Definition

The piriformis muscle and sciatic nerve both exit the pelvis through the greater sciatic notch. Numerous anatomic variations of this relationship have been well documented ( Fig. 58.1 ). Cadaver studies have described the sciatic nerve passing below the piriformis muscle, through the muscle belly, as a divided nerve above and through the muscle, and as a divided nerve through and below the muscle. More recently, a case report of piriformis syndrome described a fifth variation of an undivided nerve passing above an undivided piriformis muscle. Yeoman was the first to describe the relationship of these two structures in 1928, and Robinson first coined the term piriformis syndrome in 1947.

Three variations in the course of the sciatic nerve as related to the piriformis muscle. The sciatic nerve is shown above (3) , through (2) , and below (1) the piriformis muscle.

Piriformis syndrome describes a clinical situation whereby the piriformis muscle is compressing the sciatic nerve, resulting in a sciatic neuropathy. This may be an intrinsic injury to the piriformis muscle (primary syndrome) or a compression at the pelvic outlet (secondary syndrome).

Although the anatomic relation of these two structures is well documented, this remains a controversial diagnosis. There is no consensus among clinicians on the validity of this entity and therefore no documentation of the incidence. Nevertheless, Goldner predicted an incidence of less than 1% in an orthopedic practice. The incidence is the same for men and women.

Symptoms

The patient with piriformis syndrome will complain of buttock pain with or without radiation into the leg. This may be seen in chronic as well as in acute situations. Often, a history of minor trauma may be described, such as falling onto the buttock. Sitting on hard surfaces will exacerbate the symptoms of pain and occasional numbness and paresthesias without weakness. Activities that produce a motion of hip adduction and internal rotation, such as cross-country skiing and the overhead serve in tennis, may also exacerbate the symptoms. Because of the relationship of the piriformis muscle with the lateral pelvis wall, patients may also experience pain with bowel movements, and women may complain of dyspareunia.

Physical Examination

The physical examination will reveal normal neurologic findings with symmetric strength and reflexes. Tenderness to palpation is experienced from the sacrum to the greater trochanter, representing the area of the piriformis muscle. A palpable taut band is tender with both rectal and pelvic examination because the piriformis muscle sits in the deep pelvic floor. Passive hip abduction and internal rotation may compress the sciatic nerve, reproducing pain (a Freiberg sign). Contraction of the piriformis with resistance to active hip external rotation and abduction may also reproduce pain or asymmetric weakness (a Pace sign). A positive result of the straight-leg test may also be appreciated. Rectal examination may be performed to palpate a taut band, but is not recommended. See Table 58.1

Functional Limitations

The patient with piriformis syndrome will experience pain with prolonged sitting and with activities that produce hip internal rotation and adduction. This may include cross-country skiing and one-legged motions, such as the overhead serve in tennis and the kicking motion in soccer. Sitting on hard surfaces such as benches, church pews, or wallets kept in a back pocket (“wallet neuritis”) may exacerbate symptoms.

Diagnostic Testing

Piriformis syndrome is a clinical diagnosis. There is no gold standard in the diagnosis of piriformis syndrome. Magnetic resonance imaging (MRI) and computed tomography (CT) are primarily reserved to rule out other disorders associated with sciatic neuropathy. A few case reports have demonstrated hypertrophy of the piriformis muscle on both CT and MRI. Electrodiagnostic testing may reveal a prolonged H reflex in symptomatic cases. This was validated by demonstration of a prolongation of the H reflex with hip flexion, adduction, and internal rotation (the FAIR test) in symptomatic cases. Patients diagnosed with piriformis syndrome by this FAIR test demonstrated successful treatment outcomes with physical therapy and injections in 70% of the cases. Electrodiagnosis is also helpful in excluding piriformis syndrome during a lumbosacral radiculopathy evaluation. Diffusion tensor imaging and diffusion tensor tractography demonstrate potential as future tools for diagnosis.