Sciatica of nondisc origin and piriformis syndrome: diagnosis by magnetic resonance neurography and interventional magnetic resonance imaging with outcome study of resulting treatment

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Object. Because lumbar magnetic resonance (MR) imaging fails to identify a treatable cause of chronic sciatica in nearly 1 million patients annually, the authors conducted MR neurography and interventional MR imaging in 239 consecutive patients with sciatica in whom standard diagnosis and treatment failed to effect improvement.

Methods. After performing MR neurography and interventional MR imaging, the final rediagnoses included the following: piriformis syndrome (67.8%), distal foraminal nerve root entrapment (6%), ischial tunnel syndrome (4.7%), discogenic pain with referred leg pain (3.4%), pudendal nerve entrapment with referred pain (3%), distal sciatic entrapment (2.1%), sciatic tumor (1.7%), lumbosacral plexus entrapment (1.3%), unappreciated lateral disc herniation (1.3%), nerve root injury due to spinal surgery (1.3%), inadequate spinal nerve root decompression (0.8%), lumbar stenosis (0.8%), sacroiliac joint inflammation (0.8%), lumbosacral plexus tumor (0.4%), sacral fracture (0.4%), and no diagnosis (4.2%).

Open MR—guided Marcaine injection into the piriformis muscle produced the following results: no response (15.7%), relief of greater than 8 months (14.9%), relief lasting 2 to 4 months with continuing relief after second injection (7.5%), relief for 2 to 4 months with subsequent recurrence (36.6%), and relief for 1 to 14 days with full recurrence (25.4%). Piriformis surgery (62 operations; 3-cm incision, transgluteal approach, 55% outpatient; 40% with local or epidural anesthesia) resulted in excellent outcome in 58.5%, good outcome in 22.6%, limited benefit in 13.2%, no benefit in 3.8%, and worsened symptoms in 1.9%.

Conclusions. This Class A quality evaluation of MR neurography's diagnostic efficacy revealed that piriformis muscle asymmetry and sciatic nerve hyperintensity at the sciatic notch exhibited a 93% specificity and 64% sensitivity in distinguishing patients with piriformis syndrome from those without who had similar symptoms (p < 0.01).

Evaluation of the nerve beyond the proximal foramen provided eight additional diagnostic categories affecting 96% of these patients. More than 80% of the population good or excellent functional outcome was achieved.

Article Information

Address reprint requests to: Aaron G. Filler, M.D., Ph.D., Institute for Nerve Medicine, 2716 Ocean Park Boulevard, Suite 3082, Santa Monica, California 90405. email:

© AANS, except where prohibited by US copyright law.



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    Open MR imaging—guided piriformis muscle injection. A: A T1-weighted image. GM = gluteus maximus, IS = ischium, PM = piriformis muscle, Sa = sacrum, SN = sciatic nerve. B: Physician's finger indicating approach. C: Subcutaneous local anesthetic. D–G: Titanium Lufkin needle advanced into piriformis muscle. H & I: Marcaine injection darkens the muscle (the images in B–I were 14-second, two dimensional fast—low angle shot images.)

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    Localization for the incision and intraoperative orientation. A: Drawing of anatomy of the sciatic notch. The piriformis muscle arises on deep surface of the sacrum, passes through the greater sciatic notch, and inserts on the greater trochanter of the femur. The superior gluteal nerve typically exits above the piriformis muscle in the notch, and the inferior gluteal nerve exits inferior and posterior to the muscle. The posterior femoral cutaneous nerve typically parallels the course of the sciatic nerve. The pudendal nerve also exits the greater sciatic notch, passes over the sacrospinous ligament and then under the sacrotuberous ligament to reenter the pelvis through the lesser sciatic notch. The nerve to the piriformis muscle exits the greater sciatic notch deep to the piriformis muscle. Many of these features are subject to significant individual anatomical variation. Red lines mark the position of the incision for piriformis surgery (upper line) and for ischial tunnel surgery (lower line), both approximately 3 to 4 cm in length. For piriformis surgery, the lateral inferior end of the incision is over the tip of the greater trochanter and it proceeds medially and superiorly at a 45° angle. This ensures that one of the long retractor blades can be placed just medial to the tip of the trochanter. B: Preoperative radiographic demonstrating localization of the incision. Two 18-gauge needles are taped to the skin, pointing to the presumed position of the superior tip of the greater trochanter. C: Intraoperative image of piriformis surgery performed using open MR imaging guidance in a Siemens 0.25-tesla imager. The surgeon's finger is palpating the sciatic nerve at the level of the ischial spine. The patient is prone. D–H: Fluoro-Nav system and optical guidance images showing the sciatic nerve position. The reference marker is attached to a table-mounted Omni retractor arm, and the surgeon uses a hand-held pointer to identify the sciatic nerve course (D). Anteroposterior and lateral fluoroscopy imaging pairs with computer-generated virtual image of guidance probe superimposed in purple (E–H). Piriformis muscle attachment point on the greater trochanter (E). Sciatic nerve course as it descends below the level of the piriformis muscle (F). Position of the ischial tuberosity at a level where sciatic entrapment often occurs in ischial tunnel syndrome (G). Course of the sciatic nerve as it exits from and descends below the ischial tunnel (H).

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    Transgluteal approach to the piriformis muscle and sciatic nerve. A: Using a Bovie coagulator, the subcutaneous fatty tissue is dissected until the gluteal fascia is reached. This is coagulated along a line by using the bipolar coagulator and incised sharply using Metz scissors. The fascia will be repaired using Ovicryl sutures at the end of the operation. B: Blunt dissection is performed with the finger tip but also by using a tonsil clamp, bipolar, and Metz scissors to progress between sheets of gluteal musculature to approach the prepiriformis fascia. There is no need to cut any gluteal musculature. C: After dissection through 5 to 6 cm of gluteal musculature, the glistening, semitransparent, hard prepiriformis fascia (asterisk) is exposed; below it is fatty tissue that will typically herniate when the fascia is incised. D: The prepiriformis fascia is coagulated using bipolar cautery and then cut with Metz scissors. Evaluation with a nerve stimulator will ensure against inadvertent injury to components of the superior or inferior gluteal nerves. E: The first task after entering the prepiriformis fat pad is to use a nerve stimulator with electromyography or manual monitoring to identify and protect the sciatic nerve as it passes inferior to the piriformis muscle. The location of the gluteal nerves should also be identified. F: The margins of the piriformis muscle are then identified and a right-angle clamp is passed below the muscle to pass an O-silk tie.

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    Piriformis resection and sciatic neuroplasty. A: Two ties are passed around the piriformis muscle and tied—one proximal and one distal. The distal portion near the tendon of attachment is coagulated using bipolar cautery, in layers, and cut in layers with a Metz scissors. B: The distal end, once disconnected, can be grasped with a tonsil clamp and pulled distally while the proximal muscle is incised. This results in complete disconnection and removal of a 2-cm segment of the muscle. The nerve to the piriformis muscle is typically cut in this process as well. C: With the piriformis muscle cut, the full extent of the sciatic nerve in the region is explored. Approximately 6 in of nerve can typically be explored, extending to and through the sciatic notch as well as distally along most of its course above the ischial tunnel. D: A complete neuroplasty can include both the superficial and deep surfaces of the nerve. The posterior femoral cutaneous nerve is also identified and mobilized as it courses just superficial to and parallel with the sciatic nerve. E: Seprafilm is cut in small squares and placed along all dissected nerve surfaces. A double layer is helpful. F: Adhesion of muscle remnants to the nerve can be further inhibited by lining the cut muscle surfaces with small sheets of Seprafilm.

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    Magnetic resonance neurography findings in piriformis syndrome. A: Axial T1-weighted image of piriformis muscle size asymmetry (arrows indicate piriformis muscles). The left muscle is enlarged. B and C: Coronal and axial images of the pelvis. Arrows indicate sciatic nerves. The left nerve exhibited hyperintensity. D: Curved reformatted neurography image demonstrating left sciatic nerve hyperintensity and loss of fascicular detail at the sciatic notch (arrows).

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    Lumbar MR neurography for evaluation of distal foraminal lumbar nerve root entrapment. A: Normal linear course of lumbar spinal nerves (SN). B: The L-5 root (asterisk) in a patient with radiculopathy unchanged after two ineffective spine surgeries. The distal root shows focal narrowing and a region of hyperintensity (n). N = nerve. C: Myelogram revealing apparently normal nerve root exit.

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    Anatomy and physical examination for piriformis syndrome during a flexed internally rotated thigh adduction maneuver. A1 and A2: The maneuver pulls the piriformis muscle against the sciatic nerve at the osseous margin of ischium. B1 and B2: The patient's foot is placed lateral to the contralateral knee with resisted adduction against the examiner's hand to reproduce the symptoms. M = muscle; post = posterior; sup = superior.

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    Magnetic resonance neurography imaging diagnoses of sciatica syndromes in patients with persistent radiculopathy after surgery. A1: Flattening of root (double asterisk) by persistent foraminal disc fragment (fr). A2: Right S-1 dysesthetic pain after microdiscectomy: good decompression with hyperintense dorsal root ganglion (DRG) consistent with intraoperative mechanical trauma; no surgical treatment is recommended. B: Persistent sciatica after a fall with no improvement after discectomy. The image demonstrates inflammation around the nerve (S-1) consistent with a sacral fracture (fx) abutting the foramen. C: Persistent severe left L-5 radiculopathy exacerbated after lumbar spine instrumented fusion, and not relieved by its subsequent removal. The image demonstrates perforation of left L-5 root by the pedicle screw. No further surgical treatment is recommended. D: Sciatic nerve (SN) hyperintensity associated with adhesion to a site of a pelvic fracture in a patient with new-onset RSD. The RSD symptoms resolved after nerve release surgery. E: Inflammation in the ischium adjacent to hip joint arthritis in the acetabulum (Is/Ac) affecting the adjacent transiting sciatic nerve.

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    Sciatic nerve tumors. A1–3: After physical therapy, lumbar discectomy, and piriformis muscle sectioning, all without benefit, initial sciatic imaging revealed schwannomas (asterisk and double asterisk) in sciatic nerve near the ischial tuberosity. Symptoms resolved after tumor excision. B: Sciatic nerve (s) with mass (m) in a patient with sciatica, positive SLR test, and lumbar spondylosis. Symptoms resolved after tumor excision.

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    Variant sciatic anatomy: muscle passing through nerve. This series of axial T1-weighted images of the right pelvis progress at 3-mm intervals from the midlevel of the sciatic notch to the level of the acetabulum (A–F). The images document the passage of a variant muscle filament (VF) through the sciatic nerve (SN) at the sciatic notch. AC = acetabulum; IS = ischium; Pir = piriformis muscle.



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