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Muscle Disease

Limp can be due to muscle weakness as a result of many causes.

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 (Rickets) can cause muscle weakness and muscle pain. The vast majority of cases are due to lack of vitamin D, resulting 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.  Proximal myopathy also occurs with difficulty getting up from the floor and inability to jump. The diagnosis is confirmed by bone chemistry, parathyroid hormone levels and 25 hydroxy vitamin D levels.

Myositis is due to an inflammatory response in muscle causing pain and weakness and 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 are covered in more detail in the module 'swollen joints'. A number of infections (e.g., influenza, coxsackie B) can also result in an acute myositis.

There are many inherited myopathies; diagnosis is suspected clinically from a delay in walking or other milestones (e.g., speech, feeding) and is 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.

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy. DMD affects 1 in 3500-5000 newborn boys. In DMD the CK is typically raised 10-100 fold; normal CK levels exclude DMD.  Some children with DMD may also have learning difficulties and speech delay. DMD must be considered in boys with delayed walking (by 18 months), waddling gait, frequent falls, or speech delay. Mutations can be sporadic (in about 30% of cases) so there may not be a family history. 

DMD is caused by a fault (mutation) in a gene (the dystrophin gene) resulting in a lack of the protein dystrophin which is important in muscle fibres; absence results in muscle weakness which worsens over time because muscle cells break down and are eventually lost. The dystrophin gene is found on the X-chromosome, therefore Duchenne usually only affects boys (girls have two X chromosomes so if one of these is unaffected it can compensate for the faulty one). Boys have one X chromosome therefore if that single copy of the gene is faulty then they will show the clinical features of DMD. Girls who have one affected and one unaffected gene are called carriers and do not usually display symptoms. DMD runs in families; a carrier mum has a 50:50 chance of having a son who is affected.

Most common early features of DMD are difficulty with walking, they might walk later than their peers. Typically they have enlarged calf muscles. This is ‘pseudohypertrophy’ as muscle cells are broken down and replaced by fat cells (giving the appearance of enlarged “muscular” calves). Boys with DMD develop difficulties with running, jumping and climbing stairs due to progressive muscle weakness. A classical sign of DMD is the “Gower sign” – ask a child to lie flat on his back on the floor and ask the child to stand up quickly. A boy with Duchenne will roll onto his tummy and uses his hands and arms to climb up his body in order to stand up. Speech delay, learning and behavioural difficulties can also be a clinical feature of DMD. A normal Gower’s test is illustrated in pREMS-gait

Becker muscular dystrophy is a milder form of muscular dystrophy. It is related to DMD as both result from a mutation in the dystrophin gene. However in contrast to DMD, children with Becker muscular dystrophy still have some dystrophin being produced and expressed in the muscles (in Duchenne there is no dystrophin); this results in milder symptoms and signs. The disease onset is later and there is a much slower rate of progression. Loss of ambulation (walking) may not occur until the patient is in his fifties. Patients with Becker muscular dystrophy however are still at risk to develop cardiomyopathy even without a significant skeletal muscle weakness.

Typically boys with DMD lose the ability to walk between the ages of ten and fourteen. In their late teens they progressively lose the strength in their upper body, leading to difficulties raising their arms for example for brush their teeth or for feeding. The disease also affects the heart and respiratory muscles and eventually boys with DMD will need help with breathing. Life span is reduced in DMD due to cardiac or respiratory failure but in recent years steroids have significantly slowed down the disease progression and delayed the onset of respiratory complications. Whilst there is no cure for DMD, several potential new therapies are currently being tested in clinical trials. Early diagnosis is important to optimise outcomes and provide families with support (http://www.treat-nmd.eu/care/dmd/diagnosis-management-DMD/). 

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