Mechanisms of abnormal uterine bleeding.

Over the past 10 years there has been an upsurge of interest in the mechanisms underlying normal and disturbed menstrual bleeding. These studies have particularly focused on the mechanisms underlying the common problems of menorrhagia associated with ovulatory and anovulatory dysfunctional uterine bleeding (DUB) and of unpredictable breakthrough bleeding during hormonal contraceptive use. A wide range of abnormalities of endometrial morphology and function have been demonstrated, but it is still not clear how all the pieces of this complex jigsaw puzzle fit together. Ovulatory DUB is predominantly associated with decreased endometrial vasoconstriction and vascular haemostatic plug formation, leading to defective control of the volume of blood which is lost during menstruation. By contrast, breakthrough bleeding is associated with a wide range of molecular disturbances which appear to result in unpredictable vessel breakdown through disturbed endometrial angiogenesis, increased vascular fragility and loss of the integrity of the endothelial, epithelial and stromal supporting structures. Anovulatory DUB is very poorly understood, but may be associated with disturbed angiogenesis, fragile vessels and defective haemostatic processes. Little is known about the actual mechanisms of the common problem of abnormal bleeding associated with specific genital tract pathologies such as uterine myomata.