Myotonic dystrophy is a multi-system disorder characterised by the presence of myotonia - failure of immediate muscle relaxation after voluntary contraction has stopped. It can be demonstrated by:
Striking a muscle with the tendon hammer and watching the resultant 'dimple' persist for a while before filling up. Asking the patient to grip an object then suddenly release it. The slow relaxation and opening of the hand grip will make the object appear 'stuck' to the fingers.
Physiologically, myotonia is due to instability of the Na+ and Cl~ channels of the muscle membrane with repetitive discharges following a short period of contraction. Although suggestive of myotonic dystrophy, myotonia may occur in other muscle disorders due to a similar defect. Clinical features
Myotonic dystrophy is an autosomal dominant inherited disorder transmitted by a mutation (repeat of three base pairs) in the myotonin protein kinase gene on chromosome 19. The incidence is 5 per 100 000 with the onset occurring between 15 and 40 years. The facial appearance is typical: Frontal baldness Myopathic face with ptosis ''' Jaw hanging and wasting of muscles of mastication "" resulting in hollowing of temporal fossae and cheeks Wasting of neck and shoulder girdle muscles also is evident As the disease progresses, myotonia becomes less apparent and may disappear.
In the limbs -weakness and wasting are distal though the hands are spared until late.
Mental retardation is frequent.
These may all predate or dominate over muscle disease
In affected mothers, the disease may present in the neonate with hypotonia, contractures and mental retardation. The mechanism is obscure.
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