Abstract
Purpose: :
To correlate the effects of acute elevation of the intraocular pressure (IOP) on the retinal nerve fibre layer (RNFL) and the retinal ganglion cell (RGC) population.
Methods: :
Adult male Norway rats (200-225 g), received a mean of 111 laser impacts (532nm wavelength, 200mW power, 0.1 second duration, 100 µm diameter spot size) over the limbar veins of the left eyes. The IOP was measured in both eyes with a Tonolab prior to and at regular intervals after laser treatment. To investigate the RNFL, the eye-fundus was photographed prior to and every 2 weeks after laser treatment with a Canon CF-60 Fundus Camera equipped with a Canon EOS-1D MARK II (495 nm) blue filter. The RGC population was labelled with 3% Fluorogold applied over the surface of both superior colliculi one week before processing. Seven weeks after lasering, retinas were prepared as whole-mounts, examined and photographed under a fluorescence microscope equipped with a digital high-resolution camera and a motorized stage, connected to an image analysis system with an automatic frame-grabber device (Image-Pro Plus, ver. 5.0.1.11, IPP). Retinas were imaged by adjacent, non-overlapping frames captured in raster pattern. Each of these frames was counted with the image analysis software IPP, and an isodensity map was generated for each retina (SigmaPlot 2002 for Windows, ver 9.0).
Results: :
Laser treatment induced an increase in mean IOP of approximately 120% over baseline. Basal IOP values were (mm Hg; mean ± SD) 15.3 ± 1.1 (n=17), increased to 37.0 ± 4.1 by 24 hours and decreased to basal values five days after lasering. Mean RGC numbers in the lasered eyes (40,648 ± 21,008; n=13) represented 49% of the values found in the contralateral control eyes (82,946 ± 5,166; n=14). RGC loss was both focal and diffuse within the retina. However, neither localized nor diffuse deffects were found in the RNFL photographs.
Conclusions: :
Laser application over the perilimbar veins results in acute elevation of the IOP that causes loss of approximately 51% of the RGC population within 7 weeks but not RNFL deffects in eye fundus photographs. It is possible that after acute elevation of the IOP, RGC loss preceeds RNFL loss.
Keywords: intraocular pressure • ganglion cells • nerve fiber layer