A Study to Determine if There is a Possible Association Between NAION and Phosphodiesterase Type 5 (PDE5) Inhibitors

Inclusion Criteria: * Adult male subjects, at least 18 years of age, who are willing to participate in the study * Participants who experienced abrupt visual loss in one eye (defined as typically less than a 1-day period or visual loss noted upon awakening) and presented for an initial visit within 45 days of onset of NAION symptoms to an ophthalmologist that resulted in a diagnosis of Suspected NAION by the investigator Exclusion Criteria: * Previous history of NAION * Previous history of arteritis (anywhere in the body) or clinical or diagnostic testing evidence of temporal arteritis * History of glaucoma in either one or both eyes * History of multiple sclerosis or diagnostic testing evidence of optic neuritis * Have dementia or other reasons for memory impairment in the opinion of the investigator * Have participated in other non-observational studies within 3 months of NAION onset