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Diagnosis Myotubular and other centronuclear myopathies

Clinical signs and symptoms are usually the first indication of problems with the muscles. These signs are often not specific, so a muscle biopsy is required to support the potential diagnosis of CNM and to differentiate it from other types of myopathy. Genetic testing and identification of a mutation in a disease gene will then confirm the diagnosis.

Muscle Biopsy

Muscle biopsies are done in one of two ways:

  • an open biopsy, where a small piece of muscle is taken under a general anaesthetic, or
  • a needle biopsy to remove a small muscle sample with a special needle through a small incision. This requires a local anaethetic

The samples will then be analysed under a microscope. The appearance of muscle from people with myotubular myopathy is similar to that seen in foetal muscle. The nuclei are centrally located (‘centronuclear’), instead of being at the outer edges of the muscle fibres (see figure 1 below). Our muscle biopsies factsheet will give you more information.

Diagram of showing muscle biopsy results under the microscope of someone with Centronuclear Myopathy and someone with healthy muscles. e. The nuclei are centrally located (‘centronuclear’), instead of being at the outer edges of the muscle fibres which is where they should be

The pink circles are muscle fibres and the dark purple spots are the nuclei of the muscle fibres. The white arrows highlight the difference between healthy muscle (top) and muscle affected by CNM (bottom). You’ll see in the healthy muscle, the nuclei are on the outer edges of the muscle fibres. In muscle affected by CNM (bottom), you’ll see some of the nuclei are in the centre of the muscle fibres.

Genetic Testing

This is currently available as a diagnostic service for some of the genes in autosomal recessive CNM. It is likely that more genes will be identified in future. Genetic testing involves taking a blood sample and analysing the DNA for the presence of a mutation (or alteration). This process can take a few weeks up to several months to complete, depending on the size of the gene. Analysis of the TTN gene in particular can be challenging, as this is the biggest gene we have. Sometimes further investigations are needed to clarify the mutation that is responsible for the condition. Once the gene mutation has been identified in the affected person, it is possible to test other family members who might be carriers.

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