Retinal nerve fibre layer thickness floor and corresponding functional loss in glaucoma

Aim To estimate the floor of retinal nerve fibre layer (RNFL) thickness measurements and the corresponding retinal sensitivity loss in glaucoma. Methods Visual field (VF), Spectralis RNFL (83 patients and 37 healthy subjects) and RTVue RNFL data obtained separately (56 patients and 36 healthy subjects) were reviewed. Global and quadrant residual layer thicknesses and corresponding VF losses were estimated using two Bayesian change point models. Results The respective residual thicknesses from change point model 1 (CPM1) on Spectralis and RTVue (respectively) were 49.9 and 70.6 mu m globally, 57.1 and 83.7 mu m superiorly, 55.2 and 79.0 mu m inferiorly, 43.1 and 60.5 mu m nasally, and 40.1 and 59.5 mu m temporally. Corresponding VF losses ranged between -25.1 and -21.7 dB (Spectralis) and between -21.8 and -3.4 dB (RTVue). From CPM2, RNFL thinning reached horizontal asymptotes at VF losses between -18.0 and -10.7 dB (Spectralis) and between -12.1 and -2.5 dB (RTVue). There were no significant differences between postchange point residual layer thicknesses from CPM1 and CPM2 on Spectralis (37.0-50.8 mu m vs 38.3-56.0 mu m) and RTVue (60.6-80.5 mu m vs 58.4-88.8 mu m). Conclusions Global RNFL thinning reaches the floor at a smaller VF loss level with Spectralis than with RTVue. The nasal and temporal quadrants retain thinner residual layers than superior and inferior quadrant RNFL. Measuring RNFL below their minimums will not yield useful clinical information.