Physical Examination Findings in Pelvic Sciatic Entrapment Syndromes

Sciatic entrapments at the sciatic notch often affect all five toes (multiple dermatomes) rather than just the lateral toes (S1 radiculopathy) or medial toes (L5 radiculopathy), which is most commonly seen in those with herniated lumbar disks. The pain of sciatic entrapment more commonly extends primarily only as far as the knee, ankle, or heel—not reaching the toes at all. Straight leg raising is generally negative, but resisted abduction or adduction of the flexed internally rotated thigh usually reproduces the symptoms. Sciatic notch tenderness is present in all patients with piriformis syndromes (Fig. 238-2), so other variant types of pelvic sciatic entrapment should be considered when this examination feature is not present. Trochanteric bursitis responsive to injection of the bursa occurred in 7% of patients diagnosed with muscle-based piriformis syndrome.

FIGURE 238-2 Physical examination performed during magnetic resonance imaging distinguishes piriformis muscle from sacrotuberous ligament tear as pain source (axial T1-weighted acquisition in open magnetic resonance imaging). (*), Examiner’s finger; FH, femoral head; Pir T, piriformis tendon; Sa, sacrum; STL, sacrotuberous ligament.

Patients with entrapments at the level of the ischial tuberosity (ischial tunnel syndrome) have tenderness to palpation at the lateral surface of the ischial tuberosity, which is about 4 inches below the level of the sciatic notch.

Neurography Results for Sciatica of Nondisk Origin

Until recently, objective tests for the existence of piriformis syndrome were very limited, there was no reliable effective treatment, and the pathophysiology was not well understood.¹ Piriformis syndrome remains a subject of significant debate within the neurosurgical community.^2,3 However, magnetic resonance neurography has proved helpful in providing objective diagnostic criteria.⁴ Figure 238-3 demonstrates the appearance of the sciatic nerve in a magnetic resonance neurography image in a typical pelvic sciatic entrapment case. Ipsilateral piriformis muscle hypertrophy is a common image finding in piriformis syndrome.^5,6 Ipsilateral muscle atrophy occurs in some patients as well. Edema or hyperintensity in the ipsilateral sciatic nerve relative to the contralateral nerve occurs in 88%.⁴

FIGURE 238-3 Typical findings of irritative abnormalities of sciatic nerve in piriformis syndrome. A, Sciatic nerve bowed over medial surface of piriformis muscle. IM, ischial margin. B, Increase in nerve image intensity as nerve passes between piriformis tendon and the ischial margin. FH, femoral head. C to F, Image intensity increase persists as nerve descends through ischial tunnel. IS, ischial spine. G to I, Progressive normalization of nerve image intensity becoming isointense with surrounding muscle as it descends into the upper thigh. IT, ischial tuberosity. Arrows indicate sciatic nerve (“nerve perpendicular” oblique view, magnetic resonance neurographic acquisition sequence).

In addition to entrapment at the sciatic notch, the sciatic nerve may be compressed by fibrous bands and by dilated venous varices when the dilated vein arises inside the perineurial sheath. It may suffer entrapment owing to fixation by an artery passing through the nerve and by tendon or muscle passing through the nerve (Fig. 238-4). In some individuals, the sciatic nerve does not contact the tendon of the obturator internus, but in others, it may be entrapped by that tendon (Fig. 238-5). Entrapment also occurs in the lower ischial tunnel adjacent to the hamstring attachments or at the quadratus femoris muscle.

FIGURE 238-4 Division of sciatic nerve by piriformis muscle segments. A1 to A6, T1-weighted image sequence showing major portion of piriformis muscle passing between peroneal and tibial components of the proximal sciatic nerve (“nerve perpendicular” oblique view—T1 sequence). B and C, Separated peroneal and tibial components in patient (B1 and B2, “nerve perpendicular” oblique view; C, modified coronal view, neurographic sequence). FH, femoral head; GM, gluteus maximus; il, ilium; IM, ischial margin; IS, ischial spine; Pe, peroneal component of sciatic nerve; Ti, tibial component of sciatic nerve; Pir-1 and Pir-2, the two components of the piriformis muscle; Sa, sacrum.

FIGURE 238-5 Variable relation of sciatic nerve to obturator internus tendon (magnetic resonance neurographic acquisition sequence). A and B, Sciatic nerve impinged by obturator internus tendon (see B3 and B4). (A, coronal; B, “nerve perpendicular” oblique axial). C, Sciatic nerve descending without contacting the obturator internus tendon (“nerve perpendicular” oblique axial). Note that in C, the ischial spine is directed more posteriorly, and the greater trochanter (GT) is more anterior. This results in a more anteriorly angled course of the obturator internus tendon. FH, femoral head; IS, ischial spine; IT, ischial tuberosity; OI, obturator internus muscle; OIT, obturator internus tendon; Sci, sciatic nerve.

Open Magnetic Resonance Imaging–Guided Injections for Piriformis Syndrome

Open MRI-guided injections can be carried out in a low field (0.25 Tesla) imager that has a small diameter and in-room monitor. Injection is done using a 22-gauge, 15-cm titanium Lufkin needle. The injection should be about 10 mL of 0.5% bupivacaine and 1 mL of betamethasone (Celestone) (6 mg/ml) into the piriformis muscle.

Because of the large volume of bupivacaine, procedures should be carried out in a surgicenter setting. The injection is monitored with fast (10 to 15 seconds) three-slice image sets—a working image and the two adjacent images slices. The needle advance must be maintained in the center slice of the three-slice set so that an accurate depiction of needle depth is seen. Because the piriformis muscle is often no more than 1 to 2 cm thick (Fig. 238-6), and because the bowel is often adjacent just deep to the muscle, it is necessary to reimage once or twice with each movement of the needle. Lower-accuracy alternatives to open MRI include x-ray fluoroscopy with the needle directed toward the space below the sacroiliac joint, computed tomography (CT) guidance, or electromyography with ultrasound.⁷

FIGURE 238-6 Open magnetic resonance–guided injection of very thin piriformis muscle adjacent to underlying bowel (axial T1-weighted acquisition in open magnetic resonance imaging). C, colon; HR, hip replacement artifact in ischium; L, Lufkin titanium needle; Pir, piriformis muscle; Sci, sciatic nerve.

Patients obtaining complete or nearly complete and specific relief of symptoms and who fail to respond to spinal injections with similar agents can be considered to have confirmed muscle-based piriformis syndrome. If the symptoms recur within 1 week, the patients can be referred for piriformis surgery. If the symptoms recur after 2 weeks, additional injections can be carried out. Botulinum toxin injections (100 units in 6 mL preservative-free saline) can help some patients achieve longer-lasting relief from injection.

Minimal Access Surgery for Pelvic Entrapments of the Sciatic Nerve

Surgery is carried out generally on an outpatient basis using a 3-cm incision for a minimally invasive transgluteal approach with piriformis muscle resection, neuroplasty of the sciatic nerve and of the posterior femoral cutaneous nerve,⁸ and then placement of Seprafilm (Genzyme, Cambridge, MA) as an adhesiolytic agent. A similar approach is used for sciatic entrapments at the level of the ischial tuberosity.

For piriformis surgery, placement of the 3-cm incision is based on locating the superior medial edge of the greater trochanter of the femur with an posteroanterior hip radiograph. The patient is positioned prone on bolsters so that the knee falls below the level of the hip; this provides a relative elevation of the greater trochanter in the surgical site aiding access to the piriformis tendon.

After opening the gluteal fascia with bipolar cautery and Metz scissors, blunt finger dissection through the leaves of gluteal musculature minimizes exposure trauma and helps ensure outpatient management. Exposure is maintained with a Shadow-Line retractor system (V. Mueller)—this is an anterior cervical-type retractor that has a blade-retractor connection providing good rigidity under strong tension, but allowing for rapid replacement of blades as the depth of surgery progresses. A set of blades up to 100 mm in length (custom-made by V. Mueller) is sufficient for nearly all patients.

Safety for the sciatic nerve is ensured by carefully progressing through the muscle layers until the hard, clear prepiriformis fascia is reached with dense yellow fat behind it. The retractor blades are then reset, and the fascia is opened carefully with bipolar cautery and Metz scissors. An electrodiagnostic system with electromyographic monitoring of multiple superior gluteal, inferior gluteal, tibial nerve and peroneal nerve innervated muscles set at 0.5 to 10 mA is used to identify nerves before their exposure in the piriformis fat pad.

The sciatic nerve is partially mobilized and used, together with the greater trochanter and the sciatic notch, to identify and confirm the borders of the piriformis muscle. The femoral neck varies from nearly horizontal to nearly vertical, and the height of the greater trochanter is variable as well. In a patient with a horizontal femoral neck and large greater trochanter, the piriformis muscle can be nearly perpendicular to the sciatic nerve. By contrast, in a patient with a vertical femoral neck and short greater trochanter, the piriformis muscle is nearly parallel with the sciatic nerve.

Many patients have multipartite piriformis muscles, and in some, the superior border may appear fused with deep gluteal muscles. Some patients have an accessory piriformis muscle compressing the more proximal portion of the sciatic nerve, and this is sectioned and removed as well. Special attention and caution is required for patients in whom preoperative imaging demonstrated a split piriformis traversed by a split sciatic nerve. The slip of muscle passing between the tibial and peroneal sciatic components must be removed as well. It is always helpful to ensure by electrodiagnostic stimulation that both tibial and peroneal portions of the sciatic nerve are in view before any piriformis resection is started.

One or two silk ties are placed around the muscle so that bipolar cautery and Metz scissors can be used to fully transect the muscle in two locations with ongoing complete hemostasis. Anesthetic injection of the muscle immediately before resection can help minimize postoperative discomfort. Removal of a segment of muscle about 2 cm in length helps ensure against readhesion of the separated segments that can occur when a single cut is made. This procedure also generally severs the nerve to the piriformis muscle, resulting in subsequent atrophy of any remaining components.

Neuroplasty of the distal lumbosacral plexus, sciatic nerve, and posterior femoral cutaneous nerve is then carried out by blunt dissection generally using DeBakey pickups and a tonsil clamp. Specifically, this entails separating any abnormal fibrous covering from the nerve so that the nerve is free and fully mobile at the end of the dissection. In many cases, fibrovascular bands cross or compress the sciatic nerve and can be cut. Gentle dissection technique, liberal use of electrodiagnostic stimulation when nerve locations are in question, and meticulous, precise hemostasis with bipolar cautery before cutting any tissue help protect the neural tissues.

By swinging the retractor system, ready access can be achieved to the sciatic nerve from the top of the ischial tuberosity to well inside the sciatic notch, allowing for full mobilization of at least 12 cm of the nerve course. Decompression and muscle resection inside the pelvis through the sciatic notch is not recommended on a routine basis because of higher risk to autonomic fibers in the presacral area.

Dexamethasone, 10 mg given intravenously, can be administered at the start of the procedure. Powder-free gloves must be used to further reduce the risk for postoperative fibrosis. Only bipolar cautery is used once the gluteal fascia is reached. Meticulous and complete hemostasis is ensured before closure. Upon completion of the neuroplasty, the wound should be irrigated copiously with antibiotic irrigation maintained at body temperature in a solution warmer. Seprafilm (Genzyme) pieces should be placed in layers on all dissected nerve surfaces as an adhesiolytic agent.

Marcaine (0.5% without epinephrine) is applied to the Seprafilm and dissected nerves and is instilled in gluteal muscles along the line of approach. The gluteal fascia is then closed with 0-Vicryl sutures. Usually, no drain is placed. Patients are allowed to ambulate immediately. Patients experiencing significant muscle spasm or local pain are provided pain management in the facility overnight.

Entrapments of the Pudendal Nerve and the Nerve to the Obturator Internus

Recognition and treatment of pudendal nerve entrapment (PNE) syndrome has an extended history,^9,10 and the first large-scale study was published in 1998.¹¹ There are now numerous general publications covering various aspects of diagnosis and treatment.^12–17 However, the success rate in treating pudendal neuralgias has not been satisfactory^18–20 until recently.²¹ One problem in the past was a lack of appreciation that there are several subtypes with different anatomic bases and presentations.^22–27 Further, the surgical anatomy of the pudendal nerve is subject to significant individual variation.²⁸

Targeting of pudendal nerve injections by electrodiagnostic technique,²⁹ C-arm fluoroscopy,^30,31 and CT scan has been described.^32–34 Diagnosis has also relied on pudendal nerve latency testing,^35,36 but diagnostic imaging has really played very little role in the past. A recent large scale study on treatment of this condition, using magnetic resonance neurography,^37–39 open MRI–guided injections⁴ (Fig. 238-7), and new minimal access surgical techniques has provide significant clarification.²¹

FIGURE 238-7 Open magnetic resonance–guided injections in upper and lower regions of posterior pelvis. A, Superior gluteal nerve—three successive image planes. Arrows indicate injectate along nerve course (A1, A3) and the injection needle (A2). B, Ischial tunnel. Arrow indicates injected medication in ischial tunnel just lateral to sciatic nerve, with asterisk on ischial tuberosity. C, Pudendal nerve block at entry to Alcock’s canal. Arrows indicate needle (C1) and injected medication just medial to Alcock’s canal entry opening, with asterisk on ischial tuberosity. D, Obturator injections. D1, The needle (arrow) approaches obturator internus. D2, Medication is injected in obturator internus (arrow). D3, Medication is in the obturator internus and externus, with the needle passing into pectineus (arrow). D4, Arrows show pectineus, obturator externus, and obturator internus medication in place. Asterisk marks ischial tuberosity. (Images are axial T1-weighted acquisition in open magnetic resonance imaging scanner, except D4, which is axial T2-weighted acquisition in open magnetic resonance imaging scanner.)

Pudendal nerve entrapment can be categorized into four major categories based on the location of the entrapment.²¹ These include type I—exclusively at the level of the piriformis muscle in the sciatic notch only (3.5%), type II—at the level of the ischial spine and sacrotuberous ligament (9%), type III—in Alcock’s canal on the medial surface of the obturator internus 78.5%, and type IV—at distal branches (9%).²¹

Presentation of Pudendal Syndromes

The symptoms of PNE involve pain, dysfunction, and numbness in all or part of the distribution of the nerve. This includes the saddle area between the legs, including genitalia, rectum, and terminal urinary tract, as well as pelvic floor musculature. In addition, sexual dysfunction and sphincter dysfunction are often seen. Sexual dysfunctions included female continuous arousal as well as lack of sensation, male impotence, and dyspareunia. Aggravation by sitting is common when the obturator internus or piriformis muscles are involved, but there is not always a positional trigger. Most cases are unilateral, even though some patients have difficulty identifying laterality of pain in midline urogenital structures, but bilateral pudendal entrapment occurs in more than 25% of cases. Patients reported in the 2008 study²¹ ranged in age from 8 to 82 years, and there was no significant male or female preponderance.

Physical Examination Findings in Pudendal Syndromes

The most common finding is sensitivity to palpation at the obturator internus muscle elicited by manual compression on the deep medial aspect of the ischial tuberosity. Reproduction of symptoms with straight leg raising is rare; however, adduction of the thigh in a seated position, passive internal or external rotation of the hip joint, and resisted abduction or adduction of the flexed internally rotated thigh⁴ also elicited symptoms in some individuals.

Open Magnetic Resonance Imaging–Guided Injections for Pudendal Syndromes

Depending on the location of the entrapment, injection targets include piriformis muscle, obturator internus muscle, and pudendal nerve at the ischial margin or ischial spine, at the sacrotuberous ligament, or at small branch takeoffs from Alcock’s canal, including the rectal branch in the ischiorectal fossa, at the urogenital diaphragm, and at the “crow’s foot” distal branching area at the outlet of Alcock’s canal.

In patients obtaining no alteration of symptoms from dense proximal pudendal nerve block, open MRI can be used to guide injection through a lateral subcoccygeal approach to block the ganglion impar to rule out autonomically driven, nonpudendal saddle area pain syndromes. This approach is similar to paramedian approaches⁴⁰ as opposed to transcoccygeal approaches.^41,42

Pudendal Entrapment Surgical Management

The need to include components of the operation vary based on the etiologic subtype: piriformis muscle resection, partial section of the sacrotuberous or sacrospinal ligament, neuroplasty of the nerve to the obturator internus in the retrosciatic space, and dilation of Alcock’s canal at the obturator internus and distal pudendal operations are variably needed. Intraoperative electromyographic monitoring of muscles innervated by the pudendal nerve, sciatic nerve, nerve to the obturator internus, and inferior gluteal and superior gluteal nerves is essential for nerve identification.

Surgery can be done using minimal access approaches on an outpatient or overnight-stay basis. The surgical approach for patients with piriformis muscle involvement is similar to what is described earlier for piriformis surgery.⁴ Following that approach and any resection of the piriformis muscle, a retrosciatic dissection is commenced. Using an approach over the posterior and inferior surface of the sciatic nerve, and relying on intraoperative stimulation, the nerve to the obturator internus is identified and then tracked proximally into the greater sciatic notch, where it runs deep to the sciatic nerve where progressive release of any adhesions is carried out. The pudendal nerve is then identified through direct nerve stimulation in its typical position adjacent and posterior to the nerve to the obturator internus inside the greater sciatic notch. Neuroplasty of the pudendal nerve from this point and continuing inferiorly across the ischial spine and into the lesser sciatic notch is then carried out. From this access, it is generally possible to dilate Alcock’s canal to a depth of 2 to 3 cm along its course using gentle application of a long tonsil clamp. Care is taken not to disrupt the veins and arteries in the retrosciatic space, but any bleeding should be managed initially with gentle pressure and placement of thrombin-soaked Gelfoam to avoid excessive coagulation near the numerous nerve elements.

A medial transgluteal approach with a 3-cm incision centered over (directly posterior to) the midpoint of the acetabulum is used for patients with ischial spine or Alcock’s canal syndromes that have no piriformis involvement on physical examination. This approach requires careful identification of the sciatic nerve and then a direct retrosciatic dissection to identify and track the nerve to the obturator internus and the pudendal nerve. The more medial and inferior position of this incision allows for greater access to the sacrotuberous ligaments. Most patients with pudendal entrapment at the level of the ischial spine have a medially placed pudendal nerve, and it is often adherent to the anterior surface of the sacrotuberous ligament. The nerve is identified electrodiagnostically and carefully mobilized from the sacrotuberous ligament so that a partial resection of the ligament can be carried out. Some of the patients with entrapment in this location have a variant fibrous septum sealing the entrance to the lesser sciatic notch that is perforated by the nerve. This septum should be carefully opened or resected to improve the mobility of the nerve as it enters the lesser sciatic notch.

This medial transgluteal approach also offers excellent “magic angle” access along Alcock’s canal because the line of approach is parallel to the canal. It is therefore possible to access the canal noninvasively to dilate it, relying on careful electrodiagnostic guidance.

In patients with distal branch entrapments (e.g., isolated penile numbness), the incision is centered between the ischial spine and the ischial tuberosity; this approach takes the surgeon directly to the sacrotuberous ligament. After the ligament is exposed, the pudendal nerve can be accessed distally along the course of Alcock’s canal as well as along its proximal approach to the lesser sciatic notch. In patients who have a dense adhesion of the pudendal nerve to the deep surface of the sacrotuberous ligament, it is possible to carry out partial or complete section of the ligament to access the nerve, as some have advocated,²⁰ but most entrapments can be released without sectioning this major structural ligament. Hruby and colleages²⁶ also described direct anterior approaches for decompression of small distal sensory branches of the pudendal nerve.