Muscle Disease

• Muscle weakness and / or muscle pain (myalgia), can be due to a wide variety of causes affecting any part of the neuromuscular system.
• Presentations are variable and include limp or altered gait (e.g., waddle), muscle pains and weakness. Proximal myopathy occurs with difficulty getting up from the floor (Gower's sign) and inability to jump. Systemic features may occur. 
• A list of potential causes and mode of onset (acute and subacute / chronic) is available.
• Further information about clinical assessment of muscles is available. 
• Further information about approach to suspected muscle disease is available.
• The differential diagnosis spans infections, trauma, drugs and toxins, metabolic disturbances, inherited myopathies and inflammatory disease (the juvenile idiopathic inflammatory myopathies).
• There are many inherited myopathies / dystrophies with Duchenne Muscular Dystrophy (DMD) being the most common. Typically boys are affected with DMD as the condition is X linked. 
• The diagnosis of inherited myopathies is suspected clinically from a delay in walking or other milestones (e.g., speech, feeding.  Proximal weakness may be observed (Gower's sign) and they may have enlarged calf muscles (pseudohypertrophy).
• Blood tests show raised muscle enzymes, particularly creatine kinase and the diagnosis is confirmed by genetic testing to identify specific mutation in the dystrophin gene. 
• Becker Muscular Dystrophy is a milder form of muscular dystrophy and affected children still have some dystrophin being produced.  
• Juvenile dermatomyositis (JDM) is the most common chronic idiopathic inflammatory myopathy.
• An inflammatory myositis may occur in other systemic inflammatory conditions (e.g., juvenile systemic lupus erythematosus, systemic sclerosis, mixed connective tissue disease and overlap disorders).
• A number of infections can result in an acute myositis presenting with acute limp; these may be viral (e.g., Influenza and coxsackie B), parasitic (e.g., Toxoplasmosis infection may result in a myositis resembling JDM) or bacterial (e.g., pyomyositis from Staphylococcal aureus).
• The acute myositis that can occur with Influenza B infection has onset in the recovery phase of the 'flu illness, and presents as severe, bilateral pain and tenderness, especially of the calves. Blood tests will show raised muscle enzymes. Spontaneous resolution usually occurs over a few days.  
• Endocrine and metabolic disturbances can present with limp, myalgia and proximal weakness; muscle enzymes can be elevated. Examples include:
• Hypothyroidism and Hyperthyroidism, Parathyroid disease (hypocalcaemia).
• Periodic paralysis due to high or low potassium disturbances may occur due to an inherited condition and can also be seen in thyrotoxicosis. 
• Vitamin D deficiency (Rickets) can cause muscle weakness and muscle pain. The vast majority of cases result from poor sunlight exposure or dietary inadequacy. Cases can also occur with malabsorption (e.g., coeliac disease or inflammatory bowel disease). Clinical features include bowing of the legs, joint (metaphyseal) swelling, bossing of the forehead, limb pain and irritability.  The diagnosis is confirmed by bone chemistry, parathyroid hormone levels and 25 hydroxy-vitamin D levels.
• Further information about muscle disease is provided in the clinical cases: 

• Frequent Falls - DMD

• Delayed Walking - DMD

• Fatigue and Muscle Pain - JDM

• Further information about muscle disease is available - https://childmuscleweakness.org
• Top Tips about Muscle Disease is available