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Vulvodynia: A Common and Under-Recognized Pain Disorder in Women and Female Adolescents -- Integrating Current Knowledge into Clinical Practice

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The vulva is the external female genitalia. It includes the mons pubis, labia majora, labia minora, prepuce or clitoral hood, clitoris, and vulvar vestibule. The vulva is the center of a woman’s sexual response.  The anterior and posterior boundaries of the vulva extend from the mons pubis to the anus, respectively. Its lateral boundaries lie at the genito-crural folds. The mons pubis is comprised of a fat pad at the anterior of the vulva, which is covered in pubic hair.  The labia majora are derived embryologically from labioscrotal swellings. They fuse posteriorly and attach anteriorly to the mons pubis. The labia mi-nora, hairless folds of skin embryologically derived from urethral folds, lie within the labia majora. The labia minora fuse anteriorly, forming the prepuce (hood) of the clitoris, and extend posteriorly to either side of the vaginal opening. They fuse posteriorly at the vulvar vestibule, creating a fold of skin called the posterior fourchette. The mons pubis, perineum and labia are derived from the embryonic ectoderm. Vulvar skin is a keratinized, stratified, squamous epithelial structure that contains sebaceous glands and sweat glands. The keratin thickness of vulvar skin decreases progressively from the labia majora, over the clitoris to the labia minora. The vulvar vestibule extends from the frenulum of the clitoris anteriorly to the fourchette posteriorly. Hart’s line marks the juncture of nonkeratinized epithelium of the vestibule and the keratinized epithelium of the inner surface of the labia minora. The vulvar vestibule, derived from the urogenital sinus endoderm, contains the major vestibular glands (Bartholin’s and Skene’s) and the minor vestibular glands. The vestibular glands secrete mucous during sexual arousal and orgasm. The clitoris is located under the prepuce and is embryologically derived from the genital tubercle. It is formed of erectile corpora cavernosa tissue, which becomes engorged with blood during sexual stimulation (Farage, 2006; Krantz, 1977; Woodruff, 1985).

 

  

The vulva is innervated by the anterior labial branches of the (A) ilioinguinal nerve (L1); (B) genitofemoral nerve (L1-2); and (C, D) branches of the pudendal nerve (S2-4). Near the medial aspect of the ischial tuberosity, the pudendal nerve divides into 3 branches: (C) the dorsal nerve of the clitoris (shown deeper as dashed lines in muscles of the urogenital diaphragm), (D) the perineal nerve, which innervates the labia majora and perinuem, and (E) the inferior rectal nerve, which innervates the perianal area. The pudendal nerve also innervates the external anal sphincter and deep muscles of the urogenital triangle (Peng 2009). For a thorough review of the neurobiology of the urogenital tract, see Wesselmann, 1997.

 

  

The pelvic floor muscles are divided into 3 categories. The superficial pelvic floor muscles (bulbocavernosus or bulbospongiosus, ischiocavernosus, superficial transverse perineal/perineus muscle) are collectively known as the urogenital diaphragm. The function of the urogenital diaphragm muscles includes a role in sexual function, e.g., clitoral engorgement, vaginal closure, reflexive response to enhance sexual pleasure, and facilitating closure of the urethra and anus for continence. The middle layer is comprised of the deep transverse perineal muscle and sphincter urethra. The deep pelvic floor muscles, sometimes called the anal triangle, include the levator ani (pubococcygeus, iliococcygeus, and puborectalis), and coccygeus.  Other associated pelvic and hip muscles include the piriformis, obturator internus muscles, and gluteus maximus. The perineal body is the central tendon and attachment site for the superficial, middle, and deep pelvic floor muscles.

The internal pudendal artery, vein and nerve, which pass through Alcock’s canal, provide neurovascular function to the pelvic floor musculature.  Alcock’s canal is comprised of fascia or connective tissue from the obturator internus.  Although the pudendal nerve is known to innervate the levator ani muscles there have been more recent studies describing innervation by the levator ani nerve and direct nerve roots S3 and/or S4 (Barber 2002, Grigorescu 2008). Specifically, the pubococcygeus muscle is innervated by the levator ani nerve S3-5 and the perineal branch of the pudendal nerve S3-4. The iliococcygeus is innervated by the levator ani nerve S3-4 and the puborectalis is innervated by the inferior rectal branches of the pudendal nerve S2-S4.  The coccygeus is innervated by direct nerve roots S3-4. The function of the deep pelvic floor muscles includes: supporting the abdominal viscera or organs, providing pelvic and spinal stability, assisting in respiration, and providing sphincteric closure for bowel and bladder function. They also play a role in sexual function. (For a thorough review of the anatomy and physiology of the pelvic floor, see Herschorn 2004.)

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