Applanation tonometry is a common and widely accepted method used by eye care professionals to measure the pressure inside your eye, known as intraocular pressure (IOP). This diagnostic test involves gently flattening a small area of the cornea, the transparent front surface of the eye, to determine the force needed to achieve this flattening. It is a quick and generally painless procedure performed in an eye clinic setting.
Purpose of the Test
Applanation tonometry serves a primary role in screening for and monitoring conditions linked to elevated eye pressure, most notably glaucoma. Glaucoma is a group of eye diseases that can cause damage to the optic nerve, potentially leading to irreversible vision loss if left untreated. In many cases of glaucoma, the fluid that normally bathes the eye drains too slowly, causing pressure to build up inside the eye. Elevated IOP is a significant risk factor for developing glaucoma, a condition often called the “silent thief of sight” because it can progress without noticeable symptoms in its early stages. Regular screenings are particularly important for individuals over 40 years old or those with a family history of glaucoma, as early detection allows for timely treatment to preserve sight.
How Applanation Tonometry is Performed
Before the procedure, patients are advised to remove contact lenses. The eye care professional will instill numbing eye drops, such as proparacaine, into each eye to ensure comfort. These drops may cause a brief stinging sensation that usually subsides within about ten seconds. A fluorescein dye, often combined with anesthetic drops, is also applied to the eye; this dye glows under a special blue light, making the tear film visible during the measurement.
The patient rests their chin on a chin support and their forehead against a headband on a slit lamp microscope, a specialized instrument for examining the eye. The Goldmann tonometer, considered the gold standard for IOP measurement, is mounted on this slit lamp.
The tonometer has a small, flat-tipped probe, or biprism, which is gently advanced toward the cornea. The examiner looks through the slit lamp eyepieces, guiding the probe until it lightly touches the central part of the cornea. The probe flattens a small, fixed area of the cornea, and the force required to do so is measured. This procedure is quick, taking only a few seconds per eye. Patients may feel a slight pressure or sensation of the probe touching the eye, but no pain due to the anesthetic.
Understanding Your Results
The numerical readings from applanation tonometry are expressed in millimeters of mercury (mmHg), quantifying the pressure inside your eye. For most individuals, a normal intraocular pressure typically falls within the range of 10 to 21 mmHg. This range is widely recognized, with 21 mmHg often serving as the upper limit for normal.
A single reading higher than this range does not automatically mean a glaucoma diagnosis. It indicates that further evaluation may be needed to determine the cause of the elevated pressure and assess the optic nerve’s health. An eye care professional will interpret these results within the broader context of a comprehensive eye examination, considering factors such as your medical history, family history, and the appearance of your optic nerve. Regular monitoring of IOP is recommended, especially for those at increased risk of glaucoma, as pressure can fluctuate throughout the day.
Factors Affecting Accuracy
Several factors can influence the accuracy of applanation tonometry readings. One significant factor is central corneal thickness (CCT), as the Goldmann tonometer was designed assuming an average corneal thickness. Thicker corneas can lead to an overestimation of IOP, resulting in higher readings than the actual pressure, while thinner corneas may lead to an underestimation. A separate measurement called pachymetry can determine CCT, allowing for potential adjustments to the IOP reading if necessary.
Patient cooperation also plays a role, as pressing on the eyelids or making sudden movements during the test can artificially elevate the intraocular pressure. Additionally, the examiner’s technique, including proper alignment and consistent application of force, is important for obtaining reliable measurements. Conditions affecting the cornea, such as corneal edema (swelling) or scarring, can also impact the accuracy of the readings. Despite these influencing factors, applanation tonometry remains a reliable and widely used method for assessing intraocular pressure in clinical practice.