Study H6D-MC-LVHQ is an observational, non-interventional, multi-center, prospective, case-crossover study to evaluate the possible association between the use of phosphodiesterase type 5 (PDE5) inhibitors and the risk of acute nonarteritic anterior ischemic optic neuropathy (NAION) in males. Subjects with newly diagnosed NAION will be asked via a structured questionnaire about their use of PDE5 inhibitors and other risk factors prior to the onset of their vision loss.

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

Potential cases of acute NAION will be prospectively identified by approximately 20 neuro-ophthalmology centers in the US