Muscle disease includes a spectrum of inherited myopathies, inflammatory disease, endocrine and metabolic causes. Hypothyroidism and parathyroid disease can present with myalgia and proximal weakness - the muscle enzymes can be elevated. Osteomalacia can also cause muscle weakness and muscle pain. A number of infections (e.g., influenza, coxsackie B) can result in a muscle inflammatory response (myositis). In addition, myositis may be part of a number of systemic inflammatory conditions (e.g., juvenile dermatomyositis, juvenile systemic lupus erythematosus and systemic sclerosis/mixed connective tissue disease - these conditions are covered in more detail in the module 'swollen joints').
Muscle disease may be investigated or monitored by:
Blood tests - muscle enzymes CK [creatine kinase], AST [aspartate aminotransferase], ALT [alanine aminotransferase], LDH [lactate dehydrogenase].
Imaging - (US, MRI, capillaroscopy) - US and especially MRI of muscles are valuable tools in detecting muscle disease and can differentiate between activity and damage. Nail fold capillary changes correlate strongly with disease activity in inflammatory muscle disease and connective tissue disease. Nail fold capillaroscopy is a useful, non-invasive investigation. Changes of the finger nail fold capillaries reflect microvascular abnormalities in many rheumatological conditions (see pREMS - hands).
Electromyogram - measures the muscle’s response to stimulation of its nerve supply (nerve conduction study) and examines the electrical activity of the muscles (needle electrode examination). This test is helpful to differentiate whether muscle weakness is due to a muscle disease or a nerve disease.
Genetic testing. This involves analysing the DNA of the blood cells to see whether there is a mutation (fault) in the genes. There are several types of muscular dystrophy and each of them is caused by a mutation in a different gene. DNA testing for the different types of muscular dystrophy are often only available in specialised centres. Duchenne muscular dystrophy, (DMD) has X -linked recessive inheritance and the DNA is analysed to see whether there is a mutation in the dystrophin gene and if so, where it occurs. DNA analysis can also be used to identify carriers of Duchenne muscular dystrophy. This is important as carriers are often asymptomatic but can pass the condition on to their offspring.
Tissue or muscle biopsy - during this test a small piece of muscle tissue is removed and sent to a pathology lab. The sample is analysed for changes in the structure of muscle cells (e.g., size of muscle cells and atrophic/dystrophic appearance) and to study the expression of different proteins involved in muscle structure and function. Muscle biopsy is not required to diagnose Duchenne muscular dystrophy which is diagnosed by genetic testing. However, a muscle biopsy can still help to diagnose other types of muscle disease. This can also evaluate for the presence of inflammatory cells, e.g., lymphocytes that may be present with an inflammatory process.
Muscle strength - proximal weakness may be observed (see pREMS - hips). Examples of waddling gait or Gower's sign for proximal weakness are available. The image below demonstrates Gower's sign.
DMD must be considered in boys with delayed walking (by 18 months), waddling gait, frequent falls, or speech delay. Some children with DMD may also have learning difficulties. Mutations can be sporadic (in about 30% of cases) so there may not be a family history. Where DMD is suspected, the first step is a blood test for creatine kinase (CK); boys with DMD have very high levels of CK (10-100 times normal). If the CK is normal then DMD can be excluded. If the CK is high then a referral to paediatric neurology (or Neuromuscular Disease centre) is required. The diagnosis is confirmed by genetic testing, which analyses the DNA in order to identify the specific mutation in the dystrophin gene. In most cases this gives the diagnosis, only rarely these days a muscle biopsy is needed for the diagnosis of DMD.
There are many inherited myopathies; diagnosis is suspected clinically and confirmed with raised muscle enzymes (creatine kinase, CK) and genetic tests (which can also be used to detect carriers - mothers - in families). Creatine kinase is abundant in skeletal muscle and when muscles are damaged, CK leaks into the blood.