Case Presentation: Six-year-old female ZT presented for evaluation of knee pain which turned into back pain after several weeks duration. ZT was initially seen in the outpatient setting for bilateral knee pain where she was sent home with supportive care. She then started developing body aches, falling, and difficulty standing despite home therapy. ZT was reevaluated in the outpatient setting and then went to the ED for further evaluation. When she presented to the hospital, labs were notable for elevated CK >5000, AST >200, and ALT >100. ZT’s X-ray findings of the hips and knees bilaterally were reassuring. There was concern for neurological process at the time of admission. Once ZT was admitted, the physical exam was notable for significant proximal muscle weakness of both the upper and lower extremities. She was unable to walk or lift her upper and lower extremities during this time. She then developed swelling of the extremities which was a new finding. The swelling was in both upper and lower extremities, but no other skin findings or rash was present. Though initially swelling was intermittent, she developed significant facial and neck swelling and discomfort swallowing, concerning for airway compromise. Neck imaging was reassuring. Symptoms were more concerning for autoimmune process as opposed to a neurological process given both proximal upper and lower muscle weakness and concern for swelling. A full autoimmune work up was obtained, and she was started on IV pulse steroids. Swelling immediately improved as well as extremity weakness. MRI of Brain, C spine, pelvis, and thighs were obtained and notable for inflammation of the thighs, abductor muscles, and gluteal muscle. Muscle biopsy was obtained which showed severe perifascicular muscle atrophy and necrosis. During the hospitalization, she was started on IVIG and continued to improve. She was transitioned to oral steroids and started on Methotrexate and Folic Acid prior to discharge with close Rheumatology follow up. Myositis Specific Antibody and Myositis Associated Antibody panel were negative. Further workup for HMGGCR antibody is being considered due to the degree of atrophy and presence of necrosis. She has continued to improve on her current regimen.
Discussion: ZT’s significant improvement with steroids, IVIG, and Methotrexate leads us to believe that her symptoms were autoimmune, particularly Juvenile Polymyositis due to lack of dermatologic findings. This case is unique in that while CK, aldolase, AST were all elevated which is seen in polymyositis, MSA/MAA panels were negative. This did not include Anti-HMGCR ab which can be seen in Immune Mediated Necrotizing Myopathy. According to Anti-HMGCR myopathy: clinical and histological features, and prognosis, anti-HMGCR antibody positive immune-mediated necrotizing myopathy is present in only 1% of juvenile patients.
Conclusions: The case demonstrates an interesting initial presentation of polymyositis in that patient presented with common physical ailments such as knee pain and back pain initially for an uncommon disease in the pediatric population. Typically, findings of Juvenile Polymyositis include muscle weakness, fatigue, fevers, and sometimes joint pain but less commonly back pain. Her findings eventually progressed to proximal weakness which is more common in polymyositis. This case highlights the importance of thorough evaluation and differentiating causes of muscle weakness to ensure proper treatment.