American Family Physician - Update on Parkinson’s Disease
Parkinson’s disease is a progressive degenerative disorder of the central nervous system. The hallmark physical signs are tremor, rigidity and bradykinesia. Idiopathic Parkinson’s disease is caused by the progressive loss of dopaminergic neurons in the substantia nigra and nigrostriatal pathway of the midbrain. Secondary parkinsonism may be caused by certain drugs (e.g., metoclopramide and haloperidol) or by cerebrovascular disease (e.g., multiple lacunar strokes). The disease can usually be diagnosed based on the history and physical findings. Dopamine replacement is still considered the most efficacious treatment for Parkinson’s disease, but dopamine agonists, formerly prescribed only as adjunctive therapy, are emerging as useful initial therapy. Other pharmacologic treatments include drugs that inhibit dopamine-metabolizing enzymes (monoamine oxidase-B and catechol O-methyltransferase). Injections of botulinum toxin can be helpful in patients with associated dystonia or blepharospasm. Surgery may be indicated for certain patients or when symptoms do not respond to medical therapy. Additional adjunctive therapies include physical therapy, nutritional counseling and techniques to help patients manage emotional and cognitive changes related to the disease.
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Parkinson’s disease, a progressive disorder of the central nervous system (CNS), is caused by the degeneration of dopaminergic neurons in the substantia nigra of the midbrain. These neurons normally project to the striatum, consisting of the caudate and putamen nuclei, whose neurons bear dopamine receptors. This projection of neurons is just one component of the complex network of interconnections among the deep gray-matter structures known as the basal ganglia (Figure 1). Neurochemical or structural pathologic conditions affecting the basal ganglia result in diseases of motor control, classified as movement disorders.
Parkinson’s disease has been reported to affect approximately 1 percent of Americans over 50 years of age,1 but unrecognized early symptoms of the disease may be present in as many as 10 percent of those over 60 years of age.2 Early- onset Parkinson’s disease, which often affects persons in their 20s, is receiving more attention because of its impact on employability. Epidemiologic studies conducted in the United States have found that Parkinson’s disease is more prevalent in men than in women (approximate ratio: 3:2).2
The disease was once thought to affect primarily whites, but recent studies have demonstrated equal prevalence in African Americans and whites living in the same geographic area.3 To date, no studies have determined the prevalence or incidence of Parkinson’s disease in Hispanics, and retrospective epidemiologic studies performed in various major cities have yielded contradictory information.4 Variations in the prevalence of the disease in individual racial groups in different geographic areas have suggested an increased risk associated with rural living.5 Pesticides and other toxins have been suspected, but none has been proved to be a definite causative factor.
On the other hand, the search for genetic causes has yielded at least four independent gene loci in various forms of familial Parkinson’s disease. The autosomal dominant adult-onset type is linked to a site on chromosome 4q,6 and the gene for autosomal recessive juvenile parkinsonism maps to chromosome 6q.7
Because most patients do not have a clear history of either familial or environmental risk factors, the disorder may be due to a combination of genetic and environmental “influences” or “causes.”
Parkinson’s disease severely compromises quality of life. Patients with this illness can find it difficult to read, write and drive. With advanced disease, they often cannot manage basic activities of daily living. Thus, Parkinson’s disease can result in loss of employment and, ultimately, loss of personal autonomy.
Clinical Presentation
The first step in evaluating a patient with problems suggestive of Parkinson’s disease is to determine which components of motor control are affected. The patient’s signs and symptoms are then clustered to determine whether the diagnosis is Parkinson’s disease or another movement disorder (Table 1).
Like some other CNS degenerative disorders, Parkinson’s disease begins insidiously. Persons close to the patient may notice the problem before the patient does. The patient’s facial expression may appear “depressed” or “apathetic,” and the voice may become softer in volume and monotonous in tone. The patient may complain of muscular “weakness” or “stiffness.” Involuntary movements, such as tremor or the turning in of a foot (dystonia), may become a problem. The symptoms may be noticed during routine activities, or they may be present only at certain times, such as when the patient is walking or writing.
In the initial stages of Parkinson’s disease, many patients do not have movement problems. Instead, they may complain of anxiety and difficulty sleeping. However, signs of motor system dysfunction become apparent on neurologic and physical examination.
