EP.19Anti-HMGCR positive necrotizing myopathy presenting in childhood

Abstract

A 12-year-old girl, previously healthy, was hospitalized because of progressive proximal weakness for a few months. CK was 17000U/L. There were only minor skin changes, but an initial muscle biopsy was reported to have changes possibly compatible with dermatomyositis. Treatment was started with prednisolone and methotrexate, without adequate improvement. A biopsy two years later did not exclude the diagnosis of a muscular dystrophy. The fast development of weakness did not however really fit clinically. A neuromuscular gene panel analysis (328 genes) did not reveal the etiology, and ANA- and ANCA tests were negative. She gradually became weaker. At the age of 15,5y, she could not walk any longer, her upper extremities were weak, and she could not shift position in bed. She also had pain in her legs, and breathing and swallowing were affected. She was still on prednisolone, but methotrexate had been stopped. A new biopsy showed more extensive, chronic myopathic changes and acute changes, with fibers in different stages of necrosis. CK was 2600 U/L. MRI of total body disclosed generalized edema and patchy infiltration of fat in all visible muscles, combined with muscle atrophy. It was then suggested to test for HMGCR antibodies which were positive. The patient was started on IVIG infusions in addition to previous therapy and restarted methotrexate. She improved gradually. After one year she could walk with assistance, swallowing was normal, and respiration was much improved. HMGCR associated necrotizing autoimmune myopathy (NAM) may well occur without statin exposure. However, the disease is rare in childhood. A muscle biopsy with scattered necrotic fibers without inflammatory infiltrate should raise the suspicion of the disease. Because it is treatment responsive, HMGCR associated NAM is an important differential diagnosis also in children when the clinical picture, imaging and laboratory findings are in the borderline between a myositis and a muscular dystrophy.