Case Presentation: A 66-year-old woman with systemic lupus erythematosus and MDA5-positive dermatomyositis on hydroxychloroquine and chronic corticosteroids presented with several weeks of progressive arthralgias, joint swelling, and fatigue. She had recently been hospitalized for abnormal uterine bleeding, during which pelvic imaging revealed multiloculated gluteal and peritrochanteric fluid collections of unclear etiology. A hysteroscopy with drainage of one fluid collection was performed with the fluid growing multiple organisms such as E. coli, Pseudomonas aeruginosa, Strep anginosus, E. faecium, and B. fragilis. She was discharged on empiric antimicrobial therapy.She re-presented with persistent pelvic fluid collections, despite completing her antimicrobial therapy. A repeat attempt at drainage was unsuccessful, and at this time, she also reported worsening swelling and pain in her left knee, left ankle, and right shoulder. Given multifocal joint involvement and persistent fluid collections, the working differential favored autoimmune flare versus progression of her inflammatory myopathy; rheumatology was consulted for assistance.To better characterize her joint swelling, an MRI of the shoulder was obtained. This revealed destructive joint changes with marrow involvement and periarticular fluid collections, raising concerns for septic arthritis and osteomyelitis. Around this time, cultures from her initial hospitalization resulted, identifying Mycobacterium avium-intracellulare (MAC). Given these new data, her right shoulder and left ankle were drained with targeted sampling confirming acid-fast bacilli on smear, unifying her multifocal disease under the diagnosis of disseminated MAC infection. She was initiated on rifampin, azithromycin, and ethambutol.
Discussion: This case illustrates an atypical presentation of disseminated Mycobacterium avium-intracellulare (MAC) infection. MAC typically presents with fever and systemic symptoms such as weight loss, night sweats, fatigue, and gastrointestinal complaints. Our patient had extensive musculoskeletal involvement, including septic arthritis and periarticular collections. As evidenced here, MAC can masquerade as an autoimmune flare in immunocompromised hosts, leading to diagnostic delay.A key lesson to be learned is the danger of cognitive bias, particularly anchoring on a rheumatologic etiology when patients with known autoimmune disease present with new or worsening joint symptoms. In this case, the presence of acid-fast bacilli across multiple joints was pivotal in shifting the diagnosis from inflammatory to infectious. Recognition of such atypical infectious presentations is crucial to preventing misdiagnosis and ensuring timely initiation of therapy.
Conclusions: Disseminated Mycobacterium avium-intracellulare can present as extensive musculoskeletal infection and mimic autoimmune flares in immunosuppressed patients. Clinicians should hold a high index of suspicion for opportunistic infections in this population and guard against anchoring bias, especially when diagnostic elements do not neatly align with a single disease process.