The specific diagnosis, however, is generally made by looking at a piece of muscle or skin (muscle or skin biopsy – see our Muscle biopsies factsheet).
Before doing a muscle biopsy, which involves taking out a small piece of muscle, usually from the thigh, a few other tests may be done. One of these tests is a blood test to measure the level of a muscle protein, called creatine kinase (CK). In patients with UCMD, the CK level is either normal, or mildly elevated.
A muscle ultrasound test may also help to detect abnormalities in the muscle. This technique is very simple – similar to the ultrasound tests carried out in pregnancy – and may provide further evidence of the involvement of the muscle.
Muscle magnetic resonance imaging (MRI), as with muscle ultrasound, can assist by highlighting patterns of muscle involvement, which can be specific to particular muscle conditions.
A muscle biopsy enables a sample of muscle to be studied under the microscope, looking for signs which might indicate a muscular dystrophy; these include a variation in muscle fibre size and the replacement of some fibres by fat and fibrous tissue. It is also possible, under the microscope, to look at the production of collagen VI in the muscle. There are specific ‘markers’ or ‘tags’ that detect whether or not collagen VI is present at normal or reduced levels. A reduction in collagen VI in a child with some of the symptoms of the condition strongly suggests the possibility of UCMD.
As collagen VI is normally present in both muscle and skin, a skin biopsy (taking a small piece of skin) can also help to confirm the diagnosis of UCMD. In some cases, it is easier to detect a reduction of collagen VI in skin cells than it is in muscle cells. A skin biopsy, however, does not provide some of the information that can be obtained from a muscle biopsy and, therefore, it is important to have both muscle and skin biopsies, when UCMD is suspected.
Genetic tests, looking for mutations in one of the three genes responsible for UCMD (COL6A1, COL6A2 and COL6A3), are now available in the UK at a specific designated centre, and can provide a definitive diagnosis. If you would like to have these genetic tests, ask your neurologist consultant for a referral to the nearest place to you.
Prenatal diagnosis may be possible in UCMD. In families who already have a child with UCMD and decide to have another baby, it is possible to detect whether or not the foetus (unborn child) has a deficiency of collagen VI and/or the same gene mutation(s), early in the pregnancy.