Despite the fact that tardive dyskinesia has existed for over 50 years, its etiology is poorly understood due to the limited research conducted on psychiatric drug side effects. The cause of tardive dyskinesia appears to be related to damage to the system that uses and processes the neurotransmitter dopamine. The most compelling line of evidence suggests that tardive dyskinesia may result primarily from neuroleptic-induced dopamine supersensitivity in the nigrostriatal pathway, with the D2 dopamine receptor being most affected. Neuroleptics act primarily on this dopamine system, and older neuroleptics, which have greater affinity for the D2 binding site, are associated with high risk for tardive dyskinesia.The D2 hypersensitivity hypothesis is also supported by evidence of a dose-response relationship, withdrawal effects, studies on D2 agonists and antagonists, animal studies, and genetic polymorphism research.
Given similar doses of the same neuroleptic, differences among individuals still exist in the likelihood of developing tardive dyskinesia. Such individual differences may be due to genetic polymorphisms, which code for D2 receptor binding site affinity, or prior exposure to environmental toxins. Decreased functional reserve or cognitive dysfunction, associated with aging, mental retardation, alcohol and drug abuse, or traumatic head injuries, has also been shown to increase risk of developing the disorder among those treated with neuroleptics.
The available research seems to suggest that the concurrent prophylactic use of a neuroleptic and an antiparkinsonian drug is useless to avoid early extrapyramidal side-effects and may render the patient more sensitive to tardive dyskinesia. Since 1973 the use of these drugs have been found to be associated with the development of tardive dyskinesia. Since some of the symptoms of tardive dyskinesia can be interpreted as schizophrenia by doctors, they may prescribe additional neuroleptic drugs to treat it, leading to increased risk of more prevalent tardive dyskinesia. Several studies have indicated that long-term neuroleptic use is associated with both cognitive deterioration and atrophy of the brain.
FDA Announces that Reglan / Metoclopramide Usage May Increase Risk of Tardive Dyskinesia
Metoclopramide works by speeding up the movement of the stomach muscles, thus increasing the rate at which the stomach empties into the intestines. It is used as a short-term treatment of gastroesophageal reflux disease in patients who have not responded to other therapies, and to treat diabetic gastroparesis (slowed emptying of the stomach's contents into the intestines). It is recommended that treatment not exceed three months.
Metoclopramide is available in a variety of formulations including tablets, syrups and injections. Names of metoclopramide-containing products include Reglan Tablets, Reglan Oral Disintegrating Tablets, Metoclopramide Oral Solution, and Reglan Injection. More than two million Americans use these products.
Recently published analyses suggest that metoclopramide is the most common cause of drug-induced movement disorders. Another analysis of study data by the FDA showed that about 20 percent of patients in that study who used metoclopramide took it for longer than three months. The FDA has also become aware of continued spontaneous reports of tardive dyskinesia in patients who used metoclopramide, the majority of whom had taken the drug for more than three months. SEE: FDA Reglan Warning
