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 hypertrophyChang A, Ly N, Varacallo M. Piriformis Injection. [Updated 2023 Aug 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK448193/ 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.
Diagnosis Piriformis syndrome does not yet have a validated set of diagnostic criteria; however, the diagnosis is primarily clinical, involving a physical examination and an evaluation of patient history. Imaging can assist in excluding other conditions with similar symptoms, such as lumbar disc herniation and spinal stenosis. Diagnostic injections of anesthetic into the piriformis may be used to confirm the diagnosis. Often piriformis syndrome is a diagnosis of exclusion.
A complete exam of low back, pelvis, buttocks, lower extremities may be necessary to rule out differential diagnoses. Sciatica secondary to conditions to be ruled out include spinal disc herniation, facet arthropathy, spinal stenosis, lumbar muscle strain, wallet neuritis (sitting on a thick wallet), endometriosis, pelvic tumors, gluteal varicosities, and inferior gluteal artery aneuyrism. Pathology in the sacroiliac joint region, Sacroiliac joint dysfunction and Sacroiliitis are other conditions that present with pain in the low back and hip regions, which may radiate down along the back of the thigh, rarely going down below knee.
The clinical signs may involve unilateral or bilateral buttocks pain that fluctuates throughout the day, the absence of lower back pain, buttocks or sciatica pain when in the sitting position (especially for prolonged periods), sciatic pain with fluctuating periods without pain throughout the day, buttocks pain near the piriformis. The pain may be exacerbated with any activity that causes flexion of the hip such as stooping or lifting.
In a physical exam, the piriformis can be palpated externally though it is so deep that it may not be possible to assess tenderness. If the piriformis can be located it may feel cord-like. Internal palpation is also possible which allows better access to assess piriformis muscle tenderness. There are a number of maneuvers that can be done in an attempt to provoke sciatic nerve compression. These tests either stretch the piriformis or cause the piriformis to contract. The most common tests used are the Freiberg test, FAIR test (flexion, adduction, internal rotation), Beatty test, Pace test, seated piriformis stretch test, and straight leg raise (Lasegue sign).
Diagnostic modalities such as EMG, x-rays, ultrasound, CT, and MRI are mostly used to exclude other conditions. For example, and MRI of the spine can rule out conditions like radiculopathy and spinal stenosis. MRI of the pelvis can rule out conditions like pelvic tumors. MRI and ultrasound can be used to observe side-to-side differences, a common finding where the symptomatic side often exhibits increased piriformis size measured as increased thickness and cross-sectional area. Specialized sequences and protocols of MRI can be used for sciatic nerve imaging, namely MRN (magnetic resonance neurography) and DTI (diffusion tensor imaging). MRN can assess changes in the path, thickness, and signal intensity of the sciatic nerve. DTI uses the anisotropic diffusion of water to assess structural and functional properties of nerves. Both MRN and DTI can localize nerve lesions, their extension, and their spatial distribution due to conditions such as entrapment. MRN and DTI have also been used to visualize sciatic nerve lesions to diagnose of piriformis syndrome. However, 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 into the piriformis muscle can assist in the diagnosis. Injections usually involve delivering anesthetic to the piriformis muscle to paralyze it. In the event of a piriformis muscle spasm causing sciatic nerve compression, paralyzing the piriformis muscle will temporarily relieve the symptoms. If successful, there should be a complete or near-complete relief of pain for 4–6 hours. Needle guidance can be done with fluoroscopy, ultrasound, CT, or MRI. Ultrasound is a popular choice due to its low cost, lack of radiation, and accessibility, but lacks the spatial resolution of CT/MRI. While there is no gold standard test to diagnose piriformis syndrome, in deep gluteal syndrome, the generalization of extra-spinal sciatica in the deep gluteal space, diagnostic blocks are considered the gold standard for differentiating alternate sources of pain.
Prevention