A hypothesis to explain the occurrence of inner myometrial laceration causing massive postpartum hemorrhage

Background. Inner myometrial lacerations were found in three patients who d eveloped uncontrollable postpartum massive bleeding despite the usual treat ment for uterine atony. Because all the patients suffered from hemorrhage s hock and their medical status deteriorated, their uteri were surgically rem oved to stop bleeding. After removal, one of them died. Postpartum hemorrha ge was caused by inner myometrial laceration. We hypothesized a cause of in ner myometrial laceration, using the three resected uteri, an assumed model of the uterine body, and 34 women. Methods. The subjects were 37 women, of whom three were patients with inner myometrial laceration, 23 were women without inner myometrial laceration w ho underwent cesarean section, and 11 were women in the first stage of labo r. The three resected uteri were examined bath macroscopically and microsco pically. We measured the thickness of the wall of the uterine muscle at the widest point of the uterine corpus and the thickness of the myometrial wal l at a transverse section of the uterine cervix, as well as the radius of t he inner lumen at the widest point of the uterus in 23 women during cesarea n section. We also measured the thickness of the myometrial wall at the wid est point of the uterine corpus in 11 women at the end of the first stage o f labor during ultrasonic examination. The data were then used to estimate the stress on the uterine muscle. Results. The stress on the uterine cervix was stronger than that on the ute rine corpus during labor. When the stress on the uterine muscle is stronger than a specific value, inner myometrial lacerations develop on the right a nd/or left side of the uterine cervix. These lacerations may involve large vessels. Conclusions. We have discovered another cause of postpartum hemorrhage whic h we have named inner myometrial laceration. These lacerations appeared to result from a strong stress on the uterine cervix caused by an abnormal ris e in intrauterine pressure during labor.