Factors Affecting IOP Measurements

Feb 22, 2021 | Optometry

Factors Affecting IOP Measurements

One of the most important tasks any eye doctor has is to screen for glaucoma—the gradual loss of vision due to pressure on the optic nerve. Most of this pressure comes from an inability of the eye to drain intraocular fluid. Measuring intraocular pressure, or IOP, using an instrument known as the tonometer has been a critical aspect of eye exams. But what if the measurement is not accurate? Even with the best tonometers, pinpointing IOP is harder than it may seem due to some of these factors affecting IOP measurements.

Time of Day

Did you know your feet get bigger at the end of the day? In fact, your body’s measurables are constantly fluctuating, including intraocular pressure. For most people, IOP is highest in the early morning hours, but it ebbs and flows throughout the course of the day depending on stress levels, diet, physical activity, and just about anything else that occurs. The eyes are some of the most delicate and sensitive parts of the body, and the quickest to respond. An IOP reading is only capturing one moment in time.

Corneal Thickness

The Goldmann applanation tonometer is renowned for its accuracy, but even the Goldmann tonometer alone cannot account for variations in the thickness of the corneas, or the surface of the eyes. Thicker corneas report higher IOP through tonometry, while thinner corneas report lower pressure than is accurate. A pachymeter is a good failsafe against differences in corneal thickness and can allow ophthalmologists to adjust readings accordingly.

The Valsalva Maneuver

Its name may sound like a chess move, but the Valsalva maneuver is not a good strategy for patients who are undergoing eye exams. Exhale heavily while closing your nostrils. You will notice the pressure you’re placing on your body as you do this—it’s a common remedy for opening the ears during air travel. But that pressure manifests itself in patients’ intraocular pressure as well, so as nervous patients tighten up and bear down, they give IOP readings that suggest higher pressure than normal. The imposing structure and procedure of the Goldmann tonometer may contribute to this, but the invasive nature of any eye exam can cause a patient to get tense. Encourage patients to stay relaxed, and if the prospect of using Goldmann applanation has them visibly nervous, suggest the Tono-Pen, equipped with disposable tonometer tips for safe and easy use.

Tight Clothing

The clothing patients wear to the office can affect the results of their exams. Tight clothing, especially around the collar, puts pressure on the body, and the eyes are among the first places to feel it. This can lead to higher IOP readings than would be accurate, especially in conjunction with other factors affecting IOP measurements such as nervousness or the time of the appointment. Encourage loose-fitting clothing with no tight collars to eliminate one complicating factor of many.