The GP may carry out some initial tests to explain the symptoms, but since these conditions are often difficult to diagnose, the individual will usually be referred to a specialist – typically a neurologist with a specialist interest in neuromuscular conditions – for further investigation and diagnosis.
The specialist will use a range of tools and tests to reach a clinical diagnosis that best explains the symptoms and test results, and this will be used as the basis for genetic testing.
This factsheet describes some of the more common diagnostic tests and looks in more detail at genetic testing.
People with a suspected neuromuscular condition are likely to have their blood tested for creatine kinase (CK) levels (also called the CPK test).
CK is a protein – more specifically, an enzyme – that is normally found in muscle but can leak into the blood following muscle damage. The test measures the level of CK in the blood and clinicians can use this as a measure of muscle damage.
Although raised levels of CK in the blood point towards muscle damage, the test is not specific for damage caused by neuromuscular conditions. A particularly hard session in the gym can also lead to increased CK levels, so the test result cannot provide an accurate diagnosis by itself.
Magnetic Resonance Imaging (MRI)
MRI is a fast and non-invasive technique that can produce images of the inside of our bodies. Unlike an X-ray, which only shows our bones, MRI images can show the structure of different types of tissue such as muscle, fat and bone (see Figure 1).
Figure 1: Example of an MRI image of muscle
Muscles of the pelvis (top), the thighs (middle) and lower legs (bottom). The left-hand images are from a healthy person and the right-hand images are from someone with an RYR1-associated myopathy. While muscle appears as a dark grey colour, fat appears as white. Most of the muscle in the RYR1 patient has been replaced by fat.
An MRI scan can highlight which muscles are affected by a condition and can help a surgeon choose the most suitable muscle on which to perform a biopsy.
A muscle biopsy is a small piece of muscle removed through a small cut or with a hollow needle, usually from the leg or arm. The tissue sample enables clinicians to examine the muscle structure in detail under a microscope.
Using dyes to stain specific proteins, a clinician can also identify proteins that are missing from the muscle. In some cases this may provide a diagnosis, for example, the lack of dystrophin in Duchenne muscular dystrophy.
However in other conditions, where several genes may play a role in the biological pathway that produces the protein, genetic testing will be required to identify the precise cause of the condition. (For more information, please read our factsheet on muscle biopsies).
Functional test of muscles and nerves
The clinician may also use other tests like electromyography (EMG) or tests of nerve function. These tests examine electrical activity in nerves and muscles.
By comparing when someone is at rest and when the muscles are contracting, clinicians can pinpoint the location of underlying problems – either in the muscles themselves, or in the nerves carrying signals to the muscles.