P3.206: CheCKing for Clues: Recurrent Elevated CK Isn't Always Exertional Rhabdomyolysis

Introduction: Mixed Connective Tissue Disease (MCTD) is an autoimmune condition with overlapping features of several rheumatologic diseases. Anti-U1 RNP antibodies, in addition to clinical features, confirm the diagnosis. MCTD can be difficult to diagnose due to its varied presentations, most commonly involving Raynaud’s phenomenon, arthritis, and muscle disease. It is rare in both adults and children, making diagnosis difficult.1 This case highlights the importance of maintaining a broad differential for adolescents with recurrent muscle enzyme elevations.

Case Description: A previously healthy 16-year-old female was re-admitted for fevers, muscle pain, and elevated CK without an inciting trigger. This was her second presentation for this within 4 months. During the prior admission, she had CK of 7,200 IU/L and CTAP demonstrating pleural effusions. She was diagnosed with presumed rhabdomyolysis and discharged with CK ~7,000 IU/L. Upon re-presentation, she was tachycardic to 130 with normal blood pressure and had diffuse muscle tenderness without rashes, oropharyngeal lesions, arthritis, or pulmonary findings. Labs demonstrated ESR >130 mm/hr, CRP 1.6 mg/dL, mild anemia, CK 4543 IU/L, and mild AKI. UA showed no blood and 1+ protein. The echocardiogram demonstrated a small pericardial effusion. Fever, elevated CK, anemia, and high ESR without corresponding CRP elevation, made autoimmune conditions the highest on the differential. Workup revealed ANA positive >1:1280, positive Anti-SS-A and Anti-RNP (>240). MR pelvis showed diffuse muscle edema. Genetic workup was unrevealing.

Discussion: This case highlights the need for a broad differential for low-level CK elevation, particularly with systemic symptoms. When evaluating CK elevations it is critical to consider infection, trauma, drugs, and endocrinopathies. Inborn errors of metabolism, congenital myopathies, and inflammatory myopathies are vital considerations for recurrent cases. Our patient had mildly elevated CK levels without a clear inciting event aside from a routine dance class. In retrospect, the mildly elevated CK likely reflected myositis rather than acute rhabdomyolysis, which typically presents with CK >50,000 IU/L2. Additionally, trace effusions identified on CTAP should have raised suspicion for a rheumatologic cause. This case demonstrates the importance of recognizing subtle signs of systemic disease and revisiting diagnostic assumptions with evolving clinical presentations. Per the AAP, recurrent rhabdomyolysis is uncommon with a reported pediatric incidence of ∼5%–8%, so an underlying myopathy should be considered in recurrent cases.3 Ultimately, positive titers confirmed MCTD, providing a unifying diagnosis for her fevers, myositis, elevated inflammatory markers, and serositis.

Conclusion: While the degree of CK elevation can vary between individuals with the same etiology, it can provide diagnostic clues for the undifferentiated patient. In our case, intermediate CK elevation with proximal muscle edema on MRI supported an insidious inflammatory myopathy. This case emphasizes the importance of considering MCTD as an overlap syndrome in patients without classic features of more common diagnoses like SLE or JDM.