A tonometer is a specialized medical device used by eye care professionals to measure the internal fluid pressure within the eye, known as intraocular pressure (IOP). This measurement helps assess the firmness of the eyeball, much like checking the air pressure in a tire. The primary function of tonometry is to detect conditions that affect eye pressure.
The test is a standard part of a comprehensive eye examination. It is generally quick and causes no pain, providing a numerical value that helps eye doctors understand your eye’s internal pressure.
The Purpose of Tonometry
Maintaining balanced intraocular pressure (IOP) is important because the eye is filled with a clear fluid called aqueous humor. This fluid helps maintain the eye’s shape and nourishes internal structures, constantly flowing in and out through a delicate drainage system.
If the drainage system does not function properly, aqueous humor can build up, increasing IOP. Sustained high intraocular pressure is a significant risk factor for glaucoma, a group of eye conditions that damage the optic nerve. Damage to the optic nerve, which transmits visual information to the brain, can lead to permanent vision loss or blindness.
Regular tonometry helps detect elevated IOP, often before symptoms appear. This makes it important for early detection and ongoing monitoring, allowing for timely intervention to help preserve vision. The test also assesses the effectiveness of glaucoma treatments.
Types of Tonometers
Eye care professionals use various types of tonometers to measure intraocular pressure, each with a distinct approach. The choice often depends on the clinical situation or patient comfort.
Non-Contact Tonometry (NCT)
Non-contact tonometry, often called the “air puff” test, measures eye pressure without directly touching the eye’s surface. During this test, you rest your chin and forehead on a support while looking into the machine. The device then emits a brief, gentle puff of air onto the cornea.
The tonometer measures the time it takes for the air to flatten the cornea slightly and bounce back, calculating the IOP based on this response. This method is quick, does not require numbing eye drops, and is frequently used for initial screenings due to its non-invasive nature. While convenient, it is considered less accurate than contact methods for definitive diagnosis.
Contact Tonometry
Contact tonometry involves a gentle touch to the eye’s surface and is considered more precise. Goldmann applanation tonometry (GAT) is the “gold standard” for accuracy in measuring IOP. For this procedure, numbing eye drops are applied, sometimes with a yellow fluorescein dye, to ensure comfort and aid visualization.
The patient rests their chin and forehead on a slit lamp machine, a specialized microscope used for eye examination. A small, flat-tipped probe from the tonometer then gently touches the anesthetized cornea. The examiner uses a blue light to view two green semicircles and adjusts a dial until their inner edges align, indicating the force required to flatten a specific area of the cornea. Other contact tonometers, like the portable Tono-Pen or rebound tonometers, also gently touch the cornea and are useful when a patient cannot sit at a slit lamp.
The Tonometer Test Procedure
The tonometer test is a straightforward and quick part of an eye examination, designed to be comfortable for the patient. Your eye care professional will guide you through each step.
For non-contact tonometry, you will be asked to lean forward and place your chin on a padded rest, with your forehead against a support. You will then focus on a target light inside the machine. A soft puff of air will be directed at your eye, and you may feel a brief, cool sensation or mild pressure. The test is performed several times for each eye to ensure an accurate reading.
For contact tonometry, such as Goldmann applanation, the procedure begins with numbing eye drops to prevent discomfort. A small amount of yellow fluorescein dye may also be applied to help the doctor visualize the cornea. You will place your chin and forehead on the supports of a slit lamp microscope. The doctor will then gently bring the tonometer probe forward until it lightly touches the surface of your eye. This contact is brief and should not cause pain due to the numbing drops.
Understanding Your Results
Intraocular pressure (IOP) readings are measured in millimeters of mercury (mm Hg). A normal range for IOP is typically between 10 and 21 mm Hg, with an average around 15-16 mm Hg. However, this range can vary between individuals; what is healthy for one person might differ for another.
A single high IOP reading does not automatically mean a glaucoma diagnosis. Eye pressure can fluctuate throughout the day due to various factors, including time of day, body position, and certain medications. Your eye doctor considers the tonometer reading as one piece of a larger puzzle.
The doctor combines the IOP measurement with other diagnostic information, such as corneal thickness, optic nerve appearance, and visual field test results. These additional assessments help determine if there is actual optic nerve damage or other glaucoma risk factors.