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.