Diagnostic Utility of Novel and Established Spinal MRI Signs in AQP4-IgG-Seropositive NMOSD Myelitis

Myelitis is a common manifestation of aquaporin-4 immunoglobulin G-seropositive (AQP4-IgG+) neuromyelitis optica spectrum disorder (NMOSD), with longitudinally extensive transverse myelitis (LETM) as a characteristic MRI feature. Bright spotty lesions (BSLs) and a novel "double-contoured LETM" sign have also been increasingly observed, but their prevalence and diagnostic utility are unclear. This study aimed to characterize spinal cord MRI findings and evaluate the diagnostic performance of established and novel MRI signs in AQP4-IgG+ NMOSD myelitis.

We retrospectively reviewed spinal cord MRI scans from adults with AQP4-IgG+ NMOSD myelitis at a university-based hospital across acute (<30 days), subacute (1-3 months), and remission (>3 months) stages. MRI features were descriptively analyzed. Acute scans were assessed for specific MRI signs, including the H sign, double-contoured LETM sign, BSLs, and sagittal ring enhancement and compared with other causes of myelopathy. Sensitivity, specificity, and likelihood ratios were calculated.

Among 217 patients with AQP4-IgG+ NMOSD (median age at onset 44.5 years, 95.9% women), 170 (78.3%) experienced myelitis. A total of 331 NMOSD spinal MRI scans were reviewed. The comparison group included 72 scans, most commonly from patients with spinal cord infarction (39) and idiopathic transverse myelitis (14). Restricting the analysis to first-ever myelitis (53 scans), LETM was observed in 88.7%, and sensitivities of BSLs, the double-contoured LETM sign, and sagittal ring enhancement were 64.0% (95% CI 50.1%-75.9%), 47.2% (95% CI 34.4%-60.3%), and 60.0% (95% CI 46.2%-72.4%), respectively, with specificities of 98.6% (95% CI 92.2%-99.7%), 98.6% (95% CI 92.4%-99.8%), and 96.4% (95% CI 87.9%-99.0%). Combining these signs increased specificity further but reduced sensitivity to approximately 30%. When acute first-ever and recurrent myelitis were analyzed together (161 scans), the prevalence of these signs decreased to 56.5%, 19.9%, and 44.1%, respectively. During remission, complete radiologic resolution was observed in 12.4% of 129 scans, whereas spinal cord atrophy was present in 56.6%.

BSLs, the double-contoured LETM sign, and sagittal ring enhancement demonstrate high specificity and may aid in differentiating NMOSD from other causes of longitudinally extensive myelopathy. The double-contoured LETM sign represents a novel imaging feature, particularly in first-ever acute myelitis. However, the comparison cohort was enriched with noninflammatory myelopathies, which may limit generalizability. Further validation in independent cohorts is warranted.