Dystonia is usually defined as "a syndrome of sustained muscle contractions, frequently causing twisting and repetitive movements, or abnormal postures." (Fahn et. al., 1987) In fact, the muscle contractions related to dystonia may be quite rapid and not sustained; the movements may not be repetitive and not lead to fixed postures.

Dystonia may be considered as the production of one pattern of muscle activity when a different pattern was intended.

Therefore, it is a proven challenge to establish a single definition of dystonia. An alternative definition is to consider dystonia as the production of one pattern of muscle activity when a different pattern was intended. This emphasizes the fact that dystonia usually occurs only during voluntary movement or with voluntary maintenance of a posture of the limbs or body. For example, attempted flexion of the fingers to hold a pen may lead to flexion of additional fingers, extension of the wrist, or movements of the opposite hand.

Characteristic postures that are frequently seen in dystonia include...

• "Spooning," during which the fingers of the hand are bent backward with the wrist flexed
• Elbow and wrist flexion with the hand held near the body
• Foot in-turning or inversion at the ankle, which is frequently pronounced with walking
• Extension of the great toe
• Turning of the neck or torticollis
• Jaw or facial contortions

It is important to realize that, despite its name, there is often no abnormal muscle tone in children with dystonia. This means that, upon examination, a dystonic limb may or may not have increased resistance to movement.

Dystonia is described according to the part or parts of the body that are affected. If only one body part is involved, such as a hand, foot, or the neck, then this form is termed a "focal dystonia." If two contiguous parts are involved, such as the face and neck, then it is termed a "segmental dystonia." If two noncontiguous parts of the body are involved, such as the face and one leg, it is termed a "multifocal dystonia." If one half of the body is involved, it is "hemidystonia," and if both legs, as well as one additional body part are involved, then it is termed "generalized dystonia." A focal dystonia that progresses to become generalized or generalized dystonia itself are the most common patterns observed in children.

Dystonia may occur at rest or with action. A feature of dystonia that distinguishes it from most other movement disorders is that a dystonic movement of one limb may be triggered by an attempted movement of a different limb. For example, a dystonic posture of the right hand may occur while the left hand is performing a rapid movement, or a dystonic posture of the foot may occur during walking. The triggering movements may be very specific; for example, walking forward may be a trigger, while walking backward may not be a trigger. In adult focal, task-specific dystonias, the trigger may be as specific as writing or playing an instrument; this type of task-specific dystonia occurs only rarely in children.

When dystonia is due to another identified disease, then it is called "secondary dystonia." When dystonia is not due to another disease, it is termed "primary dystonia." Primary dystonia includes the genetic dystonias and some adult-onset, focal dystonias. Secondary dystonia may be due to a wide variety of causes, many of which are listed here.