Despite the high prevalence of PACG in ACS, what makes ACS more susceptible to PACG development in comparison to other canine breeds remains unknown. In the present study, GWAS failed to identify candidate loci associated with PACG in ACS. This result in combination with the high incidence of PACG in ACS reiterates that canine PACG could be a multifactorial disease having complex genetic traits.5–7 While this study included a modest number of samples for a GWAS, the samples were thoroughly phenotyped and the results are consistent with a previous study with a greater number of samples (> 100 samples for cases and controls, respectively) in this breed where no significant association was identified (unpublished data). Given that canine PACG has a female predilection and is considered an aging disease that does not typically develop until 4–10 years of age,7 matching controls further based on sex and selecting non-affected controls much older than affected cases may be helpful.13 In the present study, the ocular morphologic traits demonstrated in non-affected ACSs suggest that the entire ACS breed itself may be susceptible to PACG development and that this disease is incompletely penetrant.
This study demonstrated that the ACS has a more crowded anterior segment morphology with a significantly shallower ACD and thinner but more anteriorly located lens versus the Beagle, although their overall globe size is not statistically different. In humans, shallow ACD, short axial globe length, and thick, anteriorly located lens are known risk factors for acute primary angle closure, all of which contribute to the formation of crowded anterior segment anatomy.3,14 Although the role of the lens has been understudied in veterinary-based ophthalmology, an increase in thickness and a more anterior position of the lens are closely associated with PACG in humans.3,14,15
It is interesting to note that the ACS has a thinner lens, which is more anteriorly located in comparison to Beagles. In humans, anterior lens position appears to be a more critical and significant risk factor than the lens thickness for angle closure development.16 More specifically, the lens vault or the magnitude of the lens anterior to the scleral spur plane measured with spectral domain anterior-segment optical coherence tomography (AS-OCT) was a significant predictor of angle closure, and more predictive than traditional lens biometric parameters such as LT and RLP.17–19 The greater lens vault as well as the anterior position of the lens could lead to anterior displacement of peripheral iris, directly aggravating ICA and CC narrowing.17–19 It would be worthwhile to investigate the lens vault in various breeds to determine its role in canine PACG development. As dogs do not possess a scleral spur, another landmark is required to represent the plane of angles. In addition, their larger eyes in comparison to humans do not allow capture of the entire anterior ocular segment in a single image with spectral domain AS-OCT. Given these anatomical and technical difficulties, a novel canine-specific parameter needs to be defined to assess how much of the lens is anteriorly protruded relative to other structures.
It is expected that the more anterior location or protrusion of the lens increases pupillary block via increased iridolenticular contact.18 In the present study, contact between the posterior iris surface and the anterior lens capsule was significantly greater in ACS. This increased ILC reinforces the one-way valve at the pupillary margin that traps more aqueous in the anterior chamber, inducing the concave sigmoidal-shaped iris as quantified by the more negative ID parameter in ACS versus Beagle.20 The co-existence of the concave iris and angle narrowing/closure is a unique finding in dogs. In humans, PACG is associated with relative pupillary block characterized by anterior bowing (convex) of iris causing narrowing of the angle.11,21 The reverse pupillary block characterized by posterior bowing (concavity) of the iris is observed in pigment dispersion syndrome and pigmentary glaucoma, which is considered secondary open-angle glaucoma caused by pigment accumulation on trabecular meshwork with increased outflow resistance.22 Iris concavity was observed in 38–54% of humans affected with pigment dispersion syndrome,23,24 45% with latent pigmentary glaucoma and 86% with active pigmentary glaucoma.24 However, reverse pupillary block is not pathognomonic for pigment dispersion syndrome and pigmentary glaucoma as blinking, accommodation, and exercise have been reported to cause this phenomenon.22 Distinguishing between physiological and pathological reverse pupillary block can be challenging but will typically persist in the eyes affected with pigment dispersion syndrome and pigmentary glaucoma while it is temporary in normal human eyes post-exercise.25
While IOP was significantly lower in normal ACSs versus Beagles, all dogs had an IOP within the normal range. This may not be clinically relevant, but we can postulate on some possible causes. One possible scenario could be an association with a low-grade subclinical intraocular inflammation that was not detected by routine ophthalmic exams. This hypothesis, in turn, highlights the histopathological findings that lymphoplasmacytic inflammation as well as pigment dispersion were prevalent in canine eyes affected with goniodysgenesis-related glaucoma.26 Additionally, it corroborates the increased expression of COX-2 in trabecular meshwork, angular aqueous plexus and ciliary body of glaucomatous dog eyes and increased VEGF concentration in aqueous humor of dogs affected with primary glaucoma.27,28 Indeed, uveoscleral outflow was also more enhanced in humans affected with pigment dispersion syndrome to compensate for the pigment dispersion process compromising the trabecular outflow facility and to maintain a normal IOP.29 Thus, measurement of uveoscleral outflow facility and sampling of aqueous humor to measure inflammatory mediators in the ACS might help explain the underlying mechanism of lower normal IOPs in this breed in comparison to the Beagle.
Given the small sample size, the finding of lower normal IOP in normotensive ACS needs to be verified in a larger populational study. In addition, it is unclear if the inflammatory evidence reported in the literature thus far suggests a primary trigger of glaucoma or is a secondary response to the released pigments.26 Furthermore, it is still unknown whether there is a physical friction between the iris and the lens liberating pigments in vivo despite the observed iris configurations of ACS typical for reverse pupillary block. To elucidate the role of pigment dispersion and inflammation in PACG development of dogs, the use of more sophisticated measures such as laser flare-cell photometry may be needed to quantify the number of melanin granules or cells floating in the anterior chamber.30 Moreover, the ACS population enrolled in this study was comprised of ophthalmoscopically healthy, normotensive dogs although the presence of CC narrowing could be categorized as the latent and asymptomatic phase of PACG.4 Thus, it is unknown how many of these ACSs will develop glaucoma. A longitudinal study assessing the entire anterior ocular segment anatomy including the position and relationship of all anatomical structures would be highly informative to identify the contributing factors for the longer disease-free interval as well as main risk factors for PACG development in the ACS.
In summary, normotensive ACS with normal eyes had a narrowed CC and more crowded anterior ocular segment consisting of a shallow anterior chamber and a more anteriorly located lens in comparison to Beagles. The narrow CC existed concomitantly with increased ILC and posterior bowing of iris, typical features for reverse pupillary block. The resting IOP of the present ACS population was significantly lower than that of Beagles despite the aforementioned unfavorable anatomical features, which might suggest an association with a low-grade inflammatory response to the pigment dispersion process. The nonsignificant results of a case-control GWAS in glaucomatous and normal ACSs could be attributed to the predisposition of all ACSs for PACG due to their intrinsic ocular morphology. Finally, companion animals have a more heterogenous genetic background compared to laboratory animals and share the same environmental factors with humans.31 Given that PACG is a complex multifactorial disease with variable expression and low penetrance,2 the ACS may hold potential to serve as an animal model of naturally occurring PACG. The co-existence of posterior iris bowing with angle narrowing, however, should be kept in mind when utilizing the ACS in translational research.