Tumours involving the retrobulbar space in cats: 37 cases.

OBJECTIVES: The aim of this multicentre retrospective study was to describe the clinical presentation, imaging findings, diagnosis and outcomes of cats with retrobulbar neoplasia. METHODS: A total of 37 cats that were diagnosed with retrobulbar neoplasia and underwent advanced imaging were recruited from searches of the clinical records of two referral hospitals. All cats had neoplasia confirmed via cytology or histopathology. Data relating to the signalment, presentation, results of investigations, treatment and outcome were recorded. A review of imaging studies was performed where possible. RESULTS: In total, 23 cases (62%) were presented with respiratory signs. Exophthalmos was the most common ophthalmological examination finding, present in 18 cases (49%). Thirty-two cases (86%) had secondary extension of neoplasia to the retrobulbar space (most commonly from the nasal cavities), present in 20 cases (54%), of which 12 were lymphoma. In cases where contrast was administered, 28/35 (80%) had contrast-enhancing masses. Orbital extension was detected in 21 cases (57%), exophthalmos in 22 (59%), globe deformation in 12 (32%) and local lymphadenomegaly in 22 (61%). In total, 36 (97%) retrobulbar tumours were malignant. Thoracic imaging, where it was performed, was concerning for metastasis in 8/25 cases (31%), with abdominal imaging suggestive of metastasis in 5/12 (42%). The most common diagnosis was lymphoma with 19 cases (51%), with nasal lymphoma representing 12 of these, followed by carcinoma in 10 (27%). The median survival time, for cases where death was recorded, was 85 days (range 1-263 days). CONCLUSIONS AND RELEVANCE: To the authors' knowledge, this is the largest study of neoplasia affecting the feline retrobulbar space. Retrobulbar tumours in cats are overwhelmingly malignant, and commonly due to secondary extension of tumours originating elsewhere. Lymphoma, particularly arising from the nasal cavities, was the most common cause. Cats presenting with signs suggestive of retrobulbar disease should be assessed for disease affecting any of the structures of the head.

Changes in retinal vascular diameters in senior and geriatric cats in association with variation in systemic blood pressure.

OBJECTIVES: Early diagnosis of arterial hypertension is essential to prevent target organ damage. In humans, retinal arteriolar narrowing predicts hypertension. This blinded prospective observational study investigated the retinal vessel diameters in senior and geriatric cats of varying systolic blood pressure (SBP) status and evaluated retinal vascular changes in hypertensive cats after treatment. METHODS: Cats with a median age of 14 years (range 9.1-22 years) were categorised into five groups: group 1, healthy normotensive (SBP <140 mmHg; n = 40) cats; group 2, pre-hypertensive (SBP 140-160 mmHg; n = 14) cats; group 3, cats with chronic kidney disease (CKD) and normotensive (n = 26); group 4, cats with CKD and pre-hypertensive (n = 13); and group 5, hypertensive cats (SBP >160 mmHg, n = 15). Colour fundus images (Optibrand ClearView) were assessed for hypertensive lesions. Retinal vascular diameters and bifurcation angles were annotated and calculated using the Vascular Assessment and Measurement Platform for Images of the Retina annotation tool (VAMPIRE-AT). When available, measurements were obtained at 3 and 6 months after amlodipine besylate treatment. RESULTS: Ten hypertensive cats had retinal lesions, most commonly intraretinal haemorrhages and retinal exudates. Arteriole and venule diameters decreased significantly with increasing age (-0.17 ± 0.05 pixels/year [P = 0.0004]; -0.19 ± 0.05 pixels/year). Adjusted means ± SEM for arteriole and venule diameter (pixels) were 6.3 ± 0.2 and 8.9 ± 0.2 (group 1); 7.6 ± 0.3 and 10.1 ± 0.4 (group 2); 6.9 ± 0.2 and 9.5 ± 0.3 (group 3); 7.4 ± 0.3 and 10.0 ± 0.4 (group 4); and 7.0 ± 0.3 and 9.8 ± 0.4 (group 5). Group 1 arteriole and venule diameters were significantly lower than those of groups 2 and 4. Group 2 arteriole bifurcation angle was significantly narrower than those of groups 1 and 3. Post-treatment, vessel diameters decreased significantly at 3 and 6 months in seven hypertensive cats. CONCLUSIONS AND RELEVANCE: Increased age was associated with reduced vascular diameters. Longitudinal studies are required to assess if vessel diameters are a risk indicator for hypertension in cats.

Biocompatibility of a Novel Microfistula Implant in Nonprimate Mammals for the Surgical Treatment of Glaucoma.

PURPOSE: The purpose of this study was to evaluate the ocular safety of a novel microfistula implant and its composite materials in an animal model. METHODS: The anterior chambers of 12 rabbit eyes were injected with either glutaraldehyde cross-linked porcine gelatin extract or balanced salt solution and were followed by serial slit lamp examinations over 3 days. The eyes of 18 canines underwent microfistula implantation or a sham procedure. The animals were monitored over the subsequent 12 months, using serial slit lamp examinations, indirect ophthalmoscopy, tonometry, specular microscopy, and high-resolution ultrasonography. Ocular tissues were examined histopathologically on postoperative days 7, 30, 90, 180, and 365. RESULTS: Glutaraldehyde cross-linked porcine gelatin did not induce significant intraocular inflammation in the rabbit model. The microfistula implant was well tolerated and did not stimulate significant tissue response in the canine eye. The microfistula tube did not undergo structural change or degradation over the course of the study. CONCLUSIONS: In nonprimate mammals, the material composing the microfistula implant and the implant itself do not induce significant inflammation or tissue reaction.

Effects of antifungal drugs and delivery vehicles on morphology and proliferation of equine corneal keratocytes in vitro.

OBJECTIVE: To evaluate the effects of topical antifungal drugs and delivery vehicles on the morphology and proliferation rate of cultured equine keratocytes. STUDY POPULATION: 16 corneas obtained from 8 apparently ophthalmologically normal horses < 0.5 hours after euthanasia for reasons unrelated to the study. PROCEDURES: Primary cultures of equine keratocytes were obtained from corneal stroma and were exposed to several concentrations of 3 commonly used, topically applied antifungals: natamycin, itraconazole, and miconazole. In addition, effects of drug delivery vehicles DMSO, benzalkonium chloride, and carboxymethylcellulose and a combination vehicle composed of polyethylene glycol, methylparaben, and propylparaben were also evaluated. Morphological changes and cellular proliferation were assessed 24, 48, and 72 hours after application. RESULTS: At the highest concentrations tested, all antifungals caused marked cellular morphological changes and inhibited proliferation. At low concentrations, natamycin and miconazole induced rounding, shrinking, and detaching of the cells with inhibition of cellular proliferation. Natamycin caused the most severe cellular changes. Itraconazole, at the low concentrations, caused minimal morphological changes and had a minimal effect on proliferation. All vehicles tested had significantly less effects on cellular morphology and proliferation when compared with the antifungals, except for the combination vehicle, which caused severe morphological changes and inhibited proliferation, even at low concentrations. The DMSO had minimal effects on cellular morphology and proliferation, even at high concentrations. CONCLUSIONS AND CLINICAL RELEVANCE: Itraconazole had significantly less cytotoxic effects on equine keratocytes in culture than did natamycin or miconazole. Natamycin had severe cytotoxic effects in vitro.

Immunohistochemical study of matrix metalloproteinases-2 and -9, macrophage inflammatory protein-2 and tissue inhibitors of matrix metalloproteinases-1 and -2 in normal, purulonecrotic and fungal infected equine corneas.

OBJECTIVE: Determine the effects of matrix metalloproteinases (MMPs)-2, -9, macrophage inflammatory protein-2 (MIP-2), tissue inhibitors of matrix metalloproteinase (TIMP)-1 and -2 by immunohistochemical expression in fungal affected and purulonecrotic corneas. PROCEDURE: Paraffin-embedded equine corneal samples; normal (n = 9), fungal affected (FA; n = 26), and purulonecrotic without fungi (PN; n = 41) were evaluated immunohistochemically for MMP-2, -9, MIP-2, TIMP-1 and -2. The number of immunoreactive inflammatory cells was counted and statistics analyzed. Western blot was performed to detect MMP-2, MMP-9, TIMP-1 and TIMP-2 proteins. RESULTS: Matrix metalloproteinases-2, -9, MIP-2, TIMP-1 and -2 immunoreactivity was identified in corneal epithelium of normal corneas, and in corneal epithelium, inflammatory cells, keratocytes, and vascular endothelial cells of both FA and PN samples. Inflammatory cell immunoreactivity was significantly higher in FA and PN samples than in the normal corneas. There was positive correlation between MMP-2 and MIP-2, MMP-9 and MIP-2, and MMP-9 and TIMP-1 in inflammatory cell immunoreactivity in FA samples. There was positive correlation between MMP-9 and MIP-2, MMP-9 and TIMP-2, MIP-2 and TIMP-1, and MIP-2 and TIMP-2 in inflammatory cell immunoreactivity in PN samples. Western blot confirmed the presence of all four proteins in equine corneal samples. CONCLUSION: Increased immunoreactivity of MMP-2 and -9 in FA and PN samples is indirectly related to MIP-2 through its role in neutrophil chemo-attraction. Tissue inhibitors of matrix metalloproteinase-1 and TIMP-2 are up-regulated in equine purulonecrotic and fungal keratitis secondary to MMP-2 and MMP-9 expression. The correlation between MMPs -2 and -9, MIP-2, TIMPs -1 and -2 suggests that these proteins play a specific role in the pathogenesis of equine fungal keratitis.

Establishing a reproducible method for the culture of primary equine corneal cells.

OBJECTIVE: To establish a reproducible method for the culture of primary equine corneal epithelial cells, keratocytes, and endothelial cells and to describe each cell's morphologic characteristics, immunocytochemical staining properties and conditions required for cryopreservation. PROCEDURES: Corneas from eight horses recently euthanized for reasons unrelated to this study were collected aseptically and enzymatically separated into three individual layers for cell isolation. The cells were plated, grown in culture, and continued for several passages. Each cell type was characterized by morphology and immunocytochemical staining. RESULTS: All three equine corneal cell types were successfully grown in culture. Cultured corneal endothelial cells were large, hexagonal cells with a moderate growth rate. Keratocytes were small, spindloid cells that grew rapidly. Epithelial cells had heterogeneous morphology and grew slowly. The endothelial cells and keratocytes stained positive for vimentin and were morphologically distinguishable from one another. The epithelial cells stained positive for cytokeratin. Keratocytes and endothelial cells were able to be cryopreserved and recovered. The cryopreserved cells maintained their morphological and immunocytochemical features after cryopreservation and recovery. DISCUSSION: This work establishes reproducible methods for isolation and culture of equine corneal keratocytes and endothelial cells. Cell morphology and cytoskeletal element expression for equine corneal epithelial cells, keratocytes, and endothelial cells are also described. This has not previously been reported for equine corneal cells. This report also demonstrates the ability to preserve equine keratocytes and endothelial cells for extended periods of time and utilize them long after the primary-cell collection, a feature that has not been reported for veterinary corneal cell culture.

Repulsive axon guidance molecule Slit3 is a novel angiogenic factor.

Slits are large, secreted repulsive axon guidance molecules. Recent genetic studies revealed that the Slit3 is dispensable for neural development but required for non-neuron-related developmental processes, such as the genesis of the diaphragm and kidney. Here we report that Slit3 potently promotes angiogenesis, a process essential for proper organogenesis during embryonic development. We observed that Slit3 is expressed and secreted by both endothelial cells and vascular smooth muscle cells in vasculature and that the Slit cognate receptors Robo1 and Robo4 are universally expressed by endothelial cells, suggesting that Slit3 may act in paracrine and autocrine manners to regulate endothelial cells. Cellular function studies revealed that Slit3 stimulates endothelial-cell proliferation, promotes endothelial-cell motility and chemotaxis via interaction with Robo4, and accelerates endothelial-cell vascular network formation in vitro with a specific activity comparable with vascular endothelial growth factor. Furthermore, Slit3 stimulates neovessel sprouting ex vivo and new blood vessel growth in vivo. Consistent with these observations, the Slit3 knockout mice display disrupted angiogenesis during embryogenesis. Taken together, our studies reveal that the repulsive axon guidance molecule Slit3 is a novel and potent angiogenic factor and functions to promote angiogenesis in coordinating organogenesis during embryonic development.

Effects of topical 2% dorzolamide hydrochloride alone and in combination with 0.5% timolol maleate on intraocular pressure in normal feline eyes.

OBJECTIVE: To evaluate the effect of topical 2% dorzolamide alone, and in combination with topical 0.5% timolol, on intraocular pressure (IOP) in normal cats. ANIMALS: Twenty-four healthy Domestic Short-haired cats. PROCEDURE: Baseline values of IOP were established at 7 am, 10 am, 1 pm, 5 pm and 9 pm during pretreatment phase (days 1-2). During treatment phase (days 3-10) cats received 2% dorzolamide HCl q 12 h in group A (n = 6), q 8 h in group B (n = 6), and combined with 0.5% timolol maleate q 12 h in group C (n = 6). Cats in control group D (n = 6) received artificial tears q 8 h. During treatment phase IOP measurements were continued at the same time-points as in the pretreatment phase. RESULTS: Mean pretreatment IOP in all cats was 18.46 +/- 2.99 mmHg. Mean IOP decreased significantly (P < 0.0086) in all treatment groups compared to pretreatment values (group A: 16.40 +/- 0.49 mmHg, group B: 16.04 +/- 0.49 mmHg, group C: 17.76 +/- 0.49 mmHg). IOP did not decrease in control group D (18.55 +/- 0.49 mmHg). The difference in IOP between treatment groups (A, B, C) was not statistically significant, but comparison of IOP between each treatment group and the control group was statistically significant (A-D; P = 0.0057; B-D, P = 0.0012; C-D, P = 0.0212). CONCLUSION: Topical 2% dorzolamide significantly lowers IOP in normal cats but the effect is mild. Concomitant application of 2% dorzolamide and 0.5% timolol does significantly decrease IOP, but the effect is not significantly greater than q 8 h administration of dorzolamide alone.

Identification of ocular matrix metalloproteinases present within the aqueous humor and iridocorneal drainage angle tissue of normal and glaucomatous canine eyes.

PURPOSE: The purpose of this study was to identify the most active matrix metalloproteinases (MMP) within the aqueous humor and iridocorneal angle tissue involved in the normal canine eye, and to compare these results to the MMP activity in dogs with glaucoma. ANIMAL STUDIED: Aqueous humor samples from 32 normal eyes and 26 glaucomatous eyes were obtained through aqueous centesis and analyzed for MMP activity. Iridocorneal angle tissue from 16 enucleated normal canine eyes and 5 enucleated glaucomatous eyes were dissected and homogenized into solution. PROCEDURE: Bradford total protein assays were determined for aqueous humor samples and iridocorneal angle tissue samples. Substrate gelatin zymography was performed using 0.2 microL volumes of an MMP-2/MMP-9 control, 2 microL volumes of aqueous humor samples, and 10 microg weights of total protein from iridocorneal angle tissue. The presence of MMP gel bands were identified visually and measured quantitatively by densitometry technique. A statistical analysis was performed on the data using Student's t-test, multiple logistic Wald's chi-squared regression, Pearson correlations, and repeated measures analysis. RESULTS: Within the aqueous humor of canine eyes, MMP-2 latent form was found to be the most relevant MMP. The quantity of latent MMP-2 within the aqueous humor of the glaucoma samples was significantly increased compared to the normal aqueous samples (P < 0.0001). Glaucoma occurrence was associated with elevated aqueous humor latent MMP-2 (P = 0.0002). Within the canine iridocorneal angle tissue, MMP-9 latent form and MMP-2 active form were found to be the most relevant MMPs. MMP-2 active form was found to be significantly increased in the glaucoma tissue samples when compared to the normal tissue samples (P = 0.0044). MMP-9 latent form was also found to be significantly increased in glaucomatous tissue when compared to the normal eyes (P = 0.0002). Tissue MMP-9 latent form was found to be associated with glaucoma status (P = 0.042). CONCLUSION: Glaucoma aqueous humor samples expressed a statistically increased latent MMP-2 when compared to normal eyes. Iridocorneal angle tissue from glaucomatous eyes expressed a statistically significant increase in active MMP-2 and latent MMP-9 when compared to normal eyes. These data demonstrate that there is an association between elevated levels of intraocular MMP-2 and MMP-9 and the presence of glaucoma.

Endosurgical treatment of a retrobulbar abscess in a rabbit.

CASE DESCRIPTION: A 1-year-old sexually intact female Netherland dwarf rabbit was examined because of a 3-week history of signs of lethargy, decreased appetite, left unilateral exophthalmia, a previous draining sinus from a left maxillary facial abscess, and bilateral nasal discharge. CLINICAL FINDINGS: The rabbit weighed 1.0 kg (2.2 lb) and had a body condition score of 1.5/5. Physical examination revealed generalized muscle atrophy, bilateral mucopurulent nasal discharge, and severe left-sided exophthalmia. Diagnostic investigation revealed anemia, neutrophilia, severe dental disease, a superficial corneal ulcer of the left eye, and a retrobulbar abscess. TREATMENT AND OUTCOME: Stomatoscopy-aided dental trimming, tooth removal, and abscess debridement were performed. Antimicrobials were flushed into the tooth abscess cavity, and antimicrobial treatment was initiated on the basis of cytologic findings and results of bacterial culture and susceptibility testing. Two months after the initial surgery, minimal exophthalmia was evident and no further physical, radiographic, or ultrasonographic changes were evident. CLINICAL RELEVANCE: Stomatoscopy is a valuable technique that can facilitate diagnosis, treatment, and serial reevaluation of rabbits with dental disease.

Effects of human recombinant alpha-2b interferon and feline recombinant omega interferon on in vitro replication of feline herpesvirus-1.

OBJECTIVE: To evaluate the effects of recombinant human interferon alpha-2b (rHuIFN-alpha2b) and recombinant feline interferon omega (rFeIFN-omega) on in vitro replication of feline herpesvirus (FHV)-1. SAMPLE POPULATION: Cultures of Crandell-Rees feline kidney (CRFK) cells. PROCEDURES: CRFK cells were treated with rFeIFN-omega or rHuIFN-alpha2b at concentrations ranging from 100 to 500,000 U/mL. Cultures were then inoculated with FHV-1. Constant concentrations of interferon products were maintained throughout the study. Reductions in the number and size of plaques were used as indicators of antiviral activity. Six plaque reduction assays were performed in duplicate. A 3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide assay was used to detect cytotoxic effects of interferon. A 1-way ANOVA and Dunnett test were used to determine significant differences. RESULTS: Treatment with rFeIFN-omega at various concentrations resulted in significant reductions in the number of plaques (100,000 U/mL, 54.7%; and 500,000 U/mL, 59.8%) and in plaque size (100,000 U/mL, 47.5%; 250,000 U/mL, 81.0%; and 500,000 U/mL; 70.5%). Treatment with various concentrations of rHuIFN-alpha2b resulted in a significant reduction in plaque size (100,000 U/mL, 56.0%; 250,000 U/mL, 75.7%; and 500,000 U/mL, 69.0%). None of the tested concentrations of interferon caused significant cellular toxicosis. CONCLUSIONS AND CLINICAL RELEVANCE: At some of the higher concentrations, the antiviral effect of rFeIFN-omega was greater than the antiviral effect of rHuIFN-alpha2b. Reduction in plaque size appeared to be a good indicator of the antiviral activity of interferon against FHV-1.

In vitro efficacy of a buffered chelating solution as an antimicrobial potentiator for antifungal drugs against fungal pathogens obtained from horses with mycotic keratitis.

OBJECTIVE: To determine whether a novel third-generation chelating agent (8 mM disodium EDTA dehydrate and 20 mM 2-amino-2-hydroxymethyl-1, 3-propanediol) would act as an antimicrobial potentiator to enhance in vitro activity of antifungal medications against fungal isolates obtained from horses with mycotic keratitis. SAMPLE POPULATION: Fungal isolates (3 Aspergillus isolates, 5 Fusarium isolates, 1 Penicillium isolate, 1 Cladosporium isolate, and 1 Curvularia isolate) obtained from horses with mycotic keratitis and 2 quality-control strains obtained from the American Type Culture Collection (ATCC; Candida albicans ATCC 90028 and Paecilomyces variotii ATCC 36257). PROCEDURE: Minimum inhibitory concentrations (MICs) against fungal isolates for 4 antifungal drugs (miconazole, ketoconazole, itraconazole, and natamycin) were compared with MICs against fungal isolates for the combinations of each of the 4 antifungal drugs and the chelating agent. The Clinical and Laboratory Standards Institute microdilution assay method was performed by use of reference-grade antifungal powders against the fungal isolates and quality-control strains of fungi. RESULTS: Values for the MIC at which the antifungal drugs decreased the growth of an organism by 50% (MIC50) and 90% (MIC90) were decreased for the control strains and ophthalmic fungal isolates by 50% to 100% when the drugs were used in combination with the chelating agent at a concentration of up to 540 microg/mL. CONCLUSIONS AND CLINICAL RELEVANCE: The chelating agent increased in vitro activity of antifungal drugs against common fungal pathogens isolated from eyes of horses with mycotic keratitis.

Evaluation of a technique of inducing hypertensive renal insufficiency in cats.

OBJECTIVE: To compare 2 techniques of inducing combined renal insufficiency and systemic hypertension in cats. ANIMALS: 22 cats 6 to 12 months of age. PROCEDURES: Cats were randomly assigned to 1 of 3 groups. Control (C) group cats had 2 intact kidneys, remnant kidney (RK) group cats underwent unilateral partial renal infarction and contralateral nephrectomy, and remnant-wrap (W) group cats underwent unilateral partial renal infarction and partial abtation and wrapping of the contralateral kidney. Systemic arterial blood pressure (BP) was measured continuously by use of implanted radiotelemetric devices. Renal function was assessed via determination of glomerular filtration rate, measurement of serum creatinine and BUN concentrations, and determination of urine protein-to-creatinine ratio (UP/C). Serum aldosterone concentration and plasma renin activity were measured on day 75. RESULTS: Systolic BP was significantly higher in groups RK and W than in group C, and systolic BP was significantly higher in group W than in group RK. Serum aldosterone concentration and plasma renin activity were significantly higher in group W, compared with groups C and RK. Glomerular filtration rate was significantly lower in groups RK and W, compared with group C. Histologic indices of renal injury and UP/C were significantly higher in group W, compared with groups C and RK. CONCLUSIONS AND CLINICAL RELEVANCE: Hypertensive renal insufficiency in group W was characterized by marked sustained systemic hypertension, decreased renal function, proteinuria, activation of the renin-angiotensin-aldosterone axis, and renal structural injury. Results support the hypothesis that marked systemic hypertension, activation of the renin-angiotensin-aldosterone axis, and proteinuria may damage the kidney of cats with preexisting renal insufficiency.

Effects of treatment with and without adjuvant radiation therapy on recurrence of ocular and adnexal squamous cell carcinoma in horses: 157 cases (1985-2002).

OBJECTIVE: To determine the effects of treatment with and without adjuvant radiation therapy on recurrence of ocular and adnexal squamous cell carcinoma (SCC) at specific anatomic locations in horses. DESIGN: Retrospective study. ANIMALS: 91 horses. PROCEDURES: Medical records of horses with histologically confirmed ocular and adnexal SCC evaluated from 1985 to 2002 were reviewed. Sex, breed, age, type of treatment, location, and recurrence of SCC were recorded. Two treatment groups determined by recurrence of SCCs treated with and without adjuvant radiation therapy were established. RESULTS: The anatomic site with the highest recurrence rate was the limbus (junction of the cornea and sclera) or bulbar conjunctiva (477%), independent of treatment group. There was a significant difference in recurrence rates of ocular and adnexal SCCs between the 2 treatment groups, independent of anatomic location. Recurrence rates of SCCs treated with and without adjuvant radiation therapy were 11.9% and 44.1%, respectively. Recurrence rates for SCCs of the eyelid, limbus or bulbar conjunctiva, and cornea treated with adjuvant radiation therapy were significantly different from those for SCCs treated without adjuvant radiation therapy. The most frequently represented anatomic site for ocular and adnexal SCCs was the eyelid (28.7%). Coat color, breed, and the interaction of age and breed had a significant effect on tumor recurrence regardless of treatment type and anatomic location. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that ocular and adnexal SCCs treated with adjuvant radiation therapy had a significantly lower recurrence rate, compared with SCCs treated without adjuvant radiation therapy, independent of anatomic location.

Effects of transcorneal iridal photocoagulation on the canine corneal endothelium using a diode laser.

OBJECTIVE: To investigate the potential damage to the canine corneal endothelium following transcorneal iridal laser photocoagulation using a semiconductor diode laser. ANIMALS STUDIED: Sixteen young mongrel dogs. PROCEDURES: Baseline corneal endothelial cell counts and corneal thickness were measured in the central and temporal quadrants using a noncontact specular microscope under general anesthesia. Transcorneal iridal photocoagulation was applied using a semiconductor diode laser in a continuous mode with the use of an operating microscope. Fifteen dogs were treated, and the sixteenth dog served as a control. Fifteen different treatment combinations were randomly assigned to the 30 eyes; the fellow eye was treated differently. Three treatment factors were investigated: (1) laser energy intensity, (2) target tissue to endothelial distance, and (3) laser application duration. After 3 weeks the dogs were euthanized, specular microscopy was repeated, and the cornea was examined by scanning electron microscopy. RESULTS: Dyscoria and focal iris darkening were noted in all eyes immediately following laser treatment. Focal corneal edema (n = 2) and an incipient anterior capsular cataract (n = 1) were also noted. Baseline mean corneal endothelial cell densities were 2530 cells/mm2 centrally and 2607 cells/mm2 temporally. Postlaser corneal endothelial cell densities were 2499 cells/mm2 centrally and 2523 cells/mm2 temporally. Mean prelaser corneal thickness measurements were 0.555 mm centrally and 0.549 mm temporally. Postlaser corneal thickness measurements were 0.580 mm centrally and 0.554 mm temporally. Statistical analyzes revealed no significant changes in endothelial cell densities (P > 0.05) or corneal thickness (P > 0.05) induced by any treatment combination. Aside from tissue handling and processing artifacts, scanning electron microscopy revealed no endothelial cell damage. CONCLUSIONS: Our study demonstrated by specular and scanning electron microscopy that diode laser iridal photocoagulation had no significant effect on the canine corneal endothelium within the parameters described. However, one must take into consideration the young age of the dogs and the potential for corneal endothelial cell regeneration in young dogs, and the relatively short period of postoperative study.