A dyskinesia is one of many similar disorders of voluntary muscular movement. Sometimes these are characterized by an impaired ability to execute voluntary movements like walking or standing up, but they also occur as involuntary muscular spasms causing sudden tics. Many are symptoms of neurological disorders like Parkinson's or Huntington's disease. A small number of psychiatric medications that affect neurotransmitter function in the brain can cause dyskinesias as side effects in individuals who have certain mental illnesses and genetic risk factors.
Movement disorders that impair or reduce voluntary movement or cause involuntary muscular contractions are called dyskinesias. They frequently are caused by neurological conditions, including neurodegenerative diseases, especially ones that afflict the brain's basal ganglia and cerebellum. Some dyskinesias, called dystonias, cause unusual movements even in still or resting patients. Dystonias cause muscular contractions powerful enough to contort limbs into abnormal, twisted postures. Hypokinesias make up a subclass of disorders entailing an inability to move, called akinesia, and slow movements, called bradykinesia.
The choreas are dyskinesias with random, abrupt movements, which may be brief or may become long and violent bursts of activity. They derive from ailments including metal poisoning, Huntington's disease, and various pathologies of the brain's basal ganglia and cerebellum. Named after the Greek word dance, a chorea may manifest as an inability to sustain a desired posture, dropping of objects, and especially random, dance-like movements. Some of these are characterized by slow, writhing motions, while other forms, called ballisms, can be intense to the point where patients thrash or jump.
Parkinson's disease is accompanied by various dyskinesias, including walking difficulties, that occur as the neurological damage of the condition progresses. This happens in part because of cell death in brain regions like the substantia nigra, one of the pathways controlling the coordination and execution of movement. In many patients, disruption of cerebellar guidance pathways in the brain causes shuffling and impaired gait along with loss of balance. Further, many Parkinsonian individuals treated with the drug levadopa develop a secondary, progressive dyskinesia after several years on this medication. Hemiballismus, a condition with similarities to Parkinson's, is rarer and has a unique patheology where patients experience involuntary, violent flinging of their limbs.
Tardive dyskinesias are involuntary movements of muscle groups, and arise in certain patients as a side effect of treatment with psychiatric medications that block the effects of dopamine, an important chemical messenger for the brain's regulation of movement stability. The incidence of tardive dyskinesia varies greatly among patients and is highest among schizophrenics. Another class of motion disorders are tics, sudden and repetitive movements of muscle groups, which are sometimes accompanied by loss of vocal and not just muscular control. Serious motor tics can arise in conditions like Tourette's syndrome and genetic disorders like Huntington's disease.