Three Signs Suggest Conjoined Nerve Root on Axial MRI

Background
Conjoined nerve roots consist of two adjacent nerve roots that share a common dural sleeve as they exit the dural sac. The reported incidence is up to 2% in surgical studies, but it is much higher in symptomatic patients, which is understandable because a doubled nerve root occupies much more space than a single nerve root. The most common location is the L5-S1 level.

Failure to preoperatively identify this anomaly may result in nerve root injury during discectomy. There is an increased incidence with minimally invasive surgical techniques.

The objective of a recent study was to evaluate imaging features suggestive of a conjoined nerve root on routine axial MRI.

The study has concluded that T1-weighted axial images increase the diagnostic accuracy and confidence of conjoined nerve roots.

Methodology
Three surgically detected cases of conjoined nerve roots with preoperative MRIs were included. The MRIs were reviewed, and three possible signs were found:

• corner sign (asymmetry of the anterolateral corner of the dural sac);
• fat crescent sign (extradural fat between the asymmetric dural sac and the conjoined nerve root);
• parallel sign (parallel course of the entire nerve root at the disc level).

These three signs were then prospectively looked for on routine lumbar spine MRIs. If one or more of these signs were detected, coronal images were added to the routine protocol for confirmation.

Results of the Study
9 patients had one or a combination of the signs for conjoined nerve roots. The prevalence was 0.6%. Two cases were surgically confirmed. The three previously found cases were included, so the total number of cases was 12.

The conjoined nerve root was at S1/2 in nine cases, at L5/S1 in two cases, and at L 3/4 in one case. Six cases were on the right, and six cases were on the left.

T1 images demonstrated the crescent and corner sign significantly better, but there was no difference for the parallel sign. The prevalence was 100% for the corner sign (n=12), 50% for the crescent sign (n=6), and 67.7% (n=8) for the parallel sign.

Reviewer’s Comments
The corner sign appears to be the most sensitive sign for conjoined nerve roots. However, it can also be seen with epidural lipomatosis, spinal stenosis, etc. The other signs may be essential in increasing diagnostic accuracy.

Conclusions
The fat crescent, corner, and parallel signs are helpful in prospectively diagnosing conjoined nerve roots on routine axial L-spine MRIs.

Author: Cornelia Wenokor, MD

Reference:
Song SJ, Lee JW, et al. Imaging Features Suggestive of a Conjoined Nerve Root on Routine Axial MRI.
Skeletal Radiol; 2008; 37 (February): 133-138