Diagnosis of Microscopic Polyangiitis by EBUS-Guided Transbronchial Mediastinal Cryobiopsy: A Case Report.

Background and Clinical Significance: Isolated mediastinal lymphadenopathy is an exceptionally rare and diagnostically challenging initial manifestation of microscopic polyangiitis (MPA), often mimicking malignancy or infection. This case highlights the pivotal role of an innovative minimally invasive technique in achieving a definitive diagnosis. To the best of our knowledge, this is the first reported case of MPA diagnosed via EBUS-TMC. Case Presentation: A 55-year-old male livestock farmer from a rural area with a history of recurrent pneumonia presented with four weeks of persistent fever, significant weight loss (7 kg), myalgia, and asthenia. Physical examination revealed fever and cachexia. Notable findings included leukocytosis (17,000/μL), normocytic anemia, thrombocytosis (672,000/μL), highly elevated inflammatory markers (CRP 145 mg/L, ESR 120 mm/h), and strongly positive MPO-ANCA (>134 U/mL). Serological testing was significant for IgG antibodies against Coxiella burnetii (Phase I 1:64, Phase II 1:256). PET-CT imaging demonstrated hypermetabolic bilateral hilar and mediastinal lymphadenopathy. Diagnostic challenges included overlapping serological findings suggestive of past Coxiella burnetii exposure. Endobronchial ultrasound-guided transbronchial mediastinal cryobiopsy (EBUS-TMC) of a subcarinal lymph node was performed, providing a high-quality sample that revealed neutrophilic small-vessel vasculitis with fibrinoid necrosis, definitive for MPA. Immunosuppressive therapy with high-dose corticosteroids and rituximab (1000 mg on days 1 and 15) was initiated, leading to the complete resolution of all constitutional symptoms. Conclusions: This case illustrates that EBUS-TMC is a safe and highly effective diagnostic tool for obtaining critical histological evidence in systemic vasculitides with atypical presentations. This technique should be considered in the diagnostic algorithm for unexplained mediastinal lymphadenopathy to avoid more invasive surgical procedures.