AQP4 and optic neuritis: The anti-aquaporin-4 antibody (AQP4-Ab) has been detected in 25% of patients who present with recurrent or simultaneous bilateral optic neuritis (2), and 3–5% of optic neuritis cases have a recurrent course in the absence of a neurologically or systemically-based disease (2), termed relapsing optic neuritis (RON).