Abstract
Purpose :
Animal models have been fundamental to understanding the aetiology of aberrant eye growth that underpins the development of myopia. Classically, many of these models have depended on high frequency ultrasound to measure the length of the ocular components within the eye, and measures are typically undertaken under anaesthesia. Since anaesthesia affects IOP, here we studied the changes that occur with extended time of anaesthesia and compared the thickness of retina, choroid and sclera to measures using OCT under awake conditions.
Methods :
On-axis ocular biometry was undertaken in 15 tri-coloured guinea pigs aged either 21, 90 or 160 days using high-frequency A-scan ultrasonography under gaseous isoflurane (5% in 1.5L/min O2).1 The time from anaesthesia induction was recorded and analogue ultrasound traces continuously collected. Peaks were selected from the ultrasound traces corresponding to the front of the cornea and the front and back of the crystalline lens, retina, choroid and sclera. HD-OCT (Zeiss Cirrus Photo 800, HD 5-line raster scans, 128 x 4096) of the posterior layers were taken in awake animals. Layer boundaries were differentiated based on the peak inflections in the signal intensity and thickness assessed about the central visual axis.
Results :
In ultrasound measures, the only axial depth that did not change with anaesthesia was the thickness of the retina. Changes were well fitted by exponential functions (Fig. 1). The cornea swelled by 22% after 20 mins, during which the anterior chamber decreased by 7%, and these changes were independent of age. In contrast, the crystalline lens depth increased with time under anaesthesia more in older animals and correlated with shrinkage in the vitreous chamber depth (r2=.99). The choroid thinned dramatically over the first 15 mins before stabilising (Fig. 1e). Highly significant correlations were found between OCT and ultrasound measures although choroidal OCT measures tended to be overestimated and requires independent calibration against a known standard.
Conclusions :
Ocular distances can be dramatically changed by anaesthesia and should be adjusted to render true values. OCT systems should be calibrated for distance particularly if true values of choroid thickness are required.
1. McFadden SA, Howlett MH, Mertz JR. Vision Res. 2004;44(7):643-53.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.