INNERVATION OF EXTERNAL GENITALIA AND PERINEUM
The musculature and integument of the perineum are innervated mainly by the pudendal nerve. Derived from the anterior rami of the second, third, and fourth sacral nerves, it leaves the pelvis through the greater sciatic foramen, between the piriformis and coccygeus muscles, and crosses beneath the ischial spine on the mesial side of the internal pudendal artery. It then continues within Alcock canal in the obturator fascia on the lateral wall of the ischiorectal fossa, toward the ischial tuberosity. The pudendal nerve divides into three branches: (1) The inferior hemorrhoidal nerve pierces the medial wall of Alcock canal, traverses the ischiorectal fossa, and supplies the external anal sphincter and perianal skin. (2) The perineal nerve runs for a short distance in Alcock canal and divides into a deep and a superficial branch. The deep branch sends filaments to the external anal sphincter and levator ani muscles and then pierces the base of the urogenital diaphragm to supply the superficial and deep perineal muscles, the ischiocavernosus and bulbocavernosus muscles, and the membranous urethral sphincter. The superficial branch divides into medial and lateral posterior labial nerves, which innervate the labium majus. (3) The dorsal nerve of the clitoris passes through the urogenital diaphragm to the glans of the clitoris.
A number of nerves innervate the perineal skin. The anterior labial
branches of the ilioinguinal nerve (L1) emerge from the external inguinal ring
to be distributed to the mons veneris and the upper portion of the labium
majus. (Extreme flexion of the leg during childbirth or vaginal operative
procedures can result in temporary or permanent loss of function of this nerve.)
The external spermatic branch of the genitofemoral nerve (L1, 2) accompanies
the round ligament through the inguinal canal and sends twigs to the labium.
The perineal branches of the posterior femoral cutaneous nerve (S1, 2, 3) run
forward and medial in front of the ischial tuberosity to the lateral margin of
the perineum and labium majus. Branches of the perineal nerve (S2, 3, 4)
include the dorsal nerve of the clitoris and the medial and lateral posterior
labial branches to the labium majus. The inferior hemorrhoidal branch of the
pudendal nerve (S2, 3, 4) contributes to the supply of the perianal skin and
accounts for the sensory portion of the “anal wink” reflex. The perforating
cutaneous branches of the second and third sacral nerves perforate the sacrotuberous
ligament and turn around the inferior border of the gluteus maximus to supply
the buttocks and contiguous perineum. The anococcygeal nerves (S4, 5, and
coccygeal nerve) unite along the coccyx and then pierce the sacrotuberous
ligaments to supply the anococcygeal area.
The course and distribution of the pudendal nerve make it an ideal
candidate for safe and effective regional nerve
blockade. A pudendal nerve block can be accomplished through either a
transcutaneous or transvaginal approach, though the former has generally fallen
out of favor. In the transcutaneous approach, intradermal wheals are made
bilaterally, midway between the rectum and the ischial tuberosities. With the
middle and index fingers of the left hand in the vagina, a 10-cm needle is
guided to a point just under and beyond the ischial spine, where 10 to 15 mL of
a 0.5% to 1.0% local anesthetic solution is deposited.
This blocks the internal pudendal nerve, as it passes dorsal to the spine just
before entering Alcock canal.
In the transvaginal approach to a pudendal nerve block, the needle is
placed within a needle guide and directed to the ischial spine by traversing
the lateral vaginal wall. This approach is often faster and better tolerated than a transcutaneous route.
