Progressive diaphyseal dysplasia mimicking childhood myopathy: clinical and biochemical response to prednisolone.

Two siblings presented with symptoms suggestive of myopathy: the true diagnosis of Engelmann's disease was delayed until X-rays revealed the characteristic sclerosis and widening of long bone diaphyses. Dominant inheritance from their father had not been suspected because of his lack of symptoms. Biochemical studies of bone metabolism did not elucidate the underlying mechanism. Prednisolone dramatically improved the clinical condition, particularly in the more severely affected child. While research into the biochemical basis must continue, paediatricians should remember to X-ray the long bones of 'myopathic' children, and offer steroids when the clinical condition warrants.