A 68 year old man presented with 10 days of progressive lower limb myalgia and weakness. Past medical history was significant for ischaemic heart disease, type 2 diabetes mellitus and stage 4 chronic kidney disease, and with notable medication history of rosuvastatin 40mg nocte for the last 5 years.
He was found to have a CK of 33775 U/L, and severe acute on chronic kidney injury, with an eGFR of 6mL/min/1.73m2, from a baseline of 31mL/min/1.73m2. A muscle biopsy of the right vastus lateralis showed features initially believed to be consistent with immune-mediated necrotising myositis (IMNM), with immunohistochemistry for C5b9 showing prominent granular membranous reactivity.
Based on this diagnosis, the patient was immunosuppressed with a combination of intravenous immunoglobulin, high dose steroids and rituximab. Haemodialysis was also commenced in the setting of end stage renal failure.
The myositis antibody panel was requested, including 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) antibody which returned negative several weeks into the admission. This, alongside minimal clinical improvement with immunosuppression, prompted review of the initial diagnosis.
A review of the literature suggested that the finding of granular membrane deposits of C5b9 on non-necrotic muscle fibres is not specific for IMNM but can also be seen in regenerating fibres in other conditions [1-3]. Overall, the clinical presentation was felt to be more in keeping with a toxic necrotising myopathy and rhabdomyolysis, caused by a combination of statin use and long standing type 2 diabetes mellitus, with recovery impaired in the setting of severe renal failure.
The patient was therefore weaned off immunosuppression, and was managed conservatively with a focus on rehabilitation and optimisation of his comorbidities. The diagnosis of IMNM and subsequent immunosuppression should be made in the right clinical context, given non-specific muscle biopsy features and slow turnaround time of HMGCR antibody result.