The microbead occlusion model: a paradigm for induced ocular hypertension in rats and mice

Glaucoma is a common optic neuropathy characterized by the degeneration of retinal ganglion cells and their axons, which comprise the optic nerve. Although age is the most significant risk factor for the disease,1 elevated intraocular pressure (IOP) remains the only modifiable risk factor.2 In humans, whether open or closed angle, glaucoma is characterized by excavation of the optic disc and defects in the nerve fiber layer that are often accompanied by elevations in IOP.3 Lowering IOP slows glaucomatous progression, even in patients with IOP in the nominally normal range (15–21 mm Hg).2 Given the impact of IOP on disease outcome, elevated IOP or ocular hypertension is the predominant feature of most animal models of glaucoma. Rodent species offer convenience as model systems for inducing ocular hypertension because they provide the opportunity to conduct large-scale experimental studies and the potential for genetic manipulations. The three most commonly used models elevate IOP substantially (50%–100% above baseline) by altering aqueous fluid dynamics in the anterior segment.4,5 These have been applied primarily to rats, most likely because of the technical difficulty of manipulating the significantly smaller mouse eye.4–6 Their merits and weaknesses have been reviewed extensively elsewhere.5 Briefly, episcleral vein injection of hypertonic saline elevates IOP by altering aqueous outflow through sclerosis.7,8 This elevation persists as long as 200 days but varies dramatically in magnitude between individual animals.5,7,9 Laser photocoagulation of either the trabecular meshwork alone or in concert with episcleral veins elevates IOP for far shorter durations: <21 days in the rat10,11 and 6 weeks in the mouse.12,13 For these models, both interanimal variability and variability in the magnitude of elevations are limiting factors, necessitating relatively large numbers of experimental cohorts.5 In contrast, cauterization of two or three episcleral veins alone elevates IOP by reducing venous outflow, thereby increasing aqueous fluid pressure.8,14–17 Although relatively consistent between animals, the potential for ocular ischemia and neovascularization can confound interpretation of this model.5,9 These most widely used models are limited by the absence of a repeatable internal control in which an equivalent insult in the fellow eye would not result in extended elevated IOP. Such a control would allow the assessment of IOP-induced pathology without confounding factors, such as injury induced by the surgical procedure (e.g., vessel cauterization and sclerosis). Although the potential for uncontrolled confounds is well recognized, there have been few alternatives. Additional rodent systems for ocular hypertension are also available but have been less extensively used. These include ligation of extraocular veins18 and intra-aqueous injection of hyaluronic acid.19 We sought to establish a rodent model of ocular hypertension in which both IOP magnitude and duration could be manipulated relatively simply with minimal confounding factors and with an internal control for each animal. Here we describe a procedure for injecting small volumes of polystyrene microbeads into the anterior chamber to impede aqueous outflow and elevate IOP. This model builds on a limited number of earlier studies using microbeads to elevate IOP in primates,20 rats,21 and rabbits.22 For each animal, one eye is injected with microbeads and other is injected with an equivalent volume of saline. For maximal usefulness as a rodent model, we demonstrate our procedure for use in both rats (2.5- to7-μL injections) and mice (1-μL injections). We show that IOP after microbead injection is highly consistent both in duration and magnitude with interanimal variability approximately 5% of the mean, whereas saline injection affected IOP very little regardless of volume and duration. Overall, the flexibility and consistency of this model, with a simple internal control, affords the opportunity for specific assessment of IOP-induced pathology.