Continuous Measurement of Intraocular Pressure in Rabbits

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 and allows it to move without being distorted. Many internal structures remain in place because of intraocular pressure. In a glaucoma patient, the intraocular pressure is usually higher than normal. It can increase to 20 to 30 mmHg or higher, and it does not on its own return to normal. 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. Glaucoma is characterized by three symptoms, an increased intraocular pressure, changes in the optic nerve in the region where it leaves the eye, and the progressive loss of vision. Ophthalmologists treat glaucoma by lowering intraocular pressure, often through medicine taken as eye drops, and sometimes by surgery. When intraocular pressure returns to normal, the loss of sight usually slows or stops, but vision that has already been lost cannot be restored. Intraocular pressure is measured with a device called a tonometer. Some tonometers blow a puff of air at the cornea to measure pressure, but most are lightly pressed on the cornea after it has been numbed with anesthetic drops. Because of the complexity of tonometers, intraocular pressure is usually measured in the ophthalmologist’s office. Often, it would be helpful to know how intraocular pressure changes between office visits. Although intraocular pressure in some patients is not particularly high when measured in the office, the appearance of their optic nerve indicates that it may be high at other times. We do not know if intraocular pressure increases (and if so~ by how much it increases) during exercise, excitement, rest, sleep, or other daily activities in these patients. A tonometer that measures intraocular pressure at regular intervals throughout the day and night would indeed be a useful device for understanding the relationship between intraocular pressure and damage to the optic nerve. An ideal device would measure intraocular pressure without the patient being aware of its presence or that the measurement had been made. It would measure pressure at fixed intervals while the patient is awake and asleep, and during all activities, in a way similar to portable heart monitors. A device of this kind is not available for use in human subjects. In this study we will test a small pressure-measuring device that has been used to monitor blood pressure, to see if it is suitable to measure intraocular pressure. Because this device. cannot be used in humans, it will be tested in rabbits. The device consists of a small capsule that contains an element to measure pressure and a miniature radio transmitter. It will send pressure information to a nearby receiver and computer at regular intervals, and will measure intraocular pressure continuously, day and night, for up to four months. The animal will be free to move around and will not be aware that it is being studied. Since all measurements are controlled by a computer, our personnel need not even be in the room. This experiment will tell us how reliably the device measures intraocular pressure and how intraocular pressure changes over a 24-hour period when these animals are not disturbed. Animals often become suspicious and tense when investigators measure pressure by touching the eye with a tonometer. The implanted device will tell us if pressure changes just before or after these measurements. If this device works with sufficient precision, the technique will provide a new tool for studying normal changes in pressure in the eye. It will allow us to measure pressure without influencing the measurements. It will also provide a better monitor of pressure when testing new drugs and other treatments that may someday be used to lower intraocular pressure in glaucoma patients.