Similarly, three patients with optic neuritis due to serologically confirmed syphilis and probable neurosyphilis (Treponema pallidum specific AI < 1.5) but pleocytosis (3/3), blood–CSF barrier dysfunction (2/3), and intrathecal IgM synthesis (1/3) had CSF-CXCL13 levels surpassing the cut-off (5600 pg/mL, 1865 pg/mL, 959 pg/mL). Here, CXCL13 is linked to optic neuritis.