Pudental Nerve

AJR:181, August 2003

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udendal nerve entrapment is arecognized cause of chronicperineal pain [1, 2], typically pre-

senting as pain in the penis,

scrotum, labia,perineum, or anorectal region. Pudendalnerve entrapment is a clinical diagnosismade in patients with the typical history ofperineal pain aggravated by sitting, relievedby standing, and absent when recumbent orsitting on a toilet seat. No widely acceptedconfirmatory test is available, although aneurophysiologic examination may confirmnerve damage.

The symptoms of pudendal nerve entrap-ment overlap considerably with those as-cribed to chronic nonbacterial prostatitis,which is the most common symptomatictype of prostatitis, or chronic pain syndrome.In the United States, chronic prostatitis is thereason for an estimated 7.8 million physicianvisits per year [3]; approximately 95% ofmen with chronic prostatitis do not have aninfection [4]. The cause of chronic nonbacte-rial prostatitis–chronic pelvic pain syn-drome remains unclear, but the syndrome has

never been scientifically shown to be prima-rily a disease of the prostate or the result ofan inflammatory process [5]. Chronic non-bacterial prostatitis causes substantial mor-bidity and affects the health of a patient tothe same degree as do conditions such asacute myocardial infarction, unstable angina,and acute ulcerative colitis [6]. Proper diag-nosis and treatment of pudendal nerve en-trapment with CT-guided pudendal nerveperineural injection offer some patients withchronic nonbacterial prostatitis–chronic pel-vic pain syndrome a chance of long-termpain relief.

Anatomy

The pudendal nerve enters the gluteal re-gion through the lower part of the greater sci-atic foramen (Figs. 1 and 2). The nerve isaccompanied by the internal pudendal arteryand is surrounded by a venous complex; to-gether this group of structures is referred to asthe neurovascular pudendal bundle. The pu-dendal bundle hooks around the sacrospinous

ligament near its attachment to the ischialspine; the pudendal bundle first enters theperineum through the lesser sciatic foramen(Figs. 2A and 2B) and courses through the is-chioanal fossa and then through the pudendal(Alcock’s) canal that is formed by the dupli-cation of the obturator fascia on the lateralwall of the ischioanal fossa. Either just beforeentering the pudendal canal or just within it,the pudendal bundle gives rise to the inferiorrectal (inferior anal) nerve, which crosses theischioanal fossa toward the anal canal and theexternal anal sphincter muscle. Within the pu-dendal canal, the pudendal nerve divides intotwo terminal branches, the perineal nerve andthe dorsal nerve of the penis or clitoris (Figs.2A and 2B).

Mechanism of Pudendal Nerve Entrapment

The pudendal nerve is predisposed to en-trapment at the level of the ischial spine andwithin the pudendal canal [1, 2]. At the ischialspine, the nerve can be compressed between

Chronic Perineal Pain Caused by Pudendal NerveEntrapment:

Anatomy and CT-Guided PerineuralInjection Technique

David M. Hough

1

, Keith H. Wittenberg

1,2

, Wojciech Pawlina

3

, Timothy P. Maus

1

, Bernard F. King

1

,Terri J. Vrtiska

1

,Michael A. Farrell

1

, Stanley J. Antolak, Jr.

4

Received October 9, 2002; accepted after revision December 31, 2002.

1

Department of Radiology, Mayo Clinic, 200 First St. S.W., Rochester, MN 55905. Address correspondence to D. M. Hough.

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.

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2003;181:561–567 0361–803X/03/1812–561 © American Roentgen Ray Society

Pictorial Essay

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Fig. 1.—Schematic anatomy of deep dissection of gluteal region. Most of gluteus maximus and mediusmuscles have been removed. Segment of sacrotuberous ligament also has been removed, revealingpudendal nerve. Pudendal nerve emerges from pelvis inferior relative to piriformis muscle and entersgluteal region medial relative to sciatic nerve, superficial relative to sacrospinous ligament, and deeprelative to sacrotuberous ligament. After coursing around sacrospinous ligament, pudendal nerve re-enters pelvis. (Courtesy of the Mayo Foundation)

Fig. 2.—Schematic anatomy of pudendal nerve. (Courtesy of the Mayo Foundation)A, Drawing illustrates pudendal nerve arising from sacral nerve roots S2–S4, exit-ing pelvis to enter gluteal region through lower part of greater sciatic foramen andreentering pelvis through lesser sciatic foramen. Pudendal nerve gives rise to in-ferior rectal nerve, perineal nerve, and dorsal nerve of penis or clitoris.B, Drawing shows pudendal nerve in pudendal (Alcock’s) canal. Inferior rectal nervearises from pudendal nerve before entering canal. Note location of falciform processof sacrotuberous ligament, which is possible site for pudendal nerve entrapment.

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CT-Guided Treatment of

Pudendal Nerve Entrapment


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Fig. 3.—Cadaver of 77-year-old man with diabetes mellitus. CT scanswere obtained with cadaver prone.A and B, Thin-slice CT scans obtained at level of ischial spine showsacrospinous (short arrows, A) and sacrotuberous (long arrows, A) liga-ments and calcified internal pudendal artery (arrowhead, A) marking lo-cation of pudendal bundle. In B, transgluteal needle is visible, andinjected contrast agent is seen filling interligamentous space.C–E, In thin-section CT scans obtained at level of pudendal canal, calci-fied internal pudendal artery (arrowhead, C) marks site of pudendal bun-dle in canal, and fat plane between neurovascular bundle and obturatorinternus muscle is clearly seen (arrow, C). Scan (D) obtained 2.5 mm cau-dal to C shows transgluteal needle in fat plane lateral relative to neu-rovascular bundle. Contained medially by obturator fascia, injectedcontrast agent fills pudendal canal, obliterating fat plane (E).

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Fig. 4.—Photograph of model shows view of bony pel-vis from below, with sacrotuberous ligament(arrowhead) and ischial tuberosity (asterisk) displayed.

Fig. 5.—Photographs of gross dissection of cadaveric pudendal canal.A, 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 for-ceps. Pudendal canal and pudendal bundle (arrow) are stained with methylene blue.B, Close-up of dissection shown in A with obturator internus fascia reflected, showing methylene blue–stainedpudendal bundle and embolization coil (arrow) that was placed in contact with neurovascular bundle under CTguidance.

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Fig. 6.—Images of frozen cadaveric left hemipelvis.A, Photograph of axial section acquired at level of ischial spine after CT-guided injection of fluorescein dye and insertion of embolization coil shows sacrospinous (shortarrow) and sacrotuberous (long arrow) ligaments and pudendal neurovascular bundle stained yellow in interligamentous space. At this level, sciatic nerve (asterisk) isclose to pudendal nerve.B, Radiograph of axial slice shown in A reveals embolization coil (arrow) adjacent to calcified pudendal artery in interligamentous space at level of ischial spine.

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the sacrotuberous and sacrospinous liga-ments. Sometimes, the nerve is ensheathedby ligamentous expansions that form aperineural compartment. At the pudendal ca-nal, the pudendal nerve can be compressed bythe falciform process of the sacrotuberousligament (Fig. 2B). If thickened, the duplica-tion of the obturator fascia also may act as anentrapment site [1].

CT-Guided Treatment of Pudendal Nerve Entrapment

At the Mayo Clinic, patients with symp-toms of pudendal nerve entrapment who stillhave persistent, significant pain after receiving6 weeks of conservative therapy (includingamitriptyline hydrochloride, antiinflamma-tory medication, and self-care) are referredfor pudendal nerve perineural injection. ACT-guided technique is used because it ismore accurate than fluoroscopically guided orfree-hand clinical techniques.

The needle tip is positioned adjacent to thepudendal nerve at the ischial spine in the inter-ligamentous space or at the pudendal canal. Along-acting local anesthetic (bupivacaine hy-drochloride) and a corticosteroid (methylpred-nisolone) are injected to provide immediatepudendal anesthesia. The injections may alsobring a long-term response because the antiin-flammatory effects of the steroid and steroid-induced fat necrosis can reduce inflammationin the region around the nerve and decreasepressure on the nerve itself. This treatmentmay be effective in 65–73% of patients [1, 7];however, to our knowledge, no prospectivestudies on this topic have been published. Sur-gical treatment with pudendal nerve neurolysisand fasciotomy of the pudendal canal also maybenefit patients with this condition [1, 8].

Anatomic Study

We undertook an anatomic study to ensurethat we had a correct understanding of the anat-omy of the pudendal nerve and to confirm thatwe could accurately localize the pudendal nervewith CT guidance. The unembalmed cadaver ofa 77-year-old man who had had diabetes melli-tus was scanned with CT (slice thickness, 2.5mm) in the prone position. With CT guidance,we advanced 18-gauge needles to the expectedlocation of the pudendal nerve at the ischialspine (Figs. 3A and 3B) and placed the needlesbilaterally at the pudendal canal (Fig. 4).

At the ischial spines, we placed an angio-graphic embolization coil and 1 mL of fluo-

rescein; we injected 0.25 mL of iopamidol(Isovue 300, Bracco Diagnostics, Princeton,NJ) on the left side at the ischial spine. At thepudendal canal, a mixture of methylene blue(0.1 mL), Isovue 300 (0.25 mL), and saline(1 mL) was injected on each side, and an em-bolization coil was inserted on the right(Figs. 3C–3E). The pelvis was then hemi-sected, and the right hemipelvis was dis-sected for identification of the pudendalnerve at the ischial spine. We then dissectedthe obturator fascia to identify the pudendalnerve in the pudendal canal (Figs. 4 and 5).

The left hemipelvis was frozen and cut into5-mm-thick slices that we examined and radio-graphed (Fig. 6). The tissues deep relative tothe obturator internus fascia in the expectedposition of the pudendal canal were stainedwith methylene blue. Dissection of the areadeep relative to the obturator fascia in the sliceof interest confirmed the presence of the pu-dendal nerve and internal pudendal vesselssurrounded by loose connective tissue. Weconcluded that CT would allow us to accu-rately identify and localize the pudendal nerveat the ischial spine and at the pudendal canal.

CT-Guided Pudendal Nerve Perineural Injection

Our protocol is to administer three sets ofinjections 4 weeks apart; the first two sets aregiven at the level of the ischial spine, and thethird is given at the pudendal canal [1]. Usu-ally the injections are bilateral; in some pa-tients with unilateral symptoms, only thesymptomatic side is injected.

For injections at the ischial spine (Figs. 7and 8), the patient is scanned in the prone po-

sition with 2.5- to 3-mm collimation from theacetabular roof to the pubic symphysis. Theischial spine, sacrospinous and sacrotuberousligaments, and pudendal bundle are identi-fied. Using the standard CT-guided needleplacement technique, we advance a 22-gaugespinal needle toward the pudendal bundle atthe caudal portion of the ischial spine on eachside. When each needle tip appears to be cor-rectly positioned in the interligamentous space(between the sacrospinous and sacrotuberousligaments and as close as possible to the cau-dal portion of the ischial spine), 0.75 mL ofdiluted contrast agent (5% solution of Isovue300) is injected, and the CT examination isrepeated. The contrast agent should be seensurrounding the pudendal bundle in the inter-ligamentous space and may track inferiorlyand anteriorly into the posterior portion of thepudendal canal. After repositioning and (ifnecessary) rescanning the patient, we inject amixture of methylprednisolone (1 mL of 40mg/mL solution) and bupivacaine (3 mL of0.25% solution) on each side.

For injections at the pudendal canal (Fig.9), CT scans are obtained through the levelof the pubic symphysis. The pudendal bun-dle is identified at the medial aspect of theobturator internus. We advance a 22-gaugeneedle obliquely via a transgluteal approach,aiming for needle placement in the small fatplane that usually can be seen immediatelylateral to the obturator fascia. We confirmthat the needle tip is in the correct positionby injecting 0.75 mL of diluted contrastagent and then inject the mixture of local an-esthetic and steroid.

We have found pudendal nerve perineuralinjection to be a safe procedure and have en-

Fig. 7.—Axial CT scan of 43-year-oldman obtained during pudendal nerveperineural injection at ischial spine.On right side of body, needle tip isshown correctly positioned in interlig-amentous space. On left side of body,contrast agent is seen in interliga-mentous space surrounding puden-dal neurovascular bundle (arrow).

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countered no serious complications. Minorcomplications have included bruising at theinjection site, transient worsening of painlasting a few days, and transient sciatic nerve

block lasting a few hours. The procedure iseasily performed in 20–30 min and has bothdiagnostic and therapeutic value. Some pa-tients report a dramatic resolution of symp-

toms immediately after the procedure. Amongour patients, 65% have had a distinct short-term response (unpublished data); long-termdata are not yet available.

Fig. 8.—Axial CT scans of 31-year-old man obtained during pudendal nerve perineural injection at ischial spine.A, Incorrectly positioned needle tip is seen adjacent to ischial spine.B, 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 neurovas-cular bundle (arrow) in interligamentous space.C and D, Needle was withdrawn by several millimeters and diluted contrast agent was again injected. Contrast agent is seen surrounding neurovascular bundle in interligamentousspace (C), while on contralateral side, correctly positioned needle is seen with small amount of contrast agent at tip (D).

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In conclusion, perineal pain resulting frompudendal nerve entrapment is a disablingcondition that can be treated with medica-tion, a self-care program, and CT-guided pu-dendal nerve perineural injection. Theprocedure is quick, safe, and easily per-formed, and it may offer considerable reliefof symptoms.

References

1. Robert R, Prat-Pradal D, Labat JJ, et al. Anatomicbasis of chronic perianal pain: role of the puden-dal nerve.

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4. Roberts RO, Lieber MM, Bostwick DG, JacobsenSJ. A review of clinical and pathological prostati-tis syndromes.

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ment Strategies, November 5–6, 1998, Washing-ton, DC. Available at: www.prostatitis.org/newdef.html. Accessed April 17, 2002

6. Wenninger K, Heiman JR, Rothman I, BerghuisJP, Berger RE. Sickness impact of chronic non-bacterial prostatitis and its correlates.

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ObstetGynecol

2000;95:306–3098. Mauillon J, Thoumas D, Leroi AM, Freger P, Mi-

chot F, Denis P. Results of pudendal nerve neurol-ysis-transposition in twelve patients sufferingfrom pudendal neuralgia.

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Fig. 9.—Axial CT scans of 31-year-old man obtained during pudendal nerve perineural in-jection at pudendal canal.A, Needle tip positioned in fat plane between obturator internus muscle and fascia on rightside; on left side, needle tip (arrow) is positioned slightly too medially.B, After injection of 0.75 mL of diluted contrast agent, needle tip is confirmed to be in goodposition in pudendal canal on right side. On left side, contrast agent is incorrectly locatedin ischioanal space (arrow).C, Scan obtained caudal to B shows contrast agent surrounding neurovascular bundle inright pudendal canal (arrow).

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Pudental Nerve