二尖瓣脱垂被定义为收缩期瓣叶脱垂，二尖瓣的1或2个瓣叶可脱垂进入左心房。[1]Nishimura RA, Otto CM, Bonow RO, et al. 2014 AHA/ACC guideline for the management of patients with valvular heart disease: a report of the American College of Cardiology/American Heart Association task force on practice guidelines. Circulation. 2014 Jun 10;129(23):e521-643.http://circ.ahajournals.org/content/129/23/e521.longhttp://www.ncbi.nlm.nih.gov/pubmed/24589853?tool=bestpractice.com[2]Freed LA, Benjamin EJ, Levy D, et al. Mitral valve prolapse in the general population: the benign nature of echocardiographic features in the Framingham heart study. J Am Coll Cardiol. 2002 Oct 2;40(7):1298-304.https://www.sciencedirect.com/science/article/pii/S0735109702021617?via%3Dihubhttp://www.ncbi.nlm.nih.gov/pubmed/12383578?tool=bestpractice.com
阅读更多二尖瓣瓣叶在舒张期增厚5mm或更多反映了其黏液瘤样变性，这被认为是经典的二尖瓣脱垂，同时也是并发症的预兆。[3]Marks AR, Choong CY, Sanfilippo AJ, et al. Identification of high-risk and low-risk subgroups of patients with mitral-valve prolapse. N Engl J Med. 1989 Apr 20;320(16):1031-6.http://www.ncbi.nlm.nih.gov/pubmed/2927482?tool=bestpractice.com[4]Nishimura RA, McGoon MD, Shub C, et al. Echocardiographically documented mitral valve prolapse: long-term follow-up of 237 patients. N Engl J Med. 1985 Nov 21;313(21):1305-9.http://www.ncbi.nlm.nih.gov/pubmed/4058522?tool=bestpractice.com
相关的二尖瓣反流程度由无到重度。
二尖瓣瓣叶及瓣环是马鞍形的。[5]Levine RA, Triulzi MO, Harrigan P, et al. The relationship of mitral annular shape to the diagnosis of mitral valve prolapse. Circulation. 1987 Apr;75(4):756-67.http://www.ncbi.nlm.nih.gov/pubmed/3829339?tool=bestpractice.com[6]Levine RA, Stathogiannis E, Newell JB, et al. Reconsideration of echocardiographic standards for mitral valve prolapse: lack of association between leaflet displacement isolated to the apical four chamber view and independent echocardiographic evidence of abnormality. J Am Coll Cardiol. 1988 May;11(5):1010-9.https://www.sciencedirect.com/science/article/pii/S0735109798900596?via%3Dihubhttp://www.ncbi.nlm.nih.gov/pubmed/3281989?tool=bestpractice.com 二尖瓣脱垂的超声心动图学定义是在胸骨旁长轴位或心尖位的二维影像上，瓣叶在心脏收缩期脱垂超过瓣环水平2mm或以上。
[Figure caption and citation for the preceding image starts]: 胸骨旁长轴位上二尖瓣的超声心动图像描述了瓣叶后叶脱垂（红色箭头）来自 Brian Griffin, MD 的个人资料；经许可后使用 [Citation ends].
在诸如心尖四腔切面等其他超声心动图切面上，即使二尖瓣瓣叶在整个瓣膜水平以下，其仍可能表现为脱垂。 在既往的研究中，这种现象常导致二尖瓣脱垂的误诊。[7]Warth DC, King ME, Cohen JM, et al. Prevalence of mitral valve prolapse in normal children. J Am Coll Cardiol. 1985 May;5(5):1173-7.https://www.sciencedirect.com/science/article/pii/S0735109785800218?via%3Dihubhttp://www.ncbi.nlm.nih.gov/pubmed/3989128?tool=bestpractice.com[8]Savage DD, Garrison RJ, Devereux RB, et al. Mitral valve prolapse in the general population. 1. Epidemiologic features: the Framingham study. Am Heart J. 1983 Sep;106(3):571-6.http://www.ncbi.nlm.nih.gov/pubmed/6881031?tool=bestpractice.com
