A cardiac debut: Myocarditis revealing seronegative primary Sjögren's syndrome in a young woman-A case report.

Myocarditis as the inaugural manifestation of primary Sjögren's syndrome is exceptionally rare, and its occurrence in a seronegative patient without prominent sicca features represents a particularly challenging diagnostic scenario. We report a 20-year-old woman presenting with chest pain and inflammatory polyarthralgia, in whom echocardiography revealed moderate biventricular systolic dysfunction and cardiac MRI confirmed active myocarditis fulfilling the 2018 Lake Louise criteria. In the absence of anti-SSA, anti-SSB, and ANA, the diagnosis of seronegative primary Sjögren's syndrome was established through minor salivary gland biopsy demonstrating stage III chronic sialadenitis and a positive Schirmer's test confirming objective ocular dryness, fulfilling the 2016 ACR/EULAR criteria with a total score of 4. Treatment with hydroxychloroquine, methylprednisolone, bisoprolol, and an ACE inhibitor resulted in complete myocardial recovery at 3 months, with normalization of ejection fraction and global longitudinal strain. This case highlights that in young patients with unexplained myocarditis, systematic autoimmune screening including minor salivary gland biopsy is the decisive diagnostic step when serology alone is unrevealing, and that timely diagnosis enables targeted treatment and full cardiac recovery.