AQP4 and optic neuritis: Optic neuritis (ON) is an acute, inflammatory condition primarily involving the optic nerve and is frequently observed in IDD, although with different patterns.2, , –5 In MS, ON is often unilateral, characterised by short lesions and tends to recover well.6,7 In contrast, AQP4 + NMOSD-associated ON can be unilateral or bilateral, more frequently associated with severe visual loss and limited recovery if untreated.