Movement of retinal vessels toward the optic nerve head after increasing intraocular pressure in monkey eyes with experimental glaucoma
A shift or displacement of the retinal blood vessels (RBVs) with neuroretinal rim thinning indicates the progression of glaucomatous optic neuropathy. In chronic open angle glaucoma, individuals with RBV positional shifts exhibit more rapid visual field loss than those without RBV shifts. The retinal vessels reportedly move onto the optic nerve head (ONH) in response to glaucoma damage, suggesting that RBVs are pulled toward the ONH in response to increased cupping. Whether this phenomenon only applies to RVBs located in the vicinity or inside the ONH or, more generally, to RBVs also located far from the ONH, however, is unclear. The aim of this study was to evaluate the movement of RBVs located relatively far from the ONH edge after increasing intraocular pressure (IOP) in an experimental monkey model of glaucoma.
Fundus photographs were obtained in 17 monkeys. High IOP was induced in the monkeys by laser photocoagulation burns applied uniformly with 360° irradiation around the trabecular meshwork of the left eye. The right eye was left intact and used as a non-treated control. Considering the circadian rhythm of IOP, it was measured in both eyes of each animal at around the same time-points. Then, fundus photographs were obtained. Using Image J image analysis software, an examiner (N.E.) measured the fundus photographs at two time-points, i.e. before laser treatment (time 1) and the last fundus photography after IOP elevation (time 2). The following parameters were measured (in pixels): 1) vertical diameter of the ONH (DD), 2) distance from the ONH edge to the first bifurcation point of the superior branch of the central retinal vein (UV), 3) distance from the ONH edge to the first bifurcation point of the inferior branch of the central retinal vein (LV), 4) ONH area, and 5) surface area of the cup of the ONH. We calculated the ratios of UV to DD (UV/DD), LV to DD (LV/DD), and the cup area to disc area ratio (C/D).
The mean UV/DD at time 1 (0.656 ± 0.233) was decreased at time 2 (0.542 ± 0.192) (p < 0.01), and the mean LV/DD at time 1 (0.642 ± 0.151) was decreased at time 2 (0.534 ± 0.171) (p < 0.01). The mean C/D at time 1 (0.303 ± 0.035) was increased at time 2 (0.556 ± 0.110) (p < 0.01). The mean IOP at time 1 was 19.8 ± 2.5 and that at time 2 was 54.2 ± 15.8. The amount and rate of the change in LV/DD and C/D between time 1 and time 2 were significantly correlated (r = −0.654 and −0.536, p = 0.004 and 0.026, respectively).
Therefore, in an experimental monkey model of glaucoma, RBVs located relatively far from the ONH were pulled toward the ONH as cupping increased.