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OBJECTIVE To describe outcomes in patients with cor triatriatum sinister (CTS). DESIGN Retrospective review of patients with CTS followed at Mayo Clinic Rochester from 1990 to 2016. Clinical notes, operative reports, and baseline imaging studies were reviewed including echocardiogram, magnetic resonance imaging, computed tomography, and cardiac catheterization. RESULTS Fifty-seven patients (median age 34 years; men 32 (56%)) were enrolled. Definitive or suspected CTS diagnosis was made by transthoracic echocardiogram in 41 (72%) patients, and additional multimodality imaging was required in 39 (68%) patients. Of these 57 patients, initial diagnosis was made in adulthood in 35 (61%) patients, and 33 of 57 (58%) patients had additional congenital heart disease (CHD) diagnosis. A total of 27 (47%) patients required surgical resection of CTS membrane during median follow-up of 76 months, and these patients were younger at the time of CTS diagnosis (26 vs 41, P = 0.01) and more likely to have associated CHD (55% vs 45%, P = 0.02). There was one perioperative mortality and no late mortality. There was no recurrence of CTS membrane obstruction in the patients that underwent surgery. Similarly there was no significant increase in CTS membrane gradient in the patients that were managed conservatively. CONCLUSIONS The natural history of CTS is stability without progressive left atrial obstruction, especially in patients with isolated CTS and in those with initial CTS diagnosis made in adulthood. In patients requiring surgical membrane resection due to flow obstruction, surgery is safe and effective with very low risk of recurrence.