopamine terminals. It has similar symptoms to PD but does not respond to dopamine replacement therapy.
- Shy-Drager syndrome is characterised by very low blood pressure, with dizziness on standing, a rigid akinetic state, impaired sexual function and loss of bladder control (inability to initiate urination). A few cases are familial.
- Steele-Richardson-Olszewski syndrome or progressive supranuclear palsy (PSP): first described in the 1960s, this symmetrical disease is characterised by rigidity, falls (backward), a fixed facial expression, inability to look down and speech and swallowing problems. Tremor is rare and then only during movement.
- Cortico-basilar degeneration: severe cell loss in the pre- and post-central parietal area. Symptoms include stiffness, ataxia, choreiform movements, flexion dystonia, vertical gaze palsy, parkinsonian features and cerebellar and pyramidal tract signs.
Last Reviewed: 17 July 2002
