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Case Report |
1 Mallinckrodt Institute of Radiology, Washington University School of Medicine,
510 S Kingshighway Blvd., Box 8131, St. Louis, MO 63110.
2 Department of Anesthesiology, Washington University School of Medicine, MSK
Health, Wellness, and Rehabilitation, 11652 Studt Ave., St. Louis, MO
63141.
Received March 11, 2003;
accepted after revision October 21, 2003.
Address correspondence to E. Y. Lee
(Leeed{at}mir.wustl.edu).
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At physical examination, the lumbar spine was not tender and had full range of motion. All radicular provocation test findings were normal. Flexing the hip at 90° and adducting across the midline with deep digital palpation produced exquisite tenderness in the right piriformis muscle that reproduced the patient's pain. Passive internal rotation of the right thigh caused pain (Friberg's sign), as did resistance to abduction and external rotation of the right thigh (Pace's sign) and voluntary adduction, flexion, and internal rotation of the hip (Lasègue's sign). Neurologic and vascular examinations showed no abnormality.
Imaging studies were all unremarkable, including conventional radiographs of the pelvis, lumbar spine, sacrum, and coccyx; a triple-phase bone scan; and MR images of the lumbar spine. The findings of electromyography were normal. The patient also underwent a diagnostic trigger point injection with little or no benefit. It was believed that the patient's symptoms were likely the result of either local spasm of the piriformis muscle or piriformis syndrome.
The patient was referred for an MRI of the sacral plexus with attention focused on the piriformis muscle, to narrow the possibilities. This study showed an anomalous sacral attachment of the right piriformis muscle, with accessory muscle fibers extending medially over the S2 foramen and crossing over the right S2 nerve (Fig. 1A, 1B). The piriformis muscle and sciatic nerve were normal in signal intensity on T1- and T2-weighted sequences. No other abnormalities in anatomic relationships with the surrounding structures were found.
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The patient underwent surgical exploration through an incision extending from the posterior superior iliac spine to the greater trochanter. The sciatic nerve was normal in appearance and location. However, it was compressed by the piriformis muscle crossing anterior to the right S2 nerve root. The S2 nerve was surgically released and appeared slack after approximately 1 cm of the piriformis tendon was resected near its insertion at the piriformis fossa.
The patient reported complete relief of symptoms immediately after the operation. The postoperative course was uneventful. One month after surgery, the patient's lower back pain and right lower limb symptoms had not returned. The patient had a minimally reduced range of right hip flexion, adduction, and internal rotation, but these maneuvers did not elicit the previous symptoms. Five years later, the patient's symptoms have not recurred.
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No definite causative factors are known for this syndrome, but the usual source is thought to be traumatic injury to the piriformis muscle that results in spasm, edema, and contractures of the muscle and causes subsequent compression and entrapment of the sciatic nerve [5]. Other possible causes of this syndrome include reflex spasm of the piriformis muscle and an abnormal course of the sciatic nerve through the piriformis muscle [6] or its tendon [1]. Altered biomechanics resulting from leg length discrepancy leading to stretching and shortening of the piriformis muscle also can be associated with piriformis muscle syndrome [6].
The diagnosis of piriformis syndrome was previously thought to be purely clinical, and the role of imaging techniques has been largely ignored. However, MRI can be a valuable noninvasive diagnostic test, typically revealing an enlarged piriformis muscle [7, 8]. MRI can help to correctly diagnose piriformis syndrome and also to differentiate piriformis syndrome from other possible causes of lower lumbar pain and sciatica, such as lumbar disk herniation, lumbar stenosis, and mass lesions in the region of the piriformis muscle [5].
Unlike other patients described by Jankiewicz et al. [7] and Rossi et al. [8], our patient had a piriformis muscle of normal size, but with an anomalous attachment crossing over a sacral nerve at the foramen. In our patient, the muscle did not impinge directly on the nerve at the time of MRI, and the sciatic nerve seen on preoperative MRI was normal in size and signal characteristics. This could be due to the fact that piriformis syndrome is a functional entity in which the nerve becomes compressed during prolonged sitting, walking, running, or other exercise. It is possible that the sciatic nerve was not compromised by the piriformis muscle while the patient was lying comfortably on the MRI table. Furthermore, our patient had been limiting physical activities that might precipitate his symptoms for approximately 3 weeks before MRI. This may have prevented signal change in the nerve on MRI. Another possible explanation for the lack of MRI signal change in the sciatic nerve may be the chronic nature of this syndrome.
The treatment of piriformis syndrome may include the administration of nonsteroidal antiinflammatory agents and corticosteroids, injection of local anesthetics, and physical therapy [9]. For patients with symptoms refractory to these conservative treatments, surgical release of the piriformis muscle is often recommended and has been reported to be effective in relieving the symptoms [9]. Our case was unlike the typical surgical situation because it required release of an anomalous sacral attachment of the piriformis muscle that was depicted preoperatively on MRI.
In summary, the rarity, nonspecific clinical symptoms, and absence of definitive diagnostic tests may cause the diagnosis of piriformis syndrome to be missed or delayed. MRI can be used to make a correct diagnosis, to specify anatomic relationships for preoperative planning, and to differentiate piriformis syndrome from the more common causes of lower back pain and sciatica. Therefore, familiarity with the appearance of piriformis syndrome on MRI is important to facilitate appropriate diagnosis and treatment.
Acknowledgments
We thank Mehdi Poustchi-Amin for reading our initial manuscript.
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