The relationship between nerve fiber layer and perimetry measurements

PURPOSE. Losses of retinal ganglion cells (RGCs) in glaucoma are the cause of visual field defects and thinning of the retinal nerve fiber layer (RNFL), but methods of correlating these events have not been developed. The present study was conducted to investigate the relationship between standard automated perimetry ( SAP) measures of RGCs and optical coherence tomography (OCT) measures of the ganglion cell axons entering the optic nerve from corresponding visual field locations. METHODS. SAP and OCT data from normal monkeys were used to develop methods for estimating neuron counts and mapping SAP visual field locations onto the optic nerve head (ONH). The procedures developed for normal eyes were applied to monkeys with experimental glaucoma. RESULTS. The number of neurons derived from SAP and OCT data for normal eyes were in close agreement. The estimates of the number of RGCs in retinal areas of the Humphrey Field Analyzer 24-2 (Carl Zeiss Meditec, Inc., Dublin, CA) visual field and the axons entering the ONH were both approximately 1.5 million. The neural losses derived from subjective and objective measurements in monkeys with early experimental glaucoma correlated highly, with a mean +/- SD difference of 0.6% +/- 22% between the two estimates in control eyes and 3% +/- 24% in laser-treated eyes. CONCLUSIONS. SAP measures of visual field defects and OCT measures of RNFL defects are correlated measures of glaucomatous neuropathy. The normal intersubject variability and the dynamic ranges of the measurements suggest that RNFL thickness may be a more sensitive measurement for early stages and perimetry a better measure for moderate to advanced stages of glaucoma.