The piriformis syndrome is characterized by nondisco-genic, extrapelvic, sciatic nerve compression in the area of the greater sciatic notch. The symptoms include pain and dysesthesias isolated to the buttock region, radiating to the hip or posterior thigh, and/or occurring distally as radicular pain.42 The symptoms of piriformis syndrome are thought to be caused by entrapment of one or more divisions of the sciatic nerve by the piriformis muscle.43,44 The original description of this condition dates back to 1928 when Yeoman45 first described the possibility of a pathologic relationship between the sciatic nerve and the piriformis mus cle. Edwards46 described it as "neuritis of branches of the sciatic nerve, caused by pressure of an injured or irritated piriformis muscle." Freiberg and Vinke43 were the first to describe the classic findings of Lasegue's sign and tenderness at the sciatic notch over the piriformis muscle. The common peroneal division of the sciatic nerve is thought to be more frequently affected. Anatomic variants of the sciatic nerve as it courses along the piriformis muscle are thought to be contributory factors; however, this association is not grounded in diagnostic studies or surgical observa-tion.47 Historically, diagnostic studies have not been reliable; therefore, the diagnosis was made strictly on clinical grounds. This fact has severely compromised the credibility of this entity as a valid diagnosis. A fundamental problem lies in the fact that the piri-formis muscle has not been proven as the singular structure compressing the sciatic nerve in this syndrome; therefore, the nomenclature piriformis syndrome is possibly inaccurate. Any of a number of lesions around the greater sciatic notch may injure or cause dysfunction to the sciatic nerve. This condition is analogous to carpal tunnel syndrome, which is a similar peripheral entrapment neuropathy of the median nerve with an array of causes. Similar to the sciatic nerve, entrapment of the superior and inferior gluteal nerves, posterior femoral cutaneous nerve, and the pudendal nerve can cause symptoms indistinguishable from the piriformis syndrome. It is also theoretically possible that the obturator internus-gamelli complex is an alternate cause of neural compression.48 The nomenclature is therefore vague, confusing, and possibly erroneous. A new definition, deep gluteal syndrome, has emerged in the sports medicine literature. McCrory48 has recommended adopting this new nomenclature to better reflect the complex and elusive nature of this condition.
Compression of the sciatic nerve around the piriformis muscle can be caused one of a number of reasons. The sciatic nerve, like other peripheral nerves, is prone to injury and dysfunction if it is compressed by any of the surrounding tissues along its path. It is reasonable to infer that sciatic pain may be caused by compression anywhere along the nerve's length, from the spinal root level to the popliteal fossa. At the level of the piriformis muscle the sciatic nerve is particularly prone to injury because of its proximity to a vast array of structures including vessels, muscle, fascia, and bony structures, which at any time may become pathologic factors. Any disease affecting the structures that surround the sciatic nerve and its branches, such as aneurysm, tumors, infections, and hypertrophic conditions, may potentially affect the sciatic nerve function. The piriformis muscle syndrome has been reported after inferior and superior gluteal artery an-
eurysms,42 after prolonged surgery in the sitting posi-tion,49 after total hip replacements,50 and secondary to space-occupying lesions.5l,52 Trauma has also been reported as an important etiological factor.l2,53-55). This observation may be because the sciatic nerve lies within the gluteal area, a region prone to trauma from falls or direct injurious forces. Trauma to the muscles leads to irritation, inflammation, spasm, adhesion, and hypertrophy of the muscle. These traumatic changes may lead to dysfunction of the sciatic nerve and its branches. In a large series by Benson,l2 patients treated surgically had a definite history of an injurious force over their gluteal area. Intraoperative findings in this group of patients revealed piriformis muscle adhesions causing compression of the sciatic nerve.
Hypertrophy or aberrant fibrous bands of the piri-formis muscle could theoretically compress the sciatic nerve or any branch within the muscle belly.53,56 Hypertrophy may be due to increased strain on the hip abductors such as from abnormal gait mechanics or increased lumbar lordosis.44 Other possible causes for piriformis muscle hypertrophy may include repetitive exercise-induced trauma and from chronic low-energy trauma as a result of sitting on a hard surface for extended periods.
Aberrant anatomy of the sciatic nerve and the piriformis muscle is speculated to play a role in the genesis of piriformis muscle syndrome.52 Beaton and Anson47 studied the anatomic variations of the relationship of the sciatic nerve to the piriformis muscle in cadaveric dissection. Typically, the sciatic nerve exits the greater sciatic foramen, passing below the belly of the piriformis (Figure 5.6). In l5% of autopsy cases, the nerve actually passes through the belly of the mus-cle.47 Many other variations in the anatomy exist, including bipartite piriformis muscle belly, divisions of the sciatic nerve into its peroneal and tibial divisions occurring superior or within the piriformis muscle, and a piriformis muscle completely anterior to the sciatic nerve.47,52,57 These anomalies can be seen in asymptomatic individuals and are rarely observed in most surgeries for exploration and decompression of the piriformis muscle.58-61 Therefore, surgical release of the piriformis muscle in patients with anatomic anomalies and no other objective evidence is not recommended, and an alternative diagnosis should be first sought.
Any mass, tumor, or other space-occupying lesions can have neurologic manifestations when they compress adjacent neurologic structures. Space-occupying lesions known to have manifested as piriformis muscle syndrome include tumors, aneurysms/pseudoaneurysms, persistent sciatic artery, vena comitantes, large tortuous veins, abscesses, and myositis ossificans.42,51-53
Knowledge of the anatomy of the sciatic nerve and it relationship to the piriformis muscle and gluteal ves sels is fundamental to understand the clinical picture of entrapment of the nerve. The piriformis muscle originates along the ventrolateral surface of the sacrum, where its origin interdigitates with the sacral nerve roots S2, S3, and S4, and continues to run laterally through the greater sciatic notch to exit the pelvis. It then inserts along with both gamelli and the obturator internis tendon into the piriformis fossa of the superior/posterior aspect of the greater trochanter. Collectively, this cluster of muscles forms the short external rotators of the hip. Thus, the piriformis muscle is an external rotator of the extended hip and an abductor when it is flexed. Nerve branches from L5, Sl, and S2 innervate the piriformis muscle. The sciatic nerve is formed by the L4, L5, Sl, S2, and S3 sacral roots. It exits the pelvis to enter the gluteal region through the greater sciatic notch. At the distal edge of the notch, the nerve is found deep to the piriformis muscle and above the gamelli and obturator internus muscle. The superior and inferior gluteal nerves and arteries are found directly superior and inferior to the piriformis, respectively.
Clues for diagnosing piriformis muscle syndrome can be gained through a detailed and focused clinical examination. Interestingly, the clinical presentation does vary widely between published case reports; therefore, no single examination or criterion is available to confirm the diagnosis. Piriformis muscle syndrome should be suspected in cases of sciatica or posterior gluteal/thigh pain and nondiagnostic MRI of the spine. The symptoms of piriformis muscle syndrome are nonspecific and include pain and dysesthesias occurring in the gluteal region radiating to the hip or posterior thigh, and sometimes in a radicular pain pattern down the leg. The athlete may experience cramping, burning, or aching in the buttock or posterior thigh, making the symptoms indistinct from a hamstring tear or intraarticular hip problems. Robinson,55 who coined the term piriformis muscle syndrome, described a sausage-shaped tender mass over the area of the piriformis muscle as one of the key features of the syndrome. The physical examination also shows tenderness and reproduction of the radicular pain with deep palpation of the piriformis muscle. On rectal or pelvic examination, palpation of the piriformis and sciatic notch may reproduce and aggravate the symptoms. The radicular pain may also be reproduced by passively raising the straightened leg, Lasegue's sign. Frequently, flexion, adduction, and internal rotation (FAIR) of the hip exacerbate the symptoms. Freiberg's sign58 (pain on passive internal rotation of the hip in neutral extension) and Pace's sign (weakness and pain on resisted abduction-external rotation of the thigh) may be present.53,54 The neurologic examination can show abnormalities such as abnormal reflexes or motor weakness42; however, deficits are rare. There may also be evidence of superior gluteal nerve (weakness of the gluteus medius and minimus muscles), inferior
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.