Guinea pigs have become increasingly popular models for studying human myopia, which carries an increased risk of glaucoma.3 Therefore, it is important to have data on the normal development of the eyes of guinea pigs. The main goals of this study were to set the normal ranges for both morphological and vascular parameters of the ONH in normal guinea pigs and to analyse the factors influencing blood perfusion in the ONH.
Our results showed that no significant differences were found in the CDR, area of the rim, area of the disc, and cup volume between the 3-week and 4-week groups. However, as no related analyses have been reported, we were unable to compare them to any corresponding reference values. Jnawali et al. had reported that the BMO area of 2.5-year-old pigmented guinea pigs is 0.192 ± 0.023 mm2 and that the diameter is 493.79 ± 31.89 µm.2 RNFL thickness is an important clinical diagnostic measure in glaucoma.13 In our study, the cpRNFL thickness in the 3-week group (55.4 ± 16.27 µm) was significantly larger than that in 4-week group (51.3 ± 13.45 µm). Jnawali et al. had reported that the RNFL thickness in 2.5-year-old pigmented guinea pigs is 59.2 ± 4.5 µm,2 which is larger than our result.
Patel et al. had reported that VD is greater within the BMO of rhesus monkeys and significantly decreases with increasing eccentricity.14 Our study also focused on the vasculature of the ONH. The vessel area, VD, total number of junctions, total vessel length, total number of endpoints, and vascular diameter were 924.64 ± 226.329 pixels, 48.86 ± 7.116%, 6.84 ± 3.540, 180.70 ± 49.435 pixels, 7.93 ± 3.198, and 5.25 ± 1.027 pixels, respectively. Previously, it had been reported that the VD within BMO of rhesus monkeys was 42.7% (10 mmHg), 43.5% (20 mmHg), and 43.3% (30 mmHg).14 However, there were no significant differences in the above parameters between the 3-week and 4-week groups.
Patel et al. had reported that the peripapillary inner retinal capillary density inrhesus monkeys gradually decreases with increasing IOP, reaching statistical significance when pressure exceeds 50 mmHg but returns to normal when IOP is reduced.14 Similarly, age-adjusted linear regression analysis revealed that cpRNFL thickness is positively associated with total vessel length, and that IOP is negatively associated with total vessel length. Manalastas et al. had reported that the thicknesses of cpRNFL and the macular ganglion cell complex are more strongly associated with VD of the ONH (r = 39.2% and 26.7%, respectively) of glaucoma patients, glaucoma suspects, and healthy individuals.15 Moreover, age-adjusted linear regression analysis revealed that VD is positively associated with vessel area and total number of junctions and is negatively associated with total vessel length, total number of endpoints, and vascular diameter.
Our results revealed that the AL, refractive error, and IOP were 7.68 ± 0.205 mm, 2.38 ± 2.095 D, and 17.4 ± 3.04 mmHg, respectively. Jnawali et al. had reported that the mean AL was 9.997 ± 0.12 mm in guinea pigs aged 2.5 years.2 In addition, two previous studies on normal guinea pigs (age unknown) reported that the IOP was 16.5 ± 3.2 mmHg among 100 normal guinea pigs16 and 18.27 ± 4.55 mmHg among 31 normal guinea pigs.17 In our study, the IOP in the 3-week group was significantly lower than that in the 4-week group (16.7 ± 3.19 mmHg vs. 17.8 ± 2.88 mmHg). Similarly, Jnawali et al. had reported that IOPs showed an early small increase and thereafter appeared to stabilise with age.2
Our study showed that the refractive error in the 3-week group was significantly larger than that in the 4-week group (3.02 ± 1.266 D vs. 1.96 ± 2.373 D). Jnawali et al. had also reported that young guinea pigs were hyperopic, with a mean spherical equivalent refraction of + 3.50 ± 0.77 D.2 These results may support the suggestion that IOP is a driving force for the process of emmetropisation.
No difference was found in AL between the 3-week and 4-week groups (7.67 ± 0.190 mm vs. 7.69 ± 0.215 mm). Similarly, Zhou et al. had reported that the increase in AL is relatively rapid during the first week, with little change during the next three weeks, and then, rapidly increases after five weeks.10 The emmetropisation process in guinea pigs is mainly related to the increase in the vitreous chamber length, and the axial development of the vitreous chamber in guinea pigs appears to be associated with posterior scleral tissue growth.10
This study has some limitations. First, due to an increase in retinal pigmentation with age, the accuracy of judgment during retinoscopy for older guinea pigs would have been affected, which might have resulted in a fluctuating variance of the refractive data. Second, we did not measure the refractive power of guinea pigs at birth. These limitations should be addressed in further research.
Nevertheless, this is the first report on morphological and vascular characteristics of the ONH in normal guinea based on in vivo OCT/OCTA imaging and quantification of ONH parameters. Hence, our results may contribute to further research on myopia and glaucoma using a guinea pig model.