Case Report: Adult fibrinous bronchitis associated with lymphatic reflux: a probable diagnosis based on convergent evidence.

Background: Fibrinous bronchitis, also known as plastic bronchitis, is a rare airway disorder characterized by branching bronchial casts that replicate the architecture of the bronchial tree. Adult cases are uncommon and frequently misdiagnosed because clinical manifestations and imaging findings are nonspecific. Diagnosis is particularly challenging in the absence of bronchoscopic or histologic confirmation of casts. Case presentation: We present a 52-year-old man with a 2-year history of recurrent cough and expectoration of branching mucoid casts. Serial chest imaging showed fluctuating infiltrates without bronchiectasis. Bronchoscopy did not identify casts, presumably because the procedure was performed outside the active expectoration phase. Microbiologic findings were intermittent and not associated with sustained response to antimicrobial therapy. Transbronchial biopsy demonstrated chronic mucosal inflammation with fibrinous exudation, consistent with a fibrin-predominant airway process. Radionuclide lymphoscintigraphy and direct lymphangiography showed mediastinal lymphatic dilatation and pulmonary lymphatic reflux. Based on a convergent evidence framework, a probable diagnosis of fibrinous bronchitis was made. This was based on reproducible cast morphology, supported by indirect histopathologic findings, objective lymphatic abnormalities, and the mismatch between symptoms and treatment response. The patient remained asymptomatic with no recurrence at 8 months following targeted lymphatic embolization. Discussion: This case highlights the diagnostic challenge of adult fibrinous bronchitis in the absence of direct bronchoscopic or histologic confirmation of casts. It suggests that reproducible morphologic features, when interpreted alongside supportive histologic findings and mechanism-oriented imaging, can provide a practical basis for diagnosis. Recognition of symptom-treatment discordance may further prompt reconsideration of non-infectious mechanisms, including lymphatic dysfunction. Conclusion: In the absence of direct confirmation, a probable diagnosis of fibrinous bronchitis can be supported by converging evidence. Reproducible morphologic patterns, integrated with mechanism-oriented imaging, offer a practical basis for diagnosis and management in rare airway diseases and may facilitate earlier recognition.