Sciatic neuromuscular variants on MR neurography: frequency study and interobserver performance

Abstract

Objective: To evaluate the frequency of sciatic neuromuscular variants on MR neurography and determine the interobserver variability.

Methods: A retrospective evaluation of 137 consecutive lumbosacral plexus magnetic resonance neurography examinations was performed. All examinations were performed using nerve selective 3D imaging and independently reviewed by two readers for the presence of sciatic neuromuscular variants and piriformis muscle asymmetry. Inter- and intraobserver performance were evaluated.

Results: There were a total of 44/268 (16.4%) extremities with sciatic neuromuscular variants. The interobserver performance in the identification of sciatic nerve variants was excellent (kappa values from 0.8-0.9). There was a total of 45/134 (33.6%) patients with piriformis muscle asymmetry. Of these, 7/134 (5.2%) had piriformis muscle atrophy and 38/134 (28.4%) had piriformis muscle hypertrophy. The interobserver performance in the identification of piriformis muscle atrophy and hypertrophy was moderate to good (kappa values from 0.39-0.61). The intraobserver performance revealed kappa values of 0.735 and 0.821 on right and left, respectively.

Conclusion: Sciatic neuromuscular variants and piriformis muscle asymmetry are frequent on lumbosacral plexus MRN with moderate to excellent interobserver performance. Advances in knowledge: Sciatic neuromuscular variants and piriformis asymmetry on MR neurography are frequent and the prevalence is similar to cumulative prevalence from available scientific series. Interobserver performance for identification of sciatic neuromuscular variants is excellent, and moderate-good for piriformis muscle asymmetry.

Figure 1.

Axial T1W (a), axial T2 SPAIR (b), coronal SHINKEI (c) and coronal reconstructed DWI (inverted grey scale, (d) show that the common peroneal division (small arrows) exits through and the tibial division (large arrows) exits below the piriformis muscle, respectively. DWI, diffusion weighted imaging; T₁W, T1 weighted.

Figure 2.

Type 2 variant. Axial T1W (a), axial T2 SPAIR (b), coronal SHINKEI (c) and coronal reconstructed DWI (inverted grey scale, (d) show that the common peroneal division (small arrows) exits above and the tibial division (large arrows) exits below the piriformis muscle, respectively. DWI, diffusion weighted imaging; T₁W, T1 weighted.

Figure 3.

Type 4 variant. Axial T1W (a), axial T2 SPAIR (b), coronal SHINKEI (c) and coronal reconstructed DWI (inverted grey scale, (d) show that common peroneal (small arrows) and tibial (large arrows) divisions exit below muscle but are divided by a fibrous slip. DWI, diffusion weighted imaging; T₁W, T1 weighted.

Figure 4.

Piriformis muscle asymmetry. Axial T1W (a,b) images from different patients showing the right sided piriformis muscle hypertrophy (arrow in a) and atrophy with mild fatty infiltration (arrow in b). T₁W, T1 weighted.

Figure 5.

(a) Interobserver disagreement—left sciatic nerve: catagorized as Type 1 by Observer 1 (attending) and type 4 by Observer 2 (fellow). (b) Intraobserver disagreement—right sciatic: catagorized as Type 4 by first read and not split by second read.

Figure 6.

Simplified classification system: Type 1 (a), Type 2 (b), Type 3 (c) and Type 4 (d).