INTRODUCTION

What is congenital muscular dystrophy?

The congenital muscular dystrophies are a group of conditions which share early presentation and a common muscle pathology. In the great majority of cases of congenital MD the initial symptoms are present at birth or in the first few months.

Babies with congenital MD often have hypotonia (low muscle tone or floppiness), and may have reduced movements. Other common signs are contractures (tightness) in the ankles, hips, knees and elbows. The contractures can sometimes be severe and affect several joints (known as arthrogryposis). They happen because the baby has not had the muscle strength to move freely enough in the womb. Some of these babies may also have respiratory problems because of weakness of breathing muscles.

In some children who do not have contractures the first problems are only noted after a few months because of difficulties in holding the head or delay in learning how to sit unaided, stand or walk.

Congenital MD is inherited and the pattern of inheritance is known as 'autosomal recessive'. This means that both parents are carriers of the condition (although clinically unaffected) and they have risk of 25%, or 1 in 4 in each pregnancy, of passing the condition on to their children (see diagram). Occasionally a case may be 'sporadic' which means is a one-off with little risk of recurrence in other children. There is no accurate way of predicting who is and who is not a carrier.

Congenital MD is a very heterogeneous group of conditions. These are generally grouped under two main types:

1. Children who only have muscle weakness involving all muscles but have normal intelligence
2. Children who have muscle weakness and learning difficulties, with or without seizures. Learning difficulties may be subtle, moderate or severe.

While this classification is helpful in most cases, an overlap between different categories can occur. A lot of effort has gone into identifying separate entities within each group and in locating the gene responsible for each form. A number of specific conditions can now be recognised but for others a final diagnosis is still not possible.

Genetic advances

There have been recent developments in the genetics of congenital MD which have resulted in a better understanding of this group of disorders.

The first gene abnormality to be discovered was that of the LAMA2 (laminin alpha-2 chain) gene, the gene responsible for merosin. This form affects approximately 40% of the children with congenital MD. More recently several other abnormal genes have been identified and today we know nine genes responsible for separate forms of congenital MD, each of them with specific clinical features.