A case report of post-COVID polymyositis in a child

Abstract

Introduction: Over the past few years, several post-COVID-19 autoinflammatory conditions have been reported. One of them, polymyositis has been described in adults [1,2], but there are no cases described in children so far. We present a severe pediatric polymyositis with possible relation to previous SARS-CoV-2 infection. Results: Following unspecified respiratory viral infection, 7-year-old boy complained of increasing leg muscle pain for about a month and was worsening: limping, inable to climb stairs, NSAIDs were ineffective. Signs of tonsillitis, leg and face edema, painful leg muscles on palpation and pain during passive movements were seen on admission. A patient had no fever and no rashes. Laboratory studies showed thrombocytopenia (60x109 /l), normal CRP, high ESR (34mm/h) and liver enzymes (ALT 2059IU/l, AST 674IU/l, GGT 220IU/l), low albumin (30.1g/l) and increased levels of creatin-kinase (CK) (1148IU/l). The patient progressed to acute renal failure, anemia (Hgb 97g/l) and polymyositis in a few days, with increasing levels of CK to 28045IU/l and myoglobin (1297.6→8987.9μg/l). Infectious agents such as EBV, CMV, atypical bacteria were disproved. From anamnesis, the boy was not vaccinated against SARS-CoV-2 and had no PCR proven COVID19 infection during all pandemic years. However, IgG antibodies against SARS-CoV-2 were positive (140.3BAU/ml, positive >31.5BAU/ ml). Immunodeficiency, hemophagocytic lymphohistiocytosis and neurological disorders were excluded. Immunological tests (musclespecifc tyrosine kinase, ANA, anti-DNR, ANCA and myositis related autoantibodies) were negative as well as genetic acylcarnitine profile analysis. Taking into account all the exams performed, previous SARS-CoV-2 infection was the most likely cause of polymyositis. Methylprednisolone 20mg/kg pulses were given for 5 days, then switched to prednisolone 2mg/kg/d. Despite of the initiation of glucocorticoids, the condition progressed to dysphagia, leading to intubation and mechanical ventilation. Head MRI and chest CT scan showed edema of neck and chest muscles. Intravenous cyclophosphamide 750mg/ m2 once a month has been added. After 5 days with immunosuppressants CK and myoglobin levels started to decrease and the patient was extubated. In the next few weeks the patient regained ability to walk, eat and talk independently. Myoglobin and CK levels normalized in one month. During the follow up period of 2 months the patient is in remission (on prednisolone tapering and cyclophosphamide) and regains normal age-related activity. Conclusion: This clinical case reveals a relatively new and poorly studied post-infectious variant of severe polymyositis in a child with previous COVID-19 infection.