Pudendal nerve

The pudendal nerve is the main nerve of the perineum. It is a mixed (motor and sensory) nerve and also conveys sympathetic autonomic fibers. It carries sensation from the external genitalia of both sexes and the skin around the anus and perineum, as well as the motor supply to various pelvic muscles, including the male or female external urethral sphincter and the external anal sphincter.

If damaged, most commonly by childbirth, loss of sensation or fecal incontinence may result. The nerve may be temporarily anesthetized, called pudendal anesthesia or pudendal block.

The pudendal canal that carries the pudendal nerve is also known by the eponymous term "Alcock's canal", after Benjamin Alcock, an Irish anatomist who documented the canal in 1836.

Structure

Origin

thumbnail|Image showing the [[greater sciatic foramen (large foramen), and the lesser sciatic foramen, separated by the sacrospinous ligament. The pudendal nerve exits the pelvis through the greater sciatic foramen, passes over the ligament, and then reenters the pelvis through the lesser sciatic foramen.]]

The pudendal nerve is paired, meaning there are two nerves, one on the left and one on the right side of the body. Each is formed as three roots immediately converge above the upper border of the sacrotuberous ligament and the coccygeus muscle.

Course and relations

The pudendal nerve passes between the piriformis muscle and coccygeus (ischiococcygeus) muscles and leaves the pelvis through the lower part of the greater sciatic foramen. with fibers originating in Onuf's nucleus in the sacral region of the spinal cord. Consequently, damage to the sciatic nerve can affect the pudendal nerve as well. Sometimes dorsal rami of the first sacral nerve contribute fibers to the pudendal nerve, and even more rarely .

Function

The pudendal nerve has both motor (control of muscles) and sensory functions. It also carries sympathetic autonomic fibers (but not parasympathetic fibers).

Sensory

The pudendal nerve supplies sensation to the penis in males, and to the clitoris in females, which travels through the branches of both the dorsal nerve of the penis and the dorsal nerve of the clitoris. Branches also supply sensation to the anal canal.

Motor

Branches innervate muscles of the perineum and the pelvic floor; namely, the bulbospongiosus and the ischiocavernosus muscles respectively (including the Iliococcygeus, pubococcygeus, puborectalis and either pubovaginalis in females or puboprostaticus in males) the external anal sphincter (via the inferior anal branch), and male or female external urethral sphincter.

As it functions to innervate the external urethral sphincter it is responsible for the tone of the sphincter mediated via acetylcholine release. This means that during periods of increased acetylcholine release the skeletal muscle in the external urethral sphincter contracts, causing urinary retention. Whereas in periods of decreased acetylcholine release the skeletal muscle in the external urethral sphincter relaxes, allowing voiding of the bladder to occur. (Unlike the internal sphincter muscle, the external sphincter is made of skeletal muscle, therefore it is under voluntary control of the somatic nervous system.)

It is also responsible for ejaculation.

Clinical significance

The pudendal nerve may be tested by elicitation of the anocutaneous reflex ("anal wink").

Anesthesia

thumb|Approximate area of "saddle anesthesia" seen from behind (yellow highlight)|419x419px

A pudendal nerve block, also known as a saddle nerve block, is a local anesthesia technique used in an obstetric procedure to anesthetize the perineum during labor. In this procedure, an anesthetic agent such as lidocaine is injected through the inner wall of the vagina about the pudendal nerve. Abnormal loss of sensation in the same region as a medical symptom is also sometimes termed saddle anesthesia.

Damage

The pudendal nerve can be compressed or stretched, resulting in temporary or permanent neuropathy. Injury to the pudendal nerve manifests more as sensory problems (pain or alteration/loss of sensation) rather than loss of muscle control. and is sometimes associated with professional cycling.

Systemic diseases such as diabetes and multiple sclerosis can damage the pudendal nerve via demyelination or other mechanisms.

External beam radiotherapy (RT), commonly employed to treat prostate cancer, has been linked with late (≥ 2 years) post-RT gastrointestinal issues and pudendal nerve injury. Those treated present a 5-fold higher incidence rate of deficient anorectal motor and sensory function, anal sphincter morphology (thickness losses in both internal and external anal sphincters), and worse quality-of-life bowel symptoms. RT doses > 60 Gy to rectal and anal structures and the pudendal nerve have an increased risk of pudendal nerve dysfunction.

Unilateral pudendal nerve neuropathy inconsistently causes fecal incontinence in some, but not others. This is because crossover innervation of the external anal sphincter occurs in some individuals.

Imaging

thumb|Illustration of imaging from a CT-guided injection of the pudendal nerve at the pudendal canal.

The pudendal nerve is difficult to visualize on routine CT or MR imaging, however under CT guidance, a needle may be placed adjacent to the pudendal neurovascular bundle. The ischial spine, an easily identifiable structure on CT, is used as the level of injection. A spinal needle is advanced via the gluteal muscles and advanced within several millimeters of the ischial spine. Contrast (X-ray dye) is then injected, highlighting the nerve in the canal and allowing for confirmation of correct needle placement. The nerve may then be injected with cortisone and local anesthetic to confirm and also treat chronic pain of the external genitalia (known as vulvodynia in females), pelvic and anorectal pain.

Nerve latency testing

The time taken for a muscle supplied by the pudendal nerve to contract in response to an electrical stimulus applied to the sensory and motor fibers can be quantified. Increased conduction time (terminal motor latency) signifies damage to the nerve. 2 stimulating electrodes and 2 measuring electrodes are mounted on the examiner's gloved finger ("St Mark's electrode"). The pudendal canal is also known by the eponymous term "Alcock's canal", after Benjamin Alcock, an Irish anatomist who documented the canal in 1836. Alcock documented the existence of the canal and pudendal nerve in a contribution about iliac arteries in Robert Bentley Todd's "The Cyclopaedia of Anatomy and Physiology".

Additional images

File:Gray829.png|The male pelvis, showing the pudendal nerve (centre right)

File:Grant 1962 214.png|Schematic showing the structures innervated by the pudendal nerve

File:Grant 1962 215.png|Diagram of the course of the pudendal nerve in the male pelvis

</gallery>

References

External links

- "Inferior view of female perineum, branches of the internal pudendal artery."
Diagnosis and treatment at www.nervemed.com
www.pudendal.com
Pudendal nerve entrapment at chronicprostatitis.com
CT sequence showing a pudendal nerve block.