Movement disorders are a group of diseases and syndromes affecting the ability to produce and control movement.
Though it seems simple and effortless, normal movement in fact requires an astonishingly complex system of control. Disruption of any portion of this system can cause a person to produce movements that are too weak, too forceful, too uncoordinated, or too poorly controlled for the task at hand. Unwanted movements may occur at rest. Intentional movement may become impossible. Such conditions are called movement disorders.
Abnormal movements themselves are symptoms of underlying disorders. In some cases, the abnormal movements are the only symptoms. Disorders causing abnormal movements include:
- Parkinson's disease
- Parkinsonism caused by drugs or poisons
- Parkinson-plus syndromes (progressive supranuclear palsy, multiple system atrophy, and cortical-basal ganglioric degeneration)
- Huntington's disease
- Wilson's disease
- Inherited ataxias (Friedreich's ataxia, Machado-Joseph disease, and spinocerebellar ataxias)
- Tourette syndrome and other tic disorders
- Essential tremor
- Restless leg syndrome
- Cerebral palsy
- Poisoning by carbon monoxide, cyanide, methanol, or manganese.
Movement is produced and coordinated by several interacting brain centers, including the motor cortex, the cerebellum, and a group of structures in the inner portions of the brain called the basal ganglia.
Both the cerebellum and the motor cortex send information to a set of structures deep within the brain that help control involuntary components of movement (basal ganglia). The basal ganglia send output messages to the motor cortex, helping to initiate movements, regulate repetitive or patterned movements,and control muscle tone.
Motor cortex damage can cause weakness of paralysis, and may lead to spasticity. Cerebellar disorders cause inability to control the force, fine positioning, and speed of movements (ataxia). Disorders of the cerebellum may also impair the ability to judge distance so that a person under- or over-reaches thetarget (dysmetria). Tremor during voluntary movements can also result from cerebellar damage.
Basal ganglia damage causes a variety of movement disorders, depending on theportion of the basal ganglia that is damaged. Parkinson's disease, the mostcommon basal ganglia disorder, is caused by cell death in the portion known as the substantia nigra. Disruptions in other portions of the basal ganglia are thought to cause tics, tremors, dystonia, and a variety of other movement disorders, although the exact mechanisms are not well understood.
Some movement disorders, including Huntington's disease and inherited ataxias, are caused by inherited genetic defects. Some disease that cause sustainedmuscle contraction limited to a particular muscle group (focal dystonia) areinherited, but others are caused by trauma. The cause of most cases of Parkinson's disease is unknown, although genes have been found for some familial forms.
Abnormal movements are broadly classified as either hyperkinetic--too much movement--and hypokinetic--too little movement. Hyperkinetic movements include:
- Dystonia. Sustained muscle contractions, often causing twisting or repetitive movements and abnormal postures. Dystonia may be limited to one area(focal) or may affect the whole body (general). Focal dystonias may affect the neck (cervical dystonia or torticollis), the face (one-sided or hemifacialspasm, contraction of the eyelid or blepharospasm, contraction of the mouthand jaw or oromandibular dystonia, simultaneous spasm of the chin and eyelidor Meige syndrome), the vocal cords (laryngeal dystonia), or the arms and legs (writer's cramp, occupational cramps). Dystonia may be painful as well as incapacitating.
- Tremor. Uncontrollable (involuntary) shaking of a body part. Tremor may occur only when muscles are relaxed or it may occur only during an action or holding an active posture.
- Tics. Involuntary, rapid, nonrhythmic movement or sound. Tics can be controlled briefly.
- Myoclonus. A sudden, shock-like muscle contraction. Myoclonic jerks may occur singly or repetitively. Unlike tics, myoclonus cannot be controlled even briefly.
- Chorea. Rapid, nonrhythmic, usually jerky movements, most often inthe arms and legs.
- Ballism. Like chorea, but the movements are muchlarger, more explosive and involve more of the arm or leg. This condition, also called ballismus, can occur on both sides of the body or on one side only(hemiballismus).
- Akathisia. Restlessness and a desire to move to relieve uncomfortable sensations. Sensations may include a feeling of crawling, itching, stretching, or creeping, usually in the legs.
- Athetosis. Slow, writhing, continuous, uncontrollable movement of the arms and legs.
Hypokinetic movements include:
- Bradykinesia. Slowness of movement.
- Freezing. Inability to begin a movement or involuntary stopping of a movement before it is completed.
- Rigidity. An increase in muscle tensionwhen an arm or leg is moved by an outside force.
- Postural instability. Loss of ability to maintain upright posture caused by slow or absent righting reflexes.
Treatment of a movement disorder begins with determining its cause. Physicaland occupational therapy may help make up for lost control and strength. Drugtherapy can help compensate for some imbalances of the basal ganglionic circuit. For instance, levodopa (L-dopa) or related compounds can substitute forlost dopamine-producing cells in Parkinson's disease. Conversely, blocking normal dopamine action is a possible treatment in some hyperkinetic disorders,including tics. Oral medications can also help reduce overall muscle tone. Local injections of botulinum toxin can selectively weaken overactive muscles in dystonia and spasticity. Destruction of peripheral nerves through injectionof phenol can reduce spasticity. All of these treatments may have some sideeffects.
Surgical destruction or inactivation of basal ganglionic circuits has proveneffective for Parkinson's disease and is being tested for other movement disorders. Transplantation of fetal cells into the basal ganglia has produced mixed results in Parkinson's disease.