Abstract
Purpose: :
To determine whether acutely elevated IOP alters optic nerve head (ONH) structures characterized in vivo using spectral domain optical coherence tomography (SDOCT).
Methods: :
Five rhesus macaques (age range = 1 - 10 years) were tested under isoflurane anaesthesia. SDOCT images of the ONH of both eyes were acquired 30 minutes after IOP was stabilized to 10 mmHg by anterior chamber manometer and 60 minutes after IOP was elevated to 45 mmHg. The internal limiting membrane, Bruch’s Membrane/retinal pigment epithelium, neural canal opening (NCO) and anterior lamina cribrosa surface (ALCS) were delineated using custom software. Differences in a series of SDOCT structural parameters (NCO depth, rim width, rim volume, ALCS depth, prelaminar tissue thickness) between the two levels of IOP were assessed by Wilcoxon signed rank test. Additionally, in 6 eyes of three animals, images were acquired after 10 minutes of IOP stabilization at 10 mmHg and then at 30 minutes at 10mmHg, in order to serve as a control for assessment of inter-image variability in the absence of IOP elevation.
Results: :
Acute IOP elevation resulted in a reduction in the prelaminar tissue thickness in all 10 eyes (mean = -12%, SD = ± 5%, p = 0.0001, range = -3% to -21%). A decrease in both rim volume (-15% ± 5%, p = 0.001, range = -11% to -25%) and rim width (-11% ± 2%, p = 0.001, range = -8% to -13%) was detected in all 10 eyes. An increase in NCO depth relative to peripheral Bruch’s membrane was also detected in all 10 eyes (65% ± 25%, p = 0.001, range = 37% to 106%). The increase in ALCS depth was negligible but statistically significant (5% ± 6%, p = 0.01), with one eye demonstrating no change, another eye a negative change (-15%) and a range of 3 to 19% in the remaining 8 eyes. No significant parameter changes were detected in the 6 control experiment eyes.
Conclusions: :
Acute IOP elevation results in changes in the prelaminar ONH parameters (prelaminar tissue thickness and neuroretinal rim parameters) but with a negligible posterior displacement of the ALCS. A significant displacement of the NCO from peripheral Bruch's membrane may indicate a change in peripapillary scleral shape.
Keywords: optic nerve • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • intraocular pressure