Chorea consists of jerky, quasi-purposive and sometimesexplosive movements, following each other but flitting  from one part of the body to another. The causes of chorea are listed in Table 18.38; the commoner conditions are outlined below.
Huntington’s disease
Relentlessly progressive chorea and dementia in middle life are the hallmarks of this inherited disease. The prevalence of the disease is about 1 in 20000. It occurs worldwide. Inheritance is as an autosomal dominant trait with full penetrance; the children of an affected parent have a 50% chance of inheriting the disease. The family history of the disease in previous generations is often concealed, either by design or default. A mutation has been identified at the distal short arm of chromosome  4 with a sequence of randomly repeated trinucleotides(CAG). The messenger RNA for the gene is expressed in many tissues and its protein product has been termed huntingtin.

Huntington’S diseaseSydenham’s choreaBenign hereditary chorea Abetaiipoproteinaemia and chorea Choreas associated with:
Drugs-phenytoin, levodopa, alcohol Thyrotoxicosis, pregnancy and oral contraceptive pill Systemic lupus erythematosus
Polycythaemia vera
Encephalitis lethargica
Stroke (basal ganglia)
Rarities (tumour, trauma, subdural haematoma, carbon monoxide poisoning)

PATHOLOGY. There is cerebral atrophy with marked loss of small neurones in the caudate nucleus and putamen. Three important changes in neurotransmitters occur:
1 There is reduction in the enzymes synthesizing acetylcholine and GABA in the striatum.
2 There is depletion of GABA, angiotensin-converting enzyme and met-enkephalin in the substantia nigra.
3 Somatostatin levels are high in the corpus striatum. These changes may be secondary to the cell damage. In contrast to Parkinson’s disease, dopamine and tyrosine hydroxylase activity are normal.
MANAGEMENT AND COURSE. There is steady progression, of both the dementia and chorea. No treatment arrests the disease, although phenothiazines may reduce the chorea by causing drug-induced parkinsonism. Tetrabenazine or sulpiride may help to control the chorea.
Death usually occurs between 10 and 20 years after the onset.
Mutation analysis, which is accurate and specific, is ecoming available for presymptomatic testing. This raises ethical problems and centres performing these tests have adopted a common protocol for counselling.
Sydenham’s chorea (St Vitus’ dance) Thomas Sydenham described this transient chorea of adolescence and childhood in 1686. Fewer than half of the cases follow within 3 months of rheumatic fever (se ep. 591). It may recur, or appear, in adult life during pregnancy (chorea gravidarum) or in those taking oral contraceptives. In each case there is a diffuse mild encephalitis. The onset of the chorea is usually gradual. Irritability, insolence and inattentiveness herald the onset of fidgetymovements, which are sometimes predominantly unilateral. A mino rity of patients are confused. Although rheumatic heart disease is sometimes found, the child usually does not have a fever or other features of rheumatic fever. The antistreptolysin-O (ASO) titre and erythrocyte sedimentation rate (ESR) are usually normal. Recovery occurs spontaneously within weeks or months.
Hemiballismus (also called hemiballism) describes violent swinging movements of one side of the body caused usually  y infarction or haemorrhage in the contralateral subthalamic nucleus.
Myoclonus is the sudden, involuntary jerking of a single muscle or a group of muscles. It occurs in a wide range of disorders and is sometimes provoked by a sudden stimulus such as a loud noise.
Benign essential myoclonus
Nocturnal myoclonus, i.e. sudden jerking of a limb or the body on falling asleep, is extremely common and not pathological.

Paramyoclonus multiplex’ is widespread, random muscle jerking usually occurring in adolescence. Fits do not occur.
Myoclonic epilepsy
Muscle jerking is a feature of many different forms of epilepsy.
Progressive myoclonic epilepsy
These very rare conditions include various familial and metabolic disorders where myoclonus accompanies a progressive encephalopathy.
An example is Lafora body disease, a syndrome of myoclonus, epilepsy and dementia, with mucopolysaccharide inclusion bodies in neurones, liver cells and intestinal mucosa.

Static myoclonic encephalopathy

Myoclonus may follow a severe brain insult such as severe erebral anoxia following cardiac arrest. Tics
Repetitive twitching movements of the face, neck or hand are part of our normal motor gestures. Patients or their relatives seek advice about them when they become too frequent or irritating. Simple transient tics, e.g. sniffing or a particular facial grimace, are common in childhood,but may persist into adult life. The rare Gilles de la Tourette syndrome is  the occurrence of multiple tics accompanied by sudden explosive barking and grunting with the utterance of sexuallyrelated obscenities. The condition develops in childhood  or adolescence, in either sex, and is usually lifelong. This is thought to be due to an organic disorder of the basal ganglia. Treatment with haloperidol is sometimes helpful.

Torsion dystonias

Dystonia implies a movement caused by a prolongedmuscular contraction when a part of the body is thrown  into spasm. A classification of these unusual conditions Generalized dystonia Dystonia musculorum deformans Drug-induced dystonia (e.g. metoclopramide)Symptomatic dystonia (e.g. after encephalitis lethargica
or in Wilson’s disease) Paroxysmal dystonia (very rare, familial, with marked fluctuation)
Focal dystonia
Spasmodic torticollis
Writer’s cramp
Oromandibular dystonia
Hemiplegic dystonia (e.g. following stroke)

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