However, before doing a muscle biopsy, a few other tests may be done.
A blood test can measure the level of a muscle protein (called creatine kinase or CK). In people with Bethlem myopathy, the CK level is either normal or mildly elevated.
A muscle biopsy involves taking out a small piece of muscle, usually from the thigh. The piece of muscle is then studied under the microscope, looking for signs which might indicate the type of abnormality that characterises the condition. It is also possible to look at the expression of collagen VI in the muscle under the microscope. There are specific ‘markers’ or ‘tags’ which can detect if collagen VI is normally present or reduced.
Genetic tests looking for abnormalities in one of the three genes responsible for Bethlem myopathy (COL6A1, COL6A2 and COL6A3) are now available in the UK. These are done in a specific, nationally-designated central laboratory (NSCT Centre for CMD and congenital myopathies, London) and can provide a definitive diagnosis.