Last modified: 2018-09-09
Abstract
Immune-mediated necrotizing myopathy (IMNM) is a recently characterized disease, which is rarely reported in children. Autoantibodies to signal recognition particle (Anti-SRP) and 3-hydroxy-3-methylglutaryl–CoA reductase (anti-HMGCR) have been associated with juvenile and adult-onset necrotizing myopathy. However, no patient under 18 years of age with immune-mediated antibody-negative necrotizing myopathy has been described.
We present an 11-year-old boy who was brought by his mother with a 3-week history of acute-onset progressive proximal symmetric weakness in the upper and lower limbs, without associated rash or previous illness. He had an elevated creatine kinase (CK) level of 13006 U/L (cutoff 220 U/L).
An extensive investigation, including blood work, serologic testing, MRI, and muscle biopsy, led to a diagnosis of an antibody-negative immune-mediated necrotizing myopathy. The patient had no response to pulse-steroid treatment, but improved mildly with subsequent IVIG treatments and CK level decreased to 3342 U/L.
A high index of suspicion is necessary to diagnose an immune-mediated necrotizing myopathy in children. Our case report expands the phenotypical spectrum of IMNM in children and highlights the importance of earlier recognition and correct choice of treatment that can prevent rapid and severe progression.