| | Measurement of Motor Nerve Conduction Velocity of the Sciatic Nerve in Patients With Piriformis Syndrome: A Magnetic Stimulation StudyAbstract Chang C-W, Shieh S-F, Li C-M, Wu W-T, Chang K-F. Measurement of motor nerve conduction velocity of the sciatic nerve in patients with piriformis syndrome: a magnetic stimulation study. ObjectiveTo assess the motor nerve conduction of the sciatic nerve by a magnetic stimulation method in patients with piriformis syndrome. SettingAn electrodiagnostic laboratory in a university hospital. ParticipantsTwenty-three patients with piriformis syndrome and 15 healthy persons for control. InterventionsNot applicable. Main Outcome MeasuresMotor nerve conduction velocity (MNCV) of the sciatic nerve was measured at the gluteal segment by magnetic stimulation proximally at L5 and S1 roots and distally at sciatic nerve at gluteal fold and recording at the corresponding muscles. Diagnostic sensitivities were measured in the magnetic stimulation method and the conventional nerve conduction, long latency reflex, and needle electromyography studies. ResultsThe mean MNCV of the sciatic nerve ± standard deviation at the gluteal segment in L5 component was 55.4±7.8m/s in patients with piriformis syndrome, which was slower than the mean value of 68.1±10.3m/s obtained in healthy controls (P=.014). The MNCV of the sciatic nerve in S1 component showed no significant difference between the patients and controls (P=.062). A negative relation was found between the disease duration and the MNCV values of sciatic nerves in patients with piriformis syndrome (r=−.68, P<.01). The diagnostic sensitivity by magnetic stimulation is .467. ConclusionsMagnetic nerve stimulation provides a painless, noninvasive, and objective method for evaluation of sciatic nerve function in patients with piriformis syndrome. PIRIFORMIS SYNDROME IS defined by a loose cluster of symptoms arising from the entrapment of the sciatic nerve passing the sciatic notch, and it may lead to buttock or hamstring pain. Causes for anatomic abnormalities of the piriformis muscle and the sciatic nerve may result in irritation of the sciatic nerve by the piriformis muscle. Clinically, it often mimics sciatica caused by the spine. Although some available evaluation methods, including imaging studies of piriformis syndrome as shown by Rossi1 and Filler2 and colleagues, have greatly enhanced diagnostic specificity and sensitivity to anatomic abnormalities, there are still limited electrodiagnostic tools to examine functional entrapment of the sciatic nerve at the crossing level of piriformis muscle. In the previous study by Fishman and Zybert,3 the H-reflex was used to evaluate patients with piriformis syndrome. Their study showed that H-reflex was normal at rest, but there were delayed latencies in the FAIR position (hip flexion, adduction, and internal rotation). Nakamura et al4 reported another method for the diagnosis of piriformis syndrome by using a recording evoked potential from an epidural electrode at the lumbar spine and stimulation of the peroneal nerve at the fibular head. The previously reported methods are all valuable. However, their results are limited in sensory evaluation, and there is difficulty in specifying the lesion localization because of the delayed latency in the long reflex course. Chang and Lien5 showed an invasive method for spinal nerve root stimulation to evaluate patients with sciatica and lumbosacral radiculopathy. They used a monopolar needle for nerve root stimulation proximally and near sciatic nerve stimulation distally at the gluteal fold. Motor nerve conduction velocity (MNCV) of the sciatic nerve at gluteal segment could be measured by this method, and it is useful for evaluating radiculopathy and could possibly be used in patients with piriformis syndrome. However, electric stimulation with a needle is invasive and somewhat painful. In the present study, we propose a new method for assessment of motor nerve conduction at the gluteal segment of the sciatic nerve by magnetic stimulation in patients with piriformis syndrome and compare it with the conventional electrodiagnostic methods. Methods  Patient Profile Twenty-three patients with a clinical diagnosis of piriformis syndrome, drawn from outpatients referred to the electrodiagnostic laboratory of a university hospital from 2003 to 2005, participated in this study. They included 15 women and 8 men, with ages ranging from 40 to 67 years (mean, 53.8y). The clinical diagnosis of piriformis syndrome was according to the criteria of Fishman et al,6 including the accepted signs of (1) sciatica or gluteal pain in the FAIR position, (2) local tenderness at the gluteal area between the piriformis muscle and the sciatic nerve, and (3) positive Lasègue sign at supine position. Patients with diabetes, neuritis, traumatic nerve injury, spinal operation, hip joint replacement, or blood dyscrasia were excluded. Among the 23 patients with piriformis syndrome, 16 had unilateral symptoms and 7 suffered from bilateral involvement. The disease duration from the onset of the symptoms to the nerve conduction studies was between 2 and 26 months (mean, 6.6mo). In addition, 15 healthy persons ranging from 36 to 60 years (mean, 51.1y) were included as control subjects. Therefore, there were 30 affected limbs and 16 unaffected limbs in the patients with piriformis syndrome and 30 limbs in the controls for the present study. Selection of Nerve Stimulation and Muscle Recording The sciatic nerve is a major nerve arising from the lumbosacral plexus, originating from the ventral rami of L4 to S3 roots. The peroneal portion is composed of posterior divisions of L4 to S2 roots, and the tibial portion is composed of the anterior divisions of L4 to S3 roots. The nerve root contributions fuse to form a single trunk within the pelvis, which then exits through the sciatic notch inferior to the piriformis muscle.7 The L5 root is the major component of peroneal nerve portion supplying dorsiflexors of ankle and toes extensors. The tibialis anterior muscle is dominantly supplied by the L5 component and was selected for recording in the present study. The S1 root is the major component of the tibial nerve portion supplying the hamstring and the calf muscles. The medial head of the gastrocnemius muscle is mainly supplied by the S1 component and was also used for recording in the present study. Magnetic Stimulation Study Magnetic nerve stimulation was performed by using a Magstim 220 stimulator,a which was connected to a figure-of-8 coil of 70mm in diameter (type 9925a) for each external circular coil. The calibration of the shield was performed at the room temperature of 24°C by using a Mag-03MS magnetometerb that was cross-calibrated with the Magstim system. The stimulation intensity was set at 100% of the maximum output with peak magnetic field of 2.2T and peak electric field strength of 660V/m. The distribution of the magnetic fields excited by the figure-of-8 coils yielded higher values of the induced electric field in the regions below the coil surface. The radial displacement from the coil center was 12mm, and the spatial distribution on depth was up to 30mm and for instances to 100mm.8 For studying the L5 spinal nerve component of sciatic nerve, the stimulating coil was focused on the L5 root at the midpoint between the L5 spinal process and the posterior iliac crest, whereas the distal stimulation was placed over the sciatic nerve at the gluteal fold, which is located at the midpoint between the ischial tuberosity and the greater trochanter of the femur. The paired recording electrodes of a Neuroline 710c with a self-adhesive ring and Ag-AgCl content were placed on the tibialis anterior muscle and its tendon by a standard belly-tendon method. For studying the S1 spinal nerve component of sciatic nerve, the stimulating coil was focused on the S1 root at 2cm lateral to the S1 spinal process, whereas the distal stimulation was over the sciatic nerve at the gluteal fold. The active recording electrode was placed on the medial head of the gastrocnemius muscle and the reference electrode was placed on the Achilles’ tendon. The ground electrode was placed on the posterior aspect of the midthigh. The magnetic stimulation was reproduced 5 times for each site. The interval between the applications of single stimulation was 5 seconds, during which the magnetic stimulator could be fully charged. The shortest conduction latency and the largest amplitude of the compound muscle action potential (CMAP) were selected. From the distance measured by an obstetric caliper between the nerve root and the gluteal fold, the MNCV of sciatic nerve at gluteal segments in L5 and S1 components then could be obtained by the latency difference between the proximal and distal stimulations. Conventional Electrodiagnosis Motor and sensory nerve conduction and long latency reflex studies were performed in each patient with piriformis syndrome and controls. The examinations included the motor nerve conduction studies of tibial and peroneal nerves at leg segments, sensory nerve conduction studies of superficial peroneal and sural nerves, F wave of tibial and peroneal nerves, and H-reflex in bilateral tibial nerves. The Synergy T2d electromyography and nerve conduction system was used in the present study. The motor nerve conduction studies of tibial nerves were performed by using a standard belly-tendon method for recording on the abductor hallucis muscle and a supramaximal stimulation of the tibial nerve distally at the ankle and proximally at the popliteal fossa. The peroneal nerves were evaluated by recording on the extensor digitorum brevis muscles and stimulation of the peroneal nerves distally at the ankle and proximally at the fibular head. Sensory nerve conduction studies of the sural nerves were evaluated by an antidromic method by placing the recording electrodes at the lateral heel and the stimulating electrodes on the sural nerves at the middle third of the posterior aspect of the calf. Conduction studies of the superficial peroneal sensory nerves were performed by placing the recording electrodes on the medial dorsal cutaneous nerves at the ankle and the stimulating electrodes on the superficial peroneal nerves at the lower third of lateral leg. Skin temperature of the ankle was kept constantly above 31°C by using an infrared lamp, if necessary. Electromyography with a disposable concentric needlee was performed in each patient with piriformis syndrome. The examined muscles included the paraspinal muscles at L5 and S1 levels, gluteal maximus, gluteal medius, tibialis anterior, and medial head of gastrocnemius muscles. Fibrillations and positive sharp waves were counted together and considered as an acute neuropathy with denervation hypersensitivity when found at more than 1 site of the testing muscle. Mean duration of motor unit action potentials greater than 20ms or fractional polyphasic waves greater than 30% in the testing muscle was also categorized as abnormal and considered as a chronic neuropathy. Statistical Analysis Statistical analysis of comparisons between the patients with piriformis syndrome and the controls were performed with the Wilcoxon rank-sum test and paired t test. A logic regression test was used to study the relation between the MNCV of sciatic nerve and the disease duration in patients with piriformis syndrome. The maximal level of significance was .05. Results The basic data and the results of motor nerve conduction studies by magnetic stimulation in patients with piriformis syndrome and controls are summarized in table 1. The mean value for MNCV of the sciatic nerve at the gluteal segment from L5 nerve root to gluteal fold was significantly slower than the mean value of the same segments in controls (P=.014). The mean value for MNCV of the sciatic nerve at the gluteal segment from S1 nerve root to gluteal fold was slower than the mean value of the same segments in controls. However, the result was not statistically significant (P=.062). The CMAP amplitude of the tibialis anterior and gastrocnemius muscles elicited by L5 and S1 root or gluteal fold stimulation did not differ significantly. In motor nerve conduction studies of sciatic nerves, no side-to-side differences were shown in MNCV and CMAP amplitude in controls. According to the analysis of individual patients with piriformis syndrome who exceeded the lowest normative values of controls, there were 14 nerves with slowed MNCV from L5 root to gluteal fold segments of sciatic nerves and 9 nerves with slowed MNCV from S1 root to gluteal fold segments of sciatic nerves. The diagnostic sensitivities were .467 and 0.3, respectively. Four nerves with reduced CMAP amplitude of the tibialis anterior muscles were found by L5 root stimulation study, and 5 nerves with reduced CMAP amplitude of the gastrocnemius muscles were found by S1 root stimulation. The diagnostic sensitivities via CMAP amplitude were less than .167. Table 2 shows that the mean MNCV of the sciatic nerve from L5 root to gluteal fold was distinctly slower in the affected limbs than in the unaffected limbs (P=.018). There were no significant differences in the mean MNCV of sciatic nerves from S1 root to gluteal fold and the mean CMAP amplitude measured between the affected limbs and the unaffected limbs. Table 3 indicates that there were no significant differences in the parameters measured in the conventional nerve conduction and reflexes between the patients with piriformis syndrome and the controls. In analysis of the individual data from the patients with piriformis syndrome who exceeded the lowest normative values of controls, there were 4 patients with slowed MNCV in the peroneal nerves and 3 patients with slowed MNCV in the tibial nerves. Five patients with slowed sensory nerve conduction velocity (SNCV) were found in the superficial peroneal nerves, and 3 patients with slowed SNCV were found in the sural nerves. The sensory nerve action potential amplitude in the superficial peroneal nerves was reduced in 4 patients with piriformis syndrome. Prolonged F-wave latencies in the peroneal nerves were found in 3 patients. Only 2 patients had a prolonged H-reflex latency. In needle electromyography examinations, 7 patients with piriformis syndrome were found to have either acute or chronic neuropathic changes. Abnormal electromyographic findings were detected in 5 tibialis anterior muscles and 2 gastrocnemius muscles. The maximal diagnostic sensitivity was .304 by the conventional tests. In correlation with the results of motor nerve conduction study and the disease duration in patients with piriformis syndrome, there was a significant negative regression relation (r=−.68, P<.01) between the MNCV values of the sciatic nerve and the disease duration (fig 1). A longer disease duration may lead to a slower MNCV of the sciatic nerve. Discussion Although piriformis syndrome was initially described by Yeoman in 1928 and was previously considered to be a clinical syndrome,9 it has remained an unclear and controversial diagnosis.10, 11 Imaging modalities including magnetic resonance imaging and ultrasound scan were helpful to specify the anatomic relations between the piriformis muscle and the sciatic nerve.12, 13 However, these were limited in morphologic findings. From the physiologic view, electrophysiologic studies thus are important for the evaluation of sciatic nerve function in patients with piriformis syndrome. Magnetic stimulation of the peripheral nervous system has been recognized as a painless, noninvasive method that gives easy access to the deep nerves.14 The advantages of magnetic stimulation satisfied the demands of our study of the deep-seated spinal nerve roots and sciatic nerves. Figure-of-8 coils used to apply the magnetic stimulation in the present study induced their highest eddy currents in which the windings join, improving stimulation accuracy and giving more focused depolarization of nerves.15 The point of nerve depolarization can be reliably determined by using anatomic reference sites. Technically, the supramaximal stimulation should be used. If the stimulation is submaximal, a varying and unknown proportion of nerve fibers are depolarized. For prevention of inconsistent current outflow for the supramaximal stimulation, several times stimulation is required to get the best result of the shortest conduction latency and the maximal muscle response as the largest CMAP amplitude. For the present study, we selected the minimal conduction latency and the largest CMAP amplitude from 5 stimuli in each site tested. Although this approach may reduce the reliability of the individual results, we consider it to be useful in the magnetic stimulation study. In the present study, we found a prominently slowed MNCV in the sciatic nerve at the gluteal segment from L5 root to gluteal fold but not at the neural segment from S1 root to gluteal fold in the patients with piriformis syndrome. There are 3 possibilities for the different results. First, our study sample size is small, in which may reduce the sensitivity of the statistical results. Second, there is a relatively large standard deviation (SD) in the present study of MNCV. This may also reduce the sensitivity of the results. The greater SD is possibly caused by the inconsistent current outflow in the different depth of the biologic tissues by the magnetic stimulation itself. Third, or its entrapment, involvement of sciatic nerve in the patients with piriformis syndrome is more prominent in the peroneal component than in the tibial component. In a recent study by Katirji and Wilbourn,15 the vulnerability of the peroneal component in sciatic nerve lesion was reported. The authors pointed out that the anatomic course of peroneal nerve predisposed it to traction injuries because it was taut and secured at the sciatic notch and the fibular neck, whereas the tibial nerve was loosely fixed. Another vulnerability of the peroneal component in the sciatic nerve may be that it has a small number of larger nerve fascicles and more closely packed16 in comparison with the tibial component. Larger nerve fascicles with less supportive tissues in the peroneal nerve render it more susceptible to external pressure than the tibial nerve. So the peroneal component is easily damaged in the sciatic nerve lesions, as in the patients with piriformis syndrome. This hypothesis agrees with the previous clinical findings by Yuen et al17 that most of the patients with sciatic neuropathy (93% of 100 patients) had electrodiagnostic signs of significant axonal loss, and the axonal loss lesions had a greater effect on the peroneal than the tibial division. In the literature review of anatomic consideration of patients with piriformis syndrome by Benzon et al,18 they found 6 possible anatomic relations between the sciatic nerve and the piriformis muscle. That report showed that both peroneal and tibial components of sciatic nerve passed inferior to the piriformis muscle in 98.5% (65/66 dissections) of the cadaver studies. One of their dissections showed the muscle to be split and the tibial component of the sciatic nerve passed inferior to the piriformis muscle, whereas the common peroneal nerve passed through the muscle. Their anatomic findings are comparable with a report by Beason and Anson19 on a large series of 120 cadavers. They found that the most common arrangement was the undivided nerve passing below the piriformis muscle (84%), followed by the divisions of the sciatic nerve passed below the muscle (12%). In another study of 130 anatomic dissections, Pecina20 reported that the undivided nerve passed below the piriformis muscle in 78% of his dissections and the divided nerve passed through and below the muscle in 21%. Most of these studies suggested that the sciatic nerve seems to be compressed from the above piriformis muscle but less entrapped through the muscle. Patients with piriformis syndrome always have clinical symptoms and usually do not have neurologic deficits in the early stages of this disease, although it may progress and cause an entrapment neuropathy of the sciatic nerve in the later course. This hypothesis suggests that our finding of a longer disease duration may lead to a slower MNCV of sciatic nerve. Conclusions The measurement of MNCV in the sciatic nerve at the gluteal segment by magnetic stimulation may provide a painless, noninvasive, and objective method for evaluation of the sciatic nerve function in patients with piriformis syndrome. Suppliers References 1. 1Rossi P, Cardinali P, Serrao M, Parisi L, Bianco F, De Bac S. Magnetic resonance imaging findings in piriformis syndrome: a case report. Arch Phys Med Rehabil. 2001;82:519–521. Abstract | Full Text |
Full-Text PDF (95 KB)
|
CrossRef
2. 2Filler AG, Haynes J, Jordan SE, et al. Sciatica of nondisc origin and piriformis syndrome: diagnosis by magnetic resonance neurography and interventional magnetic resonance imaging with outcome study of resulting treatment. J Neurosurg Spine. 2005;2:99–115. MEDLINE |
CrossRef
3. 3Fishman LM, Zybert PA. Electrophysiologic evidence of piriformis syndrome. Arch Phys Med Rehabil. 1992;73:359–364. MEDLINE |
CrossRef
4. 4Nakamura H, Seki M, Yamano Y, Takaoka K. Piriformis syndrome diagnosed by cauda equine action potentials: report of two cases. Spine. 2003;28:E37–E40.
CrossRef
5. 5Chang CW, Lien IN. Spinal nerve stimulation in the diagnosis of lumbosacral radiculopathy. Am J Phys Med Rehabil. 1990;69:318–322. MEDLINE |
CrossRef
6. 6Fishman LM, Dombi GW, Michaelsen C, et al. Piriformis syndrome: diagnosis, treatment, and outcome—a ten-year study. Arch Phys Med Rehabil. 2002;83:295–301. Abstract | Full Text |
Full-Text PDF (99 KB)
|
CrossRef
7. 7Rogers AW. In: The textbook of anatomy. Edinburgh: Churchill Livingstone; 1992;p. 402–406. 8. 8Papazov SP, Daskalov IK. Effect of contour shape of nervous system electromagnetic stimulation coils on the induced electrical field distribution. Biomed Eng Online. 2002;1:1.
CrossRef
9. 9Silver JK, Leadbetter WB. Piriformis syndrome: assessment of current practice and literature review. Orthopedics. 1998;21:1133–1135. MEDLINE 10. 10Rodrigue T, Hardy RW. Diagnosis and treatment of piriformis syndrome. Neurosurg Clin N Am. 2001;12:311–319. MEDLINE 11. 11Broadhurst NA, Simmons DN, Bond MJ. Piriformis syndrome: correlation of muscle morphology with symptoms and signs. Arch Phys Med Rehabil. 2004;85:2036–2039. Abstract | Full Text |
Full-Text PDF (162 KB)
|
CrossRef
12. 12Lee EY, Margherita AJ, Gierada DS, Narra VR. MRI of piriformis syndrome. Am J Roentgenol. 2004;183:63–64. 13. 13Barker AT. The history and basic principles of magnetic nerve stimulation. Electroencephalogr Clin Neurophysiol Suppl. 1999;51:3–21. MEDLINE 14. 14Sun SJ, Tobimatsu S, Kato M. The effect of magnetic coil orientation on the excitation of the median nerve. Acta Neurol Scand. 1998;97:328–335. MEDLINE |
CrossRef
15. 15Katirji B, Wilbourn AJ. Mononeuropathies of the lower limb. In: Dyck PJ, Thomas PK editor. Peripheral neuropathy. Philadelphia: WB Saunders; 2005;p. p 1495. 16. 16Sunderland S. Nerve and nerve injuries. Edinburgh: Churchill-Livingstone; 1978;. 17. 17Yuen EC, So YT, Olney RK. The electrodiagnostic features of sciatic neuropathy in 100 patients. Muscle Nerve. 1995;18:414–420.
CrossRef
18. 18Benzon HT, Katz JA, Benzon HA, Iqbal MS. Piriformis syndrome: anatomic considerations, a new injection technique, and a review of the literature. Anesthesiology. 2003;98:1442–1448. MEDLINE |
CrossRef
19. 19Beason LE, Anson BJ. The relation of the sciatic nerve and its subdivisions to the piriformis muscle. Anat Record. 1937;70:1–5. 20. 20Pecina M. Contribution to the etiological explanation of the piriformis syndrome. Acta Anat. 1979;105:181–187. MEDLINE Department of Physical Medicine and Rehabilitation, National Taiwan University College of Medicine and Hospital, Taipei, Taiwan.  Reprint requests to Chein-Wei Chang, MD, Dept of Physical Medicine and Rehabilitation, National Taiwan University Hospital, No.1, Chang-Te St, Taipei 100, Taiwan
No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated. PII: S0003-9993(06)00844-6 doi:10.1016/j.apmr.2006.07.258 © 2006 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation. Published by Elsevier Inc. All rights reserved. | |
|
FULL TEXT
