Suspected Muscle Disease

• Muscle disease can present acutely, or with a more subacute presentation or a chronic insidious course.
• The differential diagnosis involves a spectrum of causes including inherited myopathies, inflammatory, infections, endocrine and metabolic causes.
• A thorough history and examination are important help to determine the differential diagnosis and plan of investigation.
• The list of causes can be assigned by the site of pathology and mode of onset (acute/subacute/chronic).
• Key points in the History, Examination, Diagnostic Considerations and Initial Investigations for Muscle Weakness.
• The clinical assessment of muscles is described and includes the clinical tools for muscle testing (e.g., CMAS, MMT and MRC / Oxford Scales).
• 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 can differentiate between activity and damage. Nail fold capillaroscopy is a useful, non-invasive investigation and nail fold capillary changes correlate strongly with disease activity in inflammatory muscle disease and connective tissue disease (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. There are several types of muscular dystrophy and each of them is caused by a mutation in a different gene. 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 DMD (usually mothers). This is important as carriers are often asymptomatic but can pass the condition on to their offspring.
• Tissue or muscle biopsy - helps to detect 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 DMD. 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.
• Further information about muscle disease is provided in the clinical cases: 

• Frequent Falls - Duchenne Muscular Dystrophy
• Delayed Walking - Duchenne Muscular Dystrophy
• Fatigue and Muscle Pain - Juvenile Dermatomyositis 

• Further information about muscle disease is available - https://childmuscleweakness.org