A New Method to Measure Aqueous Humor Flow

A normal eye is round and firm because the inside has a higher pressure than the outside. This pressure, about 15 to 20 mmHg, prevents the eye from collapsing, allows it to move without being distorted, and keeps many internal structures in place. This intraocular pressure is generated by movement of a clear fluid, aqueous humor, through the anterior segment, the front part of the eye. Aqueous humor is formed by the ciliary body, just behind the iris (the part of the eye that gives it its color). It flows through the pupil into the anterior chamber, the region between the iris and the cornea. The cornea is the clear, outermost layer at the front of the eye. From the anterior chamber, aqueous humor leaves the eye by squeezing through a small opening along the outer edge of the cornea and iris called the trabecular meshwork and by working its way back to blood vessels in the surrounding tissue. Because the trabecular meshwork is tight and resists flow of aqueous humor, a certain force is needed to move this fluid from the anterior chamber to the ocular vessels. The back-pressure that develops as a result, inflates the eye and can be measured as intraocular pressure. Besides maintaining intraocular pressure, aqueous humor flow is important for nourishing the cornea, a tissue that has no blood supply of its own, and removing debris that would otherwise cloud our vision. While we are awake, it flows at a rate of 2-3 cubic mm per minute. This may not seem like a high rate, but it is enough to completely fill the anterior chamber in a little over an hour. Flow rate is relatively constant, and few natural things change flow. However, when we sleep flow drops to about half of the rate it is while we are awake. In glaucoma patients, the intraocular pressure is usually higher than normal. It can increase to 20 to 30 mmHg or higher, and it does not return to normal on its own. Over time, this increased pressure slowly pinches the optic nerve, and nerve fibers that carry information from the retina to the brain gradually die. As a result, the eye slowly loses sight. Ophthalmologists treat glaucoma by lowering intraocular pressure, often through medicine taken as eye drops, and sometimes by surgery. Treatments that lower intraocular pressure work by either decreasing flow of aqueous humor or by opening outflow pathways. When flow is suppressed, it is reduced to about the rate that is normally present at night, a rate that is high enough to nourish the cornea but low enough to reduce pressure. If we learn how the ciliary body regulates flow rate of aqueous humor, particularly between sleep and wake, and how trabecular tissues change resistance to outflow, we would be able to design more efficient and effective treatments to lower intraocular pressure in glaucoma patients. In this study we plan to develop a new method to measure aqueous humor flow in experimental animals, a method that has several advantages over conventional methods. The flow rate of aqueous humor is typically studied by applying eye drops that contain a fluorescent dye (fluorescein), and measuring how fast the dye disappears from the anterior chamber and cornea. Most of the dye is carried out of the eye by aqueous humor flow, and when we measure its rate of disappearance, we can easily determine aqueous humor flow rate. Fluorescein is measured in the anterior chamber with an instrument similar to a camera called an ocular fluorophotometer. This instrument directs a beam of blue light into the eye and measures the fluorescent light from the fluorescein that is present. With the new method, the fluorescent dye will not be applied as eye drops, but will be slowly infused into the aqueous humor through a small tube that will be implanted in the anterior chamber. Dye will be supplied to the tube through a miniature pump implanted under the skin. Because this device cannot be used in humans, it will be tested in rabbits. It will be internal and will not interfere with the animal’s normal activity. The dye will be infused at a slow, but constant rate. After mixing with the aqueous humor, the concentration of dye will change in the direction opposite of aqueous humor flow; as flow increases dye will become more dilute and as flow decreases it will become more concentrated. We will determine aqueous humor flow rate by periodically measuring dye concentration in the anterior chamber. This method will allow uninterrupted measurements for several days if necessary. In this project we will study how well we can measure aqueous humor flow by using this new approach. If the method works with sufficient precision, the technique will provide a new tool for studying normal changes in aqueous humor production in the eye. It will also provide a better monitor of aqueous humor flow when testing new drugs and other treatments that may someday be used to lower flow and intraocular pressure in glaucoma patients.