Muscle diseases impair mobility and frequently have cardiorespiratory complications. Their age of onset is extremely wide; patients may die young, or there may be lifelong motor disability with cardiorespiratory complications, or late onset muscle weakness.

About 50 000 people in the UK population of 60.5 million have one of the diverse group of genetic and acquired conditions which make up the primary muscle diseases; many more have muscle symptoms due to drugs or systemic diseases such as cancer. The important acquired muscle diseases include the inflammatory myopathies which are treatable and so the diagnosis must not be missed (or overdiagnosed and some other disorder treated inappropriately), as well as the drug-induced myopathies. Neurologists are also often asked to assess patients with secondary muscle dysfunction due to aging, immobility, critical illness and cancer.

DIAGNOSIS

Distinguishing myopathies from peripheral neuropathies, anterior horn cell diseases (eg, motor neuron disease) and neuromuscular junction disorders (eg, myasthenic syndromes) requires careful clinical evaluation supplemented by investigations, which may include peripheral neurophysiology, imaging, muscle biopsy and genetic testing.

Clues from the history

Age of onset

• Duchenne muscular dystrophy presents at <5 years, inclusion body myositis in middle life, while mitochondrial diseases at any age.

• Dermatomyositis is far commoner than polymyositis in childhood.

• Many dystrophies, such as facioscapulohumeral, myotonic and limb girdle, typically present in adolescence or early adult life, but late onset and very young onset are recognised.

Childhood features

• Reduced fetal movements in pregnancy, “floppy baby” at birth, feeding/breathing problems, delayed motor milestones and limited sporting achievements all point to a congenital or childhood onset muscle disorder. These features alone may not differentiate from neuromuscular junction or peripheral nerve disorders.

Family history

• May reveal autosomal, X-linked or mitochondrial DNA (maternal) inheritance.

• Systemic features in other family members; eg, cataracts may be the only manifestation of myotonic dystrophy in a preceding generation.

Rate of progression and temporal pattern of weakness

• May be minimal or absent (eg, some congential …