The effect of topical latanoprost on anterior segment anatomic relationships in normal dogs

Authors


Address communications to:

S. Tsai

Tel.: 970-818-6263

e-mail: sytsai@rams.colostate.edu

Abstract

Objective

Topical latanoprost 0.005% is commonly used in dogs with primary angle closure glaucoma (PACG), and marked miosis has been reported in the literature. To further explore the effect of topical latanoprost on anterior segment anatomy, we performed iridocorneal angle biometrics in normal beagle dogs.

Methods

Thirty-five normal female beagle dogs were assessed using anterior segment optical coherence tomography (AS-OCT). One eye of each dog was scanned with the AS-OCT in the superotemporal quadrant. One drop of latanoprost 0.005% was applied topically, and the OCT scan was repeated 30 min later. Images were imported into ImageJ, and pupil diameter, anterior chamber angle, angle opening distance, angle recess area (ARA), anterior chamber hemifield, and anterior chamber depth were measured.

Results

A single drop of latanoprost resulted in marked miosis, anterior bowing of the peripheral iris, narrowing of the iridocorneal angle, and shallowing of the anterior chamber. The anterior segment parameters demonstrated a significant reduction (P-value ≤ 0.001) from baseline following latanoprost with the exception of the ARA (P = 0.07).

Conclusions

Latanoprost significantly decreases pupil diameter and narrows the iridocorneal angle in normal female beagle dogs. Therefore, the utility of latanoprost as a prophylactic treatment for PACG in fellow eyes may be limited. Studies using quantitative iridocorneal angle measurements in goniodysgenic dogs are warranted to understand the changes in iridocorneal angle morphology that occur in PACG in response to topical application of latanoprost.

Introduction

Glaucoma is one of the most common causes of irreversible blindness in dogs.[1] Most cases of glaucoma are primary angle closure glaucoma (PACG), which is at least eight times more frequent than open angle glaucoma in dogs.[2] Topical latanoprost has been demonstrated to markedly lower intraocular pressure (IOP) in a matter of hours in many dogs with acute PACG[3]; however, long-term therapy for dogs with overt PACG remains elusive. In one study, despite an initial positive response, only 11% of dogs maintained an IOP of <22 mmHg for ≥1 year on latanoprost, and no dog retained vision for more than a year.[3] This is in contrast to combined gonioimplantation and cycloablation, in which approximately 50% of eyes retain vision for over 1 year,[4, 5] suggesting that both reducing inflow and maintaining outflow through the anterior segment may be important in the long-term preservation of vision in PACG. Additionally, although prophylactic therapy with either an aqueous flow suppressor (betaxolol) or a drug that increases conventional outflow (demecarium bromide) appears to result in a fourfold reduction in the risk of an overt attack of PACG in the initially normotensive fellow eye,[2] informal polling of attendees at several William Magrane Basic Science Courses in Veterinary Ophthalmology by one of us (PEM) suggests that latanoprost may not be an effective prophylactic drug in dogs with PACG. Finally, in the course of examining normotensive experimental dogs receiving topical latanoprost, we noticed that in addition to profound miosis induced by this drug,[6] the anterior chamber appeared to be shallower than normal, and the approach to the iridocorneal angle as seen on gonioscopy appeared narrowed. If these clinical observations are correct, it is possible that alterations in the relationships between various anterior segment structures may eventually lead to further reductions in the outflow facility of the eye and partially explain the relatively poor long-term response to latanoprost in dogs with PACG. To further explore the effect of topical latanoprost on anterior segment anatomic relationships, we performed iridocorneal angle biometrics in normal female beagle dogs.

Methods

Female beagle dogs (n = 35) bred by Covance Research Products, of age 8–12 months and approximately 8–11 kg, were used in this study. This study was performed at Allergan in Irvine, CA, under an approved Animal Care and Use Committee protocol. All procedures in the protocol were in compliance with the Animal Welfare Act Regulations (9 CFR 3). Routine veterinary care was done in accordance with the Animal Welfare Act, the Guide for the Care and Use of Laboratory Animals, and the Office of Laboratory Animal Welfare. Each animal had a normal ophthalmic examination on study entry, which included biomicroscopy and fundoscopy. The anterior segment iridocorneal angle biometrics were assessed using anterior segment optical coherence tomography (AS-OCT) (Visante®; Carl Zeiss Meditec, Inc., Dublin, CA). The lighting conditions in the room were dim and consistent for all animals throughout the study. Dogs were given a pre-anesthetic SQ dose of atropine (0.022 mg/kg) and then anesthetized with a combination of 6.25 mg/kg ketamine, 0.625 mg/kg xylazine, and 0.125 mg/kg acepromazine, administered IV. The horizontal anterior chamber single-scan protocol was performed on one eye of each dog. The dogs were received from the breeder and randomly assigned numbers from 1 to 35 for the order that they would be examined. The odd-numbered dogs had their right eye designated as the study eye, and the even-numbered dogs had their left eye designated as the study eye. The scan plane was perpendicular to the limbus in the superotemporal quadrant. One drop of latanoprost 0.005% (Xalatan®; Pfizer, Inc., New York, NY) was applied topically to the eye, and the OCT scan was repeated 30 min later. In our laboratory, the maximal changes in miosis and angle narrowing by gonioscopy with topical latanoprost occur 30 min postdosing (Allergan, unpublished data). Images were imported into the external image processing program (imagej; NIH, Bethesda, MD), and pupil diameter, anterior chamber angle (ACA), angle opening distance (AOD), anterior chamber hemifield, and anterior chamber depth (ACD) were measured. The ACA was defined as the angle (in degrees) that is formed between the back of the cornea and the iris surface (Fig. 1a). The AOD was defined as the distance (mm) along a line perpendicular to the end of Descemet's membrane to the iris (Fig. 1b). The angle recess area (ARA) was defined as the triangular area (mm2) formed by the AOD, the iris surface, and the inner corneoscleral wall (sides of the triangle) (Fig. 1b). The ACD was defined as the distance from the back of the central cornea to the anterior lens capsule. If the anterior capsule was not well imaged in the eyes with very miotic pupils, the posterior extent of the ACD was the posterior aspect of the center of the pupil. As shown in Fig. 2a, the anterior chamber hemifield was defined as the surface area (mm2) of half of the anterior chamber. The tracing was placed starting in the central cornea on Descemet's membrane, along the iridocorneal angle posteriorly, and anterior iris to the center of the pupil, then straight anteriorly to the starting point on Descemet's membrane. The OCT unit did not have a large enough field of view to image a complete section of the large anterior chamber of the dog's eye. The anterior chamber hemifield was used as a surrogate to determine whether there was a change in anterior chamber volume after latanoprost dosing. A 3.0-mm caliper was placed in the image in the Visante AS-OCT analysis software and used to calibrate the image in the external image processing software.

Figure 1.

Methods of measuring iridocorneal angle parameters on an OCT Image. (a) Anterior chamber angle. (b) Angle opening distance and angle recess area.

Figure 2.

Method of measuring the anterior chamber hemifield on an OCT Image. (a) Before latanoprost. (b) After latanoprost. Note the marked miosis, anterior bowing of the peripheral iris, and shallowing of the anterior chamber following latanoprost.

The IOP was measured using a TonoVet rebound tonometer (Paragon Medical, Coral Springs FL) at baseline and after 5 days of QD unilateral dosing of topical latanoprost 0.005%. In our laboratory, topical latanoprost achieves the maximal IOP reduction in all dogs of a large cohort after 5 days of dosing (Allergan, unpublished data). Dosing occurred once a day at 9 a.m., and IOP measurements were made at baseline (day 0, prior to 9 a.m. dosing-trough) and day 5 (prior to 9 a.m. dosing – trough response, and 6 h postinstillation – peak response[7])

The paired t-test was used to test the difference between pre- and postdosing in AOD, ACA, pupil diameter, ARA, ACD, anterior chamber hemifield, and IOP.

Results

A single drop of latanoprost resulted in marked miosis, a characteristic iris configuration with anterior bowing of the peripheral iris, narrowing of the iridocorneal angle, and shallowing of the anterior chamber (Figs 2 and 3). In general, the peripheral iris configuration can be best described by Spaeth,[8] where the peripheral iris has an R (regular) configuration prelatanoprost and develops a S (steep) configuration postlatanoprost (Fig. 4).

Figure 3.

An OCT image of the anterior chamber. (a) Before latanoprost. (b) After latanoprost.

Figure 4.

The Spaeth Gonioscopic Grading System for the human eye. This human grading system includes a scale to measure the angulation of the peripheral iris relative to the trabeculum as R regular, S steep, Q concave.[8]

Table 1 lists the summary statistics of measured parameters and P-values comparing post- and predose measurements using paired t-test. Following latanoprost, the pupil diameter, ACA, and AOD (means) decreased 84%, 14%, and 16%, respectively (Figs 5, 6 and 7). There was variability in the degree of iridocorneal angle narrowing postlatanoprost between animals with 8 (23%) demonstrating an extreme amount of narrowing (≥40% reduction of AOD). The anatomical changes were accompanied by a 23% and 35% decrease in IOP measured at the trough and peak times, respectively. The anterior chamber shallowed postlatanoprost, as reflected by the anterior chamber hemifield (Fig. 8) and ACD (Fig. 9), which showed significant reductions of 8% and 9%, respectively. Interestingly, although the ACA and AOD showed significant reductions postlatanoprost, the ARA reduction was nominal (3%), as the iridocorneal angle recess appeared to extend more posteriorly compensating for the lost area from a closer approximation of the cornea and iris (i.e., AOD) after latanoprost dosing (Fig. 2).

Table 1. Summary of measured parameters (in millimeters)
ParameterTimeMeanSDSEMinMaxP-value
Angle opening distancePre-dose0.880.200.030.531.36<0.001
Postdose0.700.190.030.401.19
Anterior chamber anglePredose38.645.530.9328.0650.98<0.001
Postdose32.825.320.9022.4947.41
Pupil DiameterPredose3.761.050.181.066.28<0.001
Postdose0.560.240.040.241.24
Angle recess areaPredose0.510.230.040.161.320.07
Postdose0.440.180.030.140.83
Anterior chamber depthPredose3.520.230.043.044.04<0.001
Postdose3.180.220.042.653.61
Anterior chamber hemifieldPredose20.833.050.5215.8827.220.001
Postdose18.792.440.4211.4922.10
Intraocular pressurePredose16.873.110.5311.5023.00 
Day 5 a.m.12.832.180.378.0017.50<0.001
Day 5 p.m.10.791.980.337.0015.00<0.001
Figure 5.

The pupil diameter before and after a single drop of latanoprost. Horizontal lines represent the mean and solid boxes or circles represent individual measurements (which are often superimposed over each other) Mean pupil diameter was significantly reduced after application of a single drop of latanoprost, < 0.001.

Figure 6.

Anterior chamber angle (ACA) before and after a single drop of latanoprost. Horizontal lines represent the mean solid boxes or circles represent individual measurements (which are often superimposed over each other) The mean geometric ACA was significantly reduced after application of a single drop of latanoprost, < 0.001.

Figure 7.

Angle opening distance (AOD) before and after a single drop of latanoprost. Horizontal lines represent the mean solid boxes or circles represent individual measurements (which are often superimposed over each other) The mean AOD was significantly reduced after application of a single drop of latanoprost, < 0.001.

Figure 8.

Anterior chamber hemifield before and after a single drop of latanoprost. Horizontal lines represent the mean solid boxes or circles represent individual measurements (which are often superimposed over each other) The mean anterior chamber hemifield was significantly reduced after application of a single drop of latanoprost, < 0.001.

Figure 9.

Anterior chamber depth (ACD) before and after a single drop of latanoprost. Horizontal lines represent the mean solid boxes or circles represent individual measurements (which are often superimposed over each other) The mean ACD was significantly reduced after application of a single drop of latanoprost, < 0.001.

Discussion

The results of this study demonstrate that topical latanoprost results in marked miosis, crowding of anterior chamber structures, and significant narrowing of the iridocorneal angle without initially inducing significant changes in the ARA in normal beagles. In fact, in approximately 25% of normal eyes, there was a ≥ 40% reduction in AOD. Nevertheless, IOP was significantly reduced in the normal dogs used in this study, even though these alterations in anterior segment anatomy would, according to common sense, tend to increase IOP by limiting aqueous humor flow through the trabecular meshwork. This finding, plus the observation that latanoprost increases episcleral venous pressure in dogs[9] (and hence may increase resistance to aqueous humor outflow via the trabecular meshwork), suggests that latanoprost lowers IOP in normal dogs via mechanisms other than increasing conventional outflow. Such mechanisms may include increasing uveoscleral outflow[10] (reported in other species, although the dogs have yet to be studied) or reducing aqueous humor production[11]in dogs. Nevertheless, we speculate that latanoprost may not be an effective prophylactic drug to treat fellow normotensive eyes in dogs with PACG, as the drug may pharmacologically aggravate iris apposition to the peripheral cornea and accelerate the progression to complete iridocorneal angle closure.

Species variations in the pharmacologic responses to topical prostaglandins are common.[12] In contrast to the marked miosis induced by latanoprost in the dog, pupil changes in both non-human primates and in glaucomatous human eyes receiving topical latanoprost are minimal (http://www.pfizer.ca/en/our_products/products/monograph/221).[13] In non-human primates, prostaglandin F2α and latanoprost also have been shown to either have no effect on the ciliary muscle or to result in a mild relaxation of the precontracted ciliary muscle in vitro.[14, 15] Studies on young humans receiving topical latanoprost also demonstrate either no effect or mild reduction in accommodation, suggesting the drug has a neutral effect on the contractility of the ciliary muscle in humans,[16, 17] and pupil changes are minimal (http://www.pfizer.ca/en/our_products/products/monograph/221).[13] These fundamental differences between the effect of topical latanoprost on the iris sphincter and possibly the ciliary muscle in dogs vs. that of humans may explain why there have been no reports, to the author's knowledge, of iridocorneal angle narrowing with topical latanoprost in humans.

The exact mechanism of iridocorneal angle narrowing in the dog's eye to topical latanoprost is not known. We hypothesize that, unlike in non-human primates where latanoprost has no effect or relaxes the precontracted ciliary muscle,[15] the canine ciliary muscle, like the canine iris sphincter muscle to other FP receptor agonists,[18] contracts in response to latanoprost. This contraction would also be expected to result in accommodation for near vision where the lens changes shape and becomes more spherical. Although the data are lacking, dogs may accommodate for near vision in a fashion similar to raccoons, which do so by moving the lens anteriorly instead of changing the shape of the lens as do primates.[19] Anterior translocation of the lens would result in the AS-OCT findings noted here, that is, an anteriorly bowed appearance to the peripheral iris, a reduction in the depth of the anterior chamber, a narrowing of the geometric angle between the iris and cornea, and a smaller AOD. Unfortunately, the extremely miotic pupil induced by latanoprost in this study precluded objective determination of whether the anterior movement of the lens in this study was due to a change in the lens shape or a movement of the lens forward without a change in shape. Future studies are planned with magnetic resonance imaging to examine the whole lens geometry pre- and post-latanoprost to improve our understanding of the mechanism of angle narrowing in the dog's eye with topical latanoprost.

It is important to note that despite the marked anterior segment crowding that occurred after application of latanoprost, the overall area of the ARA remained essentially unchanged as the iridocorneal angle recess appeared to be pulled more posteriorly. Previous studies in dogs have demonstrated that contraction of the ciliary muscle pulls the bileaved, funnel-like arrangement of the uveal trabecular meshwork posteriorly, resulting in elongation and expansion of this tissue.[20] This alteration may be an evolutionary adaptation to allow for aqueous humor to continue to flow through the trabecular meshwork at a relatively constant rate, despite the shallowing of the anterior chamber and narrowing of the approach to the iridocorneal angle induced by accommodation. In this model, the pectinate ligament is believed to act like a ‘strut’ and prevent the iridocorneal angle and ciliary cleft from over-expanding. In dogs with pectinate ligament dysplasia, the abnormally thickened and often narrowed pectinate ligament may limit the expansion of the ciliary cleft during accommodation and may facilitate further narrowing of the iridocorneal angle over time.

Parasympathomimetics such as pilocarpine, carbachol, and demecarium bromide also have been shown to cause contraction of the iris sphincter (i.e., miosis) and ciliary muscle (i.e., accommodation and expansion of the trabecular meshwork) in humans[21] and a variety of other species.[20, 22] Parasympathomimetics, however, are far less effective than latanoprost at breaking an attack of acute PACG in dogs (although they can do so in humans [23]). Parasympathomimetics stimulate the ciliary muscle through muscarinic receptors and have been shown by high-resolution ultrasound to shift the human ciliary body forward resulting in an axial lens thickening and shallowing of the anterior chamber.[21] Pilocarpine produces miosis in dogs[24] and subjectively narrows the approach to the iridocorneal angle in dogs, but definitive studies of effect of topical parasympathomimetics on the relationships between various anterior segment structures in dogs are lacking. One potential explanation for the possible differences in the response of dogs to topical parasympathomimetics such as demecarium bromide and latanoprost is that latanoprost produces an approximate fivefold greater maximal miosis compared with 4% pilocarpine in normotensive beagles.[25] This exaggerated, and perhaps supra-physiologic response may more effectively break an acute attack of PACG in dogs, but the potentially more extensive shallowing of the anterior chamber and narrowing of the approach to the iridocorneal angle may ultimately reduce latanoprost's effectiveness as a prophylactic drug compared with demecarium bromide. It is likely that the differences in receptor binding between demecarium bromide and latanoprost (i.e., muscarinic vs. prostaglandin FP receptor) accounts for the different pharmacologies between these two drugs.

The anesthetic regimen alone without latanoprost does not cause miosis and narrowing of the angle in normal beagle dogs.[26] Normative values of mean AOD without the use of latanoprost eye drops in 25 normal female beagles is within 5% of the prelatanoprost dosing AOD values obtained in this study.[27] This suggests that it is unlikely that we selected an unusual strain of beagles for this study that had an aberrant response to latanoprost.

A limitation of this study is the fact that we used normotensive female beagle dogs only. We have previously reported[27] that compared with normotensive male beagles, females have smaller iridocorneal angle parameters by OCT that may render them more susceptible to closure and may partially explain the 2:1 female/male predisposition to PACG.[28] Therefore, the results of this study, where we biased the study population by including female dogs only, may not be generally applicable to male dogs. In addition, the results may not be generally applicable to all dog breeds or to glaucomatous dogs.

In conclusion, our study results demonstrate that topical latanoprost significantly decreases pupil diameter and narrows the iridocorneal angle in normal eyes of beagle dogs. Therefore, the utility of latanoprost as a prophylactic treatment for PACG in fellow eyes may be limited. Studies using quantitative iridocorneal angle measurements in goniodysgenic dogs are warranted to understand the changes in iridocorneal angle morphology that occur in PACG in response to topical application of latanoprost.

Financial Disclosures

Alexandra Almazan, Susan S. Lee, Huajiang Li, Paul Conforti, James A. Burke, and Michael R. Robinson were full-time employees of Allergan, Inc. at the time this study was conducted. Susan Tsai is at Colorado State University and is a consultant for Allergan, Inc..

Ancillary