Critical flicker fusion frequency in demyelinating and ischemic optic neuropathies

Critical flicker fusion (CFF) frequency is a well-studied test for evaluating demyelinating optic neuritis (ON). Its use in evaluating other optic neuropathies is not well established. The purpose of this study was to compare CFF in ON and non-arteritic anterior ischemic optic neuropathy (NAION).

We performed a retrospective review to compare multiple variables for eyes with ON or NAION using two-sided T tests and Chi-square tests. A multivariate linear regression was performed for the dependent variable CFF. A receiver operating characteristic (ROC) curve was used to define a CFF threshold for distinguishing these entities.

Unaffected eyes had an average CFF value of 31.5 Hz. CFF values for ON (20.7 Hz ± 7.36) and NAION (24.3 Hz ± 9.03) were not significantly different from each other (P = 0.06). However, the CFF for ON, 18.27 Hz ± 9.29, was significantly lower than for NAION, 23.92 Hz ± 7.02, P = 0.02 when limiting the comparison to moderate and severe disease. An ROC curve demonstrates that a CFF value of ≤24 Hz is 71% sensitive for ON and that a value >24 Hz is 74% specific for excluding ON. A multivariate linear regression model demonstrated that ON contributed to approximately an 8 Hz decrease in CFF compared to NAION. Patients with a CFF of ≤24 Hz have a 2.89 odds ratio (95% CI 1.76-4.01) of having ON.

CFF values in eyes with ON were significantly lower compared to eyes with NAION when evaluating moderate and severe disease. Twenty-four hertz may be a useful CFF threshold value when trying to distinguish between these two entities.