A 14-year-old boy presented with a one year history of progressive proximal muscle weakness with intermittent calf pain. There were no developmental issues and he was on no medication. Family history was unremarkable.

On examination, he had asymmetrical proximal muscle weakness affecting upper limbs more than the lower.

Creatine kinase (CK) was 24800 U/L (NR < 250). Echocardiography was normal. Neuromuscular gene panel was negative.

In view of continuing progression of his weakness and an increasing CK, a myositis-specific autoantibody screen was performed. The panel, including Ro52, EJ, OJ, PL-12, PL-7, SRP, Jo-1, PM-Scl 75, PM-Scl 100, Ku and Mi-2 autoantibodies was negative.

Magnetic resonance imaging revealed symmetric T2 signal abnormality in the rotator cuff musculature and symmetric involvement of the adductor muscles, glutei and the hamstrings.

Rectus femoris  biopsy showed variation in fibre size with focal myonecrosis, regenerating fibres,   mild endomysial inflammation, occasional vacuoles with sarcolemmal features,  p62 positivity, endomysial and perimysial fibrosis and MHC-I positivity. Electron microscopy showed autophagosomes (autophagic myopathy).

Progressive weakness, elevated CK and myonecrosis were suggestive of an immune-mediated necrotizing myopathy. HMGCR autoantibody testing was positive on further testing. He received high dose steroids, intravenous immunoglobulin and Rituximab. Muscle strength has improved and the CK levels have dropped.

Anti-HMGCR antibody associated necrotising myopathy is rare in the paediatric population. A high index of suspicion is required especially in slowly progressive weakness which may mimic late onset muscular dystrophies. Muscle biopsy can be useful in centres where routine screening for myositis associated antibodies is not possible.