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

ARTICLE HIGHLIGHTS

Research background

Redundant nerve roots (RNRs) of the cauda equina are often defined as the development of elongated, enlarged, and tortuous nerve roots at the superior and/or inferior of the lumbar canal stenosis and as secondary to it due to degenerative processes. Clinically, they can lead to lower back and leg pain, paresthesia, and neurogenic claudication in patients.

Research motivation

The radiological diagnosis of RNRs of the cauda equina was previously made with conventional myelography, while magnetic resonance imaging (MRI) findings have been more commonly defined in recent years. Nevertheless, this condition has been relatively under-recognized in radiological practice. Therefore, there is a need to keep this issue on the agenda by discussing it in light of the literature.

Research objectives

In this study, lumbar MRI findings of RNRs of the cauda equina were evaluated in spinal stenosis patients. Cross-sectional area (CSA) of the dural sac at the stenosis level that could lead to RNRs of the cauda equina and how the cauda equina nerve roots are affected by this stenosis (redundant segment length and extensions, etc.) were investigated.

Research methods

On lumbar MRI of patients with stenosis, dural sac CSA levels of less than 100 mm² at the intervertebral disc space were considered stenosis, and levels leading to lumbar stenosis were determined. Statistical differences between the CSA levels that led to RNRs of the cauda equina and those that did not lead to RNRs were investigated. Relative length (RL) was calculated by dividing the length of RNRs on sagittal T2-weighted images by the vertebrae corpus height adjacent to the stenotic segment superior. The relationships of herniation type into the dural sac (soft or sharp margins) and spondylolisthesis with CSA and RL were investigated.

Research results

RNRs of the cauda equina were observed in 42% of patients with spinal stenosis. Mean 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 (P < 0.001). Using a cut-off value of 55.22 mm² for CSA leading to RNRs of the cauda equina, the sensitivity was 96.4%, specificity 96.1%, positive predictive value (PPV) 89.4%, and negative predictive value (NPV) 98.7%. RL varied from 0.93 to 6.01 (mean: 3.39 ± 1.31). Of all RNRs, 54.5% were at the superior of stenosis level, 32.8% at both superior and inferior of stenosis level, and 7% at inferior of stenosis. Soft margin disc type was observed in 29 and sharp margin type was found in 26 of the disc herniations at the stenosis levels that led to RNRs of the cauda equina. Disc herniation type and spondylolisthesis were not significantly associated with RL or CSA of the dural sac with stenotic levels (P > 0.05). As the CSA of the dural sac decreased, the frequency of RNRs at the superior of the stenosis level increased (P < 0.001).

Research conclusions

RNRs of the cauda equina are not uncommon in patients with lumbar spinal canal stenosis. Although RNRs of the cauda equina are frequently observed at the superior of stenosis level, a considerable percentage of them can also be found at both superior and inferior, and at a lower rate at the inferior localization. The possibility of RNRs of the cauda equina is high in patients with dural sac CSA of 55 mm² or less.

Research perspectives

Although clinical and treatment outcomes are controversial, lumbar stenosis patients with marked reductions in CSA of the dural sac on MRI should be carefully evaluated for RNRs of the cauda equina. In these patients, tortuosity, elongation, and extension findings indicating redundancy in nerve roots should be reported as this could contribute to efficient treatment of the patients.