Tuesday, August 12, 2008

Lumbosacral Plexus

The lumbosacral plexus and its rami are well illustrated in Chapter 1 , Surgical Anatomy of the Retroperitoneum, Kidneys, and Ureters; only the pelvic courses of its nerves are reviewed here (see Figs. 2-6 and 2-12 [6] [12] and Table 2-2 ). The iliohypogastric nerve (L1) travels between, and supplies, the internal oblique and the transversus muscles and pierces the internal and external oblique muscles 3 cm above the external inguinal ring to supply sensation over the lower anterior abdomen and pubis (see Fig. 2-4 ). The ilioinguinal nerve (L1) passes through the internal oblique muscle to enter the inguinal canal laterally. It travels anterior to the cord and exits the external ring to provide sensation to the mons pubis and anterior scrotum or labia majora (see Figs. 2-4 and 2-6 [4] [6]). The genitofemoral nerve (L1, L2) pierces the psoas muscle to reach its anterior surface in the retroperitoneum and then travels to the pelvis and splits into genital and femoral branches. The latter supplies sensation over the anterior thigh below the inguinal ligament. The genital branch follows the cord through the inguinal canal, supplies the cremaster muscle, and supplies sensation to the anterior scrotum.

Table 2-2 -- Somatic Nerves of the Lower Abdomen and Pelvis
Nerve Name Origin Supplies
Iliohypogastric L1 Motor supply to internal oblique, transversus muscles, sensation over lower anterior abdominal wall
Ilioinguinal L1 Sensation over anterior pubis (mons) and anterior scrotum or labia
Genitofemoral L1, L2 Genital branch: motor supply to cremaster muscle, sensation to anterior scrotum; femoral branch: sensation to anterior thigh
Femoral L2, L3, L4 Motor supply to extensors of the knee, sensation to anterior thigh
Obturator L2, L3, L4 Motor supply to adductors of the thigh, sensation to medial thigh
Lumbosacral trunk L4, L5 Joins the sacral nerves to form the lumbosacral plexus that supplies motor and sensory innervation to the lower extremities
Posterior femoral cutaneous S2, S3 Sensation to perineum, posterior scrotum, and posterior thigh
Pudendal S2, S3, S4 Motor to levator ani, muscles of the urogenital diaphragm, anal and striated urethral sphincter, sensation to the perineum, scrotum, and penis
Pelvic somatic efferents S2, S3, S4 Motor supply to levator ani and striated urethral sphincter
Nervi erigentes S2, S3, S4 Parasympathetic fibers from the sacral cord supply the pelvic viscera

For most of its pelvic course, the femoral nerve (L2, L3, L4) travels within the substance of the psoas muscle and then exits its lateral side to pass under the inguinal ligament ( Fig. 2-19 ). It supplies sensation to the anterior thigh and motor innervation to the extensors of the knee. During a psoas hitch, sutures should be placed in the direction of the nerve (and the psoas muscle fibers) to avoid nerve damage or entrapment. Retractor blades must not rest on the psoas muscle because they can produce a femoral nerve palsy, a potentially dangerous setback after pelvic surgery. The lateral femoral cutaneous nerve (L2, L3) may be seen lateral to the psoas in the iliacus fascia.

Figure 2-19 Femoral nerve as it relates to the psoas muscle. Retractor blades may compress this nerve to produce a femoral nerve palsy. (From Burnett AL, Brendler CB: Femoral neuropathy following major pelvic surgery: Etiology and prevention. J Urol 1994;151:163-165.)

The obturator nerve (L2, L3, L4) emerges in the true pelvis from beneath the psoas muscle, lateral to the internal iliac vessels, and passes through the obturator fossa to the obturator canal. In the fossa, it is lateral and superior to the obturator vessels and surrounded by the obturator and internal iliac lymph nodes. Damage to this nerve during pelvic lymphadenectomy weakens the adductors of the thigh.

The lumbosacral trunk (L4, L5) passes into the true pelvis behind the psoas and unites with the ventral rami of the sacral segmental nerves to form the sacral plexus. This plexus lies on the pelvic surface of the piriformis deep to the endopelvic fascia and posterior to the internal iliac vessels (see Fig. 2-15 ). It leaves the pelvis through the greater sciatic foramen immediately posterior to the sacrospinous ligament (where it may be injured during sacrospinous culposuspension) and supplies motor and sensory innervation to the posterior thigh and lower leg. An exaggerated lithotomy position may stretch this nerve or place pressure on its peroneal branch at the fibular head to produce footdrop. Pelvic and perineal branches of the sacral plexus include (1) the posterior femoral cutaneous nerve (S2, S3), which, after passing through the greater sciatic foramen, gives an anterior sensory branch to the perineum and posterior scrotum (see Fig. 2-8 ); (2) the pudendal nerve (S2, S3, S4), which follows the internal pudendal artery to the perineum (to be discussed); (3) the nervi erigentes (S2, S3, S4) to the autonomic plexus; and (4) pelvic somatic efferent nerves from the ventral rami of S2, S3, and S4 ( Fig. 2-20 ). These last nerves travel on the pelvic surface of the levator ani in close association with the rectum and prostate and are separated from the pelvic autonomic plexus by the endopelvic fascia. They supply the levator ani and extend anteriorly to the striated urethral sphincter ( Lawson, 1974 ; Zvara et al, 1994 ).

Figure 2-20 Pelvic floor somatic efferent nerves extending anteriorly on the pelvic surface of the levator ani to supply this muscle and the striated urethral sphincter. (From Lawson JON: Pelvic anatomy: Pelvic floor muscles. Ann R Coll Surg Engl 1974;54:244-252.)

Pelvic Autonomic Plexus

The presynaptic sympathetic cell bodies that project to the pelvic autonomic plexus reside in the lateral column of gray matter in the last three thoracic and first two lumbar segments of the spinal cord. They reach the pelvic plexus by two pathways: (1) The superior hypogastric plexus is formed by sympathetic fibers from the celiac plexus and the first four lumbar splanchnic nerves ( Fig. 2-21 ). Anterior to the bifurcation of the aorta, it divides into two hypogastric nerves that enter the pelvis medial to the internal iliac vessels, anterior to the sacrum, and deep to the endopelvic fascia. (2) The pelvic continuations of the sympathetic trunks pass deep to the common iliac vessels and medial to the sacral foramina and fuse in front of the coccyx at the ganglion impar (see Fig. 2-21 ). Each chain comprises four to five ganglia that send branches anterolaterally to participate in the formation of the pelvic plexus.

Figure 2-21 Sympathetic and parasympathetic contributions to the pelvic autonomic nervous plexus. (From Drake RL, Vogl W, Mitchell AWM: Gray's Anatomy for Students. Philadelphia, Elsevier, 2005.)

Presynaptic parasympathetic innervation arises from the intermediolateral cell column of the sacral cord. Fibers emerge from the second, third, and fourth sacral spinal nerves as the pelvic splanchnic nerves (nervi erigentes) to join the hypogastric nerves and branches from the sacral sympathetic ganglia to form the inferior hypogastric (pelvic) plexus (see Fig. 2-21 ). Some pelvic parasympathetic efferent fibers travel up the hypogastric nerves to the inferior mesenteric plexus, where they provide parasympathetic innervation to the descending and sigmoid colon.

The pelvic plexus is rectangular and is 4 to 5 cm long, and its midpoint is at the tips of the seminal vesicles ( Schlegel and Walsh, 1987 ). It is oriented in the sagittal plane on either side of the rectum and pierced by the numerous vessels going to and from the rectum, bladder, seminal vesicles, and prostate ( Fig. 2-22 ). Division of these vessels (the so-called lateral pedicles of the bladder and prostate) risks injury to the pelvic plexus with attendant postoperative impotence ( Walsh and Donker, 1982 ; Walsh et al, 1983 ). The right and left components of the pelvic plexus communicate behind the rectum and anterior and posterior to the vesical neck. Branches of the pelvic plexus follow pelvic blood vessels to reach the pelvic viscera, although nerves to the ureter may join it directly as it passes nearby. Visceral afferent and efferent nerves travel on the vas deferens to reach the testis and epididymis (see later discussion).

Figure 2-22 Lateral view showing the left pelvic autonomic nervous plexus and its relation to the pelvic viscera. Bl, bladder; Ur, ureter. (From Schlegel PN, Walsh PC: Neuroanatomical approach to radical cystoprostatectomy with preservation of sexual function. J Urol 1987;138:1402-1406.)

The most caudal portion of the pelvic plexus gives rise to the innervation of the prostate and the important cavernosal nerves ( Walsh and Donker, 1982 ). After passing the tips of the seminal vesicles, these nerves lie within leaves of the lateral endopelvic fascia near its juncture with, but outside, Denonvilliers' fascia ( Lepor et al, 1985 ). They travel at the posterolateral border of the prostate on the surface of the rectum and are lateral to the prostatic capsular arteries and veins (see Fig. 2-22 ). Because the nerves are composed of multiple fibers not visible on gross inspection, these vessels serve as a surgical landmark for the course of these nerves (the neurovascular bundle of Walsh). During radical prostatectomy, the nerves are most vulnerable at the apex of the prostate, where they closely approach the prostatic capsule at the 5- and 7-o'clock positions. On reaching the membranous urethra, the nerves divide into superficial branches, which travel on the lateral surface of the striated urethral sphincter at 3- and 9-o'clock positions, and deep fibers, which penetrate the substance of this muscle and send twigs to the bulbourethral glands. As the nerves reach the hilum of the penis, they join to form one to three discrete bundles, related to the urethra at 1- and 11-o'clock positions, superficial to the cavernous veins, and dorsomedial to the cavernous arteries (see Fig. 2-41 ) ( Lue et al, 1984 ; Breza et al, 1989 ). With the arteries, they pierce the corpora cavernosa to supply the erectile tissue (see later discussion). Small fibers also join the dorsal nerves of the penis as they course distally. In the female, the nerves to vestibular bodies and corpora cavernosa of the clitoris travel between the anterior vaginal wall and the bladder in association with the lateral venous plexuses.

Figure 2-41 Dorsal penile arteries, veins, and nerves. (From Hinman F Jr: Atlas of Urosurgical Anatomy. Philadelphia, WB Saunders, 1993, p 445.)

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