Botulinum Toxin A Injection of the Obturator Internus Muscle Abstract: Chronic perineal pain is often a difficult condition to manage. Current treatments include
pudendal nerve injections and pudendal nerve release surgery. The obturator internus muscle has a
close relationship to the pudendal nerve and might be a potential target for therapeutic intervention.
Perspective: A case is presented of refractory perineal pain that was successfully treated by injecting
the obturator internus muscle with botulinum toxin A.

2005 by the American Pain Society
Key words: Chronic pain, perineal pain, obturator internus, botulinum toxin.
Thecauseofchronicperinealpainisoftendifficult Thepudendalnerveenterstheglutealre- to identify and the syndrome difficult to gion through the lower part of the greater sciatic fo- Musculoskeletal dysfunction might contribute to ramen below the piriformis muscle. It hooks around the signs and symptoms of chronic pelvic Al- the sacrospinous ligament near its attachment to the though the obturator internus muscle has been identi- ischial spine; the pudendal nerve reenters the pelvis through the lesser sciatic foramen and through the written about this muscle as a possible target for thera- pudendal (Alcock’s) canal that is formed by the obtu- peutic intervention. The fascia of the obturator internus rator fascia on the lateral wall of the ischioanal fossa muscle contributes to the formation of the pudendal Within the pudendal canal, the pudendal nerve canal and when thickened might become a possible en- divides into 2 terminal branches, the perineal nerve trapment Injections into the muscle, adjacent to and the dorsal nerve of the penis or clitoris.
the canal, might have beneficial effects because of ac- The pudendal canal is a fascial tunnel on the medial tions on both the muscle and the nerve in terms of de- aspect of the obturator internus muscle. The obturator creasing compression and modulation of neurotransmit- internus muscle originates within the pelvis on the obtu- Botulinum toxin has been used successfully in the rator membrane, a membrane that closes all but the su- perior border of the obturator foramen, and on those ported indicating that injection of the obturator inter- portions of the pubis and ischium that surround the ob- nus muscle with botulinum toxin might be a therapeutic turator foramen. It is a lateral rotator of the thigh. Al- option for patients with chronic perineal pain. This case though broad in origin, the muscle tapers to a narrow report received institutional review board approval, and tendon that passes through the lesser sciatic foramen the subject of the case report gave signed consent for and rides over the ischial body (over a bursa) just superior to the ischial tuberosity to attach on the medial aspect ofthe greater trochanter of the femur.
Anatomy of the Pudendal Nerve and
Obturator Internus Muscle

Case Report
Arising from the sacral plexus (S2, 3, 4), the pudendal A 64-year-old woman complained of right-sided pelvic nerve is a sensory and motor nerve to the peri- pain affecting the upper leg, vagina, and rectum. Thepain was rated at a level of 4 to 8 out of 10 on a visualanalogue scale and had been present for 4 years. The Received June 28, 2004; Revised January 3, 2005; Accepted January 3,2005.
pain was aggravated by sitting. Previous diagnoses made From the Center for Pain Management, Baylor University Medical Center, included lumbosacral radiculopathy, post-laminectomy syndrome, pudendal nerve entrapment, sacroiliac joint Address reprint requests to Noor M. Gajraj, MD, Baylor Center for PainManagement, 5575 Warren Parkway, POB1, Suite 220, Frisco, TX 75034.
dysfunction, and piriformis syndrome. The patient had received numerous medications and injection therapies in the past including pudendal nerve injections with local 2005 by the American Pain Societydoi:10.1016/j.jpain.2005.01.353 anesthetic and steroid, sacroiliac joint injections, and The Journal of Pain, Vol 6, No 5 (May), 2005: pp 333-337 the second injection 100 units of botulinum A in 3 mL0.25% bupivacaine was injected on a separate occasion.
Pudendal nerve entrapment is a recognized cause of chronic perineal typically presentingas pain in the penis, scrotum, labia, perineum, or anorec-tal region. Pudendal nerve pathology might also causeurinary incontinence, anal incontinence, and dysorgas-mia. Pudendal nerve entrapment is a clinical diagnosismade in patients with the typical history of perineal painaggravated by sitting, relieved by standing, and absentwhen recumbent or sitting on a toilet seat. No widelyaccepted confirmatory test is available, although a neu- Figure 1. Anatomy of the obturator internus muscle and pu-
rophysiologic examination might confirm nerve dam- The pudendal nerve is predisposed to entrapment at the level of the ischial spine and within the pudendalAt the ischial spine, the nerve can be com- piriformis muscle injections. Other previous therapies in- pressed between the sacrotuberous and sacrospinous cluded physical therapy and water aerobics. The patient ligaments. At the pudendal canal, the pudendal nerve also had a pudendal nerve release procedure in France 2 can be compressed by the falciform process of the sa- years previously, but with only partial relief. Medications crotuberous ligament. The fascia on the obturator in- being taken at the time of the office visit included meth- ternus contributes to the formation of the pudendal adone 20 mg twice a day, zonegran 200 mg twice a day, canal. If thickened, the obturator fascia also might act Zoloft 50 mg once daily, valium 5 mg at night, and zanaflex 12 mg at night. Physical examination showed a nerve entrapment include local anesthetic and steroid distressed woman, uncomfortable while sitting, but there were no focal neurologic signs. A vaginal examina- tion was not performed at that time but had been per- toxin have been reported in patients with cervical dys- formed by a previous physician and showed tenderness tonia and spastic disorders.More patients re- at the right anterior and right posterior lateral regions.
ported improvement in pain than dystonia, raising the The patient underwent a right obturator internus mus- possibility that pain relief might not be solely the re- cle injection with bupivacaine 0.25%, which gave 90% relief for 12 hours. The average preprocedural and post- toxin has been used successfully in pain management procedural visual analogue scale scores were 7 of 10 and 1 of 10, respectively. After a subsequent botulinun toxin A injection, the patient again reported 90% relief fromher pain for more than 3 months. Pain assessments weremade before the procedure and then at 1-hour intervalsafter the procedure by using a 0 to 10 numeric ratingscale. There were no adverse effects such as motor weak-ness or disturbance of bowel or bladder function.
Fluoroscopically Guided Obturator
Internus Muscle Injection Technique

The patient was placed in the prone position. Standard monitors were applied consisting of electrocardiogram,pulse oximetry, and an automated blood pressure de-vice. Sedation was achieved by using increments of intra-venous midazolam and fentanyl. Fluoroscopy was usedto visualize the obturator foramen. The skin and subcu-taneous tissues were anesthetized by using 1% lido-caine. With an aseptic technique and transgluteal ap-proach, a 22-gauge spinal needle was advanced to thelateral border of the obturator foramen, inferior to theischial spine Correct needle placement was con-firmed by injecting 1 mL of iohexol. For the diagnostic Figure 2. Fluoroscopic view of right obturator internus muscle
injection 5 mL of 0.25% bupivacaine was injected. For and bruising at the injection site, a flu-like syn- mouth, dysphagia, and formation of neutralizing an- num toxin not only inhibits acetylcholine release but also the release of pain neurotransmitters such as glu- tamate, substance P, calcitonin gene-related peptide, it can be very useful in refractory Botulinum toxin is usually reconstituted by mixing the contents of ter release might prevent local sensitization of noci- a vial with normal saline, although it is also common ceptors and thus reduce the perception of pain. Botu- practice to use local anesthetic as a In the linum toxin might also alter impulse transmission case reported, the pain relief obtained by the botuli- within the dorsal horn pathways responsible for hy- num toxin injection might have resulted in part from the bupivacaine, although this effect would be ex- action include changes in the sensitivity and response pected to last only hours rather than months. Injection patterns of muscle nociceptors, diminished activity in of botulinum toxin into the obturator internus muscle the gamma-motor neurons with consequent changes in muscle spindle afferents, alterations in cholinergic In conclusion, obtutator internus muscle injections control of vascular and autonomic functions, and di- might be a therapeutic option for patients with rect noncholinergic effects on pain The du- chronic perineal pain whose other treatments have ration of action of botulinum toxin correlates with failed. The precise mechanism of action and the rela- neural sprouting and reinnervation of the muscle, tive actions on the muscle and pudendal nerve remain which restores function in 1 to 4 mAdverse to be elucidated. Evidence for efficacy from a single events from botulinum toxin therapy are not generally serious and might include localized pain, tenderness, randomized controlled studies are required.
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