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Piriformis syndrome is a neuromuscular disorder that occurs when the sciatic nerve is compressed or otherwise irritated by the piriformis muscle. This causes pain, tingling and numbness in the buttocks and along the course of the sciatic nerve. The syndrome may result from anatomical variations in the muscle-nerve relationship, or from overuse or strain. Although the existence of this syndrome is disputed by some, two recent uncontrolled studies support the possibility (PMID 15739520; PMID 17030664). The need for controlled studies is supported by studies of spinal disk disease that show a high frequency of abnormal disks in asymptomatic patients (PMID 11172169 - see Table 2).
In 15% of the population the sciatic nerve passes through the piriformis muscle, rather than underneath it. These people have been reported by some studies to have a greater incidence of piriformis syndrome than does the general population. Some researchers discount the importance of this relationship in the etiology of the syndrome.
Inactive gluteal muscles also facilitate development of the syndrome. These are important in both hip extension and in aiding the piriformis in external rotation of the femur. A major cause for inactive gluteals is unwanted reciprocal inhibition from overactive hip flexors (psoas major, iliacus, and rectus femorus). This imbalance usually occurs where the hip flexors have been trained to be too short and tight, such as when someone sits with hips flexed, as in sitting all day at work. This deprives the gluteals of activation, and the synergists to the gluteals (hamstrings, adductor magnus, and piriformis) then have to perform extra roles they were not designed to do. Resulting hypertrophy of the piriformis then produces the typical symptoms.
Another purported cause for piriformis syndrome is stiffness, or hypomobility, of the sacroiliac joints. The resulting compensatory changes in gait would then result in shearing of one of the origins of the piriformis, and possibly some of the gluteal muscles as well, resulting not only in piriformis malfunction but in other low back pain syndromes as well.
In addition to causing gluteal pain that may radiate down the leg, the syndrome may present with pain that is relieved by walking with the foot on the involved side pointing outward. This position externally rotates the hip, lessening the stretch on the piriformis and relieving the pain slightly. Piriformis syndrome is also known as "wallet sciatica" or "fat wallet syndrome," as the condition can be caused or aggravated by sitting with a large wallet in the rear pocket. 
Treatment usually begins with stretching exercises and massage, and the avoidance of contributary activities such as running and bicycling. Some clinicians recommend formal physical therapy, including the teaching of stretching techniques, manual massage, and strengthening of the core muscles (abs, back, etc.) to reduce strain on the piriformis muscle. Recommended stretching exercises will usually target the piriformis muscle, but may also include the hamstrings and hip muscles, in order to adequately reduce pain and increase range of motion.
Patients with piriformis syndrome may also find some pain relief from ice and heat. Ice can be helpful right when the pain starts or immediately after an activity that usually causes pain (e.g., going up stairs). The ice may be in the form of an ice pack held to the area or an individual ice cube used in combination with a massaging motion. A heating pad may be alternated with the ice for relief from the pain.
Anti-inflammatory drugs (such as ibuprofen or naproxen), Botox, and/or corticosteroid injections can be used. Occasionally surgery may be recommended. The prognosis with treatment is generally good. Ultrasound is another option for treatment.
- National Institute of Neurological Disorders NINDS Piriformis Syndrome Information Page
- Piriformis syndrome information including NMR scans
- e-medicine Piriformis Syndrome
- Stretching and Strengthening
- Piriformis syndrome – another irritation to the sciatic nerve
- Treatment and Diagnoisis of Piriformis Syndrome Medical Paper
- Using Botox for treatment