Musculoskeletal Imaging • Original Research Lee et al. MRI of Disk Herniation
Downloaded from www.ajronline.org by 37.44.207.97 on 01/26/17 from IP address 37.44.207.97. Copyright ARRS. For personal use only; all rights reserved
Musculoskeletal Imaging Original Research
Extraforaminal With or Without Foraminal Disk Herniation: Reliable MRI Findings In Sook Lee1 Hak Jin Kim1 Jung Sub Lee2 Tae-Yong Moon1 Ung Bae Jeon1 Lee IS, Kim HJ, Lee JS, Moon TY, Jeon UB
OBJECTIVE. The purpose of our study was to evaluate spinal MR images for extraforaminal disk herniation with or without foraminal disk herniation to determine the reliable MRI findings. MATERIALS AND METHODS. Thirty-five patients with extraforaminal with or without foraminal disk herniation confirmed at radiculography or surgery between March 2005 and July 2007 underwent spinal MRI. We assessed the morphologic features of the disk, changes in nerve root thickness, epidural fat obliteration surrounding the nerve root, and displacement of the nerve root in the foraminal and extraforaminal zones. RESULTS. Mixed disk herniation was found in 23 patients, and purely extraforaminal herniation was found in 12 patients. Focal eccentricity of the disk contour was identified in 32 patients (91%). Change in the nerve root thickness was found in 30 patients (86%). The nerve roots were displaced in 22 patients (63%), and the original location was maintained in nine patients (26%). Differentiation between the disk and the nerve root was poor in four of the 35 patients (11%). Obliteration of the epidural fat surrounding the nerve root was present in all patients. CONCLUSION. The presence of extraforaminal with or without foraminal disk herniation should be ascertained on the basis of the following MRI findings: focal eccentricity of the disk contour, obliteration of epidural fat surrounding the nerve root, change in the thickness of the nerve root, and displacement of the nerve root.
E
Keywords: extraforaminal disk herniation, lateral disk herniation, MRI DOI:10.2214/AJR.08.1035 Received April 8, 2008; accepted after revision September 3, 2008. 1 Department of Radiology, School of Medicine, Pusan National University, Medical Research Institute, 1-10 Ami-Dong, Seo-Gu, Busan 602-739, Republic of Korea. Address correspondence to H. J. Kim (
[email protected]). 2 Department of Orthopaedic Surgery, College of Medicine, Pusan National University, Medical Research Institute, Pusan National University, Busan, Republic of Korea.
AJR 2009; 192:1392–1396 0361–803X/09/1925–1392 © American Roentgen Ray Society
1392
xtraforaminal, or far lateral, disk herniation with or without foraminal herniation is an uncommon cause of lumbar radiculopathy. It occurs less frequently than posterior and posterolateral disk herniation at the lumbar level [1, 2]. The extraforaminal and foraminal types of herniation cause nerve root compression at the level of or beyond the dorsal ganglion where the root enters or emerges from the foraminal canal [3]. The exact position of disk herniation within or beyond the foramen is an important criterion for surgeons, who must choose between a classic intraspinal approach (with laminectomy and facetectomy) and a paraspinal approach [4]. Making the correct diagnosis before surgery is difficult, however, because careful examination of the foraminal and extraforaminal zones often is impossible. The extent of imaging in the sagittal plane and slice thickness in the axial plane are limited. Moreover, coexisting intraspinal abnormalities, spinal canal stenosis, and disk herniation can ob-
scure far lateral entrapment of the lumbar spinal nerve [2]. Although successful visualization of protrusions of the lateral disk, which account for 6–10% of all lumbar disk herniations, has become possible with CT and MRI, prolapse of a lumbar disk in the far lateral zone can still be overlooked [5–8]. We often do not focus on the extraforaminal zone in our daily practice of spinal MRI. Specifically, it is not easy to visualize the morphologic features of the disk at the L5–S1 level because the bony structures, such as the sacral ala and iliac bone, overlap one another and because of a severe decrease in disk height due to degeneration. To our knowledge, there have been no reports on the characteristic MRI findings of extraforaminal disk herniation with or without foraminal disk herniation. Although it is important clinically, extraforaminal disk herniation tends to be overlooked. The purpose of this study was to evaluate extraforaminal with or without foraminal disk herniation on spinal MR images to determine the reliable MRI findings.
AJR:192, May 2009
MRI of Disk Herniation TABLE 1: Protocols for Spinal MRI Sequence
TR (ms)
TE (ms)
Slice Thickness (mm)
Field of View (mm2)
Matrix Size
No. of Signals Acquired
Sagittal T2-weighted
3,185 (2,400–3,500)
112 (105–120)
4.2 (4–5)
253 (240–270)
597 × 217 (256–1,024 × 180–240)
2–3
Sagittal T1-weighted Downloaded from www.ajronline.org by 37.44.207.97 on 01/26/17 from IP address 37.44.207.97. Copyright ARRS. For personal use only; all rights reserved
500 (363–613)
14 (13–15)
4.2 (4–5)
253 (240–270)
378 × 198 (256–512 × 156–230)
2–3
Axial T2-weighted
4,064 (3,000–5,240)
118 (116–120)
4.2 (4–5)
135 (120–150)
378 × 208 (256–512 × 180–256)
2–3
Axial T1-weighted
559 (480–664)
13 (11–15)
4.2 (4–5)
135 (120–150)
377 × 193 (256–512 × 98–230)
2–3
Note—Values in parentheses are ranges.
Materials and Methods Patients Although this study was approved by our institutional review board, informed consent was not required because of the retrospective nature. Thirty-five patients (20 women, 15 men; mean age, 58 years; range, 23–79 years) with extraforaminal disk herniation with or without foraminal disk herniation confirmed at radiculography or surgery between March 2005 and July 2007 were included. Patients with purely foraminal or posterolateral disk herniation were excluded because that con dition can be readily diagnosed with axial and sagittal imaging. We focused on extraforaminal disk herniation because many cases of mixed herniation exhibit extraforaminal and foraminal protrusion, and we considered the mixed type frequently overlooked and underreported. All patients underwent preoperative spinal MRI at our institution. Except for two patients who had only lower back pain, the patients presented with unilaterally radiating pain in the thigh or leg.
MRI Protocol MRI was performed with a 1.5-T system (Magnetom Vision or Sonata, Siemens Medical Solutions) with spinal coils. Axial acquisition was parallel to the lumbar disks (referred to as sagittal images). Sagittal imaging was performed within the range of the vertebral body width. All patients underwent T1- and T2-weighted imaging in the axial and sagittal planes. The protocols are summarized in Table 1.
Analysis of MR Images Two radiologists who had 20 and 6 years of experience in interpreting spinal MR images retro spectively analyzed all images independently unaware of the surgical results and clinical infor mation. If there was a discordant case, the inter pretation was made in consensus. On the basis of previous reports [3, 9], we defined the foraminal zone as the space between the medial and lateral borders of the pedicle and the extraforaminal zone as the space outside the lateral border of the pedicle. Therefore, for assessment of nerve root status in one axial slice, the nerve root in the
AJR:192, May 2009
foraminal zone was defined as the nerve root ganglion, and the nerve root in the extraforaminal zone was defined as the exiting nerve root. We defined disk herniation as focal eccentricity of the disk contour regardless of protrusion, extrusion, or sequestration. Disk herniation was classified as purely extraforam inal or as mixed (foraminal and extraforaminal herniation) according to the loca tion of the herniation. Purely extraforaminal herni ation was considered present when the herniated disk was located beyond the lateral border of the pedicle. Mixed herniation was considered present when a disk with extraforaminal herniation extended into the vertebral foramen. We assessed four parameters on axial T1- and T2-weighted images of both types of herniation. First, we evaluated whether the disk contour was eccentric at the foraminal, extraforaminal, or both zones. Eccentricity was defined as obviously focal extension of the disk beyond the boundary of the circumferential margin of the disk around the vertebral endplates. Second, we evaluated whether the epidural fat plane was obliterated by the herniated disk material in the medial, lateral, or both aspects of the nerve root. Third, we evaluated the thickness of the ipsilateral nerve root relative to the herniated disk and graded the thickness as decreased, increased, or the same compared with the thickness of the contralateral nerve root. Fourth, we evaluated whether the nerve root was displaced by the herniated disk.
Results Purely extraforaminal herniation (Figs. 1 and 2) was found in 12 patients and mixed herniation (Figs. 3–5) in 23 patients. Concurrence of the purely extraforaminal and mixed types of herniation did not occur at any level in any patient. The L4–L5 disk was involved in 15 herniations, the L5–S1 disk in 11, the L3–L4 disk in eight, and the L1–L2 disk in one herniation. The MRI findings are summarized in Table 2. Focal eccentricity of the disk contour (Figs. 1 and 3) was seen in 32 patients (91%). Obliteration of the epidural fat surrounding the nerve root was seen partially or com-
pletely in all patients. Loss of epidural fat on only the lateral aspect of the nerve root was not seen in any patient. A change in nerve root thickness was seen in 30 patients (86%). Eighteen patients (52%) had decreased thickness of the nerve root (Fig. 1), and 12 patients (34%) had increased thickness of the nerve root (Fig. 2). The nerve roots were displaced in 22 patients (63%) (Fig. 4). Eighteen patients had all four MRI findings, nine patients had three, and seven patients had two findings. One patient had only obliteration of the fat plane surrounding the nerve root. Other, minor MRI findings also were made. Diffuse disk bulging without obvious focal eccentricity of the disk contour was visible in three patients (8%) (Figs. 2 and 4). In four patients (11%), the distinction between the nerve root and the disk material was poor (Fig. 5). One patient (3%) who had consistent thickness had focal eccentricity of TABLE 2: MRI Findings in Extraforaminal With or Without Foraminal Disk Herniation (n = 35) Finding
n
Focal eccentricity of disk Present
32
Absent
3
Thickness of nerve root Decreased
18
Increased
12
Indistinct
4
Same
1
Displacement of nerve root Present
22
Absent
9
Indistinct
4
Loss of fat plane around nerve root Medial aspect of nerve root
25
Medial and lateral aspects of nerve root
10
1393
Downloaded from www.ajronline.org by 37.44.207.97 on 01/26/17 from IP address 37.44.207.97. Copyright ARRS. For personal use only; all rights reserved
Lee et al.
Fig. 1—23-year-old man with right extraforaminal disk herniation at L4–L5. Axial T1-weighted MR image shows focal eccentricity of disk contour (arrow) in right extraforaminal zone. Right-exiting L4 nerve root (arrowhead) is compressed by protruding disk material. Epidural fat is obliterated in medial aspect of right-exiting L4 nerve root.
Fig. 2—55-year-old man with left extraforaminal disk herniation at L4–L5. Axial T2-weighted MR image shows subtle asymmetry of disk contour (arrow). Thickness of left-exiting L4 nerve root (arrowhead) is increased compared with contralateral nerve root.
Fig. 3—58-year-old woman with left foraminal and extraforaminal disk herniation at L4–L5. Axial T1-weighted MR image shows prominent focal eccentricity of disk contour (arrow) in foraminal and extraforaminal zones. Displacement of left-exiting L4 nerve root (arrowhead) is not evident; however, fat plane is obliterated in medial aspect of nerve root.
the disk contour and obliteration of the fat plane surrounding the nerve root (Fig. 3). Nine patients (26%) had no displacement of the nerve root (Fig. 3).
and loss of the epidural fat surrounding the nerve root (one or both sides in the foraminal, extraforaminal, or both zones). All four MRI findings were not always seen in cases of extraforaminal with or without foraminal disk herniation. Even if a patient has only one of the four MRI findings, the possibility of extraforaminal with or without foraminal disk herniation should be considered. To our knowledge, there have been no reports of assessments of these four MRI findings with respect to extraforaminal with or without foraminal disk herniation. Several authors [3,
9–12], however, have reported surgical findings and clinical features of extraforaminal lumbar disk herniation and MRI findings of foraminal and extraforaminal disk herniation. Therefore, we assessed the characteristic MRI findings of extraforaminal with or without foraminal disk herniation on routine spinal MR images. Although we easily diagnosed focal disk herniation into the foraminal or extraforaminal zone with MRI, we often overlooked or misdiagnosed the lesion as asymmetric or simple disk bulging when the eccentricity of the disk contour was indistinct or subtle. Focal eccentricity of the disk contour was indistinct in three patients. In such patients, other MRI findings, such as a change in nerve root thickness, displacement of the nerve root, and epidural fat loss surrounding the nerve root, should be evaluated. We investigated the status of nerve roots, such as change in thickness and presence or absence of displacement compared with the contralateral nerve root at the same level. In this study, 30 patients (86%) had a change in nerve root thickness. In four cases (11%), however, the nerve root was not distinguishable from the disk. Therefore, nerve root thickness or displacement was not assessed in those four patients. According to our results, not only an alteration in nerve root thickness but also an indistinct nerve root on images may be an important factor in assessment of disk herniation in the foraminal, extraforaminal, or both zones. A normal nerve root is surrounded with epidural fat. The MRI
Discussion In this study, the common MRI findings of extraforaminal disk herniation with or without foraminal extension included focal eccentricity of the disk contour, change in nerve root thickness due to direct compression by herniated disk material or to compressive swelling, nerve root displacement,
Fig. 4—61-year-old man with left foraminal and extraforaminal disk herniation at L3–L4. Axial T1weighted MR image shows indistinct focal eccentricity, or asymmetry, of disk contour. Left-exiting L3 nerve root (arrowhead) is displaced, and epidural fat surrounding nerve root is obliterated in medial aspect. Contralaterally exiting nerve root (arrow) is surrounded with fat in medial and lateral aspects.
1394
Fig. 5—75-year-old woman with left foraminal and extraforaminal disk herniation at L5–S1. Axial T1weighted MR image shows differentiation between disk material and nerve root is indistinct (arrows) in left foraminal and extraforaminal zones. Normal epidural fat is completely obliterated in medial and lateral aspects.
AJR:192, May 2009
Downloaded from www.ajronline.org by 37.44.207.97 on 01/26/17 from IP address 37.44.207.97. Copyright ARRS. For personal use only; all rights reserved
MRI of Disk Herniation findings most suggestive of stenosis include a foramen of diminished size and a paucity of perineural fat surrounding the nerve root on T1-weighted images [13]. In all of our patients the herniated disk material or adjacent bony structures obliterated the epidural fat surrounding the nerve root on one or both sides. We found that the MRI findings did not differ between the extraforaminal and mixed types of herniation. In the case of purely extraforaminal disk protrusion, epidural fat was obliterated in the medial aspect only. In mixed herniation, especially herniation into the nerve root ganglion, epidural fat obliteration was visible on both sides of the nerve root ganglion. If the disk was herniated below the nerve root ganglion, only the exiting nerve root was affected, and the obliteration of epidural fat occurred only on the medial aspect of the nerve root. Therefore, the sites of epidural fat obliteration did not differ between the purely extraforaminal and mixed types of herniation. The other two parameters (displacement of the nerve root and thickness change) did not differ between the two types of herniation. Acquisition of axial images should be parallel to the disk (sagittal imaging) for assessment of focal eccentricity of the disk contour and asymmetric morphologic features and location of the nerve root compared with the contralateral nerve root. If acquisition of axial images is not parallel to the disk, the disk contour usually is asymmetric. In addition, bilateral nerve roots at the same level can have different sizes and asymmetric locations. In this study, all axial images were obtained parallel to the disk, and we evaluated disk contour and nerve root asymmetry without difficulty. An extraforaminal disk compresses the nerve root exiting at the same level. The characteristic clinical findings include anterior thigh and leg pain, appropriate sensory loss, absence of back pain, absence of knee jerk, and no reduction in straight leg raise [10]. The clinical symptoms, such as severe radicular pain and signs of intraforaminal and extraforaminal lumbar disk herniation, often are worse than those of intraspinal lesions primarily because of compression on the nerve root ganglion [2, 11]. Failure to recognize the presence of extraforaminal disk herniation with or without foraminal extension often has been responsible for poor outcome and persistent sciatica, even after surgery [10, 12, 13]. Because this condition often necessitates total facetectomy to achieve sufficient decompression, special precautions should be taken in making the diagnosis and choos-
AJR:192, May 2009
ing the appropriate surgical procedure [9]. Therefore, extraforaminal disk herniation with or without foraminal extension is an important pathologic entity, and the exact diagnosis before surgery is necessary. Extraforaminal lumbar disk herniation beneath or lateral to the facet joint occurs in 2.6–11.7% of all lumbar disk herniations [1, 5, 14–18]. However, detection of this herniation is generally difficult. Osborn et al. [8] misdiagnosed one third of far lateral herniations at initial interpretation. In general, most radiologists and clinicians rarely miss posterolateral disk herniation at spinal MRI. However, the diagnosis of extraforaminal disk herniation often is overlooked and has remained elusive because of the atypical clinical presentation and the inconsistent radiographic findings [12]. Therefore, to ascertain the characteristics and accessible MRI findings of extraforaminal disk herniation, we evaluated spinal MR images of patients with extraforaminal with or without foraminal extension confirmed at surgery or radiculography. When nerve root involvement is suspected in the foraminal, extraforaminal, or both zones, comprehensive evaluation of the clinical findings, CT and MRI findings, and selective nerve root infiltration and block are necessary to avoid misdiagnosis [9]. Selective nerve root infiltration and radiculography are useful diagnostic tools for identifying the exact site and degree of neural compression. However, this method is invasive, and it is difficult to examine several levels simultaneously [2]. Extraforaminal disk herniation usually is not detected with myelography [12]. The CT features of extraforaminal disk herniation are nonspecific, and CT evaluation is inappropriate [15, 19, 20]. MRI is preferred because it is noninvasive, has high soft-tissue resolution, and has multiplanar imaging capabilities [21]. Therefore, MRI may be the preferred study for evaluating symptomatic pathologic changes in the central and lateral parts of the spinal canal. Grenier et al. [3] concluded that MRI is useful for analyzing extraforaminal disk herniation. They added a 15–30° angled frontal view, oblique caudally and anteriorly, and followed the course of the nerve roots. This image, however, is not generally included in routine spinal MRI protocols. In this study, we evaluated all extraforaminal disk herniations on general axial T1- and T2-weighted images. Jenis and An [13] suggested that the use of parasagittal images allows visualization of the foramina along the length of the
lumbar spine and gives superior resolution of changes in the associated disk and vertebral body. We, however, found extraforaminal herniations with foraminal extension on sagittal images in only 16 patients (46%). Because sagittal spinal MRI at our institution is performed within the range of the vertebral body width, purely extraforaminal disk herniation was not visualized on sagittal images. However, disk herniation into the neural foramen was partially seen in the sagittal plane in the case of mixed herniations when we more carefully assessed disk contour and the nerve root on axial images. There were limitations to our study. First, the number of patients was small, but disk herniation of the extraforaminal type is rare. Second, there was no control group. Therefore, we did not perform a statistical analysis to include the sensitivity, specificity, and diagnostic accuracy of the four MRI parameters. Third, we evaluated all four parameters retrospectively on routine spinal MR images. Thus we did not add special imaging protocols for detailed evaluation of extraforaminal disk herniation. We did, however, evaluate all disk herniations on the axial images. Radiologists should consider the possibility of extraforaminal with or without foraminal disk herniation when the following MRI findings are seen: focal eccentricity of the disk contour, change in the thickness of and displacement of the nerve root, indistinct nerve root compared with the contralateral nerve root, and loss of epidural fat surrounding the nerve root. References 1. Abdullah AF, Ditto EW 3rd, Byrd EB, Williams R. Extreme-lateral lumbar disc herniations: clinical syndrome and special problems of diagnosis. J Neurosurg 1974; 41:229–234 2. Baba H, Maezawa Y, Furusawa N, Uchida K, Kawahara H, Kokubo Y. Extraforaminal lumbar disc herniation at two contiguous intervertebral levels. Spinal Cord 1997; 35:725–728 3. Grenier N, Greselle JF, Douws C, et al. MR imaging of foraminal and extraforaminal lumbar disk herniations. J Comput Assist Tomogr 1990; 14: 243–249 4. Wiltse LL, Spencer CW. New uses and refinements of the paraspinal approach to the lumbar spine. Spine 1988; 13:696–706 5. Abdullah AF, Wolber PG, Warfield JR, Gunadi IK. Surgical management of extreme lateral lumbar disc herniations: review of 138 cases. Neurosurgery 1988; 22:648–653 6. Fankhauser H, de Tribolet N. Extreme lateral
1395
Downloaded from www.ajronline.org by 37.44.207.97 on 01/26/17 from IP address 37.44.207.97. Copyright ARRS. For personal use only; all rights reserved
Lee et al. lumbar disc herniation. Br J Neurosurg 1987; 1: 111–129 7. Hood RS. Far lateral lumbar disc herniations. Neurosurg Clin N Am 1993; 4:117–124 8. Osborn AG, Hood RS, Sherry RG, Smoker WR, Harnsberger HR. CT/MR spectrum of far lateral and anterior lumbosacral disk herniations. Am J Neuroradiol 1988; 9:775–778 9. Kunogi J, Hasue M. Diagnosis and operative treatment of intraforaminal and extraforaminal nerve root compression. Spine 1991; 16:1312–1320 10. O’Hara LJ, Marshall RW. Far lateral lumbar disc herniation: the key to the intertransverse approach. J Bone Joint Surg Br 1997; 79:943–947 11. Ohmori K, Kanamori M, Kawaguchi Y, Ishihara H, Kimura T. Clinical features of extraforaminal lumbar disc herniation based on the radiographic
location of the dorsal root ganglion. Spine 2001; 26:662–666 12. Maroon JC, Kopitnik TA, Schulhof LA, Abla A, Wilberger JE. Diagnosis and microsurgical approach to far-lateral disc herniation in the lumbar spine. J Neurosurg 1990; 72:378–382 13. Jenis LG, An HS. Spine update: lumbar foraminal stenosis. Spine 2000; 25:389–394 14. Kurobane Y, Takahashi T, Tajima T, et al. Extraforaminal disc herniation. Spine 1986; 11:260–268 15. Gado M, Patel J, Hodges FJ 3rd. Lateral disk herniation into the lumbar intervertebral foramen: differential diagnosis. Am J Neuroradiol 1983; 4:598–600 16. Kornberg M. Extreme lateral lumbar disc herniations: clinical syndrome and computed tomography recognition. Spine 1987; 12:586–589
17. Leonardi M, Biasizzo E, Fabris G, Penco T, Bertolissi D. CT evaluation of the lumbosacral spine. Am J Neuroradiol 1983; 4:846–847 18. Macnab I. Negative disc exploration: an analysis of the causes of nerve-root involvement in sixty-eight patients. J Bone Joint Surg Am 1971; 53:891–903 19. Eckardt JJ, Kaplan DD, Batzdorf U, Dawson EG. Extraforaminal disc herniation simulating a retroperitoneal neoplasm: case report. J Bone Joint Surg Am 1985; 67:1275–1277 20. Schubiger O, Valavanis A, Hollmann J. Computed tomography of the intervertebral foramen. Neuroradiology 1984; 26:439–444 21. Ozturk C, Tezer M, Sirvanci M, Sarier M, Aydogan M, Hamzaoglu A. Far lateral thoracic disc herniation presenting with flank pain. Spine J 2006; 6:201–203
F O R YO U R I N F O R M AT I O N
PQI Connect is the latest addition to the ARRS Website and serves as a source for information on meeting the growing demand for quality review programs in today’s radiology practices and facilities. The interactive and easy-to-navigate site focuses on five critical topics that guide you through news items, relevant articles, and links to important information on each topic.
1396
AJR:192, May 2009
