Aqueous humor is formed in the posterior chamber by the ciliary body in an active secretory process involving carbonic anhydrase as well as by passive filtration from the vessels on the anterior surface of the iris.
Drainage of aqueous humor is via a network of connecting venous channels (includes Schlemm's canal) that empty into the superior vena cava. (Any obstruction between the eye and right atrium impedes aqueous drainage and increases intraocular pressure [IOP].)
Maintenance of Intraocular Pressure
IOP normally varies between 10 and 21.7 mm Hg but becomes atmospheric when the globe is opened. The major determinant of IOP is the volume of aqueous humor.
Any sudden increase in IOP when the globe is open may lead to prolapse of the iris and lens, extrusion of the vitreous, and blindness.
Straining, vomiting, or coughing (as during laryngoscopy and tracheal intubation) greatly increases venous pressure and IOP.
Glaucoma is characterized by increased IOP, resulting in impairment of capillary blood flow to the optic nerve.
Treatment consists of topical medication to produce miosis and trabecular stretching.
Atropine premedication in the dose range used clinically has no effect on IOP in patients with glaucoma. (Scopolamine may have a greater mydriatic effect, and its use may be avoided.)