Causes Congenital myasthenic syndrome (CMS)

There are many types of Congenital myasthenic syndrome (CMS), dependent on where the problem in sending messages from the nerve to muscle occurs.

Identifying where the problem occurs may inform which drugs to prescribe to help with the management of symptoms. CMS can fall into three categories:

  • pre-synaptic: at the nerve ending, where there is a fault in the production and release of the chemical (acetylcholine) that signals to the muscle to contract
  • synaptic: in the gap between the nerve and muscle, or
  • post-synaptic: on the muscle where there is a fault in the receptors that receive the chemical message.

Many of the defects are known to be caused by alterations in known genes and so can be identified using a DNA test. Some people require a muscle biopsy to test how the nerve-muscle junction works. There are forms of CMS that can’t yet be identified but rapid advances in research are helping to find more of the genes responsible.

Image showing how messages are sent from the nerve to muscles. The nerve endings produce and release of the chemical acetylcholine that signals to the muscle to contract
Image showing how messages are sent from the nerves to the muscles


How is CMS inherited?

Children who have inherited an AR type of CMS will pass on one copy of the faulty gene to their child. They are also unlikely to have affected children because the chances that their partner also carries a faulty CMS gene are very small, unless they marry a blood relative.

The type of CMS known as ‘slow channel CMS’ is known to be inherited in a different way. It is an autosomal dominant (AD) mutation, which means it can be passed on from either parent to child. If either parent has the condition, there is a 50 percent chance any future pregnancy will result in a child with CMS. In other rare cases, the foetus may develop a sudden new mutation in its genetic make-up without the parents’ carrying any faulty genes.