This term stems from a Greek word meaning “unfixed” or “changeable.” The condition is characterized by an inability to sustain the fingers and toes, tongue, or any other part of the body in one position. The maintained posture is interrupted by relatively slow, writhing or twisting, sinuous, purposeless movements that have a tendency to flow into one another. As a rule, the abnormal movements are most pronounced in the digits and hands, face, tongue, and throat, but no group of muscles is spared. One can detect as the basic patterns of movement an alternation between extension–pronation and flexion–supination of the arm and between flexion and extension of the fingers, the flexed and adducted thumb being trapped by the flexed fingers as the hand closes. Other characteristic movements are eversion–inversion of the foot, retraction and pursing of the lips, twisting of the neck and torso, and alternate wrinkling and relaxation of the forehead or forceful opening and closing of the eyelids. The movements appear as slower than those of chorea, but all gradations between the two are seen; in some cases, it is impossible to distinguish between them, hence the term choreoathetosis. An apt description could be of a moving dystonia (see below). Discrete voluntary movements of the hand are executed more slowly than normal, and attempts to perform them may result in a co-contraction of antagonistic muscles and a spread (overflow) of contraction to muscles not normally required in the movement. The overflow appears related to a failure of the striatum to suppress the activity of unwanted muscle groups. Some forms of athetosis occur only during the performance of projected movement (intention or action athetosis).
Athetosis may affect all four limbs or may be unilateral, especially in children who have suffered a hemiplegia at an early time in life (posthemiplegic athetosis). Many athetotic patients with destructive focal brain lesions exhibit variable degrees of rigidity and motor deficit as a result of associated corticospinal tract disease; these may account for the slower quality in these patients of athetosis compared to chorea. In other patients with generalized choreoathetosis, as pointed out above, the limbs may be intermittently hypotonic.
The combination of athetosis and chorea of all four limbs is a cardinal feature of Huntington disease and of a state known as double athetosis, a form of cerebral palsy that begins in childhood. Athetosis appearing in the first years of life is usually the result of a congenital or postnatal condition such as hypoxia or, now rarely, kernicterus. Postmortem examinations in some of the cases have disclosed a unique pathologic change, status marmoratus, of probable hypoxic etiology in the striatum (see Chap. 37). In other cases, of probable kernicteric (hyperbilirubinemic) etiology, there is a loss of nerve cells and myelinated fibers—a status dysmyelinatus—in the same regions. In adults, athetosis may occur as an episodic or persistent disorder in hepatic encephalopathy, as a manifestation of chronic intoxication with phenothiazines or haloperidol, and as a feature of certain degenerative diseases, most notably Huntington chorea but also Wilson disease, Leigh disease, and other mitochondrial disease variants; less frequently athetosis may be seen with Niemann-Pick (type C) disease, Kufs disease, neuroacanthocytosis, and ataxia telangiectasia, all of which are described in later chapters. It may also occur as an effect of excessive L-dopa in the treatment of Parkinson disease, in which case it appears to be caused by a decrease in the activity of the subthalamic nucleus and the internal segment of the globus pallidus (Mitchell et al). Athetosis, usually in combination with chorea, may occur rarely in patients with AIDS and in those taking antiepileptic drugs. Localized forms of athetosis may occasionally follow vascular lesions of the lenticular nucleus or thalamus, as in the cases described by Dooling and Adams.