Immune-mediated necrotizing myopathy in a multi-ethnic malaysian cohort

Objective: To describe the clinical features and treatment outcomes of immune-mediated necrotizing myopathy (IMNM) in Malaysian patients. Methods: We describe a cohort of IMNM patients from a tertiary medical centre in Kuala Lumpur, Malaysia, in terms of their demography, clinical features, investigations, treatments and outcome. Comparisons were made between the anti-signal recognition particle (SRP) positive, anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) positive and seronegative subgroups. Results: IMNM was seen in 23.7% of inflammatory myopathy cases in the University of Malaya muscle biopsy databank. Of these, 35 patients who underwent serological testing for IMNM myositis-specific antibodies were included in the study. 45.7% were anti-SRP positive, 20% anti-HMGCR positive, and 28.6% seronegative. Two (5.7%) patients had dual positivity. Mean age of onset was 37.1 ± 17.0 years (range of 8 to 76 years) with female predominance (74.3%). Twenty three (65.7%) patients presented subacutely (≤ 6 months) with the majority having symmetrical severe weakness (MRC grade ≤ 3), with proximal and lower limb predominance. Four (11.4%) had respiratory involvement but none had cardiac symptoms. Extramuscular manifestations included skin lesions (20%), arthralgia (5.7%), associated connective tissue disease (11.4%) and none with cancer. Anti-SRP positivity was significantly associated with muscle wasting and anti-HMGCR group with cutaneous involvement. Anti-HMGCR positivity was not significantly associated with statin use. Most patients required a combination of at least two immunotherapies and at follow up, 50% showed good recovery with minimal weakness while a third had at least moderately severe disability. Conclusion: IMNM is a common inflammatory myopathy seen in all ethnic groups in Malaysia. Its clinical characteristics are consistent with other populations. Extramuscular involvement is uncommon. Most patients require combined immunosuppressive therapy with variable outcome; about half having a good outcome with minimal disability and a third with more severe disease with significant persistent disability.