Four studies (n=944) were included in the meta-analysis: three controlled studies and one prospective cohort study. Length of follow-up ranged from two to five years. Loss to follow-up ranged from 7% to 43%.
Pooled results found no statistically significant difference between groups for risk of second eye NAION (RR 0.62, 95% CI: 0.36, 1.07, I2=66.3%; four studies, n=142). Assuming the risk for second eye NAION in untreated patients during two years was 20%, the NNT with aspirin to prevent one occurrence of second eye NAION was approximately 12. Three studies found an increase in the incidence of cerebrovascular events following NAION (3% to 7%).