AJR 2003; 181:561-567
© American Roentgen Ray Society
Chronic Perineal Pain Caused by Pudendal Nerve Entrapment: Anatomy and CT-Guided Perineural Injection Technique
David M. Hough1,
Keith H. Wittenberg1,2,
Wojciech Pawlina3,
Timothy P. Maus1,
Bernard F. King1,
Terri J. Vrtiska1,
Michael A. Farrell1 and
Stanley J. Antolak, Jr.4
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.
Received October 9, 2002;
accepted after revision December 31, 2002.
Address correspondence to D. M. Hough.
Introduction
Pudendal nerve entrapment is a recognized cause of chronic perineal pain
[1,
2], typically presenting as
pain in the penis, scrotum, labia, perineum, or anorectal region. Pudendal
nerve entrapment is a clinical diagnosis made in patients with the typical
history of perineal pain aggravated by sitting, relieved by standing, and
absent when recumbent or sitting on a toilet seat. No widely accepted
confirmatory test is available, although a neurophysiologic examination may
confirm nerve damage.
The symptoms of pudendal nerve entrapment overlap considerably with those
ascribed to chronic nonbacterial prostatitis, which is the most common
symptomatic type of prostatitis, or chronic pain syndrome. In the United
States, chronic prostatitis is the reason for an estimated 7.8 million
physician visits per year [3];
approximately 95% of men with chronic prostatitis do not have an infection
[4]. The cause of chronic
nonbacterial prostatitis–chronic pelvic pain syndrome remains unclear,
but the syndrome has never been scientifically shown to be primarily a disease
of the prostate or the result of an inflammatory process
[5]. Chronic nonbacterial
prostatitis causes substantial morbidity and affects the health of a patient
to the same degree as do conditions such as acute myocardial infarction,
unstable angina, and acute ulcerative colitis
[6]. Proper diagnosis and
treatment of pudendal nerve entrapment with CT-guided pudendal nerve
perineural injection offer some patients with chronic nonbacterial
prostatitis–chronic pelvic pain syndrome a chance of long-term pain
relief.
Anatomy
The pudendal nerve enters the gluteal region through the lower part of the
greater sciatic foramen (Figs.
1 and
2A,
2B). The nerve is accompanied
by the internal pudendal artery and is surrounded by a venous complex;
together this group of structures is referred to as the neurovascular pudendal
bundle. The pudendal bundle hooks around the sacrospinous ligament near its
attachment to the ischial spine; the pudendal bundle first enters the perineum
through the lesser sciatic foramen (Figs.
2A and
2B) and courses through the
ischioanal fossa and then through the pudendal (Alcock's) canal that is formed
by the duplication of the obturator fascia on the lateral wall of the
ischioanal fossa. Either just before entering the pudendal canal or just
within it, the pudendal bundle gives rise to the inferior rectal (inferior
anal) nerve, which crosses the ischioanal fossa toward the anal canal and the
external anal sphincter muscle. Within the pudendal canal, the pudendal nerve
divides into two terminal branches, the perineal nerve and the dorsal nerve of
the penis or clitoris (Figs. 2A
and 2B).
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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)
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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.
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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.
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Mechanism of Pudendal Nerve Entrapment
The pudendal nerve is predisposed to entrapment at the level of the ischial
spine and within the pudendal canal
[1,
2]. At the ischial spine, the
nerve can be compressed between the sacrotuberous and sacrospinous ligaments.
Sometimes, the nerve is ensheathed by ligamentous expansions that form a
perineural compartment. At the pudendal canal, the pudendal nerve can be
compressed by the falciform process of the sacrotuberous ligament
(Fig. 2B). If thickened, the
duplication of the obturator fascia also may act as an entrapment site
[1].
CT-Guided Treatment of Pudendal Nerve Entrapment
At the Mayo Clinic, patients with symptoms of pudendal nerve entrapment who
still have persistent, significant pain after receiving 6 weeks of
conservative therapy (including amitriptyline hydrochloride, antiinflammatory
medication, and self-care) are referred for pudendal nerve perineural
injection. A CT-guided technique is used because it is more accurate than
fluoroscopically guided or free-hand clinical techniques.
The needle tip is positioned adjacent to the pudendal nerve at the ischial
spine in the interligamentous space or at the pudendal canal. A long-acting
local anesthetic (bupivacaine hydrochloride) and a corticosteroid
(methylprednisolone) are injected to provide immediate pudendal anesthesia.
The injections may also bring a long-term response because the
antiinflammatory effects of the steroid and steroid-induced fat necrosis can
reduce inflammation in the region around the nerve and decrease pressure on
the nerve itself. This treatment may be effective in 65–73% of patients
[1,
7]; however, to our knowledge,
no prospective studies on this topic have been published. Surgical treatment
with pudendal nerve neurolysis and fasciotomy of the pudendal canal also may
benefit patients with this condition
[1,
8].
Anatomic Study
We undertook an anatomic study to ensure that we had a correct
understanding of the anatomy of the pudendal nerve and to confirm that we
could accurately localize the pudendal nerve with CT guidance. The unembalmed
cadaver of a 77-year-old man who had had diabetes mellitus was scanned with CT
(slice thickness, 2.5 mm) in the prone position. With CT guidance, we advanced
18-gauge needles to the expected location of the pudendal nerve at the ischial
spine (Figs. 3A and
3B) and placed the needles
bilaterally at the pudendal canal (Fig.
4).
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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.
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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.
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At the ischial spines, we placed an angiographic embolization coil and 1 mL
of fluorescein; we injected 0.25 mL of iopamidol (Isovue 300, Bracco
Diagnostics, Princeton, NJ) on the left side at the ischial spine. At the
pudendal 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 embolization coil was
inserted on the right (Figs.
3C,
3D,
3E). The pelvis was then
hemisected, and the right hemipelvis was dissected for identification of the
pudendal nerve at the ischial spine. We then dissected the obturator fascia to
identify the pudendal nerve in the pudendal canal (Figs.
4 and
5A,
5B).
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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).
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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).
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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).
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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.
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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.
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The left hemipelvis was frozen and cut into 5-mm-thick slices that we
examined and radiographed (Fig.
6A,
6B). The tissues deep relative
to the obturator internus fascia in the expected position of the pudendal
canal were stained with methylene blue. Dissection of the area deep relative
to the obturator fascia in the slice of interest confirmed the presence of the
pudendal nerve and internal pudendal vessels surrounded by loose connective
tissue. We concluded that CT would allow us to accurately identify and
localize the pudendal nerve at the ischial spine and at the pudendal
canal.
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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.
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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.
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CT-Guided Pudendal Nerve Perineural Injection
Our protocol is to administer three sets of injections 4 weeks apart; the
first two sets are given at the level of the ischial spine, and the third is
given at the pudendal canal
[1]. Usually the injections are
bilateral; in some patients with unilateral symptoms, only the symptomatic
side is injected.
For injections at the ischial spine (Figs.
7 and
8A,
8B,
8C,
8D), the patient is scanned in
the prone position with 2.5- to 3-mm collimation from the acetabular roof to
the pubic symphysis. The ischial spine, sacrospinous and sacrotuberous
ligaments, and pudendal bundle are identified. Using the standard CT-guided
needle placement technique, we advance a 22-gauge spinal needle toward the
pudendal bundle at the caudal portion of the ischial spine on each side. When
each needle tip appears to be correctly positioned in the interligamentous
space (between the sacrospinous and sacrotuberous ligaments and as close as
possible to the caudal portion of the ischial spine), 0.75 mL of diluted
contrast agent (5% solution of Isovue 300) is injected, and the CT examination
is repeated. The contrast agent should be seen surrounding the pudendal bundle
in the interligamentous space and may track inferiorly and anteriorly into the
posterior portion of the pudendal canal. After repositioning and (if
necessary) rescanning the patient, we inject a mixture of methylprednisolone
(1 mL of 40 mg/mL solution) and bupivacaine (3 mL of 0.25% solution) on each
side.
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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).
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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.
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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).
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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).
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For injections at the pudendal canal (Fig.
9A,
9B,
9C), CT scans are obtained
through the level of the pubic symphysis. The pudendal bundle is identified at
the medial aspect of the obturator internus. We advance a 22-gauge needle
obliquely via a transgluteal approach, aiming for needle placement in the
small fat plane that usually can be seen immediately lateral to the obturator
fascia. We confirm that the needle tip is in the correct position by injecting
0.75 mL of diluted contrast agent and then inject the mixture of local
anesthetic and steroid.
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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.
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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).
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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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We have found pudendal nerve perineural injection to be a safe procedure
and have encountered no serious complications. Minor complications have
included bruising at the injection site, transient worsening of pain lasting a
few days, and transient sciatic nerve block lasting a few hours. The procedure
is easily performed in 20–30 min and has both diagnostic and therapeutic
value. Some patients report a dramatic resolution of symptoms immediately
after the procedure. Among our patients, 65% have had a distinct short-term
response (unpublished data); long-term data are not yet available.
In conclusion, perineal pain resulting from pudendal nerve entrapment is a
disabling condition that can be treated with medication, a self-care program,
and CT-guided pudendal nerve perineural injection. The procedure is quick,
safe, and easily performed, and it may offer considerable relief of
symptoms.
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