Congenital Myopathies: Congenital fibre type disproportion myopathy

Written by Dr Paola Nicolaides, Consultant Paediatric Neurologist for the Muscular Dystrophy Campaign.

What is a myopathy?

The term myopathy is derived from the Greek and it means muscle disorder.

What is a congenital myopathy?

The congenital myopathies are a group of conditions affecting proximal muscles and causing weakness and wasting of the muscles. Sometimes symptoms are present at birth but at other times they are not recognised until later on in childhood or adult life.

What is congenital fibre type disproportion?

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 young children is roughly similar. In congenital fibre type disproportion type 1 fibres are smaller than type 2.

How does the disease present?

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, contractures (limitation of joint movement) or dislocation of the hips. In other cases the child may show delayed motor development (slow to crawl, stand or walk). Some children have muscle weakness affecting the trunk and extremities (arms and legs) in a more “proximal” fashion (proximal muscles are those that are near the trunk, such as the shoulder and thigh muscles).

Are the patients at risk of developing any complications?

Children with congenital fibre type disproportion are at increased risk of respiratory (chest) infections which on occasion may be severe.

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 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 there a risk with General Anaesthesia?

No, unless the patient has significant breathing difficulties

Is congenital fibre type disproportion inherited?

Yes.

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.

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) 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 slightly raised.

The other is an electrical test of the muscles and the nerves supplying the muscle. This test involves placing of a fine 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 most important test which will precisely identify the diagnosis. The muscle in this condition characteristically shows smaller type 1 than type 2 fibres.

Is the condition progressive?

The rate of progression of the condition is variable. In general the earlier the onset the more severe the disorder is. In most cases the disease is only slowly progressive. 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, the use of antibiotics to treat chest infections, or naso-gastric tube feeding when necessary.

What help is available?

Physiotherapy is one of the main forms of help. An exercise programme and regular check-ups should follow an initial physiotherapy assessment at the time of the diagnosis. 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.