Objective
We sought to characterize the distribution of the pudendal nerve branches and to correlate findings with injury risk related to common midurethral sling procedures.
Study Design
Dissections were performed in 18 female cadavers. Biopsies were obtained to confirm gross findings by histology.
Results
In all dissections, most of the clitoral and perineal nerves coursed caudal to the ventral portion of the perineal membrane. The inferior rectal nerve did not enter the pudendal canal in 44% (n = 8) of specimens. Nerve tissue was confirmed histologically in tissue sampled.
Conclusion
The clitoral and perineal branches of the pudendal nerve should be at low risk of direct nerve injury during midurethral slings and similar procedures as they course caudal to the ventral portion of the perineal membrane. The inferior rectal nerve might be at risk of injury during procedures that involve passage of needles through the ischioanal fossa.
The pudendal nerve may be susceptible to direct and indirect injury by a variety of mechanisms that may lead to immediate sensory symptoms or delayed motor deficits. The nerve is relatively “fixed” as it courses behind the sacrospinous ligament and within the pudendal canal; as such, it may be at risk of injury during downward displacement of the pelvic floor and perineal structures during childbirth. Vaginal birth has been associated with pudendal nerve injury by mode of compression and/or nerve stretch. While pelvic and/or perineal trauma may result in direct injury to the nerve or its branches, iatrogenic pudendal nerve injury is commonly attributed to pelvic reconstructive procedures and also to pudendal nerve injections. Sacrospinous ligament fixation procedures can result in entrapment or laceration of the nerve or nerve irritation from hematoma formation. The dissection involved in vaginal reconstructive procedures, such as anterior colporrhaphies and anal sphincteroplasties, has been associated with subclinical denervation injuries, as demonstrated by prolonged terminal motor latency studies. The clinical significance of such injuries is unclear but they may be implicated in delayed urethral and/or anal sphincter dysfunction that manifests as urinary or fecal incontinence later in life.
Pudendal nerve sensory deficits may manifest as neuropathic pain symptoms including hyperalgesia, allodynia, and paresthesias in the dermatomal distribution of the injured nerve. Recently, pain along pudendal nerve distribution has been described in women who underwent midurethral sling procedures and vaginal mesh reconstructive procedures. In our practice, anecdotal cases of clitoral pain following retropubic midurethral sling placement have been noted, prompting the question of whether this procedure could result in direct pudendal nerve injury. Previous reports of pain associated with retropubic midurethral sling needle placement have evaluated the suprapubic region, where injury to the ilioinguinal nerve may be possible as the needle exits the skin. The main objective of this study is to describe the distribution and course of the pudendal nerve branches relative to fixed anatomic structures such as the perineal membrane and ischiopubic rami. A secondary objective is to provide an assessment of injury risk during antiincontinence midurethral sling procedures.
Materials and Methods
Detailed dissections were performed on 14 unembalmed and 4 embalmed female cadavers obtained from the Willed Body Program at the University of Texas Southwestern Medical Center in Dallas. This study was exempt from review by the institutional review board in accordance with the Code of Federal Regulations, Title 45. Age, race, height, weight, and cause of death of the body donors were recorded.
Pudendal nerve dissections were completed bilaterally through a perineal approach in 18 cadaver specimens. Seven of the 18 specimens were also dissected through the transabdominal and gluteal approaches to confirm the origin and initial course of the nerve.
From the transabdominal approach, the sacral nerve roots that contributed to the formation of the pudendal nerve were noted and the relationship of the nerve to the ischial spine and to the pudendal vessels was recorded. The position of the nerve relative to the internal pudendal vessels was noted at the point where the neurovascular bundle exited the pelvis. Any additional nerve branches arising from the sacral plexus and exiting the pelvis along with the pudendal neurovascular bundle were noted and their course followed.
Dissections from the gluteal approach were performed as previously described to establish continuity of the pudendal nerve from its pelvic origin. The sacrotuberous ligament was transected at its sacral attachment and deflected laterally. The pudendal neurovascular bundle was identified underneath the sacrotuberous ligament as it coursed behind the sacrospinous ligament. Continuity of the pudendal nerve from its pelvic location was established by following the nerve to its pelvic origin. The pudendal neurovascular bundle was followed distally toward its entry point into the pudendal canal; the branching characteristics of the nerve before and after it entered the canal were recorded.
The perineal dissection was performed with cadavers in the supine position. The legs were transected at midthigh, and the thighs were positioned in standard lithotomy. The skin overlying the anterior (urogenital) and posterior (anal) triangles of the perineum was removed with a scalpel. The subcutaneous tissue was sharply excised until the superficial perineal and anal sphincter muscles were identified. The fatty tissue in the ischioanal fossa was bluntly and sharply removed with care not to transect pudendal nerve or vessel branches. The fatty tissue was removed superiorly and medially until the inferior surface of the pelvic floor muscles was identified and laterally until the obturator muscle and covering fascia was identified. Pudendal branches that terminated in the external anal sphincter and perianal skin were followed proximally. The course of these branches relative to the pudendal canal was noted. The point at which these branches were first noted to emerge into the ischioanal fossa was noted and their relative proximity to the ischial spine or to the ischial tuberosity was also recorded. The pudendal neurovascular bundle was identified as it exited the pudendal canal. Continuity of the pudendal nerve branches in the perineum from the pudendal nerve as it originated in the pelvis was established in those specimens where dissections were performed through all 3 approaches. In those specimens where only a perineal approach was employed, branches believed to be of pudendal nerve origin were followed to their destination. In these cases, full-thickness biopsies from selected branches were taken for histological confirmation of neural tissue. The biopsies were placed in formaldehyde solution and processed in paraffin blocks. All histologic sections were reviewed by one of the authors (K.S.C). Photographs of the dissections were taken for documentation.
The superficial perineal muscles (superficial transverse, ischiocavernosus, and bulbocavernosus) and the perineal membrane were identified in the anterior perineal triangle. The relationship of the nerves to these muscles and to the perineal membrane was noted. Special attention was given to the relationship (cephalad or caudal) of the perineal nerve and the clitoral nerve to the perineal membrane.
In 6 unembalmed cadavers, a midurethral retropubic and/or an out-to-in transobturator needle was passed in the usual fashion to establish the relationship of the needle path with the perineal membrane.
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