Progressive Changes in the Neuroretinal Rim and Retinal Nerve Fiber Layer in Glaucoma: Impact of Baseline Values and Floor Effects

Abstract

ObjectiveTo determine whether more severe baseline damage impedes measurement of minimum rim width (MRW) and peripapillary retinal nerve fibre thickness (RNFLT) change in glaucoma patients because of a floor effect. DesignProspective longitudinal cohort study in a hospital-based setting. SubjectsThe study included patients with open-angle glaucoma and healthy control subjects. Participants had at least 5 years of follow-up with optical coherence tomography every 6 months. MethodsBaseline global and sectorial MRW and RNFLT values were classified as within normal limits, borderline, or outside normal limits based on reference normative values. Regression analysis was used to determine the magnitude and significance of MRW and RNFLT change. Additionally, the follow-up period for each participant was divided into two equal halves (1st and 2nd periods) to determine whether there was attenuation of MRW and RNFLT change with follow-up time. Main Outcomes MeasuresRates of global and sectoral MRW and RNFLT changes (slopes). ResultsA total of 97 patients with glaucoma (median age, 70.3 years) and 42 healthy subjects (median age, 64.8 years) were followed for a median of 6.9 years and 7.0 years, respectively. The median mean deviation of visual field test for glaucoma patients was -4.30 dB (interquartile range: -7.81 to -2.06 dB, range: -20.68 to 1.37 dB). Statistically significant changes in global and sectoral MRW and RNFLT were detected across all baseline classifications, however, there was tendency for less change with increasing baseline damage. In glaucoma patients, RNFLT slopes, but not MRW slopes, were significantly more positive (less change) in the 2nd period compared to the 1st. There were also no differences in either MRW or RNFLT slopes in the 1st and 2nd periods in healthy subjects. Conclusions and RelevanceSignificant MRW and RNFLT changes were detected at all levels of baseline damage. However, an attenuation in the rate of RNFLT change compared to MRW indicate an earlier floor effect in RNFLT measurements globally and in equivalent sectors. Because the axonal component of these measurements should be equivalent, our results suggest important differences in tissue remodelling at the level of the optic nerve head and peripapillary retina.