Congenital Myopathies
 - Congenital fibre type disproportion myopathy

  What is myopathy?
  The term myopathy is derived from the Greek and it means muscle disorder
  (myo = muscle, pathy = disease).

  The congenital myopathies are a group of conditions which cause weakness
  and wasting of the muscles. In general symptoms are present at birth but
  may not be recognised until later on in childhood or even in adult life.

  What is congenital fibre type disproportion?
  'Congenital' refers to the onset of the myopathy and means from birth. The
  name 'fibre type disproportion' was first suggested in 1973 and refers to the
  different size of the muscle fibres. When a normal muscle is looked at under
  the microscope, two types of muscle fibres (type 1 and 2) are seen, both of
  which are needed for the proper functioning of that muscle. The size of type 1
  and type 2 fibres in the muscles of most young children is roughly similar. In
  congenital fibre type disproportion, type 1 fibres are smaller than type 2. A
  number of cases of congenital fibre type disproportion have an onset in
  infancy, and may even present at birth with hypotonia (floppiness),
  swallowing and breathing difficulties, joint contractures (limitation of joint
  movement) or dislocation of the hips.

  In other cases the child may show delayed motor development (i.e. slow to
  crawl, stand or walk). The muscle weakness is usually non-progressive or
  very slowly-progressive after the age of two years. In some cases the
  condition may actually improve after that time, although the degree of muscle
  weakness is variable. It involves the muscles of the trunk and limbs, with
  more 'proximal' involvement (proximal muscles are those that are near the
  trunk such as shoulder and thigh muscles).

  Because of this weakness children may have a variety of symptoms ranging
  from recurrent chest infections, drooping of the eyelids, drooling, swallowing
  difficulties, and weakness of the limbs and the trunk. Children with this
  condition are usually shorter than other children and in addition may develop
  curvature of the spine. Some children also suffer from constipation and this
  may be as a direct result of the muscles in the gut being affected, or caused
  by lack of exercise and insufficient mobility. Hearing, vision and intelligence
  are usually unaffected.

  Is congenital fibre type disproportion an inherited disorder?
  Yes, usually. Although the exact genetic cause is unknown, different patterns
  of inheritance are recognised; these are known as autosomal recessive and
  autosomal dominant. In the remaining cases there does not appear to be a
  clear pattern of inheritance and these are described as 'sporadic' cases
  which means they occur randomly, or as a one-off, with very little risk of other
  children in the family being affected. However, people who are themselves
  sporadic cases will be at risk of passing the condition on to their children.

  The autosomal recessive type may be clinically difficult to separate from the
  autosomal dominant form, although onset is generally earlier, and the
  condition may present as a 'floppy infant', or later on with severe muscle
  weakness. In the recessive form both parents are carriers of the condition,
  and the risk of a child of either sex being affected is 25% (1 in 4).

  In the autosomal dominant type of inheritance one (either) of the two parents
  is affected, may be only very mildly so, but carries the abnormal gene, and
  each child (of either sex) of that affected parent has a 50% (1 in 2) chance of
  being affected.

  How is congenital fibre type disproportion myopathy diagnosed?
  The diagnosis of a 'myopathy' is usually suspected from the history and
  examination. However the specific diagnosis of congenital fibre type
  disproportion is nearly always made by looking at a piece of muscle (muscle
  biopsy). Before doing a muscle biopsy (which involves taking out a small
  piece of muscle usually from the thigh muscle) a few other tests may be
  done, one of which is a blood test which measures the level of a muscle
  enzyme (creatine kinase or CK level), which is usually normal but may be
  increased. The other is an electrical test of the muscles and the nerves
  supplying the muscle; it involves the placing of a thin needle into the muscle
  which then measures the electrical activity arising from that muscle. Although
  this test may be able to show a 'myopathic' pattern of abnormal muscle
  activity (seen in any myopathy) it is unable to define the exact type. The
  muscle biopsy is therefore the important test which will make the diagnosis.
  The muscle in this condition characteristically shows smaller type 1 than type
  2 fibres.

  Is the condition progressive?
  The condition is usually non-progressive or slowly progressive and the
  muscle weakness may improve after the second year of life. In general
  however, the earlier the onset of the myopathy the more severe and
  progressive the disorder is. Some patients may become weaker in their 20s
  or 30s and may eventually lose the ability to walk. Occasionally muscle
  weakness may progress quite rapidly and these patients experience serious
  breathing problems.

  Is there a treatment or a cure?
  At the moment, there is no cure, nor any drug treatment for congenital fibre
  type disproportion myopathy. However, other very helpful measures can be
  taken such as physiotherapy (see below), the use of antibiotics to treat chest
  infections, or nasogastric tube feeding when necessary.

  What help is available?
  Physiotherapy is one of the main forms of help. An initial physiotherapy
  assessment at the time of the diagnosis should be followed by an exercise
  programme and regular check-ups. The main aim of physiotherapy is to keep
  the muscles as active as possible to prevent the formation of 'contractures'
  (muscle tendon tightness causing restriction in the range of joint movement).
  It is also important to provide good seating and to ensure a proper sitting and
  standing posture to prevent scoliosis (curvature of the spine). The other role
  of physiotherapy is to help with provision of appliances, such as splints,
  calipers, standing frames (mechanical aids helping to keep children on their
  feet) and wheelchairs where necessary. Children and adults are encouraged
  to remain as active as possible and ensure that they do not become
  overweight to limit the strain imposed on their muscles. Swimming is a
  particularly good form of exercise.

Published by the Muscular Dystrophy Campaign at: http://www.muscular-dystrophy.org
Written by Dr Paola Nicolaides, Senior Registrar and Dr Richard Appleton, Consultant Paediatric Neurologist

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