Magnetic resonance imaging findings of redundant nerve roots of the cauda equina

Abstract

Background: Redundant nerve roots (RNRs) of the cauda equina are often a natural evolutionary part of lumbar spinal canal stenosis secondary to degenerative processes characterized by elongated, enlarged, and tortuous nerve roots in the superior and/or inferior of the stenotic segment. Although magnetic resonance imaging (MRI) findings have been defined more frequently in recent years, this condition has been relatively under-recognized in radiological practice. In this study, lumbar MRI findings of RNRs of the cauda equina were evaluated in spinal stenosis patients.

Aim: To evaluate RNRs of the cauda equina in spinal stenosis patients.

Methods: One-hundred and thirty-one patients who underwent lumbar MRI and were found to have spinal stenosis between March 2010 and February 2019 were included in the study. On axial T2-weighted images (T2WI), the cross-sectional area (CSA) of the dural sac was measured at L2-3, L3-4, L4-5, and L5-S1 levels in the axial plane. CSA levels below 100 mm² were considered stenosis. Elongation, expansion, and tortuosity in cauda equina fibers in the superior and/or inferior of the stenotic segment were evaluated as RNRs. The patients were divided into two groups: Those with RNRs and those without RNRs. The CSA cut-off value resulting in RNRs of cauda equina was calculated. Relative length (RL) of RNRs was calculated by dividing the length of RNRs at mid-sagittal T2WI by the height of the vertebral body superior to the stenosis level. The associations of CSA leading to RNRs with RL, disc herniation type, and spondylolisthesis were evaluated.

Results: Fifty-five patients (42%) with spinal stenosis had RNRs of the cauda equina. The average CSA was 40.99 ± 12.76 mm² in patients with RNRs of the cauda equina and 66.83 ± 19.32 mm² in patients without RNRs. A significant difference was found between the two groups for CSA values (P < 0.001). Using a cut-off value of 55.22 mm² for RNRs of the cauda equina, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) values of 96.4%, 96.1%, 89.4%, and 98.7% were obtained, respectively. RL was 3.39 ± 1.31 (range: 0.93-6.01). When the extension of RNRs into the superior and/or inferior of the spinal canal stenosis level was evaluated, it was superior in 54.5%, both superior and inferior in 32.8%, and inferior in 12.7%. At stenosis levels leading to RNRs of the cauda equina, 29 disc herniations with soft margins and 26 with sharp margins were detected. Disc herniation type and spondylolisthesis had no significant relationship with RL or CSA of the dural sac with stenotic levels (P > 0.05). As the CSA of the dural sac decreased, the incidence of RNRs observed at the superior of the stenosis level increased (P < 0.001).

Conclusion: RNRs of the cauda equina are frequently observed in patients with spinal stenosis. When the CSA of the dural sac is < 55 mm², lumbar MRIs should be carefully examined for this condition.

Keywords: Cauda equina; Dural sac; Lumbar spine; Magnetic resonance imaging; Redundant nerve roots; Spinal stenosis.

Figure 1

Seventy-one-year-old female patient with lumbar spondylosis. A: Redundant nerve roots (arrows) secondary to the stenosis at both the superior and inferior of the stenosis at the L2-L3 level, which are more prominent at the superior, are shown; B: On the axial T2-weighted image, the cross-sectional area of the dural sac was 41.60 mm² at the stenosis level (L2-L3).

Figure 2

Seventy-one-year-old male patient with lumbar spondylosis. A: On the sagittal T2-weighted image, redundant nerve roots (arrows) secondary to the stenosis at L2-L3 level are shown at the inferior of stenosis level; B: On the axial T2-weighted image passing through L2-L3 intervertebral disc space level, marked stenosis due to ligamentum flavum and facet joint hypertrophy and disc herniation (cross-sectional area was 41.33 mm²) are shown.

Figure 3

Forty-seven-year-old female patient with lumbar spondylosis. A: On the sagittal T2-weighted image, redundant nerve roots at the superior of the stenosis level secondary to the stenosis at the L3-L4 intervertebral disc space (arrows) are shown; B: Relative length was calculated by dividing the length of redundant nerve roots (thick arrow) by the vertebra height at the superior of stenosis level (thin arrow).

Figure 4

Soft and sharp margin types of disc herniation into the dural sac. A: On the sagittal T2-weighted image, soft margin disc herniation at the level of L3-L4 intervertebral disc space and redundant nerve roots at the inferior of the stenosis are shown; B: The axial T2-weighted images of soft margin disc herniation are shown; C: On the sagittal T2-weighted image, sharp margin disc herniation at the L3-L4 intervertebral disc space and redundant nerve roots at its superior are shown; D: Axial T2-weighted image of sharp disc herniation is shown.

Figure 5

Receiver operating characteristic curve with a cut-off value of 55.22 mm² or less for the cross-sectional area of the dural sac.