|Classification and external resources|
Nerves of the right lower extremity Posterior view.
Piriformis syndrome is a neuromuscular disorder that occurs when the sciatic nerve is compressed or otherwise irritated by the piriformis muscle causing pain, tingling and numbness in the buttocks and along the path of the sciatic nerve descending down the lower thigh and into the leg. Diagnosis is often difficult due to few validated and standardized diagnostic tests, but two have been well-described and clinically validated: one is electrophysiological, called the FAIR-test, which measures delay in sciatic nerve conductions when the piriformis muscle is stretched against it. The other is magnetic resonance neurography, a sophisticated version of MRI that highlights inflammation and the nerves themselves. Some say that the most important criterion is the exclusion of sciatica resulting from compression/irriation of spinal nerve roots, as by a herniated disc. However, compression may be present, but not causal, in the setting of sciatica due to piriformis syndrome.
Uncontrolled studies have suggested theories about the disorder, however a large scale formal prospective outcome trial found that the weight of the evidence-based medicine is that piriformis syndrome should be considered as a possible diagnosis when sciatica occurs without a clear spinal cause. The need for controlled studies is supported by studies of spinal disc disease that show a high frequency of abnormal discs in asymptomatic patients.
When the piriformis muscle shortens or spasms due to trauma or overuse, it can compress or strangle the sciatic nerve beneath the muscle. Generally, conditions of this type are referred to as nerve entrapment or as entrapment neuropathies; the particular condition known as piriformis syndrome refers to sciatica symptoms not originating from spinal roots and/or spinal disc compression, but involving the overlying piriformis muscle. In 17% of an assumed normal population the sciatic nerve passes through the piriformis muscle, rather than underneath it, and in 16.2% of patients undergoing surgery for a suspected piriformis syndrome such an anomaly was found leading to doubt about the importance of the anomaly as a factor in piriformis syndrome. 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 femoris). 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 have not evolved to do. Resulting hypertrophy of the piriformis then produces the typical symptoms.
Runners, bicyclists and other athletes engaging in forward-moving activities are particularly susceptible to developing piriformis syndrome if they do not engage in lateral stretching and strengthening exercises. When not balanced by lateral movement of the legs, repeated forward movements can lead to disproportionately weak hip abductors and tight adductors. Thus, disproportionately weak hip abductors/gluteus medius muscles, combined with very tight adductor muscles, can cause the piriformis muscle to shorten and severely contract. Upon a 40% increase in piriformis size, sciatic nerve impingement is inevitable. This means the abductors on the outside cannot work properly and strain is put on the piriformis.
The result of the piriformis muscle spasm can be impingement of not only the sciatic nerve but also the pudendal nerve. The pudendal nerve controls the muscles of the bowels and bladder. Symptoms of pudendal nerve entrapment include tingling and numbness in the groin and saddle areas, and can lead to urinary and fecal incontinence.
When piriformis syndrome is caused by weak abductors combined with tight adductors, a highly effective and easy treatment includes stretching and strengthening these muscle groups. An exercise regimen targeting the gluteus medius and hip abductor muscle groups can alleviate symptoms of piriformis syndrome within days.
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.
Piriformis syndrome can also be caused by overpronation of the foot. When a foot overpronates it causes the knee to turn medially, causing the piriformis to activate to prevent over-rotating the knee. This causes the piriformis to become overused and therefore tight, eventually leading to piriformis syndrome.
Piriformis syndrome may also be associated with falling injury.
The Sciatic Nerve 
The affected nerve in piriformis syndrome is the sciatic nerve. Collectively the tibial and common fibular nerve makes up the general term sciatic nerve. Since the sciatic nerve is so close to the piriformis it is acknowledged that the muscle plays a role in irritating the nerve.
Piriformis syndrome (PS) data is often confused with other conditions  due to differences in definitions, survey methods and whether or not occupational groups or general population are surveyed. This causes a lack of group harmony about the diagnosis and treatment of PS, affecting its epidemiology. In a study, 0.33% of 1293 patients with lower back pain cited an incident for PS. A separate study showed 6% of 750 patients with the same incidence. About 6% - 8% of lower back pain occurrences were attributed to PS,  though other reports concluded about 5% - 36%. In a survey conducted on the general population, 12.2% - 27% included a lifetime occurrence of PS, while 2.2% - 19.5% showed an annual occurrence. However further studies show that the proportion of the sciatica, in terms of PS, is about 0.1% in orthopaedic practice. This is more common in women with a ratio of 3 to 1  and most likely due to the wider quadriceps femoris muscle angle in the os coxae. Between the years of 1991- 1994, PS was found to be 75% prevalent in New York, Connecticut, New Jersey, Pennsylvania; 20% in other American urban centers; and 5% in North and South America, Europe, Asia, Africa and Australia. The common ages of occurrence happen between thirty and forty, and are scarcely found in patients younger than twenty; this has been known to affect all lifestyles.
Other presentations 
In addition to causing gluteal pain that may radiate down buttocks and 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 affected side's rear pocket.
We know that Piriformis syndrome occurs when the Sciatic nerve is compressed or pinched by the Piriformis muscle. Indications include sciatica (radiating pain in the buttock, posterior thigh and lower leg) and the physical exam finding of tenderness in the area of the sciatic notch. The pain is exacerbated with activity, prolonged sitting, or walking. The diagnosis is largely clinical and is one of exclusion. In physical examination, attempts are made to stretch the irritated piriformis and provoke sciatic nerve compression, such as the Freiberg, the Pace, and the FAIR (flexion, adduction, internal rotation) maneuvers. Conditions to be ruled out include herniated nucleus pulposus (HNP), facet arthropathy, spinal stenosis, and lumbar muscle strain.
Diagnostic modalities such as CT, MRI, ultrasound, and EMG are mostly useful in excluding other conditions. However, magnetic resonance neurography is a medical imaging technique that can show the presence of irritation of the sciatic nerve at the level of the sciatic notch where the nerve passes under the piriformis muscle. Magnetic resonance neurography is considered "investigational/not medically necessary" by some insurance companies. Neurography can determine whether or not a patient has a split sciatic nerve or a split piriformis muscle – this may be important in getting a good result from injections or surgery. Image guided injections carried out in an open MRI scanner, or other 3D image guidance can accurately relax the piriformis muscle to test the diagnosis. Other injection methods such as blind injection, fluoroscopic guided injection, ultrasound, or EMG guidance can work but are not as reliable and have other drawbacks.
Symptomatic relief of muscle and nerve pain can be obtained by non-steroidal anti-inflammatory drugs and/or muscle relaxants. Conservative treatment usually begins with stretching exercises, myofascial release, massage, and avoidance of contributory activities, such as running, bicycling, rowing, etc. Some clinicians recommend formal physical therapy, including the teaching of stretching techniques, massage, and strengthening of the core muscles (abs, back, etc.) to reduce strain on the piriformis.
Stretching is recommended every 2 to 3 waking hours. For optimal stretching capacity, move the hip joint capsule anteriorly and posteriorly. The muscle is manually stretched by applying pressure perpendicular to the long axis of the muscle and parallel to the surface of the buttocks until the muscle is relaxed. Another stretching exercise is to lie on the side opposite of the pain with the hip and knee of the upper leg flexed and adducted towards the ground while the torso is rotated so that the back of the upper shoulder touches the ground. Chiropractors may suggest stretching exercises that will target the piriformis, 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 relief from ice. Ice can be helpful when the pain starts, or immediately after an activity that causes pain. This may be simply an ice pack, or ice massage.
Muscle strengthening involves working on the hip abductors, external rotators and extensors. This treatment involves three phases: non-weight bearing exercises, weight bearing exercises, and ballistic exercises. The purpose of non-weight bearing exercises is to focus on isolated muscle recruitment. Ballistic and dynamic exercises consists of plyometrics.
Failure of conservative treatments described above may lead to consideration of various therapeutic injections such as local anesthetics (e.g., lidocaine), Anti-inflammatory drugs and/or corticosteroids, botulinum toxin (BTX, BOTOX), or a combination of the three. Injection technique (discussed in above section) is a significant issue since the piriformis is a very deep seated muscle. A radiologist may assist in this clinical setting by injecting a small dose of medication containing a paralysing agent such as botulinum toxin under high-frequency ultrasound or CT control. This inactivates the piriformis muscle for 3 to 6 months, without resulting in leg weakness or impaired activity.
Rarely surgery may be recommended. The prognosis is generally good. Minimal access surgery using newly reported techniques has also proven successful in a large-scale formal outcome published in 2005.
Failure of piriformis syndrome treatment may be secondary to an underlying obturator internus muscle injury.
See also 
- [Fishman, LM, Dombi, GW, Michaelsen, C, Ringel, SV, Rosbruch, J, Rosner, B, and Weber, C (2002). "Piriformis Syndrome: Diagnosis, treatment and outcome- a ten year study". Archives of Physical Medicine and Rehabilitation 83 (3): 295–302.
- Kirschner JS, Foye PM, Cole JL (2009). "Piriformis syndrome, diagnosis and treatment". Muscle Nerve 40 (1): 10–8. doi:10.1002/mus.21318. PMID 19466717.
- Filler AG, Haynes J, Jordan SE, et al. (2005). "Sciatica of nondisc origin and piriformis syndrome: diagnosis by magnetic resonance neurography and interventional magnetic resonance imaging with outcome study of resulting treatment". Journal Neurosurg Spine 2 (2): 99–115. doi:10.3171/spi.2005.2.2.0099. PMID 15739520.
- Lewis AM, Layzer R, Engstrom JW, Barbaro NM, Chin CT (2006). "Magnetic resonance neurography in extraspinal sciatica". Arch. Neurol. 63 (10): 1469–72. doi:10.1001/archneur.63.10.1469. PMID 17030664.
- Deyo RA, Weinstein JN (2001). "Low back pain". N. Engl. J. Med. 344 (5): 363–70. doi:10.1056/NEJM200102013440508. PMID 11172169.
- Smoll NR (2010). "Variations of the Piriformis and Sciatic Nerve With Clinical Consequence: A Review". Clinical Anatomy 23 (1): 8–17. doi:10.1002/ca.20893. PMID 19998490.
- Benzon HT, Katz JA, Benzon HA, Iqbal MS (2003). "Piriformis syndrome: anatomic considerations, a new injection technique, and a review of the literature". Anesthesiology 98 (6): 1442–8. doi:10.1097/00000542-200306000-00022. PMID 12766656.
- Kuncewicz E, Gajewska E, Sobieska M, Samborski W (2006). "Piriformis muscle syndrome". Annales Academiae Medicae Stetinensis 52 (3): 99–101; discussion 101. PMID 17385355.
- Saladin, Kenneth S. (2004). Anatomy physiology : the unity of form and functio. Boston: McGraw-Hill Higher Education. ISBN 0-07-242903-8.
- Boyajian-O'Neill, LA.; McClain, RL.; Coleman, MK.; Thomas, PP. (Nov 2008). "Diagnosis and management of piriformis syndrome: an osteopathic approach.". J Am Osteopath Assoc 108 (11): 657–64. PMID 19011229.
- Hopayian, K.; Song, F.; Riera, R.; Sambandan, S. (Dec 2010). "The clinical features of the piriformis syndrome: a systematic review.". Eur Spine J 19 (12): 2095–109. doi:10.1007/s00586-010-1504-9. PMID 20596735.
- Papadopoulos, EC.; Khan, SN. (Jan 2004). "Piriformis syndrome and low back pain: a new classification and review of the literature.". Orthop Clin North Am 35 (1): 65–71. doi:10.1016/S0030-5898(03)00105-6. PMID 15062719.
- Fishman, LM.; Dombi, GW.; Michaelsen, C.; Ringel, S.; Rozbruch, J.; Rosner, B.; Weber, C. (Mar 2002). "Piriformis syndrome: diagnosis, treatment, and outcome--a 10-year study.". Arch Phys Med Rehabil 83 (3): 295–301. PMID 11887107.
- Kirschner, JS.; Foye, PM.; Cole, JL. (Jul 2009). "Piriformis syndrome, diagnosis and treatment.". Muscle Nerve 40 (1): 10–8. doi:10.1002/mus.21318. PMID 19466717.
- Piriformis Syndrome (Hip/Buttocks Pain)
- Klein, M. J. (2012). Physical medicine and rehabilitation for piriformis syndrome. In C. Lorenzo (Ed.),Medscape Reference: Drugs, Diseases and Procedures.
- Steiner, C.; Staubs, C.; Ganon, M.; Buhlinger, C. (Apr 1987). "Piriformis syndrome: pathogenesis, diagnosis, and treatment.". J Am Osteopath Assoc 87 (4): 318–23. PMID 3583849.
- Tonley, JC.; Yun, SM.; Kochevar, RJ.; Dye, JA.; Farrokhi, S.; Powers, CM. (Feb 2010). "Treatment of an individual with piriformis syndrome focusing on hip muscle strengthening and movement reeducation: a case report.". J Orthop Sports Phys Ther 40 (2): 103–11. doi:10.2519/jospt.2010.3108. PMID 20118521.
- Lang AM (2004). "Botulinum toxin type B in piriformis syndrome". Am J Phys Med Rehabil. 83 (3): 198–202. doi:10.1097/01.PHM.0000113404.35647.D8. PMID 15043354.
- JNS: Spine Table of Contents Menu
- Image sequence of piriformis muscle being injected under CT guidance
- Article on Piriformis Syndrom by Igor Marcikic
- Review article: "The Diagnosis and Management of Piriformis Syndrome: Myths and Facts" by TA Miller, KP White, DC Ross, Can J Neural Sci. 2012; 39: 577-583. Five procedures are proposed as the standard for a diagnosis of PS.
- piriformis_syndrome at NINDS