An eye pressure monitor, known as a tonometer, is a device that measures intraocular pressure (IOP) to keep track of and maintain control over glaucoma. Glaucoma refers to a number of eye diseases that result in part from increased IOP. High IOP is caused by excess eye fluid, or aqueous humor, inside the eyeball. Routine eye exams and tonometer tests are the only ways to make sure eye pressure remains at safe levels.
Usually, an eye pressure monitor measures the pressure in millimeters of mercury (mm Hg), the same as a blood pressure monitor. Normal eye pressure should fall between 10 mm Hg and 21 mm Hg, and high eye pressure, known as ocular hypertension, is diagnosed when eye pressure exceeds 21 mm Hg. The excess fluid can be a result of either increased fluid production or decreased fluid drainage in the eye. This increased eye pressure can damage the optic nerve, which transmits visual stimuli from the eye to the brain. Glaucoma can lead to decreased vision and, eventually, blindness if left untreated.
The most common type of eye pressure monitor is called an applanation tonometer. These tonometers applanate, or flatten, a portion of the cornea and measure the resistance. There are several types of applanation tonometers.
Contact tonometers are applanation tonometers that require contact with the cornea. These can be uncomfortable and scary for children and require a topical anesthetic to be applied to the eyeball before the procedure, usually in the form of eyedrops. Corneal-contact tonometers are, therefore, generally used only in a clinical setting by medical professionals.
Patients can also monitor their condition at home by purchasing their own eye pressure monitor. A home eye pressure monitor can be used by pressing the device gently against a closed or partially closed eyelid to applanate the eye until a pressure phosphene, or a swirl of color or light, appears. It's at this point that the device takes a reading of the eye pressure.
A third type of tonometer uses a puff of air instead of the device itself to applanate the eye. Because this method does not require contact with either an open or a closed eye, it does not carry the same risk of transmission of eye diseases as the other types, and it is somewhat more comfortable. For these reasons, this type of tonometer is commonly used in optometrists' and ophthalmologists' offices as a quick, easy method of screening patients for glaucoma.