Mitral valve dysplasia characterized by isolated cleft of the anterior leaflet resulting in fixed left ventricular outflow tract obstruction.

A 7-month-old, sexually intact male English toy spaniel weighing 4 kg was referred for evaluation of a subclinical cardiac murmur. Echocardiography disclosed fixed left ventricular outflow tract obstruction that was caused by attachment of a cleft anterior mitral valve leaflet to the interventricular septum. Neither atrial nor ventricular septal defects were detected. Fixed obstruction of the left ventricular outflow tract by a malformed mitral valve is rare in human beings and has not been previously reported in the dog.