Piriformis syndrome is an infrequent cause of entrapment neuropathy. A diagnosis of exclusion, piriformis syndrome occurs secondary compression or entrapment of the sciatic nerve by the piriformis muscle.
Historically, the exact definition of this syndrome has varied in the literature. In 1928, Yeoman was the first to describe pain in the sciatic nerve distribution secondary to piriformis syndrome. In 1947, Robinson coined the term piriformis syndrome and proposed six cardinal features which include:
Since this historical report, many of Robinson's proposed diagnostic criteria have since been disputed or refuted. Contemporary dogma is accepting of a much broader clinical definition accompanying a diagnosis of piriformis syndrome, defined as a syndrome consisting of isolated sciatic pain usually limited to the buttock with radiation down the thigh .
Symptoms associated with piriformis syndrome occur from compression of the sciatic nerve by the piriformis muscle. The cause of compression and subsequent nerve irritation is variable. It can result from overuse or repetitive trauma where overtraining or sitting on hard surfaces that can cause direct or indirect injury to the muscle or nerve.
Athletes and weightlifters are prone to intermittent symptom exacerbation secondary to increased training requirements and activities. In these cases, the piriformis muscle itself becomes hypertrophied and causes direct pressure on the nerve.
Acute trauma like blunt forces can also result in hematoma formation and subsequent scarring between the sciatic nerve and the short external rotators (piriformis, superior and inferior gemelli, quadratus femoris). Anomalous anatomic relationships of the sciatic nerve with the piriformis muscle have also been implicated where the nerve is more susceptible to compression because of the variant anatomy. Beaton and Anson originally described 6 different anatomic relationships between the sciatic nerve and piriformis muscle. In >80% of the population, the sciatic nerve passes deep and exits inferiorly to the inferior edge of the muscle belly/tendon . Notable variations include early (proximal) divisions of the sciatic nerve into its tibial and common peroneal components, with these branches passing through and below the piriformis muscle or above and below the muscle. These anomalous sciatic nerve variants are all a potential cause of piriformis syndrome .
There have also been numerous case reports that show that other conditions have been associated with piriformis syndrome which includes pyomyositis, aberrant fibrous bands, myositis ossificans, and pseudoaneurysms.
Piriformis syndrome may be responsible for 0.3% to 6% of all cases of low back pain and/or sciatica. With an estimated amount of new cases of low back pain and sciatica at 40 million annually, the incidence of piriformis syndrome would be roughly 2.4 million per year. In the majority of cases, piriformis syndrome occurs in middle-aged patients with a reported ratio of male to female patients being affected 1:6.
Two components contribute to the clinical presentation of piriformis syndrome: somatic and neuropathic. The somatic component is a myofascial pain component of the syndrome secondary to any of the surrounding muscles/fascia (i.e. any of the short external rotators of the hip). The neuropathic component refers to the compression or irritation of the sciatic nerve as it courses through above, near, or through, the piriformis muscle itself.
Patients may present acutely in cases of post-traumatic piriformis syndrome or insidiously as in cases of overuse or myofascial pain. Patients will report having gluteal pain with possible associated paresthesias in the back, groin, perineum, buttocks, hip, back of the thigh, calf, foot, and rectum. There is also intense and worsened pain with sitting or squatting.
Initial evaluation should start with a lumbar spine examination. The examination should take into account the patient's gait, posture, and alignment as well as any leg length discrepancies. Also, the practitioner should examine the patient's hips, pelvis, and the sacroiliac joint, noting sensory, motor, and deep tendon reflexes.
In piriformis syndrome, sensory, motor, and deep tendon reflexes are normal. The piriformis should be palpated as well just posterior to the hip joint and in the area of the greater sciatic notch. Straight leg raise may also be positive although there is variable sensitivity and specificity for this provocative maneuver.
There are individual tests that stretch the piriformis and can be used to help aid in the diagnosis of exclusion. One of these is the Freiberg sign, which is performed by putting the hip in extension and internal rotation and having the patient externally rotate against resistance. The positive exam will reproduce pain around the piriformis. This test will have positive results in up to 63% of patients. Another individual test called the Pace sign is performed by resisting abduction and external rotation of the hip while the patient is in a seated position. Pace sign occurs in 30% to 74% in piriformis syndrome. FAIR test or also known as the piriformis stretch may elicit pain. Beatty sign has also been described and is accomplished by elevating the flexed leg on the irritated side while the patient lies on the asymptomatic side. Pain and reproduction of the symptoms are a positive test result.
No definitive diagnostic criteria has been validated and established for piriformis syndrome. It remains a diagnosis of exclusion, but many of the following exam findings can aid the clinician in honing in on this diagnosis:
Standard radiographs of the pelvis and hip should be done to rule out underlying hip pathology. MRI of the lumbar spine is also recommended to evaluate for a discogenic causes of lumbar radiculopathy and to rule out other sources of compression (e.g. soft tissue tumors or masses). MRI can also evaluate for spinal stenosis, herniated disks, facet arthrosis, SI joint pathology, occult pelvic fractures, and any surrounding tendinopathies or bursitides.
EMG is often normal in patients with acute piriformis syndrome presentations, however, chronically, EMG/NCS can be useful in identifying neuropathic changes and abnormal results in muscles innervated by the sciatic nerve .
Patient Position: Prone
Fluoroscopy starting position:
Piriformis Muscle Injection with Fluoroscopic Guidance
Caution: Never inject if the patient feels a sharp pain shooting down their leg as the needle tip may be inside the sciatic nerve. The needle should be repositioned and then retry.
Treatment for piriformis syndrome begins with nonoperative modalities including:
Piriformis syndrome is an infrequent cause of entrapment neuropathy. Significant pain relief may be obtained by injection of the piriformis muscle.
An interprofessional team involving a doctor and nurse will result in the best possible outcomes.
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