Massive Left Atrial Thrombi during Dabigatran Therapy for Nonvalvular Atrial Fibrillation.

•Massive LA thrombus is rare in the absence of anticoagulation and mitral stenosis.•Direct-acting oral anticoagulants are widely used in nonvalvular AF.•Inadequate dosing of direct-acting anticoagulants may be difficult to recognize.•Consequences of inadequate dosing may be severe.