Pediatric case of immune-mediated necrotizing myopathy with anti-HMGCR antibodies and dermatomyositis skin rash

Background

Immune-mediated necrotizing myopathy (IMNM) is a subgroup of idiopathic inflammatory myopathies (IIM). The hallmark of IIM is muscle weakness associated with elevated creatine kinase (CK) levels. Juvenile IMNM is characterised by an insidious course, muscle atrophy, proximo-distal muscle weakness, high CK elevation, and resistance to treatments often involving a combination of immunosuppressants [1, 2]. Diagnosis relies on a wide range of assessments, including specific biomarkers (anti-HMGCR and anti-SRP antibodies) and muscle biopsy. This presentation in children is thought to account for 3% of IIM, versus 19% in adults [2, 3].

Case presentation

A 9-year-old girl from Mayotte (a south-east African island) presented with progressive symmetrical proximo-distal muscle weakness associated with myalgias for the past year. A skin rash appeared on the extensor aspect of elbows, hands and knees, with pruritic erythematous plaques (Figs. 1, 2 and 3). Apart from periungual telangiectasias, the other dermatomyositis classical cutaneous manifestations (e.g. violaceous erythema of the eyelids, malar rash, poikiloderma) were not found. Patient also had muscular atrophy, reduced walking ability requiring a manual wheelchair for long distances and weight loss (BMI 12.97 kg/m²). Cutaneous calcinosis was not found. Skin biopsy revealed non-specific dermal inflammatory changes with mucin deposits, consistent with dermatomyositis. CK was markedly elevated at 26,240 IU/L (N < 180). ANA and myositis dot were negative. Anti-HMGCR antibodies were present at high levels (691 CU ; N < 20), leading to the diagnosis of IMNM.

Other cases of IMNM associated with a rash suggestive of juvenile dermatomoyositis (JDM) have been reported [1, 2, 4]. In children with anti-HMGCR antibodies, this association does not seem unusual. Depending on the series, between 40 and 60% of them have skin manifestations suggestive of JDM [1, 2].

In our case, skin lesions disappeared after corticosteroid and methotrexate treatment. Muscle weakness persisted and CK levels plateaued, prompting the start of monthly IVIg courses. Rituximab will be added if no clinico-biological improvement is seen after 4 courses of IVIg. Resolution of the skin rash with myositis treatment and the presence of anti-HMGCR antibodies led us to consider this cutaneous involvement as secondary to the IMNM.

Conclusion

One teaching point from our case is that JDM skin rash can be part of the juvenile IMNM phenotype. When faced with JDM exhibiting a poor therapeutic response, it may be worthwhile to test for anti-HMGCR antibodies. As IMNM can present with JDM-like skin rash and is known to be refractory to usual treatments, this may have a significant therapeutic impact.

Fig. 1

Dorsal aspect of the right wrist showing an erythematous plaque, along with localized erythema on the MCP and IP joints, generally seen in dermatomyositis

Fig. 2

The extensor surface of the right elbow displaying an erythematous plaque

Fig. 3

Extensor surface of both knees demonstrating erythematous plaques