PURPOSE: To examine the prevalence and incidence of second eye nonarteritic anterior ischemic optic neuropathy (NAION) and associated patient characteristics in patients enrolled in the Ischemic Optic Neuropathy Decompression Trial (IONDT) Follow-up Study. DESIGN: Randomized clinical trial with observational cohort. METHODS: Patients randomized to optic nerve sheath decompression surgery or careful follow-up had a diagnosis of acute unilateral NAION, visual acuity between 20/64 and light perception, and were aged 50 years or older. Eligible patients who declined randomization or whose visual acuity was better than 20/64 were not randomized but followed as part of an observational cohort. Follow-up examinations took place at 3, 6, 12, 18, and 24 months and annually thereafter. RESULTS: Four hundred eighteen patients were enrolled; 258 randomized and 160 observed. Previous NAION or other optic neuropathy was present in the fellow eye of 21.1% (88/418) of patients at baseline. Four patients developed optic neuropathy in the fellow eye at follow up that could not be conclusively diagnosed as NAION. New NAION in the fellow eye occurred in 14.7% (48/326) of patients at risk during a median follow up of 5.1 years. Randomized patients experienced a higher incidence (35/201; 17.4%) than nonrandomized patients (13/125; 10.4%). A history of diabetes and baseline visual acuity of 20/200 or worse in the study eye, but not age, sex, aspirin use, or smoking were significantly associated with new NAION in the fellow eye. Final fellow eye visual acuity was significantly worse in those patients with new fellow eye NAION whose baseline study eye visual acuity was 20/200 or worse. CONCLUSIONS: Follow-up data from the IONDT cohort provide evidence that the incidence of fellow eye NAION is lower than expected: new NAION was diagnosed in 14.7% of IONDT patients over approximately 5 years. Increased incidence is associated with poor baseline visual acuity in the study eye and diabetes, but not age, sex, smoking history, or aspirin use.
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