The Unsung Mycobacterium: A Rare Case of Right Middle Lobe Syndrome Due to Mycobacterium Kansasii

Abstract Right Middle Lobe Syndrome (RMLS) is a rare diagnosis characterized by vague presentation and often associated with recurrent infections and impaired mucociliary clearance. Although nontuberculous mycobacteria (NTM) have been long recognized as a cause of RMLS, Mycobacterium Kansasii (M.Kansasii) is a rare cause of this condition and has not been reported before. This is a case of RMLS due to M. Kansasii in a 72-year-old lifelong non-smoker woman with who presented with a 6-month history of fatigue, cough, and upper respiratory symptoms that began following travel to Texas in November 2023. Initial workup, including chest X-ray, revealed right middle lobe opacities. She had a history of latent tuberculosis treated 20 years ago. Despite courses of antibiotics and inhaled corticosteroids, her symptoms persisted. A chest CT in June 2024 demonstrated bilateral reticular nodular infiltrates with a tree-in-bud distribution in the right middle lobe and lingula. When she did not respond to a trial of steroids. Bronchoscopy with bronchoalveolar lavage was pursued and confirmed the diagnosis of M. Kansasii pulmonary infection. Following sensitivity report, the patient was started on a regimen of Rifampin, Ethambutol, and Azithromycin. One month into therapy, she reported symptomatic improvement with well-tolerated treatment. This case highlights the need to consider NTM infections, particularly M. Kansasii, in patients with RMLS who are on long-term immunosuppressive therapy. NTM pulmonary infections can mimic other lung pathologies such as tuberculosis and chronic bronchitis. Early bronchoscopy and microbiologic testing were critical in establishing the diagnosis, allowing for targeted therapy. The patient's RA was well-controlled throughout the course of the illness, underscoring the importance of comprehensive management of both her autoimmune disease and infectious complication. Conclusion: M. Kansasii is a rare but important cause of RMLS in immunosuppressed individuals. This case underscores the diagnostic complexity and importance of early recognition and treatment of NTM pulmonary infections in patients with chronic lung disease or on biologic therapies. Clinicians should maintain a high index of suspicion for NTM in patients with protracted respiratory symptoms and radiologic findings suggestive of bronchiectasis.