Here, we present the protocol for the ACON study, which primarily aims to evaluate the effect of time to corticosteroid treatment (measured as the number of days from onset of visual loss to treatment with high-dose corticosteroids; days-to-Rx) on visual and structural outcomes in MS-ON, AQP4-IgG+ON, and MOG-IgG+ON respecting the novel diagnostic criteria for ON (18). The gene discussed is AQP4; the disease is myeloid sarcoma.