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 and intentional movement may become impossible. Such conditions are called movement disorders.
Causes of Movement Disorders
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. Sensory information provides critical input on the current position and velocity of body parts, and spinal nerve cells (neurons) help prevent opposing muscle groups from contracting at the same time.
Voluntary motor commands begin in the motor cortex located on the outer, wrinkled surface of the brain. Movement of the right arm is begun by the left motor cortex, which generates a large volley of signals to the involved muscles. These electrical signals pass along upper motor neurons through the midbrain to the spinal cord. Within the spinal cord, they connect to lower motor neurons, which convey the signals out of the spinal cord to the surface of the muscles involved. Electrical stimulation of the muscles causes contraction, and the force of contraction pulling on the skeleton cause movement of the arm, hand and fingers. Damage to or death of any of the neurons along this path causes weakness or paralysis of the affected muscles.
In addition to sight, the most important source of information comes from the "position sense" provided by the many sensory neurons located within the limbs (proprioception). Proprioception is what allows you to touch your nose with your finger even with your eyes closed. The balance organs in the ears provide important information about posture.
Both postural and proprioceptive information is processed by a structure at the rear of the brain called the cerebellum. 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 the target (dysmetria). Tremors during voluntary movements can also result from cerebellar damage.
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. Circuits within the basal ganglia are complex. Disruptions of these circuits are known to cause several distinct movement disorders. For instance, loss of substantia nigra cells, as in Parkinson's disease, increases blocking of the thalamic nuclei, preventing them from sending signals to the motor cortex. The result is a loss of movement (motor activity), a characteristic of Parkinson's. In contrast, cell loss in early Huntington's disease decreases blocking of signals from the thalamic nuclei, causing more cortex stimulation and stronger but uncontrolled movements. 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.
Diagnosis of Movement Disorders
Diagnosis of movement disorders requires a careful medical history and a thorough physical and neurological examination. Brain imaging studies are usually performed. Imaging techniques include computed tomography scan (CT scan), positron emission tomography (PET) or magnetic resonance imaging (MRI) scans. Routine blood and urine analyses are performed and A lumbar puncture (spinal tap) may be necessary. Video recording of the abnormal movement is often used to analyze movement patterns and to track progress of the disorder and its treatment. Genetic testing is available for some forms of movement disorders.
Treatment of Movement Disorders
Treatment of a movement disorder begins with determining its cause. Physical and occupational therapy may help make up for lost control and strength. Drug therapy can help compensate for some imbalances of the circuits.
Surgical destruction or inactivation of circuits has proven effective for Parkinson's disease and is being tested for other movement disorders.
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