Abstract
Purpose: :
Losses of retinal ganglion cells (RGCs) in glaucomatous neuropathy cause visual field defects and thinning of the retinal nerve fiber layer (RNFL), but methods of quantifying the relationship between these functional and structural measurements have not been developed. The present study was an investigation of the relationship between standard automated perimetry (SAP) measures of the number of ganglion cells in retinal areas representing specific visual field locations and optical coherence tomography (OCT) measures of the number of ganglion cell axons entering the optic nerve from those locations.
Methods: :
Data from normal monkeys for SAP measures of visual sensitivity and OCT measures of RNFL thickness were used to develop methods for estimating neuron counts by each procedure and for mapping SAP visual field locations onto the optic nerve head (ONH). The procedures developed for normal eyes were then applied to data from monkeys in the early stages of laser–induced experimental glaucoma.
Results: :
The numbers of neural elements derived from SAP and OCT data for normal eyes were in close agreement, except for the nasal sectors of the ONH where perimetry does not sample a sufficient amount of retina to quantify axons entering the nerve. The derived estimates for the number of RGCs in retinal areas of the HFA 24–2 visual field and the number of axons entering the ONH from these locations were both about 1.5 million. The neural losses derived from subjective and objective measurements in monkeys with early experimental glaucoma were highly correlated, with a mean ±SD difference of 2% ±20% between the two estimates of neural elements.
Conclusions: :
SAP measures of visual field defects and OCT measures of RNFL defects are correlated measures of glaucomatous neuropathy. An analysis of normal inter–subject variability and the dynamic ranges of the measurements suggest that RNFL thickness may be a more sensitive measurement for early stages of glaucoma and perimetry a better measure for moderate to advanced stages of glaucoma.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • perimetry • ganglion cells