Injection Technique for Piriformis Syndrome and Sciatic Nerve Block

Indications and Clinical Considerations

Piriformis syndrome is caused by compression of the sciatic nerve by the piriformis muscle as it passes through the sciatic notch ( Fig. 138.1 ). This painful condition can be caused by a variety of pathologic processes associated with the sciatic nerve and its relationship with the piriformis muscle ( Box 138.1 ). This entrapment neuropathy manifests as pain, numbness, paresthesias, and associated weakness in the distribution of the sciatic nerve. These symptoms often begin as severe pain in the buttocks that radiates into the lower extremity and foot. Patients with piriformis syndrome may develop altered gait, which in turn may result in developing coexistent sacroiliac, back, and hip pain, further confusing the clinical picture. If the condition is not treated, progressive motor deficit of the gluteal muscles and lower extremity can result. The onset of symptoms of piriformis syndrome usually occurs after direct trauma to the sacroiliac and gluteal region and occasionally as a result of repetitive hip and lower extremity motions or repeated pressure on the piriformis muscle and underlying sciatic nerve. Rarely, occult tumors in this anatomic area can also compress the sciatic nerve as it passes through the sciatic notch and produce symptoms identical to piriformis syndrome ( Fig. 138.2 ). Anomalous piriformis muscle can also compress the sciatic nerve, as can acute injury to the nerve as it passes through the muscle ( Fig. 138.3 ).

Physical findings include tenderness over the sciatic notch. A positive Tinel sign over the sciatic nerve as it passes beneath the piriformis muscle often is present. A positive straight-leg raising test is suggestive of sciatic nerve entrapment that may be a result of piriformis syndrome. Palpation of the piriformis muscle reveals tenderness and a swollen, indurated muscle belly. Lifting or bending at the waist and hips increases the pain symptoms in most patients with piriformis syndrome. Weakness of affected gluteal muscles and the lower extremity and, ultimately, muscle wasting often are seen in advanced untreated piriformis syndrome. The piriformis syndrome provocation test is usually positive in patients suffering from piriformis syndrome ( Fig. 138.4 ).

Piriformis syndrome often is misdiagnosed as lumbar radiculopathy or is attributed to primary hip disease. Radiographs of the hip and electromyography help distinguish piriformis syndrome from radiculopathy of pain emanating from the hip. Most patients with a lumbar radiculopathy have back pain associated with reflex, motor, and sensory changes that are associated with neck pain, whereas patients with piriformis syndrome have only secondary back pain and no reflex changes. The motor and sensory changes of piriformis syndrome are limited to the distribution of the sciatic nerve below the sciatic notch. Lumbar radiculopathy and sciatic nerve entrapment may coexist as the so-called double crush syndrome.

Electromyography helps distinguish lumbar radiculopathy from piriformis syndrome. Plain radiographs of the back, hip, and pelvis are indicated in all patients with piriformis syndrome to rule out occult bony disease. On the basis of the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, uric acid, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging of the back is indicated if a herniated disk, spinal stenosis, or a space-occupying lesion is suspected. The injection technique described later serves as both a diagnostic and a therapeutic maneuver.

Clinically Relevant Anatomy

The piriformis muscle has its origin from the anterior sacrum. It passes laterally through the greater sciatic foramen to insert on the upper border of the greater trochanter of the femur. The piriformis muscle’s primary function is to externally rotate the femur at the hip joint, and it is innervated by the sacral plexus. With internal rotation of the femur, the tendinous insertion and belly of the muscle can compress the sciatic nerve and, if this persists, cause entrapment of the sciatic nerve.

The sciatic nerve provides innervation to the distal lower extremity and foot with the exception of the medial aspect of the calf and foot, which are subserved by the saphenous nerve. The largest nerve in the body, the sciatic nerve, is derived from the L4, L5, and the S1-S3 nerve roots ( Fig. 138.5 ). The roots fuse in front of the anterior surface of the lateral sacrum on the anterior surface of the piriformis muscle. The nerve travels inferiorly and leaves the pelvis just below the piriformis muscle via the sciatic notch ( Fig. 138.6 ). Just beneath the nerve at this point is the obturator internus muscle. The sciatic nerve lies anterior to the gluteus maximus muscle; at its lower border, the sciatic nerve lies halfway between the greater trochanter and the ischial tuberosity. The sciatic nerve courses downward past the lesser trochanter to lie posterior and medial to the femur. In the midthigh the nerve gives off branches to the hamstring muscles and the adductor magnus muscle. In most patients the nerve divides to form the tibial and common peroneal nerves in the upper portion of the popliteal fossa, although in some patients these nerves can remain separate through their entire course. The tibial nerve continues downward to provide innervation to the distal lower extremity, whereas the common peroneal nerve travels laterally to innervate a portion of the knee joint and, via its lateral cutaneous branch, provides sensory innervation to the back and lateral side of the upper calf.