CMD or congenital muscular dystrophy is in fact not one single condition but a number of different inherited neuromuscular conditions that are grouped under the label “congenital” because they usually show symptoms at birth or from a very early age. It is estimated that 1 baby in every 20,000-50,000 is born with congenital muscular dystrophy. The different types of CMD are caused by mutations in different genes and can have different symptoms.
All congenital muscular dystrophies affect the muscles. Some also affect the brain. This means some children have muscle weakness involving all muscles but have normal intelligence, while others may have muscle weakness and learning difficulties. In some cases the involvement of the brain can also cause seizures. Learning difficulties may be mild, moderate or severe.
Babies with congenital muscular dystrophy often appear “floppy” (this may also be described by doctors as low muscle tone or hypotonia). Contractures (tightness) in the joints – the ankles, hips, knees and elbows – is also common. The contractures can sometimes be severe and affect several joints. This is known as arthrogryposis. Some babies may also have respiratory problems because their breathing muscles are weak. In some children who do not have contractures, the first problems may only be noticed after a few months when the child does not reach their motor milestones at the same age as other children and difficulties in holding the head up or delay in learning how to sit unaided, stand or walk are noticed.
Most forms of CMD are inherited in an autosomal recessive pattern, which means that in order for a child to be affected, both parents must be carriers of the abnormal gene and both must pass this gene on to their child. Carrier parents have a 1 in 4 chance of having an affected child, a 2 in 4 chance of having a carrier child, and a 1 in 4 chance of having a child who is completely free of the affected gene.
An overview of the common types of CMD is provided in the table below (click the table to see a larger PDF version). More information about the different types of CMD and their genetic background is also available on the website of the advocacy organization Cure CMD.