A number of disorders can cause symptoms similar to those of Parkinson's Disease (PD). People with symptoms that resemble PD but that result from other causes are sometimes said to have parkinsonism. Some of these disorders are listed below.
Postencephalitic parkinsonism. Just after the first World War, a viral disease, encephalitis lethargica, attacked almost 5 million people throughout the world, and then suddenly disappeared in the 1920s. Known as sleeping sickness in the United States , this disease killed one third of its victims and led to post-encephalitic parkinsonism in many others. This resulted in a particularly severe form of movement disorder that appeared sometimes years after the initial illness. (In 1973, neurologist Oliver Sacks published Awakenings, an account of his work in the late 1960s with surviving post-encephalitic patients in a New York hospital. Using the then-experimental drug levodopa, Dr. Sacks was able to temporarily "awaken" these patients from their statue-like state). In rare cases, other viral infections, including western equine encephalomyelitis, eastern equine encephalomyelitis, and Japanese B encephalitis, have caused parkinsonian symptoms.
Drug-induced parkinsonism. A reversible form of parkinsonism sometimes results from use of certain drugs, such as chlorpromazine and haloperidol, which are prescribed for patients with psychiatric disorders. Some drugs used for stomach disorders (metoclopramide), high blood pressure (reserpine), and epilepsy (valproate) may also produce parkinsonian symptoms. Stopping the medication or lowering the dosage of these medications usually causes the symptoms to go away.
Toxin-induced parkinsonism. Some toxins — such as manganese dust, carbon disulfide, and carbon monoxide — can cause parkinsonism. The chemical MPTP also causes a permanent form of parkinsonism that closely resembles PD. Investigators discovered this reaction in the 1980s when heroin addicts in California who had taken an illicit street drug contaminated with MPTP began to develop severe parkinsonism. This discovery, which showed that a toxic substance could damage the brain and produce parkinsonian symptoms, caused a dramatic breakthrough in Parkinson's research: for the first time, scientists were able to simulate PD in animals and conduct studies to increase understanding of the disease.