Spinal H-Sign in Syphilitic Myelitis Mimicking MOGAD

Longitudinally extensive transverse myelitis (LETM) is classically associated with aquaporin-4 antibody-mediated neuromyelitis optica spectrum disorder and myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD). The "H-sign," reflecting gray matter involvement, is considered suggestive of MOGAD. However, infectious etiologies, including neurosyphilis, may rarely mimic autoimmune inflammatory myelopathies radiologically.

A 50-year-old man presented with 6 months of erectile dysfunction and urinary retention, followed by progressive gait instability. Examination revealed spastic paraparesis with tetrapyramidal signs, saddle hypoesthesia, and impaired vibration sense. Thoracic spine MRI demonstrated LETM from T4 to T10 with central gray matter involvement forming the "H-sign." Postcontrast sequences showed intense pial and intramedullary enhancement with inversion of signal intensity ("flip-flop sign"), suggestive of inflammatory meningomyelitis. Brain MRI was normal. Serum treponemal tests were reactive, and cerebrospinal fluid analysis revealed lymphocytic pleocytosis, elevated protein, and positive VDRL, confirming neurosyphilis. MOG-IgG and AQP4-IgG were negative using validated cell-based assays. The patient received intravenous benzylpenicillin combined with high-dose corticosteroids, resulting in significant clinical improvement and partial radiologic resolution at 6 months.

Neurosyphilis is a rare but treatable cause of LETM and may radiologically mimic antibody-mediated demyelinating disorders, including MOGAD. The coexistence of the "H-sign" and the flip-flop enhancement pattern highlights the diagnostic overlap between infectious and autoimmune myelitis. Careful exclusion of infectious etiologies remains essential in patients presenting with LETM to ensure appropriate therapy and avoid unnecessary immunosuppression.