Development of a Canine Glaucoma Model with Controlled Ocular Hypertension

ABSTRACT

Objective: Traditional methods for replicating glaucoma in animal models often involve obstructing the aqueous humor outflow by injecting micro substances or applying a laser to the trabecular meshwork (TM), leading to inconsistent and unpredictable intraocular pressure (IOP) elevations. Our study aimed to establish a large animal model that induces ocular hypertension with precise control over magnitude and duration, mimicking glaucoma with a noninvasive method without intraocular intervention.

Methods: Five purpose-bred beagle dogs were anesthetized, and a vascular loop was placed around the eye, slightly posterior to the limbus, to modulate IOP externally. Dogs were divided into three groups based on the target IOP levels: 31-40 mmHg (n= 1), 36-45 mmHg (n= 3), and 41-50 mmHg (n= 1), with each condition maintained for three hours. Sessions were repeated until noticeable changes were observed. Ocular hypertension was induced to one eye of each dog, while the fellow eye served as non-treated control. The progression of glaucoma was monitored in vivo using ophthalmic examinations, vision tests with a maze, fundus photography, electroretinography (ERG), optical coherence tomography (OCT), and OCT- Angiography (OCTA) for a comprehensive evaluation. Post-experiment, histopathological examination of the eyes was conducted.

Results: The experimental study maintained targeted IOP values with minimal vascular loop adjustments while demonstrating an inverse relationship between IOP and mean ocular perfusion pressure (MOPP). Fundus photography revealed no gross abnormalities in lower and moderate-pressure dogs (31-40 mmHg, 36-45 mmHg) except for transient hypoperfusion in one dog, while the high-pressure dog (41-50 mmHg) exhibited vascular attenuation and optic nerve head (ONH) pallor. OCT analysis demonstrated significant ganglion cell complex (GCC), outer and total retinal thinning (p≤0.01) in the 36-45 mmHg group, with severe full thickness retinal degeneration in the 41-50 mmHg dog and minimal differences in the 31-40 mmHg dog. OCTA assessment of 36- 45 mmHg dogs did not show a significant reduction in vessel densities between glaucoma induced and control eyes (p > 0.05). ERG studies in 36- 45 mmHg dogs demonstrated significantly reduced pattern ERG (PERG) amplitudes (N35-P50: p = 0.04; N50-N95: p = 0.03), diminished photopic negative (PhNR) responses across multiple stimuli (p≤0.05) and reduced International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG components in glaucoma induced eyes. Functional vision assessment in 36- 45 mmHg dogs through maze testing revealed impaired performance, with delayed exit times in glaucoma-induced eyes but did not reach statistical significance (49.23 ± 0.79 vs. 9.81 ± 2.66 seconds in glaucoma-induced and control, respectively;p = 0.06. Gonioscopy and ultrasound biomicroscopy (UBM) revealed no structural alterations in any IOP groups. Histopathological analysis revealed with characteristic inner retinal thinning in the 36-45 mmHg group, mild gliosis without significant retinal changes in the 31-40 mmHg dog, and severe diffuse inner retinal atrophy in the 41-50 mmHg dog. ON axons and RGC quantifications in a dog within 36- 45 mmHg dogs confirmed a substantial reduction in ON axons (41,432 vs. 109,345) and RGCs (38,546 vs. 144,163).

Conclusions: This study demonstrates that controlled, intermittent elevation of IOP successfully induces glaucoma in dogs. The experimental approach utilizing specific IOP ranges (31-40 mmHg, 36-45 mmHg and 41-50 mmHg) resulted in distinct and IOP dependent outcomes, with the 36-45 mmHg range most effectively replicating characteristic inner retinal damage observed in glaucoma. The findings establish a valuable experimental platform for future investigations into glaucoma pathogenesis, neuroprotective strategies, and potential therapeutic interventions. This model will be particularly beneficial for studies requiring precise control over the magnitude, onset, and duration of IOP elevation.