Piriformis syndrome

Piriformis syndrome is a condition which is believed to result from nerve compression at the sciatic nerve by the piriformis muscle. It is a specific case of deep gluteal syndrome. The largest and most bulky nerve in the human body is the sciatic nerve. Starting at its origin it is 2 cm wide and 0.5 cm thick. The sciatic nerve forms the roots of L4-S3 segments of the lumbosacral plexus. The nerve will pass inferiorly to the piriformis muscle, in the direction of the lower limb where it divides into common tibial and fibular nerves. Symptoms may include pain and numbness in the buttocks and down the leg. Often symptoms are worsened with sitting or running. Causes may include trauma to the gluteal muscle, spasms of the piriformis muscle, anatomical variation, or an overuse injury. Few cases in athletics, however, have been described. Diagnosis is difficult as there is no definitive test. A number of physical exam maneuvers can be supportive. Medical imaging is typically normal. Other conditions that may present similarly include a herniated disc. Treatment may include avoiding activities that cause symptoms, stretching, physiotherapy, and medication such as NSAIDs. Steroid or botulinum toxin injections may be used in those who do not improve. Surgery is not typically recommended. The frequency of the condition is unknown, with different groups arguing it is more or less common.

Anatomy The piriformis is a flat, pear-shaped muscle. The thicker side of the piriformis is medial (closer to the center of the body). The function of the piriformis is as an external thigh rotator. When the hip is extended, such as standing, the piriformis rotates the thigh outwards. When the hip is flexed, such as sitting, the piriformis abducts the hip (spreading the legs). The piriformis originates at the anterior surface of the sacrum, passes through the greater sciatic foramen, and inserts at the greater trochanter. The sciatic nerve is the largest peripheral nerve in the body and innervates significant portions of the skin and muscles of the thigh, leg, and foot. The sciatic nerve originates from spinal nerves L4-S3. It forms in the pelvis from nerves of the sacral plexus, and exits the greater sciatic foramen just underneath the piriformis. A number of anatomic variations exist in the branching pattern of sciatic nerve around the piriformis, such as passing over, through, or under the piriformis, as well as early branching into the tibial nerve and common peroneal nerve before passing the piriformis.

Signs and symptoms Patients with piriformis syndrome may have some of the following symptoms:

Sciatica, or pain felt in the leg along the distribution of the sciatic nerve External tenderness near the greater sciatic notch Buttock pain Pain on any maneuver that increases piriformis muscle tension Pain aggravated on sitting Limitation of the straight leg raise There are also some reports of gluteal atrophy on the same side as the symptoms for chronic cases, but this is less commonly seen.

Etiology Causes of piriformis syndrome include the following

Trauma to the hip or buttock area such as a fall is the most common precipitating factor. Athletes and weightlifters overtraining or acquiring a repetitive strain injury, causing piriformis irritation or hypertrophy Sitting for prolonged periods (office workers, taxi drivers, bicycle riders) Anatomic variations which might cause piriformis syndrome have been reported (e.g. sciatic nerve branching, bipartite piriformis muscle); however, comprehensive anatomic data on the piriformis morphology is lacking. In other words, patients with piriformis syndrome sometimes have anatomic variations suggesting a cause, but it's not known if these anatomic variations actually occur more frequently in patients with piriformis syndrome. A heavily scrutinized anatomic variation involves sciatic nerve branching around the piriformis using the 6-category classification first described by Beaton and Anson. In this classification, the normal anatomy (type A) seen in about 80% of people is where the sciatic nerve passes under the piriformis muscle at the greater sciatic notch. However, the incidence of atypical anatomic variations (types B-F) has not been found to differ significantly between piriformis syndrome patients and cadavers, leading to doubt about the importance of this anatomic variation in the development of piriformis syndrome.

Pathophysiology

Under certain conditions, the piriformis muscle is believed to compress the sciatic nerve, also known as sciatic nerve entrapment, causing sciatica. The pathophysiology of piriformis syndrome is not completely understood. There are several mechanisms by which the piriformis muscle is thought to be capable of compressing the sciatic nerve, and these mechanisms are not mutually exclusive. Piriformis muscle spasm may compress the sciatic nerve. As the piriformis muscle spasms, it shortens and becomes harder, applying greater pressure on the sciatic nerve against the ischium at the inferior greater sciatic foramen. The empirical evidence supporting this is that patients can often see immediate and permanent relief from local anesthetic and the effectiveness of Botox injections as a muscle relaxer. Piriformis muscle hypertrophy may cause crowding around the greater sciatic foramen. Both the piriformis and sciatic nerve pass through the greater sciatic foramen and the deep gluteal space. An enlarged (hypertrophic) piriformis muscle may place pressure on nearby structures. The empirical evidence supporting this is that ipsilateral (same-side) piriformis hypertrophy is a common image finding in piriformis syndrome, and that botox injections reduce symptoms (by paralyzing a muscle for months the muscle shrinks). A single injury, or many smaller injuries, may predispose the piriformis muscle to fibrosis, making the tissue tougher and tighter, applying greater pressure on the sciatic nerve against the ischium at the greater sciatic foramen. Or the formation of scar tissue from a hematoma might restrict normal movement of the sciatic nerve as it passes by the piriformis muscle. The piriformis may be capable of dynamically compressing the sciatic nerve with certain hip movements. The empirical evidence supporting this is the presence of electrophysiology testing abnormalities (delayed H reflex) of the sciatic nerve during the FAIR test, as well as the improvement of these electrophysiology results after successful treatment.

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