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Chronic Perineal Pain Caused by Pudendal Nerve Entrapment: Anatomy and CT-Guided Perineural Injection Technique

¹ Department of Radiology, Mayo Clinic, 200 First St. S.W., Rochester, MN 55905.
² Present address: Department of Radiology, United Hospital, 333 Smith Ave., St. Paul, MN 55102.
³ Department of Anatomy, Mayo Clinic, Rochester, MN 55905.
⁴ Department of Urology, Mayo Clinic, Rochester, MN 55905.

View larger version (114K): [in a new window]   Fig. 1. —Schematic anatomy of deep dissection of gluteal region. Most of gluteus maximus and medius muscles have been removed. Segment of sacrotuberous ligament also has been removed, revealing pudendal nerve. Pudendal nerve emerges from pelvis inferior relative to piriformis muscle and enters gluteal region medial relative to sciatic nerve, superficial relative to sacrospinous ligament, and deep relative to sacrotuberous ligament. After coursing around sacrospinous ligament, pudendal nerve reenters pelvis. (Courtesy of the Mayo Foundation)

View larger version (115K): [in a new window]   Fig. 2A. —Schematic anatomy of pudendal nerve. (Courtesy of the Mayo Foundation) Drawing illustrates pudendal nerve arising from sacral nerve roots S2–S4, exiting pelvis to enter gluteal region through lower part of greater sciatic foramen and reentering pelvis through lesser sciatic foramen. Pudendal nerve gives rise to inferior rectal nerve, perineal nerve, and dorsal nerve of penis or clitoris.

View larger version (101K): [in a new window]   Fig. 2B. —Schematic anatomy of pudendal nerve. (Courtesy of the Mayo Foundation) Drawing shows pudendal nerve in pudendal (Alcock's) canal. Inferior rectal nerve arises from pudendal nerve before entering canal. Note location of falciform process of sacrotuberous ligament, which is possible site for pudendal nerve entrapment.

View larger version (158K): [in a new window]   Fig. 3A. —Cadaver of 77-year-old man with diabetes mellitus. CT scans were obtained with cadaver prone. Thin-slice CT scans obtained at level of ischial spine show sacrospinous (short arrows, A) and sacrotuberous (long arrows, A) ligaments and calcified internal pudendal artery (arrowhead, A) marking location of pudendal bundle. In B, transgluteal needle is visible, and injected contrast agent is seen filling interligamentous space.

View larger version (161K): [in a new window]   Fig. 3B. —Cadaver of 77-year-old man with diabetes mellitus. CT scans were obtained with cadaver prone. Thin-slice CT scans obtained at level of ischial spine show sacrospinous (short arrows, A) and sacrotuberous (long arrows, A) ligaments and calcified internal pudendal artery (arrowhead, A) marking location of pudendal bundle. In B, transgluteal needle is visible, and injected contrast agent is seen filling interligamentous space.

View larger version (98K): [in a new window]   Fig. 4. —Photograph of model shows view of bony pelvis from below, with sacrotuberous ligament (arrowhead) and ischial tuberosity (asterisk) displayed.

View larger version (135K): [in a new window]   Fig. 3C. —Cadaver of 77-year-old man with diabetes mellitus. CT scans were obtained with cadaver prone. In thin-section CT scans obtained at level of pudendal canal, calcified internal pudendal artery (arrowhead, C) marks site of pudendal bundle in canal, and fat plane between neurovascular bundle and obturator internus muscle is clearly seen (arrow, C). Scan (D) obtained 2.5 mm caudal to C shows transgluteal needle in fat plane lateral relative to neurovascular bundle. Contained medially by obturator fascia, injected contrast agent fills pudendal canal, obliterating fat plane (E).

View larger version (155K): [in a new window]   Fig. 3D. —Cadaver of 77-year-old man with diabetes mellitus. CT scans were obtained with cadaver prone. In thin-section CT scans obtained at level of pudendal canal, calcified internal pudendal artery (arrowhead, C) marks site of pudendal bundle in canal, and fat plane between neurovascular bundle and obturator internus muscle is clearly seen (arrow, C). Scan (D) obtained 2.5 mm caudal to C shows transgluteal needle in fat plane lateral relative to neurovascular bundle. Contained medially by obturator fascia, injected contrast agent fills pudendal canal, obliterating fat plane (E).

View larger version (154K): [in a new window]   Fig. 3E. —Cadaver of 77-year-old man with diabetes mellitus. CT scans were obtained with cadaver prone. In thin-section CT scans obtained at level of pudendal canal, calcified internal pudendal artery (arrowhead, C) marks site of pudendal bundle in canal, and fat plane between neurovascular bundle and obturator internus muscle is clearly seen (arrow, C). Scan (D) obtained 2.5 mm caudal to C shows transgluteal needle in fat plane lateral relative to neurovascular bundle. Contained medially by obturator fascia, injected contrast agent fills pudendal canal, obliterating fat plane (E).

View larger version (122K): [in a new window]   Fig. 5A. —Photographs of gross dissection of cadaveric pudendal canal. Photograph of dissection of cadaveric pudendal canal acquired from below (same viewpoint as in Figure 4) shows ischial tuberosity (asterisk) and sacrotuberous ligament (arrowhead). Obturator fascia is lifted by forceps. Pudendal canal and pudendal bundle (arrow) are stained with methylene blue.

View larger version (99K): [in a new window]   Fig. 5B. —Photographs of gross dissection of cadaveric pudendal canal. Close-up of dissection shown in A with obturator internus fascia reflected, showing methylene blue–stained pudendal bundle and embolization coil (arrow) that was placed in contact with neurovascular bundle under CT guidance.

View larger version (149K): [in a new window]   Fig. 6A. —Images of frozen cadaveric left hemipelvis. Photograph of axial section acquired at level of ischial spine after CT-guided injection of fluorescein dye and insertion of embolization coil shows sacrospinous (short arrow) and sacrotuberous (long arrow) ligaments and pudendal neurovascular bundle stained yellow in interligamentous space. At this level, sciatic nerve (asterisk) is close to pudendal nerve.

View larger version (146K): [in a new window]   Fig. 6B. —Images of frozen cadaveric left hemipelvis. Radiograph of axial slice shown in A reveals embolization coil (arrow) adjacent to calcified pudendal artery in interligamentous space at level of ischial spine.

View larger version (164K): [in a new window]   Fig. 7. —Axial CT scan of 43-year-old man obtained during pudendal nerve perineural injection at ischial spine. On right side of body, needle tip is shown correctly positioned in interligamentous space. On left side of body, contrast agent is seen in interligamentous space surrounding pudendal neurovascular bundle (arrow).

View larger version (169K): [in a new window]   Fig. 8A. —Axial CT scans of 31-year-old man obtained during pudendal nerve perineural injection at ischial spine. Incorrectly positioned needle tip is seen adjacent to ischial spine.

View larger version (162K): [in a new window]   Fig. 8B. —Axial CT scans of 31-year-old man obtained during pudendal nerve perineural injection at ischial spine. After 0.75 mL of diluted contrast agent has been injected, contrast agent is visible deep relative to sacrospinous ligament. No contrast agent is seen around neurovascular bundle (arrow) in interligamentous space.

View larger version (167K): [in a new window]   Fig. 8C. —Axial CT scans of 31-year-old man obtained during pudendal nerve perineural injection at ischial spine. Needle was withdrawn by several millimeters and diluted contrast agent was again injected. Contrast agent is seen surrounding neurovascular bundle in interligamentous space (C), while on contralateral side, correctly positioned needle is seen with small amount of contrast agent at tip (D).

View larger version (157K): [in a new window]   Fig. 8D. —Axial CT scans of 31-year-old man obtained during pudendal nerve perineural injection at ischial spine. Needle was withdrawn by several millimeters and diluted contrast agent was again injected. Contrast agent is seen surrounding neurovascular bundle in interligamentous space (C), while on contralateral side, correctly positioned needle is seen with small amount of contrast agent at tip (D).

View larger version (137K): [in a new window]   Fig. 9A. —Axial CT scans of 31-year-old man obtained during pudendal nerve perineural injection at pudendal canal. Needle tip positioned in fat plane between obturator internus muscle and fascia on right side; on left side, needle tip (arrow) is positioned slightly too medially.

View larger version (142K): [in a new window]   Fig. 9B. —Axial CT scans of 31-year-old man obtained during pudendal nerve perineural injection at pudendal canal. After injection of 0.75 mL of diluted contrast agent, needle tip is confirmed to be in good position in pudendal canal on right side. On left side, contrast agent is incorrectly located in ischioanal space (arrow).

View larger version (149K): [in a new window]   Fig. 9C. —Axial CT scans of 31-year-old man obtained during pudendal nerve perineural injection at pudendal canal. Scan obtained caudal to B shows contrast agent surrounding neurovascular bundle in right pudendal canal (arrow).

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