The efficacy of topical prophylactic antiglaucoma therapy in primary closed angle glaucoma in dogs: a multicenter clinical trial

Role of Inflammation in Canine Primary Glaucoma

Primary glaucoma is a painful, progressive, and blinding disease reported in many canine breeds, characterized by intraocular pressure (IOP) elevation in the absence of antecedent intraocular disease. Clinical observations of dogs with primary glaucoma suggest that many affected eyes develop concurrent intraocular inflammation in addition to elevated IOP. In this work, we summarize the current knowledge that relates inflammation to primary glaucoma in dogs, reviewing studies focused on genetics, physiology, histopathology, bioanalysis of ocular fluids, therapeutics, and clinical outcomes of glaucomatous patients. Through disruption of the blood-aqueous and blood-retinal barriers, pigment dispersion, and biochemical changes to the aqueous humor and tear film, the pathogenesis of canine primary glaucoma appears to involve inflammatory changes to various extents and with various consequences from the front to the back of the eye. Among others, inflammation further impacts IOP by reducing aqueous humor outflow at the level of the iridocorneal angle and accelerates vision loss by promoting neuronal degeneration. As such, the vicious cycle of ocular inflammation and IOP elevation might warrant the use of anti-inflammatory medications as a core component of the treatment regime for dogs with primary glaucoma, either therapeutically (i.e., actively glaucomatous eye) or prophylactically in the yet unaffected contralateral eye.

Iris angiography in ADAMTS10 mutant dogs with open-angle glaucoma (ADAMTS10-OAG)

OBJECTIVE

To evaluate anterior segment angiographic findings in hypertensive ADAMTS10-open-angle glaucoma (ADAMTS10-OAG) eyes as compared to normotensive control eyes.

ANIMALS STUDIED

Nine ADAMTS10-OAG beagles and four wild-type control dogs.

PROCEDURES

Anterior segment angiography was performed under general anesthesia following intravenous injection of indocyanine green (ICG; 1 mg/kg) and sodium fluorescein (SF; 20 mg/kg) using a Heidelberg Spectralis® confocal scanning laser ophthalmoscope. Time to onset of iridal angiographic phases and the presence/severity of dye leakage into the iris stromal and/or aqueous humor were recorded. Group findings were compared, and multiple linear regression analysis was performed to identify potential factor associations with disease status.

RESULTS

Time to onset of all angiographic phases visualized using ICG was significantly prolonged while time to onset of SF leakage into the aqueous humor was significantly reduced in glaucomatous eyes compared to controls. Only glaucomatous eyes (n = 9) demonstrated evidence of SF stromal leakage. Mean intraocular pressure (IOP) and age were significantly higher, while mean cardiac pulse was significantly lower in glaucomatous eyes compared to controls. Blood pressure and ocular perfusion pressure were not significantly different between groups. Multiple linear regression analysis, controlling for age, IOP, and pulse demonstrated glaucoma, was not predictive of the time to onset of any angiographic phase, stromal, or aqueous humor leakage. However, pulse was a significant factor contributing to the severity of aqueous humor leakage.

CONCLUSIONS

A compromised vascular supply to the anterior segment exists in dogs with ADAMTS10-OAG. These observations warrant further exploration of what role altered perfusion and/or disruption to the blood-aqueous barrier may play.

The Clinical Approach to Canine Glaucoma

This article provides a simplified approach to diagnosis and clinical decision making in cases of canine glaucoma for the general practitioner. An overview of the anatomy, physiology, and pathophysiology related to canine glaucoma is provided as a foundation. Classifications of glaucoma based on cause are described as congenital, primary, and secondary, and a discussion of key clinical examination findings is provided to guide appropriate therapy and prognostication. Finally, a discussion of emergency and maintenance therapy is provided.

A genome-wide association study to investigate genetic loci associated with primary glaucoma in American Cocker Spaniels

OBJECTIVE

To identify genetic associations with primary glaucoma (PG) in American Cocker Spaniels using a genome-wide association study (GWAS).

ANIMALS

A nationwide ambidirectional case-control cohort study was performed in American Cocker Spaniels that had an ophthalmic examination performed by a veterinarian. Ninety-four dogs with PG (cases) and 111 dogs without glaucoma (controls) met phenotypic criteria and had a blood sample collected after receiving informed owner consent.

PROCEDURES

Genomic DNA was extracted from whole blood samples and genotyped (CanineHD BeadChip, Illumina Inc). A case-control GWAS using a linear mixed model was performed, and 3 significance thresholds were calculated (1) using a Bonferroni correction on all single nucleotide polymorphisms (SNPs) included in the GWAS, (2) using a Bonferroni correction on only the unlinked SNPs from a pruned data set, and (3) using 10,000 random phenotype permutations.

RESULTS

Following genotype data quality control, 89 cases and 93 controls were included in the GWAS. We identified an association on canine chromosome (CFA10); however, it did not reach statistical significance. Potential candidate genes within the surrounding linkage disequilibrium interval include coiled-coil domain containing 85A (CCDC85A) and extracellular growth factor containing fibulin extracellular matrix protein 1 (EFEMP1).

CLINICAL RELEVANCE

Primary glaucoma in the American Cocker Spaniel is a complex heterogeneous disease that may be influenced by a locus on CFA10. The candidate genes CCDC85A and EFEMP1 within the identified linkage disequilibrium interval have been shown to be involved in human open-angle glaucoma.

Remodeling of the Lamina Cribrosa: Mechanisms and Potential Therapeutic Approaches for Glaucoma

Glaucomatous optic neuropathy is the leading cause of irreversible blindness in the world. The chronic disease is characterized by optic nerve degeneration and vision field loss. The reduction of intraocular pressure remains the only proven glaucoma treatment, but it does not prevent further neurodegeneration. There are three major classes of cells in the human optic nerve head (ONH): lamina cribrosa (LC) cells, glial cells, and scleral fibroblasts. These cells provide support for the LC which is essential to maintain healthy retinal ganglion cell (RGC) axons. All these cells demonstrate responses to glaucomatous conditions through extracellular matrix remodeling. Therefore, investigations into alternative therapies that alter the characteristic remodeling response of the ONH to enhance the survival of RGC axons are prevalent. Understanding major remodeling pathways in the ONH may be key to developing targeted therapies that reduce deleterious remodeling.

Atypical chorioretinal lesions in Siberian Husky dogs with primary angle-closure glaucoma: a case series

BACKGROUND

A number of etiologies for different canine chorioretinal lesions have been proved or suggested but some fundic lesions remain unclear in terms of an etiologic diagnosis, treatment options and prognosis. The purpose of this case series is to describe atypical chorioretinal lesions observed in dogs with primary angle-closure glaucoma (PACG).

CASE PRESENTATION

Two spayed-female Siberian Huskies (3- and 4-year-old) and one Siberian Husky/Australian Shepherd mixed breed dog (11-month-old) that had multifocal depigmented retinal lesions and PACG were included.

PROCEDURES

Ophthalmic examination, gross, and histopathologic examination findings are described. One of the dogs underwent further clinical diagnostics. Advanced clinical diagnostics on the fellow, presumed to be non-glaucomatous eye of a dog revealed: pectinate ligament dysplasia by gonioscopy, retinal thinning in the depigmented area and wedge shaped retinal thinning with delayed choroidal vascular perfusion by optical coherence tomography, confocal scanning laser ophthalmoscopy, fluorescein and indocyanine green angiography. Quantifiable maze testing for the same eye revealed mild nyctalopia but the full-field electroretinogram showed no generalized decrease of retinal function. Genetic testing for mutations within the retinitis pigmentosa GTPase regulator gene causing X-linked progressive retinal atrophy in Siberian Huskies was negative. Histopathologic evaluations on enucleated eyes in two dogs confirmed goniodysgenesis, PACG with optic nerve head cupping, and diffuse inner retinal atrophy. In addition, segmental profound retinal atrophy, loss of retinal pigment epithelium, and adhesion of the retina to Bruch's membrane was observed and coincided with multifocal depigmented lesions noted on fundic examination.

CONCLUSIONS

To our knowledge, this is the first case series with clinical and histopathologic data of chorioretinal lesions, most likely caused by severely impaired choroidal perfusion. Further studies are warranted to elucidate the etiology and pathophysiology, including its possible association with PACG.

Is it necessary to wait several minutes between applications of different topical ophthalmic solutions? A preliminary study with tropicamide eye drops in healthy dogs

OBJECTIVE

To evaluate the efficacy of topical tropicamide when placed at different time intervals before or after a saline drop.

ANIMALS STUDIED

Eight healthy Labrador and golden retriever dogs.

PROCEDURES

The effect of 1% tropicamide on pupillary diameter (PD) was measured over 240 min when administered alone (control) and then 1 and 5 min prior to, or following, application of a saline drop, with 1-week washout between each of the five trials. Data were analyzed using repeated-measures ANOVA and Tukey post hoc test.

RESULTS

Only 6/110 pairwise comparisons among the 5 trials were statistically significant (p ≤ .035), with post-hoc analysis showing no significant differences (p ≥ .14) between the overall means of all trials. In all five trials, maximal PD was reached 30 min after tropicamide application and maintained until 210 min for 180 min (p = .0005).

CONCLUSIONS

Our results suggest that waiting 1 min between applications of different ophthalmic solutions may be sufficient for maximal drug effect. Care should be taken when extrapolating these results to other species and different ophthalmic formulations.

Looking into the future: Gene and cell therapies for glaucoma

Glaucoma is a complex group of optic neuropathies that affects both humans and animals. Intraocular pressure (IOP) elevation is a major risk factor that results in the loss of retinal ganglion cells (RGCs) and their axons. Currently, lowering IOP by medical and surgical methods is the only approved treatment for primary glaucoma, but there is no cure, and vision loss often progresses despite therapy. Recent technologic advances provide us with a better understanding of disease mechanisms and risk factors; this will permit earlier diagnosis of glaucoma and initiation of therapy sooner and more effectively. Gene and cell therapies are well suited to target these mechanisms specifically with the potential to achieve a lasting therapeutic effect. Much progress has been made in laboratory settings to develop these novel therapies for the eye. Gene and cell therapies have already been translated into clinical application for some inherited retinal dystrophies and age-related macular degeneration (AMD). Except for the intravitreal application of ciliary neurotrophic factor (CNTF) by encapsulated cell technology for RGC neuroprotection, there has been no other clinical translation of gene and cell therapies for glaucoma so far. Possible application of gene and cell therapies consists of long-term IOP control via increased aqueous humor drainage, including inhibition of fibrosis following filtration surgery, RGC neuroprotection and neuroregeneration, modification of ocular biomechanics for improved IOP tolerance, and inhibition of inflammation and neovascularization to prevent the development of some forms of secondary glaucoma.

Medical anti-glaucoma therapy: Beyond the drop

Barriers to effective medical therapy are numerous and include difficulties with effective and sustained control of intraocular pressure (IOP) and adherence to prescribed anti-glaucoma drop regimens. In an effort to circumvent these challenges, a number of new anti-glaucoma therapies with sustained effects have emerged. Methods for sustained delivery of prostaglandin analogs are being intensely investigated and many are in human clinical trials. Intracameral devices include the following: Allergan's Durysta^™ Bimatoprost SR, Envisia Therapeutics' ENV515 travoprost implant, Glaukos' iDose^™ , Ocular Therapeutix's OTX-TIC travoprost implant, and Santen's polycaprolactone implant with PGE2-derivative DE-117. Other prostaglandin-based technologies include Allergan's bimatoprost ring (placed in the conjunctival fornix), Ocular Therapeutics' OTX-TP intracanalicular travoprost implant, subconjunctival latanoprost in a liposomal formulation, and the PGE2 derivative PGN 9856-isopropyl ester that is applied to the periorbital skin. Exciting breakthroughs in gene therapy include using viral vectors to correct defective genes such as MYOC or to modulate gonioimplant fibrosis, CRISPR technology to edit MYOC or to alter aquaporin to reduce aqueous humor production, and siRNA technology to silence specific genes. Stem cell technology can repopulate depleted tissues or, in the case of Neurotech's Renexus® NT-501 intravitreal implant, serve as a living drug delivery device that continuously secretes neurotrophic factors. Other unique approaches involve nanotechnology, nasal sprays that deliver drug directly to the optic nerve and noninvasive alternating current stimulation of surviving cells in the optic nerve. Over time these modalities are likely to challenge the preeminent role that drops currently play in the medical treatment of glaucoma in animals.

Association between preoperative gonioscopic status and postoperative glaucoma after phacoemulsification in dogs: A retrospective cohort study of 505 eyes

OBJECTIVE

To assess whether preoperative gonioscopy status is associated with the development of postoperative glaucoma after phacoemulsification.

PROCEDURE

Eligible dogs and eyes were selected from medical records (2007-2017), and each eye was classified as having normal or abnormal gonioscopy status. Records were examined for postoperative glaucoma, and data were collected for baseline variables including patient signalment, surgical factors, and pre, intra, and postoperative medications.

RESULTS

Of 305 dogs (505 eyes) selected for review in this study, 111 dogs (163 eyes) had abnormal gonioscopy findings, and 194 dogs (342 eyes) had normal gonioscopy findings. Postoperative glaucoma occurred in 24% (39/163) of eyes with abnormal gonioscopy and in 14% (49/342) eyes with normal gonioscopy. Eyes with abnormal gonioscopy status were at higher risk of postoperative glaucoma compared to normal eyes (P = .025). There was no important confounding due to the numerous baseline variables summarised above. Regardless of gonioscopy status, the risk of glaucoma was highest during the first 2 months after surgery.

CONCLUSIONS

Eyes with abnormal gonioscopy findings are at increased risk of postoperative glaucoma compared with eyes with normal gonioscopy findings. Gonioscopy is recommended as a part of presurgical assessment in all dogs prior to phacoemulsification. Further study is needed to assess whether the incidence of postoperative glaucoma can be reduced by increased monitoring or altered treatment protocols in cases that are known to have abnormal gonioscopy findings prior to cataract surgery.

Prophylactic anti-glaucoma therapy in dogs with primary glaucoma: A practitioner survey of current medical protocols

AIM

To examine the use of prophylactic anti-glaucoma medications in the normotensive fellow eye in dogs with unilateral overt primary glaucoma by veterinary ophthalmology clinicians.

METHODS

A survey of veterinary ophthalmology clinicians was distributed over two international list serves servicing veterinary ophthalmologists, trainees, and individuals whose practice consisted primarily of ophthalmic patients. The survey was developed following analysis of historical and currently available medical options for control of intraocular pressure and for neuroprotection.

RESULTS

Responses from 199 veterinary ophthalmology clinicians were evaluated. While a large variety of topical anti-hypertensive drugs and protocols were used, the most commonly used medications were aqueous humor production suppressors such as dorzolamide 2.0% ophthalmic solution, timolol 0.5% ophthalmic solution, and a combination product containing both drugs. Latanoprost 0.005% ophthalmic solution was used infrequently for prophylaxis by comparison. The majority of respondents do not use concurrent anti-inflammatory medications (61.22%), although a sizeable minority used prednisolone acetate, dexamethasone, or ketorolac as prophylactic treatment. Systemically administered ocular anti-hypertensive agents were rarely used. Only 40% of respondents used neuroprotectant agents; the most commonly prescribed were the calcium channel blocker amlodipine and the nutraceutical Ocu-Glo™. Recommended intervals between re-examination by the clinician ranged from one month to one year, with most re-evaluations occurring every 3 to 6 months. The majority of respondents recommended more frequent assessments of IOP at intervals between once monthly and once every 3 months.

CONCLUSIONS

Data analysis of medical therapy for the normotensive fellow eye of dogs previously diagnosed with primary glaucoma suggests that there is a great need for well-designed, prospective, controlled, multi-center studies to determine which protocols have the greatest efficacy in delaying an overt attack in the previously normotensive eye in dogs with a genetic predisposition to glaucoma. Prospective studies utilizing a carbonic anhydrase inhibitor such as dorzolamide and a prostaglandin analogue such as latanoprost would be reasonable as these two drugs are widely used in the treatment of overt glaucoma and would allow for an exploration of the impact of different mechanisms of action of lowering IOP on the pathophysiology of primary glaucoma.

The future of canine glaucoma therapy

Canine glaucoma is a group of disorders that are generally associated with increased intraocular pressure (IOP) resulting in a characteristic optic neuropathy. Glaucoma is a leading cause of irreversible vision loss in dogs and may be either primary or secondary. Despite the growing spectrum of medical and surgical therapies, there is no cure, and many affected dogs go blind. Often eyes are enucleated because of painfully high, uncontrollable IOP. While progressive vision loss due to primary glaucoma is considered preventable in some humans, this is mostly not true for dogs. There is an urgent need for more effective, affordable treatment options. Because newly developed glaucoma medications are emerging at a very slow rate and may not be effective in dogs, work toward improving surgical options may be the most rewarding approach in the near term. This Viewpoint Article summarizes the discussions and recommended research strategies of both a Think Tank and a Consortium focused on the development of more effective therapies for canine glaucoma; both were organized and funded by the American College of Veterinary Ophthalmologists Vision for Animals Foundation (ACVO-VAF). The recommendations consist of (a) better understanding of disease mechanisms, (b) early glaucoma diagnosis and disease staging, (c) optimization of IOP-lowering medical treatment, (d) new surgical therapies to control IOP, and (e) novel treatment strategies, such as gene and stem cell therapies, neuroprotection, and neuroregeneration. In order to address these needs, increases in research funding specifically focused on canine glaucoma are necessary.

Combined subconjunctival injection of dexamethasone for the management of acute primary angle closure: a randomised controlled trial

PURPOSE

To investigate whether the combined subconjunctival injection of dexamethasone can accelerate the decrease in intraocular pressure (IOP) in acute primary angle closure (APAC)-affected eyes.

METHODS

42 patients with APAC were recruited for a randomised controlled trial. These patients were separated into two groups: the injection group (21 patients) and the control group (21 patients). The injection group was subjected to a subconjunctival injection with 2.5 mg dexamethasone. Other drug treatments were the same with the control group. The follow-up was at 0, 3, 6, 12 and 24 hours after injection. The outcome measures include IOP and intraocular inflammation variables.

RESULTS

The IOP was significantly decreased in both groups after treatment. However, 24 hours after the initial treatment, the IOP of the injection group was significantly lower compared with the control group (p = 0.017). Kaplan-Meier survival curve analysis showed that the total success rate of the injection group and the control group were 79.7% and 54.9% at 24 hours after treatment (p = 0.027), respectively. For the comparison of anterior chamber inflammation, the severity of conjunctival erythema, ciliary flush and pain in the injection group was also lower than that in the control group at 24 hours after treatment(p = 0.012, p = 0.048 and p = 0.013, respectively). No statistical significance was found between the two groups regarding the anterior chamber cells, anterior chamber flare and photophobia.

CONCLUSION

The combined subconjunctival injection of dexamethasone for the management of APAC eyes can significantly accelerate the relief of high IOP, and therefore, improve the success rate of treatment.

Primary angle-closure glaucoma with goniodysgenesis in a Beagle dog

Background

Open angle glaucoma is the only type of primary glaucoma reported in Beagles. This case report describes a primary angle-closure glaucoma in a Beagle and its diagnostic and prognostic relevance.

Case presentation

A 12-year-old, neutered male Beagle presented to the Michigan State University (MSU) Comparative Ophthalmology Service for evaluation of suspected visual impairment. Complete ophthalmic examination of the left eye (OS) revealed: blepharospasm, absent menace response, moderate episcleral congestion, mild diffuse corneal edema, mydriasis, asteroid hyalosis, decreased myelination and cupping of the optic nerve head, and mild retinal vascular attenuation. Examinations of the right eye (OD) were within normal limits. Intraocular Pressure (IOP) were 24 mmHg OD and 49 mmHg OS. Gonioscopy OD revealed a narrow iridocorneal angle with moderate pectinate ligament dysplasia characterized by broad-based pectinate ligament strands (fibrae latae) and solid sheets (laminae) throughout all 4 quadrants. DNA testing revealed that the dog did not carry the Gly661Arg ADAMTS10 mutation responsible for primary open angle glaucoma (POAG) in Beagles. The OS was medically managed with latanoprost 0.005% and dorzolamide HCl 2% /timolol malate 0.5% ophthalmic solutions for 7 months and then enucleated due to uncontrolled IOP. Histopathologic evaluation was consistent with goniodysgenesis with a broad, non-perforate, sheet-like band of uveal stroma bridging from the base of the iris to the terminal arborization of Descemet’s membrane. Approximately 14 months from the initial diagnosis of glaucoma OS, OD also developed glaucoma and was enucleated. Histopathologic findings were consistent with goniodysgenesis OD.

Conclusions

To our knowledge, this is the first reported case of PACG with goniodysgenesis in a Beagle supported by clinical, genetic, and histopathologic data. It highlights the importance of gonioscopy in Beagles with glaucoma. Further studies with a larger number of dogs are warranted to characterize clinical manifestations and inheritance of PACG in this breed.

Evaluation of potential risk factors for development of primary angle-closure glaucoma in Bouviers des Flandres

OBJECTIVE To evaluate potential risk factors for development of primary angle-closure glaucoma (PACG) in Bouviers des Flandres. DESIGN Prospective, observational study. ANIMALS 98 Bouviers des Flandres. PROCEDURES All dogs underwent slit-lamp biomicroscopy, indirect ophthalmoscopy, gonioscopy, applanation tonometry, streak retinoscopy, and A-scan, B-scan, and high-resolution ultrasonography. Iridocorneal angles and degree of pectinate ligament dysplasia sheeting were graded, and an angle index was mathematically derived for each eye on the basis of these values. Ciliary clefts evaluated by high-resolution ultrasonography were classified as open, narrow, or closed. Owners were contacted by telephone 7 to 9 years after the initial examination to determine whether dogs had a subsequent diagnosis of PACG. Relationships between previously recorded variables and the development of PACG were evaluated by logistic regression methods. Available pedigrees were reviewed to assess genetic relationships among affected dogs. RESULTS 9 of 92 (9.8%) dogs with follow-up information available developed PACG. An angle index < 1 and presence of a narrow or closed ciliary cleft in 1 or both eyes were each significantly associated with development of PACG. Odds of developing PACG for dogs with an angle index < 1 (indicating marked reduction in outflow capacity through the iridocorneal angle), a narrow or closed ciliary cleft in > 1 eye, or both findings were 13, 20, and 28 times those for dogs that did not have these findings, respectively. All dogs that developed PACG shared 1 common male sire or grandsire. CONCLUSIONS AND CLINICAL RELEVANCE Several anatomic factors were significant risk factors for development of PACG in this population of dogs. Results also suggested a genetic component for the disease.

Microscopic Lesions in Canine Eyes with Primary Glaucoma

Although the clinical classification of primary glaucoma in dogs is quite simple, the phenotypes of glaucoma in most of the species are indeed multiple. Ophthalmologists can often evaluate the dynamic changes of clinical signs at different times in the course of the disease, whereas pathologists are often presented with globes that have undergone abundant therapies and are at the end stage. Therefore, an open collaboration between clinicians and pathologists can produce the most accurate interpretation in the pathology report and improve patient outcomes. This article focuses on the histomorphologic elements that characterize, and are important to, canine primary glaucomas.

Definition, Classification, and Pathophysiology of Canine Glaucoma

Glaucoma is a common ocular condition in humans and dogs leading to optic nerve degeneration and irreversible blindness. Primary glaucoma is a group of spontaneous heterogeneous diseases. Multiple factors are involved in its pathogenesis and these factors vary across human ethnic groups and canine breeds, so the clinical phenotypes are numerous and their classification can be challenging and remain superficial. Aging and oxidative stress are major triggers for the manifestation of disease. Multiple, intertwined inflammatory and biochemical cascades eventually alter cellular and extracellular physiology in the optic nerve and trabecular meshwork and lead to vision loss.

Clinical Signs and Diagnosis of the Canine Primary Glaucomas

The diagnosis of glaucoma is highly dependent on a working understanding of the clinical signs and available diagnostic procedures. Clinical signs may be attributable to increased intraocular pressure and/or complex alterations in the physiology or molecular biology of the anterior segment, retinal ganglion cells, and optic nerve. Many diagnostic procedures seek to more fully characterize these alterations and to identify which clinical features increase the risk of overt primary angle closure glaucoma (PACG) occurring. Considerable progress has been made in identifying the anatomic features that predispose an eye to PACG, and in elucidating the role of reverse pupillary block.

Chronic Glaucoma in Dogs: Relationships Between Histologic Lesions and the Gonioscopic Diagnosis of Pectinate Ligament Dysplasia

Pectinate ligament dysplasia (PLD) is a common cause of canine glaucoma and the definitive clinical diagnosis is based on gonioscopy. Although the histologic lesions of PLD have been described, it has not been determined whether these changes are specific for PLD or if similar histologic changes can develop as a consequence of secondary glaucoma. The filtration angles of 61 enucleated canine globes with chronic glaucoma were evaluated with light microscopy by 3 examiners who were masked to the clinical history, signalment, and gonioscopic results. A histologic diagnosis of PLD versus non-PLD was determined by each examiner based on previously reported morphologic criteria and compared with the clinical gonioscopic diagnosis. Of the 61 enucleated glaucomatous eyes, 40 were clinically diagnosed with PLD. For all 3 examiners, a histologic diagnosis of PLD corresponded poorly with the clinical diagnosis of PLD (range of kappa score: 0.149-0.269; range of AUC: 0.592-0.621). There was no difference between examiners in their ability to correctly diagnose PLD histologically (P = .978). A fair degree of agreement was noted among examiners in obtaining their suspected histologic diagnosis of PLD (kappa score 0.256). No individual or sets of histologic ICA features were consistent with clinical PLD. The results indicate the histologic ICA changes proposed to be characteristic of PLD are also noted in canine globes affected with chronic secondary glaucoma. Therefore, using routine histologic evaluation, a histologic diagnosis of PLD is not possible in the face of chronic canine glaucoma.

Genetics of Canine Primary Glaucomas

Primary glaucomas are a leading cause of incurable vision loss in dogs. Based on their specific breed predilection, a genetic cause is suspected to be responsible, and affected dogs should be excluded from breeding. Despite the high prevalence of primary glaucomas in dogs, their genetics have been studied in only a small number of breeds. The identification of canine glaucoma disease genes, and the development of genetic tests, will help to avoid the breeding of affected dogs in the future and will allow for earlier diagnosis and potentially more effective therapy.

Ultrasound biomicroscopic findings of the iridocorneal angle in live healthy and glaucomatous dogs

By using ultrasound biomicroscopy (UBM), the cross-sectional structures of the entire iridocorneal angle (ICA) which are unable to assess with gonioscopic examination were evaluated objectively and quantitatively in live healthy and glaucomatous dogs. The ICAs of normotensive eyes in healthy dogs with normal open angle (NOR), a predisposition to primary closed angle glaucoma (PCAG) (PREDIS) and suffering from unilateral PCAG (UNI), as well as the ICAs of hypertensive eyes with acute and chronic PCAG (ACG and CRG), were assessed. The opening of the ciliary cleft in PREDIS was smaller than that in NOR. In UNI, the opening and area of the ciliary cleft were significantly decreased compared with those of NOR and PREDIS. ACG had widespread structural abnormalities including marked decrease in the ciliary cleft and scleral venous plexus, and a thinner sclera than those in normotensive eyes, whereas the ICA collapsed in CRG with the thinnest sclera. Medical therapy-responsive glaucomatous cases had wider ciliary cleft and scleral venous plexus than unresponsive ones. These findings suggest that the ciliary cleft and scleral venous plexus of the ICA are key structures contributing to not only the pathophysiology of canine glaucoma but also the responsiveness to medical therapy in glaucomatous eyes, and cross-sectional entire structures of the ICA should be evaluated quantitatively with UBM when diagnosing and managing canine glaucoma.

Glaucomas

Canine and feline glaucomas are commonly presented as ocular emergencies. Glaucoma is a common cause of vision loss and a frustrating disorder in terms of medical and surgical treatment. Increased intraocular pressure (IOP) is a significant risk factor in the disease, leading to damage of the retina and optic nerve head. IOP measurement and gonioscopic and fundic examinations provide the instruments for diagnosis of glaucoma. The primary goal in glaucoma therapy is aimed at vision preservation. Medical treatment provides temporary relief, but alone it fails to control IOP in the long term, and surgical intervention is recommended. Surgical patient selection depends on several factors, from type and stage of glaucoma to the presence of or potential for vision. Available surgical procedures to decrease IOP consist of cyclodestructive techniques to decrease aqueous humor production and filtering techniques to increase its drainage. Even with recent surgical and medical advances, pain and blindness are still common occurrences in the disease: end-stage procedures such as enucleation, evisceration with intrascleral prosthesis, and pharmacologic ablation of ciliary bodies are then recommended to address chronic discomfort for buphthalmic and blind globes.

Autonomic drugs in the treatment of canine and feline glaucoma--Part II: Medications that lower intraocular pressure by reducing aqueous humour production

One characteristic of the most common types of glaucoma is increased intraocular pressure (IOP), which has a damaging effect on optic nerve axons, leading to progressive loss of retinal ganglion cells. Therefore, ocular hypotensive drugs are the mainstay of pharmacological therapy for glaucoma. This review article, which is the second part of a two-part series, is dedicated to autonomic drugs which lower IOP by decreasing the aqueous humour production. These agents are subdivided into two groups: β-adrenergic antagonists and selective α2-adrenergic agonists. This paper summarizes the current state of knowledge on the mechanism of action of these drugs and their effect on IOP in dogs and cats. Moreover, it discusses their possible undesirable side effects of these medications and presents the current ideas about their role and position in the medical management of glaucoma in small animals.

Autonomic drugs in the treatment of canine and feline glaucoma – Part I: Medications that lower intraocular pressure by increasing the outflow of aqueous humour

One characteristic of the most common types of glaucoma is increased intraocular pressure (IOP), which has a damaging effect on optic nerve axons, leading to progressive loss of retinal ganglion cells. Therefore, ocular hypotensive drugs are the mainstay of pharmacological therapy for glaucoma. This review article, which is the first part of a two-part series, is dedicated to autonomic drugs which lower IOP by increasing the outflow of aqueous humour. These agents are subdivided into two groups: (a) drugs that lower IOP by increasing the trabecular outflow and the uveoscleral outflow (i.e. nonselective adrenergic agonists), and (b) medications that lower IOP by opening of the drainage angle and by increasing the conventional outflow via the trabecular outflow (i.e. parasympathomimetics). This paper summarizes the current state of knowledge on the mechanism of action of these drugs and their effect on IOP in dogs and cats. Moreover, it discusses possible undesirable side effects of these medications and presents the current ideas about their role and position in the medical management of glaucoma in small animals

Pharmacology of topical prostaglandin F2 α analogs and their place in the treatment of glaucoma in small animals

A distinguishing feature of the most common types of glaucoma is an increased intra-ocular pressure (IOP), which has a damaging effect on optic nerve axons, leading to the progressive loss of retinal ganglion cells. Therefore, IOP-lowering medications are the mainstay of glaucoma therapy. Topical prostaglandin F2 α analogs (PGAs) are a relatively new class of ocular hypotensive drugs, which have made a huge impact on the treatment of glaucoma in dogs. This study summarizes the current state of knowledge on the mechanism of action of these agents and their effect on IOP in dogs and cats. It also discusses potential harmful side effects of PGAs and presents contemporary opinions about their role and place in the medical management of glaucoma in small animals.

Gender differences in iridocorneal angle morphology: a potential explanation for the female predisposition to primary angle closure glaucoma in dogs

OBJECTIVE

Female dogs have approximately twice the risk of males for developing primary angle closure glaucoma (PACG). The cause of this gender difference is unknown, but one theory proposes that the gender differences in iridocorneal angle morphology are involved in this risk differential.

PROCEDURES

Fifty beagles (25 males, 25 females) were included into this study and had normal baseline ophthalmic examinations. Normal dogs were selected so as to avoid any potentially confounding influence of goniodysgenesis. Standardized 20-MHz high-resolution ultrasound images of the iridocorneal angle were acquired from one eye of each dog with the scan plane perpendicular to the limbus in the superior temporal quadrant. Images were imported into ImageJ, and the angle opening distance (AOD) and angle recess area (ARA) were measured by a masked observer, and the analysis of variance method was used to compare differences.

RESULTS

The mean (±SD) AOD was significantly smaller for female dogs (0.847 ± 0.241 mm) vs. male dogs (1.058 ± 0.322 mm) P-value = 0.012. The mean (± SD) ARA tended to be smaller for female dogs (0.584 ± 0.278 mm) vs. male dogs (0.748 ± 0.385 mm), but this difference was not significant (P-value = 0.092).

CONCLUSIONS

Female dogs have a significantly smaller AOD vs. males. This difference may render the female iridocorneal angle more susceptible to closure and may partially explain the 2:1 female/male predisposition to PACG. Further studies using goniodysgenic dogs are warranted.

Epidemiology of canine glaucoma presented to University of Zurich from 1995 to 2009. Part 1: Congenital and primary glaucoma (4 and 123 cases)

OBJECTIVE

To investigate the epidemiology of canine congenital and primary glaucoma in the cases presented to the University of Zurich, Vetsuisse Faculty (UZH) from 1995 to 2009.

METHODS

Information was obtained from the computer database of patients examined by members of the UZH Ophthalmology Service, between January 1995 and August 2009. Congenital and primary glaucoma was diagnosed based on the age of onset, the lack of evidence of any antecedent eye conditions, and/or the presence and severity of iridocorneal angle defects. The data was evaluated for breed, gender and age at presentation.

RESULTS

A total of 5984 dogs presented to the UZH Ophthalmology service between 1995 and 2009. Four dogs of different breed were diagnosed with congenital glaucoma and 123 dogs were diagnosed with primary glaucoma. For the primary glaucomas the overall male to female ratio (M:F) was 1:1.41 and the age of onset ranged from 0.12 to 18.3 years with a mean of 7.3 ± 3.6 years. Data suggested a predisposition for primary glaucoma in the Siberian Husky, Magyar Vizsla and Newfoundland from 2004 to 2009.

CONCLUSION

The report presents the epidemiology of canine congenital and primary glaucomas presented to the UZH from 1995 to 2009. A previous suspicion of predisposition for primary glaucoma in the Newfoundland dog (n = 6) and the Magyar Vizsla breed (n = 8) was confirmed.

Cloning of canine myocilin cDNA and molecular analysis of the myocilin gene in Shiba Inu dogs

OBJECTIVE

To identify canine myocilin cDNA and compare its sequence in glaucomatous and nonglaucomatous Shiba Inu dogs with closed and open iridocorneal angles (ICAs).

PROCEDURES

Total RNA was extracted from the ciliary body of the eyes of a healthy Beagle, and the canine myocilin gene was cloned and sequenced. Of the Shiba Inu dogs tested, five were glaucomatous with closed ICA, three were nonglaucomatous with open ICA, and two were nonglaucomatous with closed ICA. The genomic DNA of these dogs was extracted from peripheral blood leukocytes. The exons of the canine myocilin gene were amplified using the polymerase chain reaction (PCR), and sequenced. The frequency of mutation in canine myocilin DNA was verified in these dogs by using the myocilin cDNA of a Beagle.

RESULTS

The canine myocilin cDNA was 1452 bp long and contained the entire open reading frame encoding 483 amino acids. A leucine zipper-like motif and olfactomedin-like domain were conserved in the amino acid residues. The presence of sequence variants in the genomic DNA of Shiba Inu dogs was independent of the occurrence of glaucoma and ICA grading.

CONCLUSIONS

Myocilin RNA was detected in the ciliary body and trabecular meshwork (TM) of a Beagle. The myocilin sequence of Shiba Inu dogs suggests that myocilin mutations are unlikely to play a significant role in the pathogenesis of primary closed-angle glaucoma in this breed. However, several mutations in the myocilin gene in exon 1 of Shiba Inu dogs may predispose them to an obstruction in the anterior aqueous outflow.

Outcomes of nonsurgical management and efficacy of demecarium bromide treatment for primary lens instability in dogs: 34 cases (1990-2004)

OBJECTIVE

To determine outcome of initial conservative management for primary lens luxation and evaluate topically administered demecarium bromide miotic treatment for prevention of anterior lens luxation, glaucoma, and vision loss in dogs.

DESIGN

Retrospective case series.

ANIMALS

34 dogs with primary lens luxation.

PROCEDURES

Medical records of affected dogs were reviewed for times to anterior luxation, luxation of the lens in the opposite eye, development of glaucoma, and vision loss.

RESULTS

At 4 to 6 weeks and at 3 months after diagnosis of lens instability (subluxation or posterior luxation), 100% (34/34 and 29/29, respectively) of conservatively managed eyes retained vision. At 1 year after diagnosis of lens instability, 80% (16/20) of conservatively managed eyes retained vision, and at 2 years after diagnosis of lens instability, 11 of 19 conservatively treated eyes retained vision. The only significant effect of miotic treatment was to delay anterior lens luxation in eyes with lens instability. Miotic treatment did not significantly affect the time from anterior lens luxation in 1 eye to anterior luxation in the other eye, time to onset of glaucoma, or time to loss of vision in eyes with an unstable lens.

CONCLUSIONS AND CLINICAL RELEVANCE

Prophylactic topically administered miotic treatment may be effective at delaying anterior luxation of an unstable lens in eyes affected by primary lens instability. Conservative medical management of dogs with primary lens instability is a reasonable alternative to surgical removal of a subluxated or posteriorly luxated lens via intracapsular lens extraction.

Corneal sensitivity and aqueous tear production in dogs undergoing evisceration with intraocular prosthesis placement

OBJECTIVE

To determine whether there is an association between evisceration with intraocular prosthesis placement via a dorsal scleral approach and decreased corneal sensitivity or aqueous tear production in dogs.

DESIGN

Prospective study.

PROCEDURE

Twenty-one dogs scheduled to undergo unilateral evisceration with intraocular prosthesis, and with a normal normotensive contralateral eye, based on slit-lamp biomicroscopy, indirect ophthalmoscopy and applanation tonometry, were included in the study. Central corneal sensitivity was measured with a Cochet-Bonnet esthesiometer prior to (week 0) and 2, 7, 14 and 28 weeks following surgery. Other pre- and postoperative data collected were Schirmer's tear test (STT I), Schirmer's tear test with topical anesthesia (STT II), corneal fluorescein retention, and intraocular pressure (IOP; postoperative control eye only). Axial globe length was determined for both eyes. Scleral incision length in the surgery eye was also recorded.

RESULTS

Corneal sensitivity was significantly lower in eyes undergoing evisceration with intraocular prosthesis than in control eyes both preoperatively and postoperatively at all time points. There was no significant difference between preoperative and postoperative corneal sensitivity within the two groups. There was no significant difference in STT I or STT II-values between control eyes and surgery eyes preoperatively. There was a significant difference between preoperative and 2-week postoperative STT I in the surgery eye. There was a significant difference between the preoperative and 28-week postoperative STT II in the surgery eye. The 28-week postoperative STT I-values differed significantly between control and surgery eyes. Corneal sensitivity tended to decrease with increased degree of buphthalmos or with increased scleral incision length, although neither relationship was statistically significant.

CONCLUSIONS

Eyes that have sustained chronically elevated intraocular pressure had decreased axial corneal sensitivity compared to normal eyes, which persisted even after intraocular pressure was reduced. The presence of buphthalmos appeared to be a factor in globes developing decreased corneal sensitivity. Evisceration via a dorsal scleral incision did not result in a significant further reduction in axial corneal sensitivity. A mild reduction in aqueous tear production occurred over time in globes following evisceration with intraocular prosthesis.

Evaluation of risk factors for development of secondary glaucoma in dogs: 156 cases (1999–2004)

OBJECTIVE

To determine the immediately antecedent cause of secondary glaucoma and the prevalence of secondary glaucoma with anterior uveitis or lens dislocation in dogs.

DESIGN

Retrospective case series.

ANIMALS

156 dogs with secondary glaucoma.

PROCEDURES

Cause of glaucoma was determined from records. Breed, age, sex, and neuter status of all dogs with secondary glaucoma were compared with the general hospital population. The prevalence of secondary glaucoma in dogs with a primary diagnosis of lens dislocation or anterior uveitis during the same period was determined.

RESULTS

Secondary glaucoma was diagnosed in 156 of 2,257 (6.9%) dogs examined because of ophthalmic disease and was bilateral in 33 (21.2%) of those dogs. In 31 (94%) bilaterally affected dogs, the antecedent cause was the same in both eyes. Common causes of secondary glaucoma were non-surgical anterior uveitis (44.9%), anterior uveitis associated with prior phacoemulsification (15.8%), and lens dislocation (15.2%). Parson Russell Terriers, Poodles, Boston Terriers, Cocker Spaniels, Rhodesian Ridgebacks, and Australian Cattle Dogs had diagnoses of secondary glaucoma more often than expected, compared with the reference population. Age, sex, neuter status, and laterality were not associated with secondary glaucoma. The prevalence of secondary glaucoma in dogs with lens dislocation or uveitis was 15% or 17%, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Glaucoma develops secondary to many intraocular diseases, particularly uveitis and lens dislocation. Diagnosis of these diseases should prompt frequent monitoring of intraocular pressure, regardless of signalment.

Possible association of glaucoma with pectinate ligament dysplasia and narrowing of the iridocorneal angle in Shiba Inu dogs in Japan

The purpose of the study was to describe the shape of the pectinate ligament (PL) and to estimate the width of the iridocorneal angle (ICA) in glaucomatous and non-glaucomatous eye of Shiba Inu dogs in Japan. One hundred fourteen Shiba Inu dogs were presented to the Veterinary Medical Center at the University of Tokyo between June 1998 and June 2003. Among these, 46 dogs had glaucoma; the remaining 68 dogs were presented for routine vaccinations or heartworm testing and were used as a control population. Complete ophthalmic examination and gonioscopy were performed in all the dogs. PL and ICA were evaluated by gonioscopy and goniophotographs. Of the 46 dogs with glaucoma, 17 (37%) were affected bilaterally, and 29 (63%) were affected unilaterally. Of the 29 dogs with unilateral glaucoma, the ICA of the normotensive eye was slightly narrow in 2 eyes (7%), narrow in 12 eyes (41%), and closed in 15 eyes (52%). Among the normotensive control group, the ICA was open in 13 dogs (19%), slightly narrow in 29 (43%), narrow in 22 (32%), and closed in 4 (6%). In dogs with narrow and slightly narrow ICAs, the PLs were thickened, and in some cases, formed a solid sheet. The majority of dogs with glaucoma had changes in both ICA and PL. A narrow or slightly narrow ICA was also detected in the majority of normotensive control dogs. This study suggests that ICA narrowing and PL thickening is a common abnormality in Shiba Inu dogs in Japan and may predispose those dogs to glaucoma.

Putative aqueous humor misdirection syndrome as a cause of glaucoma in cats: 32 cases (1997-2003)

OBJECTIVE

To characterize the clinical and morphologic aspects of aqueous humor misdirection syndrome (AHMS) in cats and provide a hypothesis regarding its pathogenesis on the basis of detailed analysis of affected cats.

DESIGN

Retrospective study.

ANIMALS

32 cats (40 eyes).

PROCEDURE

Medical records of cats in which AHMS was diagnosed from July 1997 to August 2003 were reviewed. In certain cats, results of additional diagnostic testing were also obtained, including A-scan, B-scan, and high-resolution ultrasonography; streak retinoscopy; video keratometry; and infrared neutralizing videoretinoscopy as well as results of analysis of flash-frozen sections and histologic examination of enucleated globes.

RESULTS

Cats had a uniformly shallow anterior chamber, intact lens zonules, and a narrowed approach to an open iridocorneal angle. Mean age of affected cats was 11.7 years (range, 4 to 16 years), and female cats were significantly more often affected than male cats. Clinical signs included mydriasis, decreased pupillary light reflex, decreased menace response, and blindness. Glaucomatous changes to the optic nerve, incipient cataracts, and eventual blindness were seen. Intraocular pressure was > or = 20 mm Hg (range, 12 to 58 mm Hg) in 32 of 40 eyes. Ultrasonography and histologic examination revealed a thickened anterior vitreal face interposed between the lens and ciliary body, partial ciliary cleft collapse, and cavitated vitreal regions. Various treatment modalities were used.

CONCLUSIONS AND CLINICAL RELEVANCE

AHMS affects older cats, especially females, and may result in glaucoma, vision loss, and signs of ocular pain. Topical administration of carbonic anhydrase inhibitors decreased intraocular pressure.

Incidence of Canine Glaucoma with Goniodysplasia in Japan: A Retrospective Study

The incidence of primary and secondary glaucoma in dogs was investigated. A total of 1244 dogs received ophthalmologic examinations, including tonometry and gonioscopy. Goniophotographs were taken using a goniolens to evaluate the iridocorneal angle (ICA) as well as pectinate ligament (PL). The anterior width of the ciliary cleft and the total distance from the origin of the PL to the anterior corneal surface were measured from the goniophotographs. Glaucoma was diagnosed based on the cupping of the optic nerve head, clinical signs, ocular changes, and high IOP, and it was synchronized with gonioscopic grades to differentiate between primary and secondary glaucoma. We investigated 1244 dogs of 29 breeds, including the mixed breed; among these, glaucoma was diagnosed in 127 dogs (162 eyes). Of 162 eyes, primary glaucoma was diagnosed in 129 eyes and secondary glaucoma in 33 eyes. Shiba Inu dogs (42 dogs, 33%) showed the highest incidence of glaucoma, followed by Shih-Tzu (21 dogs, 16.5%). Furthermore, all the glaucomatous Shiba Inu dogs had primary glaucoma with abnormal ICA grades and dysplastic PLs. The findings of our study reveal that the Shiba Inu breed in Japan may have a hereditary predisposition to glaucoma.

Canine goniodysgenesis-related glaucoma: a morphologic review of 100 cases looking at inflammation and pigment dispersion

PURPOSE

To investigate the role of pigment dispersion and inflammation in the pathogenesis of goniodysgenesis-related glaucoma (GDRG).

PROCEDURES

Cases of GDRG were selected when the duration of the disease was specified and there was not any confounding pathology. Cases were grouped into < or = 7-day (acute), and > 7-day (chronic) durations, based on the time required to effect end-stage retinal damage. Acute cases were further divided into < 4-day and 4-7-day groups to assess peracute changes. Slides were evaluated for four individual signs of pigment dispersion: segmental loss of posterior iris pigment epithelium, clumping of posterior iris pigment epithelium, pigmented cells in the trabecular meshwork or anterior chamber and preferential settling of pigmented cells in the ventral aspect of the iridocorneal angle. Slides were also evaluated for the presence of neutrophils and/or lymphoplasmacytic cells in the trabecular meshwork (TM). Differences between groups were analyzed statistically.

RESULTS

Of 100 cases evaluated, 34 were < or = 7-days (acute) (14 < 4-day and 20 4-7-day) and 66 were > 7-days (chronic) in duration. Of all globes examined, 96% had at least one sign of pigment dispersion, with no significant difference between groups. Two or more signs of pigment dispersion were present in 76% of all globes. The 4-7-day group was significantly more likely than the < 4-day group to have at least two signs. The difference was not significant between < or = 7- and > 7-day groups. Neutrophils were present in the TM of 86% of < 4-day and 50% of 4-7-day cases. Cases in the < or = 7-day group were significantly more [corrected] likely than > 7-day cases to have neutrophils in the TM, with 65% and 17% [corrected] positive cases, respectively. Lymphoplasmacytic inflammation was present in 53% of all cases, with no significant difference between groups. Cases in the < or = 7-day group were significantly more likely than > 7-day cases to have both types of inflammation.

CONCLUSIONS

Our results indicate that both acute inflammation and pigment dispersion may be key factors in the pathogenesis of GDRG. Pigment dispersion is prevalent at all time points and increases during the first 7 days. The finding of iris pigment epithelial loss supports the theory that pupillary block associated with iris-lens touching may be important in the pathogenesis of GDRG.

Ocular hypotensive drugs

The glaucomas are vision-threatening diseases that commonly result in blindness. In addition to knowledge of the at-risk breeds and predisposing factors for glaucoma as well as the clinical signs of the disease, regular IOP screening with tonometry is required to confirm glaucoma and to monitor the status of the eye. Early intervention with a combination of medications that are capable, in a given species, of reducing aqueous production and increasing aqueous outflow as well as potentially reducing progressive neurosensory degeneration is key to the medical management of this disease.

Compounding in veterinary ophthalmology

Compounded medications provide treatment options that can improve the clinical outcome for patients, improve compliance, and increase the potential for cure. This article provides an overview of the types and uses of compounded ophthalmic products in veterinary practice as well as criteria for selecting a pharmacist skilled in sterile compounding.

The effects of topical ocular application of 0.25% demecarium bromide on serum acetylcholinesterase levels in normal dogs

OBJECTIVE

To assess the effects of topical ocular application of 0.25% demecarium bromide on serum acetylcholinesterase (AChE) levels in normal dogs.

ANIMALS

Nine adult mixed breed dogs weighing between 18 and 27 kg.

PROCEDURES

Fifty micro L of 0.25% demecarium bromide were applied to one eye of each dog every 8 h for 6 days. Blood was analyzed for AChE levels prior to commencement of eye drops, and at 45 min, 1 h 45 min, 4 h 45 min, 1 day, 3 days, and 7 days following commencement of eye drops using a 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) reaction.

RESULTS

Acetylcholinesterase levels declined over the first 24 h following commencement of demecarium administration in most dogs. This decline was highly variable and was statistically significant by 24 h. In some individuals AChE levels were suppressed to levels approaching clinical toxicity. By day 3 AChE levels had risen to levels above baseline in most dogs.

CONCLUSIONS

Topical ocular application of demecarium causes transient suppression of systemic acetylcholinesterase levels in most dogs. Acetylcholinesterase levels generally do not fall to toxic levels, but may do so in certain individuals. Demecarium bromide eye drops generally do not cause AChE toxicity, but dogs receiving such therapy should be monitored for signs of AChE toxicity, and concomitant use of other AChE inhibitors should be avoided.