Extraforaminal disc herniations are uncommon causes of lumbar radiculopathy, and occur less frequently than posterior or posterolateral disc herniation at the lumbar level1,2. These types of herniation are responsible for nerve root compression at the level of or beyond the dorsal ganglion, where the root enters into or emerges from the foraminal canal3. Assessing the precise position of the disc herniation within or beyond the foramen is important for surgeons who must choose the appropriate surgical approach4. However, there are difficulties in making a correct diagnosis before surgery because careful observations of the foraminal or extraforaminal zone are often lacking and the extent of scanning on the sagittal plane and slice thickness of the axial plane is limited. Moreover, a coexisting intraspinal pathology, spinal canal stenosis and/or disc herniation can obscure a diagnosis of far lateral entrapment of the lumbar spinal nerve2. Although CT and magnetic resonance imaging (MRI) allow successful demonstration of protrusions of the lateral disc, which account for between 6% and 10% of all lumbar disc herniations, prolapse of a lumbar disc in the far lateral zone might be overlooked5-8. The extraforaminal zone is generally not focused on in daily practice with spine MRI, particularly in the condition of large extrusion or protrusion causing descending nerve root compression or severe spinal stenosis at other levels. In particular, it is difficult to observe the disc morphology at the L5-S1 level due to the overlapping bony structures, such as the sacral ala or iliac bone, and severe decrease in disc height by degeneration.

However, there are no reports showing the characteristics and reliable MRI findings of extraforaminal disc herniation. Although extraforaminal disc herniation is important clinically, it tends to be overlooked. Therefore, this study evaluated the reliability of the MRI findings to detect symptomatic extraforaminal disc herniation in the lumbar spine.

Of the 60 subjects enrolled in this study, 30 patients had a symptomatic extraforaminal disc herniation. The mean ages of the 30 healthy volunteers (15 males and 15 females) and 30 patients (16 males and 14 females) with symptomatic extraforaminal disc herniation were 40.5±9.8 and 38.0±9.2, respectively. The consensus revealed focal eccentricity of disc in 33 cases (55%, 28 patients and 5 controls), a change in the diameter of the nerve root in 31 cases (51.7%, 26 patients and 5 controls), and a displacement of the nerve root in 23 cases (38.3%, 19 patients and 4 controls).

The overall agreement of the three reviewers in determining the presence or absence of a symptomatic extraforaminal disc herniation was 89.4% (161/180; range 86.7~91.7%, sensitivity; 90.0%, specificity; 88.9%, positive predictive value; 89.0%, negative predictive value; 89.9%). An assessment of paired intraobserver and interobserver reliability revealed a mean Kappa statistic of 0.822 (95% CI, 0.739~0.905; SE, 0.042) and 0.667 (95% CI, 0.558~0.775; SE, 0.056), respectively.

The agreements between the three reviewers in determining the presence or absence of focal eccentricity of the disc, changes in the diameter of the nerve root and a displacement of the nerve root were 83.9% (151/180; range, 83.3~85.0%), 83.3% (150/180; range, 80.0~88.3%), and 74.4% (134/180; range, 60.0~81.7%), respectively.

An assessment of paired intraobserver reliability revealed a mean Kappa statistic of 0.833 (95% CI, 0.753~0.914; SE, 0.041) for focal eccentricity of the disc, 0.656 (95% CI, 0.545~0.766; SE, 0.056) for a change in the diameter of the nerve root, and 0.669 (95% CI, 0.557~0.782; SE, 0.057) for a displacement of the nerve root. When all MRI findings were taken into consideration, the paired intraobserver reliability had a mean Kappa statistic of 0.724 (95% CI, 0.666~0.783; SE, 0.030).

An assessment of the paired interobserver reliability revealed a mean Kappa statistic of 0.667 (95% CI, 0.558~0.775; SE, 0.056) for focal eccentricity of the disc, 0.556 (95% CI, 0.434~0.677; SE, 0.062) for a change in the diameter of the nerve root, and 0.020 (95% CI, -0.137-0.176; SE, 0.080) for a displacement of the nerve root. When all MRI findings were taken into consideration, the paired interobserver reliability had a mean Kappa statistic of 0.433 (95% CI, 0.357~0.509; SE, 0.039).

Although obvious focal disc herniation into the foraminal or extraforaminal zone can be seen on MRI, the condition is often overlooked or interpreted as asymmetric or simple disc bulging when the eccentricity of the disc contour is indistinct or subtle.

A failure to recognize the presence of extraforaminal disc herniation with or without foraminal herniation is often responsible for a poor outcome and persistent sciatica, even after surgery10-12. Therefore, extraforaminal disc herniation with or without foraminal herniation is an important pathological entity, and a precise diagnosis before surgery is essential.

Extraforaminal lumbar disc herniations beneath or lateral to the facet joint occurs in 2.6% to 11.7% of all lumbar disc herniations1,5,13-17. However, detection of these herniations is generally difficult. Osborn et al.8 reported that one-third of far-lateral herniations have an initial misdiagnosis. In general, most radiologists and clinicians can easily detect posterolateral disc herniations and rarely make a misdiagnosis on spine MRI. However, a diagnosis of the extraforaminal disc herniation can often be overlooked. This condition has remained elusive because of its atypical clinical presentation and inconsistent radiographic findings12. To our knowledge, there are no reports on the characteristics and reliable MR findings of extraforaminal disc herniation. Therefore, reliable MRI findings to detect symptomatic extraforaminal disc herniation were assessed.

When nerve root involvement in the extraforaminal zones is suspected, a comprehensive evaluation of the clinical findings, CT and MRI findings as well as a selective nerve root infiltration and block are necessary to avoid a misdiagnosis9. Although selective nerve root infiltration and radiculography are useful diagnostic tools for identifying the precise site and degree of neural compression, this method is invasive and it is difficult to examine several levels simultaneously2. Extraforaminal disc herniations are usually not detected on myelography and are often overlooked on the high-resolution CT or MR images12. The CT features of extraforaminal disc herniation are nonspecific14,18,19. Recently, a detailed evaluation of disc herniation using only CT imaging was determined to be unsuitable, and MR is preferred because it is noninvasive, has high soft-tissue resolution, and multiplanar imaging capabilities20. Therefore, MRI may be the preferred method for evaluating a symptomatic patient with both central and lateral spinal canal pathology.

Grenier et al.3 concluded that MRI is useful for analyzing extraforaminal disc herniation. They added a 15~30° angled frontal view oblique caudally and anteriorly, following the course of the nerve roots. However, this image is not generally included in routine spine MRI protocols. This study evaluated all extraforaminal disc herniations on general axial T1- and T2-weighted images. Jenis and An10 suggested that the use of parasagittal images allows visualization of the foramina along the length of the lumbar spine while allowing superior resolution of the associated disc and vertebral body changes. However, the extraforaminal herniations could not be confirmed on the sagittal images. Purely extrafominal disc herniation could not be visualized on the sagittal images because sagittal scanning of spine MRI at our institution was obtained within the range of the vertebral body width. However, the disc contour or nerve root on the axial images at the same level were assessed more carefully when disc herniation into the neural foramen was partially observed on the sagittal plane in the case of mixed type (extraforaminal with foraminal) herniations.

This study showed that the overall agreement in determining the presence or absence of symptomatic extraforaminal disc herniation using the MRI findings, including focal eccentricity of the disc, changes in the diameter of the nerve root, and a displacement of the nerve root, was 89.4% (161/180; range, 86.7~91.7%). The assessment of the paired intraobserver and interobserver reliability in determining the presence or absence of a symptomatic extraforaminal disc herniation revealed a mean Kappa statistic of excellent and substantial, respectively. The assessment of the paired intraobserver and interobserver reliability with each MRI finding revealed a mean Kappa statistic of excellent and substantial for focal eccentricity of the disc, substantial and moderate for a change in the diameter of the nerve root, and substantial and poor for a displacement of the nerve root, respectively. This study showed that the focal eccentricity of the disc was the most reliable MRI finding.

There were some limitations in our study. First, the number of patients included in this study was small. Second, other imaging protocols were not added for a detailed evaluation of extraforaminal disc herniation because this study was retrospective.

There are three possible MRI findings in determining the presence or absence of symptomatic extraforaminal disc herniation. Among these MRI findings, focal eccentricity of disc was found to be the most reliable. However, it is important to recognize that even with these MRI findings, it is possible to misdiagnose symptomatic extraforaminal disc herniation.