Necrotising autoimmune myopathy requires early, aggressive treatment

Researchers call for the prompt recognition of necrotising autoimmune myopathy (NAM) and aggressive early treatment with a combination of intravenous immune globulin (IVIG), corticosteroids and a steroid-sparing agent for 3 months.

They report in JAMA Neurology that this aggressive therapy resulted in over half of 32 patients with the condition improving markedly or back to full health over a median follow-up period of 13.5 months; with just 10% showing little or no clinical improvement.

Editorialist Matthew Meriggioli (Rush University Medical Center, Chicago, Illinois, USA) agrees, saying: “Although therapy must obviously be individualized, this is a welcome recommendation and provides a useful starting point for consideration in the clinic.”

However, researcher Margherita Milone (Mayo Clinic, Rochester, Minnesota, USA) and team point out that the relapse rate following taper or discontinuation of treatment was high, at 55%.

“Only one patient was able to discontinue immunotherapy”, they note, adding that long-term treatment with a steroid-sparing agent is likely to be necessary.

Factors predicting a favourable outcome included being male and using at least two forms of immunotherapy, the researchers report. Indeed, the findings suggest that NAM is often refractory to corticosteroid monotherapy, with 90% of 32 patients followed-up for a median of 13.5 months requiring two or more immunotherapeutic agents and an IVIG to improve muscle strength.

Although NAM is relatively rare, the team confirmed it is a severe disease. Muscle weakness was reported as severe for half of the patients studied, irrespective of whether the cause was idiopathic (n=32), statin-associated (n=22), or related to cancer (n=6) or connective tissue disease (n=3).

And in agreement with previous reports, NAM was acute to subacute in evolution (≤4 weeks), predominantly proximal and associated with markedly elevated serum creatine kinase levels. Distal weakness was seen in 41% of patients, affecting finger abduction, finger extension and foot dorsiflexion, which Meriggioli says is “surprisingly common” and awareness of this could “help to avoid diagnostic delay”.

Prominent neck weakness was also evident in two patients, and a “novel clinical feature”, notes Meriggioli.

Although in most instances NAM evolution was subacute, some patients showed slowly progressive NAM over 9 to 14 months, and the researchers highlight the need to rule out a late-onset muscular dystrophy in these patients.

The results also showed that accompanying respiratory and cardiac manifestations were a concern across all the patients, irrespective of disease aetiology.

Overall, there was little distinction among the aetiological subgroups with regard to clinical features. Dysphagia was predominantly seen in patients with idiopathic disease, occurring in 45% of the 22 patients affected, otherwise there were no significant differences including in clinical severity, response to treatment and outcome.

Autoantibody profiles offered some subtype distinction. For example, statin exposure was strongly associated with the presence of anti-3-hydroxy-3-methylglutaryl–CoA reductase antibodies – the combination of which was linked to a milder disease course – and myotonic discharges.

Anti-signal recognition particle antibodies, which are seldom present in other immune-mediated myopathies, were detected in patients with idiopathic or statin-associated NAM.

But Meriggioli concludes that, as clear clinical subsets could not be defined in the study, “it remains likely that NAM is not a single disease but several diseases leading to autoimmunity and ultimately causing a necrotizing myopathy.”

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