Myopathy with a normal creatine kinase level in juvenile myopathic dermatomyositis

Abstract

A 14-year-old boy was referred to our division with a diagnosis of ‘progressive myopathy with psoriasis’ after a normal calf muscle biopsy excluded a previously suspected diagnosis of muscular dystrophy. The referring clinician had rejected a diagnosis of inflammatory myositis, based on repeatedly normal serum creatine kinase (CK) levels. Described as a ‘lazy child’ who refused to play games such as soccer, he had developed increasing weakness over 3 years, a rash and, more recently, hard painless lumps in both axillae and groins. The weakness affected proximal limbs, resulting in difficulty with standing up from a low chair, getting out of bed and walking more than 200 meters. Climbing stairs was a challenge, and he could not carry objects such as grocery bags.

He demonstrated classic Gottron’s lesions on both hands, subcutaneous calcinosis in both axillae and groins, and a shawl sign on the back of his neck, associated with a diffuse erythematous, poikilodermatous and scleredermoid rash, with atrophic skin involving the face, arms, forearms and legs. He had bilateral elbow flexion contractures. His thigh and calf muscles were atrophic, with marked proximal myopathy and inability to stand up from chair with arms folded or to rise from a recumbent position. Nailfold capillaroscopy showed dilated, tortuous, haemorrhagic vessels with telengiectasia and drop-out. His vital capacity and other systems were normal. His serum CK was 108 (normal <140 units/litre (U/l)), and electromyography showed myopathic units (low amplitude, short duration, polyphasic) with evidence of active denervation (fibrillation potentials and positive sharp waves) consistent with an active inflammatory myositis. Dermatomyositis (DM) was diagnosed, and the patient was treated with oral prednisone, with good improvement in muscle strength and enhanced performance.