Tweetable abstract:
A line passing through the pubic symphysis to the third sacral vertebra mostly conforms to the normal upper two-thirds of the vaginal axis.
Introduction
Pelvic organ prolapse (POP) is a common, distressing condition1. An objective and accurate evaluation of POP is important for diagnosis and treatment, but physical examinations remain limited to anatomical assessments2. MRI with high soft-tissue resolution provides a multiplanar depiction of the pelvic anatomy and has been considered a promising complementary diagnostic tool3-8, but its validation is unsatisfactory9-10. As POP is clinically defined as the descent of the anterior, apex, or posterior vaginal walls11, an in situ reference line for evaluating the vagina is critical but lacking 9-10.
An assessment of the vagina in situ requires an understanding of its supporting structures, which have been divided into three levels4, 12. The upper third of the vagina is typically considered to be suspended by the cardinal and uterosacral ligaments12, 13. However, the role of the cardinal ligament is controversial but was proven to be a mesentery structure consisting of vessels and nerves14,15. The main ligament-like structure of the upper third seems to be the uterosacral ligament. In the middle third of the vagina, the anterior surface of the vagina and its lateral attachment to the arcus tendineus fasciae of the pelvis form the pubocervical fascia 12. In the lower third of the vagina, the pubocervical fascia continues anteriorly, merges with the urethra and then attaches to the pubic symphysis. The attachments in the lower third are highly dense such that the lower portion seldom suffers from POP12. In summary, the upper two-thirds of the vagina are more commonly clinically and anatomically affected by POP, and to our understanding, the structure seems to be mainly supported by the uterosacral ligament, which extends from the second to the fourth sacral vertebra region to the dorsal margin of the uterine cervix and/or the upper third of the posterior vaginal wall13, and by the pubocervical fascia, which originates from the anterior surface of the vagina and anteriorly ends at the pubic symphysis12. Therefore, we speculate that a line passing through the inferior aspect of the pubic symphysis to the third sacral vertebra might be the closest proxy for the normal upper two-thirds of the vaginal axis (Fig. 1A). Therefore, this article reviews the vaginal anatomic structures of women without POP and analyses the relationship between this line and the vagina on sagittal MR images.