Dystonia

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The term dystonia refers to sustained contractions of agonist and antagonist muscles, causing twistings and repetitive movements or abnormal postures.

The twisting nature of dystonic movements and postures is distinctive and has led to the use of the term torsion dystonia. Dystonia can be idiopathic or secondary to several causes. Structural lesions may involve the putamen but may be found in other parts of the basal ganglia. The segmental dystonias include torticollis, retrocollis writer's cramp and other craft-associated dystonias, Meige facial dys-tonia and blepharospasm. Hemifacial spasms are a form of focal dystonia.

Several inherited disorders with known metabolic defects have been identified as causes of dystonia. Wilson's disease and dopa-responsive dystonia (DRD) (or dystonia with diurnal variation or Segawa's variant) are examples of this group. Idiopathic dystonia may be generalized, or limited to a particular muscle group. In idiopathic dystonia the basal ganglia show no gross or microscopic abnormalities, and no specific biochemical abnormality that explains the motor symptoms. As could be expected, given the lack of understanding of the relevant neurochemistry involved, a wide variety of medications have been reported to be effective for some patients.

Most cases of childhood-onset dystonia are inherited, usually in an autosomal dominant pattern. In 1989,18 a DNA marker in the q32-34 region of chromosome 9 in a large non-Jewish kindred was identified. About a third of those carrying the gene express it clinically (30-40% penetrance). After the recent identification of a 3-base-pair deletion in a gene coding for a novel ATP-binding protein in the 9q34 locus, termed torsin A (resulting in the loss of a pair of glutamic acid residues), gene testing for this abnormal DYT1 gene can be carried out on individuals with dystonia.19

The gene for DRD was mapped to a locus on chromosome 14.20 The use of botulinum toxin A (Botox, Dysport) injections for focal and segmental dystonia has greatly improved the therapeutic possibilities in this type of disorder.21,22

Unfortunately, botulinum toxin injections cannot be used to treat segmental and generalized dys-tonias, because of the greater number of muscles involved and the larger doses that would be needed. Very large doses may stimulate antibody formation or even induce generalized weakness. Even doses used to treat cervical dystonia have been found to produce blocking antibodies in some patients. A list of medications used for the treatment of this disorder is presented in Table 33.2.

Since 5-10% of children respond to small doses of levodopa, this drug in small doses is the drug of first choice for the treatment of dystonia in children and

Table 33.2 Medications used in the treatment of

dystonia

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