Hallucinations are a common symptom of Parkinson's disease. Estimates for the exact frequency vary, though. One assessment suggests that a quarter of patients hallucinate regularly and about half have experienced a hallucination at some point during the course of the disease. The causes and timing of these hallucinations are complicated.
The hallucinations in Parkinson's are usually relatively minor. One of the most common reported hallucinations is simply a feeling of presence: a sensation that some being is nearby. Visual hallucinations are the next most common, followed by auditory hallucinations, which only rarely occur independently. These visual hallucinations often involve a complicated, moving, and blurry image. In this respect Parkinson's differs from other hallucinogenic illnesses such as schizophrenia. Hallucinations can, however, sometimes be part of a larger psychotic complex involving paranoid delusions.
Several different factors have been identified as causes for hallucinations in Parkinson's. The oldest explanation is that the medications for Parkinson's are responsible. It has been suggested that levdopa, which the body turns into the neurotransmitter dopamine, is the main culprit. Monoamine oxidase inhibitors have similar and perhaps more widespread effects. Of all the dopaminergic drugs, dopamine agonists seem to be the most hallucinogenic. Anticholinergic drugs, which are sometimes used to treat Parkinson's, are independently known to be associated with hallucinations.
A 2000 study by Fénelon, Mahieux, Huon, and Ziégler, published in Brain produced a set of other explanations. They found that hallucinations correlated not only with medication but also independently with a variety of other symptoms associated with Parkinson's. For example, weakened vision caused by the disease also increased the likelihood of hallucination. Depression, disability, unusual daytime sleep, and general mental deficiency are all symptoms of Parkinson's that also correlate with an increased number of hallucinations.
The study also found that hallucinations were more frequent at night and after a longer duration of the disease, although they seem to occur somewhat erratically. The researchers also caution that hallucinations in Parkinson's patients seem to be relatively underreported. This may be because patients fear insanity or because many of the episodes are minor.
Later studies have confirmed the results of the 2000 team, finding that any of several factors can cause hallucinations in Parkinson's patients. It has been found that regardless of the variety of causes, lowering doses of medication can improve the condition. Antipsychotic drugs to treat these side effects have also enjoyed more widespread use. The benefit of these drugs, however, must still be balanced against their contribution to the deterioration of motor function.
