Anatomy and function of closing muscle complex of the vagina in health and pelvic organ prolapse

Introduction. In modern literature, the pelvic floor muscles are presented by showing their anatomical location and indicating relevant attachment points, but their combined anatomical, topographic and functional significance is described insufficiently, which the current review was aimed at.

Aim: to determine the physiological significance and anatomical units of the vagina occlusor muscle complex, topography and functions in health and in pelvic organ prolapse (POP).

Materials and Methods. The search for English-published literary sources was conducted in the international publication databases PubMed/MEDLINE, Google Scholar, Cochrane Library, and Russian-language eLibrary resource by using keywords «pelvic floor», «genital fissure», «vagina», «sphincters», «pelvic organ prolapse», «urinary incontinence», «anal incontinence» with unlimited search depth. The inclusion eligibility criteria were as follows: systematic reviews, full-text original studies examining pelvic floor anatomy and physiology containing the results of pathological, clinical and instrumental methods devoted to the anatomy and physiology of the pelvic floor, as well as monographs and textbooks. Total 53 publications were included in the descriptive review.

Results. Unlike animals, as well as the closing apparatus of the urethra and rectum consisting of internal and external sphincters, the muscle complex that compresses the genital slit and the lower third of the vagina includes five muscles. The m. bulbospongiosus of the superficial layer of the pelvic floor and m. transversus perinei superficialis, located in the perineal body, allow to keep the genital fissure closed. The external urethral sphincter located in the middle layer covers the ventral surface of the urethra and ensheath the distal part of the vagina, forming m. sphincter urethrovaginalis, that contracts to narrow both the urethra and the vagina. The deep layer is presented by m. levator ani, the medial and anterior bundles of which – m. pubovaginalis, pass along the sides of the vagina, whereas the m. puborectalis located laterally bends around the rectum as a U-shaped muscular sling – both of them close the gap between the medial legs of m. levator ani, narrowing the lower third of the vagina. All of such muscles have a close anatomical connection with adjacent organs due to their common embryonic development, and determine the stability and functional activity of the entire pelvic organs complex. Altering integrity of the specified muscle complex (rupture, in-labour overstretching) or its structural degradation characterized by loss of tone and contraction force, results in failed closing genital fissure, increased distance between the medial parts of m. levator ani in the lower third of the vagina and the loss of supporting and closing functions of the pelvic floor as well as POP development. Moreover, dysfunction of the constrictor muscles anatomically associated with neighbouring organs due to their common embryonic development, contributes to emergence of urinary and anal incontinence, despite the integrity of relevant sphincters.

Conclusion. The muscle complex that compresses the genital fissure and the lower third of the vagina is characterized by the lack of the circularmuscle and consists of five muscles having a close anatomical connection with adjacent pelvic organs. Damage to this muscle complex results in combined anatomical, topographic and functional changes in the pelvic floor, manifested by descent/prolapse of the pelvic organs and urinary/anal incontinence.

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