Modeling glaucoma in rats by sclerosing aqueous outflow pathways to elevate intraocular pressure

- Retrograde injection of hypertonic saline increases outflow resistance, with gradual increase in intraocular pressure (IOP).
- Detecting IOP elevation requires understanding the effects of general anesthesia and circadian rhythms on IOP.
- Correlating IOP to nerve damage should include mean, peak, duration and fluctuation in IOP, as well as integral IOP.
- The hypertonic saline model can be used to test neuroprotective therapies and study mechanisms of axonal injury in glaucoma.
- Despite exacting surgical skill and attention to detail, hypertonic saline injection can be successfully learned.

Injection of hypertonic saline via episcleral veins toward the limbus in laboratory rats can produce elevated intraocular pressure (IOP) by sclerosis of aqueous humor outflow pathways. This article describes important anatomic characteristics of the rat optic nerve head (ONH) that make it an attractive animal model for human glaucoma, along with the anatomy of rat aqueous humor outflow on which this technique is based. The injection technique itself is also described, with the aid of a supplemental movie, including necessary equipment and specific tips to acquire this skill. Outcomes of a successful injection are presented, including IOP elevation and patterns of optic nerve injury. These concepts are then specifically considered in light of the use of this model to assess potential neuroprotective therapies. Advantages of the hypertonic saline model include a delayed and relatively gradual IOP elevation, likely reproduction of scleral and ONH stresses and strains that may be important in producing axonal injury, and its ability to be applied to any rat (and potentially mouse) strain, leaving the unmanipulated fellow eye as an internal control. Challenges include the demanding surgical skill required by the technique itself, a wide range of IOP response, and mild corneal clouding in some animals. However, meticulous application of the principles detailed in this article and practice will allow most researchers to attain this useful skill for studying cellular events of glaucomatous optic nerve damage.