Table 3

Some myopathies that may not require a biopsy for diagnosis

Myotonic dystrophy—DM1 is recognisable—genetic test
Dermatomyositis and other inflammatory myopathies—some diagnosed by myositis-specific and myositis-associated antibodies in the appropriate clinical setting
Inclusion body myositis is recognisable at the bedside, but we strongly recommend considering a biopsy
Some forms of rhabdomyolysis—note: wait 8 weeks if considering a biopsy—see text

  • Use the acronym ‘RHABDO’ to identify those in whom biopsy might help the diagnosis (see box 1)

  • Exertional rhabdomyolysis—consider metabolic myopathies and also glycogen and lipid disorders (and mimics)—possible metabolic gene panel rather than a biopsy

  • Non-exertional non-traumatic includes statin users co-prescribed medication that interferes with statin metabolism—often no need for biopsy

Endocrine causes—recognisable at bedside and next test is obviously to confirm clinical impression—for example

  • Dysthyroid

  • Cushing’s syndrome

  • Addison’s disease

Oculopharyngeal muscular dystrophy—genetic test
Non-dystrophic myotonias.
Myotonia congenita might be recognised quickly with genetic test as the next step; the others may require electrophysiology (short exercise test) and genetics
Periodic paralysis—electrophysiology (long exercise test) and genetic tests
Rippling muscle disease—genetic test, although sometimes occurs in an acquired form in myasthenia
Emery–Dreifuss syndrome and other contracture myopathies, for example, Bethlem myopathy, rigid-spine syndrome—MR imaging of muscle and genetic tests often before biopsy.
Facioscapulohumeral dystrophy—scapuloperoneal weakness—genetic test
Pseudohypertrophy myopathies—often Becker or LGMD2I (and dried blood test for α-glucosidase deficiency)—before biopsy
Myopathy with early respiratory muscle weakness—think Pompe’s disease (acid maltase deficiency)—dried blood test for α-glucosidase deficiency before other genetic tests or biopsy.
Mitochondrial disorders—often progressive external ophthalmoplegia—test POLG gene in blood and A3243G in urine but if negative then a muscle biopsy