11.5.8\ Postoperative Neuritis

11.5.8.1\ Discussion

Aseptic inflammatory neuritis can occur following lumbar spine surgery due to mechanical factors, such as compression, stretching, contusion, or transection. Diagnostic imaging is useful for excluding a mechanical etiology, such as impingement by screws or disc material. Furthermore, MRI may demonstrate enhancement of the nerve roots (Fig. 11.105). This finding has a 94% positive predictive value for residual or recurrent clinical symptoms. Furthermore, there may be an association between nerve root changes, postoperative epidural fibrosis, and the development of sciatica.

11.5.9\ Arachnoiditis

11.5.9.1\ Discussion

Postoperative arachnoiditis is an inflammatory process affecting the nerve roots that may be associated with anesthetics, chemotherapy, certain contrast agents, intradural hemorrhage, substances released from the intervertebral discs, the surgery itself, and infection. This condition produces symptoms in 6–16% of spine surgery cases. Three imaging patterns of arachnoiditis have been described, which can be well delineated on MRI:

1\ .\ “Empty sac” appearance in which the nerve roots are peripherally distributed (Fig. 11.106)

2\ .\ “Clumped” nerve roots centrally within the dural sac

\3.\ “Mass” comprised of the dural-based aggregated nerve roots

Fig. 11.105  Neuritis. The patient presented with left lower extremity weakness after surgery. Sagittal post-­ contrast T1-weighted MRI shows cauda equina nerve root enhancement (arrow)

Fig. 11.106  Arachnoiditis with “empty sac” sign. Axial T2-weighted MRI shows that the cauda equina nerve roots are adherent to the margins of the thecal sac. Laminectomy posterior fusion and interbody fusion were performed

The presence and degree of contrast enhancement is variable for any of these patterns of arachnoiditis. A rare form of arachnoiditis is arachnoiditis ossificans, which is characterized by calcified plaques or ossification forms along the leptomeninges. On MRI, arachnoiditis ossificans appears as linear or mass-like intrathecal

lesions that have high signal on T1-weighted sequences and variable signal on T2-weighted sequences. CT with multiplanar reformats is helpful for confirming the presence of arachnoiditis ossificans, which shows linear bone attenuation structures along the nerve roots (Fig. 11.107).

11.5.10  Residual/Recurrent Disc

Material Versus Epidural

Scar

11.5.10.1\ Discussion

Both postoperative epidural fibrosis (scar) and residual or recurrent disc material are rather common occurrences that can produce symptoms. Both disc material and epidural fibrosis can be hypoor isointense on T1-weighted sequences and hyperintense on T2-weighted sequences relative to the annulus. However, scar enhances early

a

and diffusely after contrast administration on MRI (Fig. 11.108). Scar can also encase and retract the thecal sac and nerve roots. On the other hand, disc material enhances peripherally but can fill with contrast on delayed imaging and tends to displace or compress the nerve roots or thecal sac (Fig. 11.109). The size of the scar decreases or remains the same after 6 months in the majority of cases. Similarly, residual disc material can also involute over time. MRI has an accuracy of 96–100% for differentiating between scar and disc.

b

Fig. 11.109  Residual/recurrent disc material. Sagittal (a) and axial (b) T2-weighted MR images show a sequestered disc fragment that compresses the thecal sac at the

level of the L5 vertebral body (arrows). Post-contrast fat-suppressed T1-weighted MRI (c) show enhancement surrounding the disc fragment (arrow)