Congenital myopathies are a group of conditions that cause weakness and wasting of the shoulder and thigh, or proximal, muscles. Symptoms are present at birth or in early infancy. The name 'fibre type disproportion', first suggested in 1973, refers to the different size of muscle fibres.

When a normal muscle is examined under the microscope two types of muscle fibre (called type 1 and type 2) can be seen. Both are usually the same size and both are needed if the muscle is to work properly. However, for people with congenital fibre disproportion the type 1 fibres are smaller than the type 2 fibres. The implications of this factor depend on when the symptoms present. Symptoms in babies may include:

  • floppiness (hypotonia)
  • difficulty breathing and swallowing

Symptoms in infancy can include:

  • delayed crawling, sitting and walking
  • muscle weakness affecting the trunk, arms and legs

Congenital fibre type disproportion is often inherited although the exact genetic cause is unknown. Two different patterns of inheritance are recognised - autosomal recessive and autosomal dominant.

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