Sciatic Nerve Decompression

Sciatic nerve decompression is a treatment option for sciatic nerve pain that is caused by certain structural spinal conditions.[1]

Sciatic neuropathies can result from nerve compression and injury in the gluteal region or thigh. Sciatic neuropathy in the gluteal region is most often caused by trauma. The nerve can be directly injured during an intramuscular injection into the buttocks.

Because of the close anatomical relationship between the nerve and the hip joint, nerve injuries can result from posterior dislocation of the hip, pelvic fracture, and accidental injury during elective hip arthroplasty. Other causes of sciatic neuropathy in this region include prolonged external nerve compression (“toilet seat neuropathy”), compression from pelvic hematoma, tumors, and endometriosis. See the images below.

Anatomic variation of the piriformis muscle may also cause compression of the nerve as it exits the pelvis. Piriformis syndrome generally includes buttocks and leg pain (sciatica) made worse by prolonged sitting. This is caused by direct compression of the sciatic nerve by the piriformis muscle as it exits the pelvis.[2]  It may be relieved by standing or walking, which is associated with signs of progressive sciatic neuropathy in severe cases. Piriformis syndrome is often misdiagnosed as disc disease and therefore may be diagnosed late.[3]

Sciatic neuropathy in the thigh is less common. It can be caused by fractures of the femur, compression by popliteal aneurysm, penetrating trauma, and nerve sheath tumors.

The indications for surgical exploration, decompression, and repair of sciatic neuropathies should take into account both the mechanism and completeness of nerve injury.

Complete traumatic axonal sciatic nerve injuries are best addressed with immediate nerve exploration and repair.

Partial traumatic lesions are usually initially managed conservatively but are referred for surgical exploration and repair if there is limited or no recovery after 3–6 months.

Patients with suspected piriformis syndrome who present with pain without neurologic deficit should initially be managed conservatively with medication and physiotherapy.[4] Surgical exploration and decompression of the sciatic nerve is usually reserved for patients who are resistant to conservative management, ideally in the presence of MRI or neurographic confirmation of the diagnosis.

All nontraumatic progressive sciatic neuropathies with neurologic deficit should be referred for surgical exploration, even if results of imaging studies are normal, since compressive lesions, including nerve sheath tumors and piriformis syndrome, can be difficult to diagnose without direct examination of the nerve.[5, 6]

Anatomy

The sciatic nerve originates from the L4 through S3 nerve roots in the inner wall of the pelvis and then passes under the piriformis muscle, out of the pelvis into the buttock, and through the greater sciatic foramen (sciatic notch). The nerve continues under the gluteus maximus muscle, close to the posterior capsule of the hip joint, deep in the posterior thigh, where motor branches innervate the hamstring muscles.[7]

The sciatic nerve then divides into the common peroneal and tibial nerve branches in the distal thigh proximal to the popliteal fossa. The common peroneal nerve continues around the fibula head into the lateral calf, innervates the peroneal muscle group (ankle dorsiflexion and eversion), and provides sensation to the lateral calf and dorsum of the foot. The tibial nerve continues through the posterior calf and innervates the gastrocnemius and soleus muscles (ankle plantar flexion), posterior tibial muscle (ankle inversion), and toe flexors and provides sensation to the posterior calf and sole of the foot. See the image below.

Although the sciatic nerve branches in the distal thigh, the nerve fibers within the proximal main nerve trunk are arranged topographically. Those fibers are destined to become the common peroneal nerve arranged as a discrete fascicle on the outside (lateral) aspect of the sciatic nerve, where they are more predisposed to compression and injury. Consequently, proximal sciatic nerve injuries can be difficult to distinguish from common peroneal nerve injuries without detailed electrophysiological and radiological assessment.[8]

In addition to a careful clinical examination, patients with suspected sciatic nerve injury or compression usually require electrodiagnostic testing and imaging studies prior to surgical management.

Electrophysiological assessment of nerve injuries is beyond the scope of this topic. However, these studies can be critical for differentiating complete and partial lesions, lateral sciatic nerve and common peroneal nerve injuries, and sciatic nerve injuries in the gluteal and thigh regions. Preoperative imaging may include radiographs of the hip and pelvis, EMG of the sciatic nerve, MRI of the lumbar spine and pelvis, and MR neurography.

Imaging of the pelvis can be used to identify a pelvic mass or hematoma, as well anatomic variations in the piriformis muscle that can cause sciatic nerve compression. See the images below.

Magnetic resonance neurography uses a high-resolution fast-spin echo T2 imaging technique to demonstrate abnormal signal within nerve trunks at sites of major nerve injuries and compression. It can be particularly useful for identifying piriformis syndrome, even when results of conventional imaging studies are normal.

There are two basic classifications of complications: systemic complications and complications specific to the procedure. Deep venous thrombosis can be prevented with serial compression devices, which may need to be placed on the upper extremities when both legs need to be approached to harvest the sural nerves for grafting. Pharmacologic prophylaxis can be used, but caution should be exercised when a complex and potentially bloody dissection is anticipated. As with any surgery, complication prophylaxis needs to be individualized per patient and for their particular medical, surgical, and pharmacologic history.

Prone positioning can be fraught with hazards, particularly with regard to pressure sores, neurapraxia, and airway issues. The airway should be well stabilized. Changes in position should not result in right main stem bronchus intubation and concomitant atelectasis. All potential neurovascular pressure points should be well padded or free floating.

Complications specific to the procedure revolve mainly around nerve injury and bleeding. There are numerous short perforating blood vessels in and around the buttocks. Bleeding may be troublesome in two ways: It may be hemodynamically significant and/or may obscure the surgical field, which can lead to inadvertent nerve injury.

Epinephrine-laden irrigation fluid can help to mitigate intraoperative bleeding. Very proximal dissection of the nerve can lead to an intrapelvic injury of the sacral venous plexus, which can easily lead to exsanguination. Bleeding of the sacral plexus is difficult to control even during open laparotomy. It is even more difficult when a prone patient needs to be emergently repositioned supine and then laparotomy performed.

Nerve injury can result from numerous factors. Difficulty in controlling bleeding and inadvertent cautery injury and/or hemostatic sutures can easily injure nerves. Overly vigorous dissection and misidentification of structures can also lead to injury. This is especially possible in light of difficult dissections at reoperative surgery. Intraoperative nerve monitoring can be very helpful in discerning structures, but there is no substitute for a good working knowledge of normal anatomy and common variations.

It is especially important to note that the area around the piriformis muscle is especially subject to variability in nerve anatomy, as compared to other nerve variants. The superior gluteal, sciatic, and inferior gluteal nerves can vary in their relationship to the piriformis muscle. They frequently pierce the substance of the muscle.

The area around the buttocks is not particularly sterile and difficult to dress properly, so infection avoidance is also important.

Careful selection of patients for surgery is necessary to obtain optimal outcomes. Three of the largest cohort studies published on sciatic nerve decompression reported that patient outcomes varied from 68.8% of patients achieving >50% relief to two-thirds of patients experiencing improvement to 100% of patients reporting excellent or good results. In a study by Ilizaliturri et al., the authors report the outcomes of 15 patients who underwent endoscopic sciatic nerve decompression. Average pain scores were estimated using a visual analog scale (VAS); decreasing from 7.4 (preoperatively) to 1.86 at 24-month follow-up. All cases demonstrated statistically significant postoperative improvement in pain and were able to return to work and perform daily activities without limitations.[9]

In most cases, standard surgical instruments alone can be used. Fiberoptic retractors and illumination are especially helpful when high dissection of the nerve is planned or other difficulties are anticipated. As with any surgery in which significant bleeding may be encountered, vascular instruments should always be on hand or readily available.

Although controversial, an endoscopic approach warrants endoscopic equipment. Specifics as to ports, scopes, and individual instruments are up to the particular surgeon's preference.

In instances involving possible nerve grafting, resection, or intrafascicular microneurolysis, an operating microscope, microsurgical instruments, microsuture, and fibrin glue should be available. Cases involving neuroma-in-continuity, tumors, or difficult dissections may require intraoperative nerve monitoring, conduction studies, and electromyography. In those instances, the proper personnel need to be available.

Anesthesia

Although it is theoretically possible to perform sciatic nerve decompression with spinal or epidural anesthesia, general anesthesia is almost always used. This is especially true if intraoperative nerve monitoring is to be performed, as regional anesthesia may limit some monitoring modalities. In addition, if nerve monitoring is to be performed, the use and timing of specific anesthetic agents should be preplanned with the anesthesia staff.

Positioning

Patients are typically placed in a well-padded prone position with moderate hip flexion. A modified lateral decubitus position can also be used. Prone cases, particularly those anticipated to be of long duration, mandate the use of a urinary catheter.

Long-term follow-up is mostly clinical. Cases that are mainly due to a neurapraxia of the nerve should improve significantly in 3 months or less. In cases in which Wallerian degeneration has occurred, a clinician should follow the migrating Tinel sign, which moves at or about the standard 1 mm per day. Stalled migration of a Tinel sign may be troublesome and may require reoperation.

Patients may also have a concomitant compression syndrome (ie, tarsal tunnel syndrome). Nerve conduction velocities may also have a role, but, ultimately, clinical assessment is key. Patients may continue to improve over the course of a year or more. Secondary procedures, such as tendon transfer, should typically not be entertained until improvement has reached a plateau.

Physical therapy is particularly important in patients with palsy. Keeping joints supple, strength building, range of motion, re-education, and desensitization are to be pursued when necessary. For patients with palsy such as foot drop, proper splinting and range-of-motion maintenance are also very important. It is important to avoid a return of function that is stunted by a plantar flexion contracture.

Hip flexion and prolonged ischial pressure should be avoided. Sitting is the least desirable position. Ambulation and prone positioning should be encouraged.

The sciatic nerve can be compressed proximally near the sciatic notch or more distally in the thigh. There are two different surgical approaches, which can also be combined.

For proximal pathology, a curvilinear incision around the posterolateral aspect of the buttocks is made, which is then carried medially distal to the gluteal fold. The area of the sciatic nerve is in the area of the ischial tuberosity just below the inferior border of the gluteus maximus muscle and between the hamstring muscles.

The gluteus maximus muscle is dissected off the iliotibial tract and the femur and dissected medially. This maneuver exposes the anatomy of the superior and inferior gluteal nerves, as well as the sciatic nerve. Care should be exercised to avoid injury to the perforating vessels.

If these vessels are to be transected, prior proximal control is recommended, as they may retract into the pelvis. At the point of exposure, nerve monitoring and direct observation can be used to discern the cause of the compression. Vascular bands can be ligated and transected. Muscular compression can be released with partial myotomy. Neurolysis can be performed for scarring.

Predissection and postdissection electromyography testing or conduction studies are often helpful in determining prognosis or the need for nerve grafting. If necessary, the tibial and peroneal contributions of the sciatic nerve can be isolated to be dealt with separately. If nerve grafting needs to be performed, sural nerves can be harvested and the microscope brought into the field.

After the nerves are decompressed and hemostasis is achieved, the gluteal muscles are reapproximated with long-term absorbable sutures and a stout suturing technique. A surgical drain may be placed. The skin and subcutis are closed with surgeon preference. Sturdy closure techniques should be used, as the patient will bear weight and flex the area.

When the pathology is further distal, the sciatic nerve is exposed with a near midposterior approach between the biceps femoris muscle and the semitendinosus muscle.. If necessary, muscles can be divided to maximize exposure. Distal to the adductor canal, the popliteal artery and vein are encountered medially and must be treated carefully. Use of an operating sterile tourniquet (at least on standby) can be helpful. The surgeon must also be cognizant of the nerves crossing genicular vessels that may be encountered near the knee joint.

Conservative treatment of sciatic nerve compression and piriformis syndrome may include the following:

• Physical therapy and stretching

• Injection of corticosteroids, botulinum toxin and/or local anesthetic into the piriformis muscle, usually performed under image guidance

• Therapy with systemic medications, such as gabapentin or pregabalin, for neuropathic pain

Other interventions for sciatic neuropathies when proximal nerve exploration and repair is not feasible are usually primarily aimed at overcoming foot drop. These interventions include the following:

• Use of an orthotic device

• Tendon transfer from the posterior tibial muscle to the anterior tibial muscle (applicable only in cases of lateral sciatic nerve trunk injury in which the former muscle may be spared)