Administration of corticosteroids is one of the commonest causes of proximal muscle weakness. However the identical effect may also occur in Cushing's disease. Synthetic glucocorticoids such as dexamethasone appear to be particularly potent cause of proximal muscle weakness.
Biopsy reveals selective atrophy of type IIb - a subtype of fast twitch - fibres. Corticosteroids may also contribute to weakness by causing hypokalaemia.
Treatment is aimed at minimising corticosteroid excess, via the use of alternative medication if appropriate, and maintaining physical activity.
Mineralocorticoid excess, as seen in Conn's syndrome, may cause muscle weakness secondary to hypokalaemia.
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