the clinical constellation of findings with or without symptoms due to prolapse of the mitral valve: a nonejection systolic click accentuated in the standing posture, sometimes multiple, sometimes with mitral regurgitation occurring relatively late in systole, and accompanied by echocardiographic evidence of the mitral valve prolapse, usually with thickened leaflets of the valve. Symptoms are nonspecific and may include vague chest pains and dyspnea on exertion. SYN: billowing mitral valve syndrome.