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Myotonia 

Myotonia

Chapter:
Myotonia
Author(s):

David Hilton-Jones

DOI:
10.1093/med/9780199204854.003.0242403_update_002

Update:

Additional comments on clinical correlation with CTG repeat expansion size in myotonic dystrophy type 1.

Revised comments concerning pregnancy and reproductive options in myotonic dystrophy.

Updated on 28 Nov 2012. The previous version of this content can be found here.
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date: 28 April 2017

Myotonia is defined at an electrical level as repetitive discharge of the muscle fibre membrane after initial activation, which occurs due to dysfunction of the membrane’s ion channels, most commonly the chloride channel, less commonly the sodium channel. This manifests clinically as stiffness of the muscle and delayed relaxation after voluntary contraction, e.g. difficulty relaxing the grip after clenching the fingers, and stiffness in the thigh muscles and difficulty walking on first moving after rest. Disabling myotonia may respond to mexiletine.

Particular myotonic disorders

Useful clinical distinction can be made between (1) myotonic dystrophies—multisystem disorders in which weakness is a significant feature, and (2) nondystrophic myotonias.

Myotonic dystrophy type 1 (Steinert’s disease)—caused by expansion of an unstable trinucleotide repeat in the myotonic dystrophy protein kinase (DMPK) gene, leading to myotonia through altered splicing of the chloride channel gene. There are four main patterns of disease: (1) congenital; (2) childhood onset; (3) classic or early adult onset; (4) late onset, asymptomatic or oligosymptomatic. The classic form of the disease is the most frequent cause of myotonia and the most prevalent muscular dystrophy in adults (c.1 in 8000). In addition to myotonia and a characteristic pattern of weakness affecting the facial muscles and (unusually for a myopathic disorder) distal limbs, other features include premature male pattern balding, cataracts, central nervous system involvement (cognitive change, excessive daytime sleepiness), cardiac conduction abnormalities (which may lead to sudden death), gastroenterological involvement (dysphagia and irritable bowel syndrome), and respiratory problems. Recurrent chest infections are common due to the combination of muscular weakness and the tendency to aspirate, and death is often secondary to pneumonia. The underlying trinucleotide repeat is unstable and increases in size during meiosis, giving rise to anticipation in which the disease has an earlier onset in the offspring of affected individuals.

Myotonic dystrophy type 2 (proximal myotonic myopathy)—caused by a quadruplet repeat expansion in the zinc finger 9 protein (ZNF9) gene that leads to disruption of normal RNA processing and altered splicing patterns of numerous genes. Clinical features are similar to type 1, but with proximal (rather than distal) weakness and less evident anticipation.

Nondystrophic myotonias—mutations affecting the skeletal muscle chloride channel (CLCN-1) gene give rise to the rare condition myotonia congenita, which can be inherited as either an autosomal dominant or recessive trait. Myotonia is striking; leg stiffness causing difficulty walking is the major feature, but persistent weakness is uncommon.

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