Piriformis syndrome: Pathogenesis, diagnosis, and treatment CHARLES STEINER, D.O., FAA() Maplewood, New Jersey CHARLES STAUBS, B.S. Lindenwold, New Jersey MICHAEL GANON, D.O. Littleton, Colorado CHRISTINE BUHLINGER, Morrison, New Jersey The failure of conservative syndrome present almost identically to treatment for lumbosacral disk lumbar disk syndrome, except for the disorders often leads to surgery. If the consistent absence of true neurologic pain is produced by sciatic neuritis findings. Diagnosis is accomplished by rather than sciatic radiculitis, operative palpation of myofascial trigger points treatment may be unavailing. This paper within the piriformis muscle. Computed describes the mechanism by which tomography, myelography, piriformis syndrome causes sciatic roentgenography, and neuritis and differentiates neuritis from electromyography are of limited radiculitis, the treatment of which often diagnostic value. Treatment, which results in the "failed disk syndrome." consists of a conservative approach Sciatic neuritis is now believed to result employing local anesthetics and from irritation of the sciatic nerve osteopathic manipulation, is without sheath, which is caused by biochemical significant risk. Reducing muscle spasm, agents released from an inflamed restoring joint motion, and keeping the piriformis muscle where the two patient ambulatory and in motion are structures meet at the greater sciatic keys to successful treatment. foramen. The symptoms of piriformis Persistent or severe pain along the distribution of and a therapeutic approach to the piriformis syn- the sciatic nerve may be produced by irritation or drome. mechanical trauma anywhere along its course. Two common causes of sciatic distribution pain are lum- Microanatomy of the sciatic nerve bar disk herniation and piriformis syndrome.-3 The sciatic nerve, a "typical" peripheral nerve, is Back pain is common to both. It has been our expe- ensheathed by epi-, peri-, and endo-neurium. The rience that direct conservative measures may be epineurium surrounds the whole nerve, the per- employed effectively in both differential diagnosis ineurium surrounds individual nerve fascicles, and and treatment of herniated lumbar disk and pi- the endoneurium surrounds individual neurons. riformis syndrome. The ambulatory conservative Below the level of the endoneurium, myelinated treatment of disk disease has been described pre- fibers are encompassed by Schwanns cells and my- viously.4 Our aim in this paper is to provide an elin sheaths; unmyelinated fibers lack the latter.5 understanding of the different symptomatology of The function of the connective tissue sheaths sur- Piriformis syndrome: Pathogenesis, diagnosis, and treatment 318/111 rounding the sciatic nerve is to provide protection The two parts of the nerve within their common against mechanical and chemical insult. The epi- sheath usually pass between the piriformis muscle neurium and perineurium provide mechanical pro- above and gemelli muscles below. It travels through tection; the perineurium also serves as a chemical the buttock and out of the pelvis through the diffusion barrier because of the absence of pi- greater sciatic foramen. The nerves usually will not nocytotic transport and the presence of occluding pursue separate courses until reaching the junctions that connect its epithelioid cells.5-7 The popliteal space 13 diffusion barrier of perineurium combined with the The piriformis muscle typically arises from the blood-nerve barrier of the endoneurial vessels anterior surface of the second through the fourth provide the sciatic nerve with an environment iso- sacral vertebra, the upper margin of the greater lated from chemical conditions outside its barri- sciatic foramen, and the sacrotuberous ligament, to ers.7 Only in cases of crushing trauma or severe insert on the superior surface of the greater ischemia are these barriers breached 78 trochanter of the femur (Fig. 1). However, variations The connective tissue sheaths that surround the exist in a significant percentage of the population. nerve roots and sciatic nerve are innervated by The contraction of the piriformis muscle produces nerve fibers called the nervi nervorum. The ventral abduction and external rotation of the thigh. Occa- nerve root connective tissue sheath receives fibers sionally, the origin is fused with the overlying glu- from the sinu-vertebral nerve, and the dorsal nerve tei muscles or underlying superior gemellus root connective tissue sheath receives fibers from muscle. In about 20 percent of the population, the spinal ganglia.9 The sciatic nerve connective tissue muscle is split and one or both parts of the sciatic sheath receives fibers from the plexuses of nutrient nerve pass through the muscle belly.° In 10 percent arteries and nerve fascicles of the sciatic nerve.1° of the population, the tibial and peroneal portions Many of the nervi nervorum fibers are unmyeli- of the sciatic nerve are not enclosed in a common nated, with free nerve endings indicative of type C sheath, and one (usually the peroneal part) or, nociceptive neurons. Histamine and bradykinin rarely, both parts may pierce the muscle 13 applied to nociceptive free nerve endings result in pain, while prostaglandin E applied to nociceptors Sciatic neuritis versus sciatic radiculitis produces hyperalgesia to mechanical and his- The processes leading to sciatic radiculitis as op- tamine stimuli." posed to sciatic neuritis are quite different. In the Because both the root sheath and peripheral lumbar spine, the integrity of the spinal nerve roots nerve sheath derive their innervation from the spi- may be influenced by the encroachment of a bulg- nal nerve roots that form the sciatic nerve, irrita- ing disk, cord tumor, osteophytes, extravasated tion of either will result in pain perceived as being blood, spinal stenosis, or pedicular kinking.14 along the somatotopic distribution of the sciatic Sciatic radiculitis caused by a bulging disk results nerve. This is due to convergence of visceral, mus- in unequivocal neurologic signs, including loss of cular, and cutaneous pain fibers on dorsal horn cells deep tendon reflexes at the patella or ankle (de- in Rexeds lamina 5 of the spinal cord, which pre- pending on whether the root involved is L3, IA, or vents the body from clearly differentiating visceral Sl, respectively) and paresis of muscles supplied by and cutaneous pain. II,I2 The tendency is to refer the involved segment.3,15 The symptoms may be visceral and subcutaneous pain to the cutaneous produced or increased by coughing, sneezing, or somatotopic map in the brain. Therefore, sciatica straining because they cause an increase in spinal can result from irritation of the sciatic nerve sheath fluid pressure. Such pressure on an already in- either at the roots or anywhere along the course of flamed root may cause pain in the low back and the peripheral nerve because of the nervi nervorum down the leg, paresthesia along the sciatic nerve nociceptors located within the sheath. distribution, and coldness of the foot. These symp- toms together with weakness in foot dorsiflexion, Gross anatomy of the sciatic nerve and foot drop, and flaccidity of the extensor hallucis piriformis muscle longus muscle lead to a diagnosis of sciatic radi- The sciatic nerve, rather than a single trunk, con- culitis.3 sists of the peroneal and tibial portions, which are, The signs and symptoms of sciatic neuritis, as in most cases, bound together by the fibrous epi- exemplified by piriformis syndrome, are distinct neurium as the nerve runs distally to the popliteal from those of sciatic radiculitis. By far, the most space. The peroneal portion usually is derived from important criterion in the differentiation of sciatic fibers of the fourth and fifth lumbar and first and pain caused by piriformis syndrome is the lack of a second sacral spinal nerves. The tibial division has true neurologic deficit. 1,2 Generally, the only relia- additional fibers from the third sacral spinal nerve. ble positive sign is point tenderness over either end 319/112 April 1987/Journal of AOA/vol. 87/no. 4 of the piriformis muscle itself; this may be found near its origin just lateral to the midsacrum and its insertion just medial to the greater trochanter. The symptoms are often intensified by the palpatory examination. The chief symptoms are pain and/or paresthesia anywhere along the course of the scia- Is it tic nerve. nerve root The qualitatively different pathogenesis of these compression Nerve roots two entities is responsible for the similar but diver- (radiculitis) gent symptomatology. In the disk syndrome, me- or Piriformis muscle chanical trauma to the nerve root results in intraneural pathology. 16,17 Compression or stretch- nerve ing of a nerve root by a bulging disk may result in compression segmental demyelination, impairment of intra- (neuritis)? neural blood flow, and intraneural edema and in- flammation.16-18 Extreme compression may even result in axonal transport block interfering with trophic functions of the axon.17 Demyelination, is- Sciatic nerve chemia, and axonal transport block result in dete- rioration of nerve function, which is observed clinically as neurologic deficits. 17 Inflammatory agents produced by the mechanical trauma to the Fig. 1. Nerve irritation in the herniated disk occurs at the root (sciatic nervous tissue and intraneural blood vessels, par- radiculitis). In the piriformis syndrome it is to the full thickness of the nerve (sciatic neuritis). ticularly bradykinin