Glaucoma: The 5 o'clock emergency (Proceedings)
Glaucoma is a syndrome in which progressive elevation in intraocular pressure (IOP) of the eye results in damage to the optic disk and retina, causing blindness. This starts occurring at IOPs above 25mmHg in dogs and above 30mmHg in cats.
The elevated IOP is due to a disharmony between production and outflow of the aqueous humor. More specifically, a reduction in the outflow rate of the aqueous humor is responsible for the elevated IOP, not the overproduction of the aqueous humor.
Aqueous humor dynamics and physiologyIn order to better understand these processes, it is first important to understand the normal fluid dynamics of the eye. The aqueous humor is formed at a constant rate of approximately 2% of the anterior chamber volume per minute. In the dog, that is about 2.5µl/min and in the cat 15µl/min.
Aqueous humor is produced in the non-pigmented epithelium of the ciliary processes located posterior to the iris and is formed via three basic mechanisms, namely: passive diffusion, ultra filtration and active transport/secretion. Active transport/secretion is the greatest contributor to aqueous humor formation, with the carbonic anhydrase enzyme catalyzing the reaction: CO2 + H2O HCO3- + H+. Thus, every time a bicarbonate ion enters the aqueous, it is associated with movement of water into the posterior chamber.
There are two mechanisms for aqueous outflow: the conventional/corneoslceral outflow and the unconventional/uveoscleral outflow.
In conventional outflow, aqueous humor passes from the posterior chamber through the pupil then into the anterior chamber. The aqueous humor exits the eye near the periphery of the iris at the iridocorneal angle, where it passes through the Spaces of Fontana between the pectinate ligaments to enter the trabecular network of the ciliary cleft. The aqueous humor is then filtered between the corneoslceral trabecular meshwork to enter a series of radially oriented, blood free collecting vessels called the aqueous capillaries or, collectively, the angular aqueous plexus.
With the uveoscleral outflow, the aqueous humor passes through the iris and interstitial spaces of the ciliary muscle to reach the supraciliary space. This outflow is independent of the IOP.