Mycobacterium avium complex causing transverse pericardial sinus infection: A case report.

Nontuberculous mycobacteria (NTM) are increasingly recognized as significant opportunistic pathogens in humans, yet they remain rarely implicated in cardiac conditions. Here, we report a rare case of Mycobacterium avium complex (MAC) infection in the transverse pericardial sinus, an unusual site previously undocumented for NTM infection. The patient, a 68-year-old male with prior cardiac surgery, presented with recurrent fever lasting for over 10 days. Positron emission tomography/computed tomography demonstrated a hypermetabolic mass-like lesion within the transverse pericardial sinus, radiologically suggestive of an infectious process. Empirical broad-spectrum antimicrobial therapy failed to achieve clinical response. Following surgical intervention to remove lesion tissue from the transverse pericardial sinus, histopathological analysis revealed granulomatous inflammation and acid-fast bacilli, indicating NTM infection. Metagenomic next-generation sequencing (mNGS) identified MAC in the tissue sample. After starting antimycobacterial therapy, the patient's body temperature gradually returned to normal, and no recurrence was noted during a 7-month follow-up via serial surveillance imaging. This case suggests that, in patients with a history of cardiac surgery who present with unexplained pericardial or mediastinal lesions and non-diagnostic routine cultures and examinations, atypical pathogens such as NTM may warrant consideration within a broad differential diagnosis. It also illustrates the potential value of surgical intervention and mNGS in diagnosing and managing such rare infections.