Notes on some terms relating to movement disorders

A number of descriptive terms are used to differentiate patterns of disordered muscle tone and movement, and may give rise to confusion among non-neurologists. Some of these terms, and their corresponding meanings, are listed below:

• Spasticity is the type of muscle hypertonia seen following a lesion of the corticospinal tract. Resistance to passive movement of an affected limb is maximal at its outset and reduced once movement is initiated ("clasp-knife" effect). Tendon reflexes are increased. The Babinski response is extensor. Clonus (rhythmic repetitive contractions) may occur.

• Rigidity is a uniform increase of muscle tone seen in extrapyramidal lesions, notably parkinsonism. Resistance to passive movement is evenly encountered throughout the range ("lead-pipe" rigidity). In cases where tremor is superimposed, rapid fluctuations in the degree of resistance may be felt ("cog-wheel" effect).

• Dyskinesia is a term used to cover the range of involuntary movements seen in extrapyramidal disturbance:

- chorea: jerky, quasi-purposive movements, typically of the face/upper limbs;

- athetosis: slower, more writhing movements;

- hemiballismus: violent excursions of an entire limb;

- dystonia: sustained, often repetitive, muscle contraction, typically giving rise to twisting movements and/or abnormal postures;

- tremor: rhythmic rapid oscillations;

- myoclonus: brief isolated jerks which may involve part of a muscle, an entire muscle, or several muscle groups.

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