Eye Pressure Checks: Are You Getting the Right Angle?
"Exploring how slight misalignments during rebound tonometry can affect your intraocular pressure readings and what it means for your eye health."
Intraocular pressure (IOP) measurement is a critical part of routine eye exams, especially for those at risk of glaucoma or other eye conditions. One of the tools used to measure IOP is the rebound tonometer, known for being quick and relatively comfortable. Unlike the traditional Goldmann applanation tonometer (GAT), rebound tonometers don't require numbing eye drops, making them a popular choice for both practitioners and patients.
However, the accuracy of any measurement tool can be affected by how it's used. This article delves into a study investigating how small misalignments during rebound tonometry can influence the readings. The goal is to help you, the patient, better understand the factors that contribute to accurate IOP measurements and what questions to ask during your eye exam.
The study we're discussing focused on the iCare TA0li rebound tonometer and examined how lateral and angular deviations of the probe affected IOP readings. By understanding these potential sources of error, we can all work together—patients and eye care professionals—to ensure the most accurate assessment of your eye health.
Goldmann Tonometry and Alternatives
Goldmann applanation tonometry (GAT) has been considered the gold standard for measuring intraocular pressure (IOP) for almost 50 years. The search for alternative methods has continued because GAT can be impractical in some settings and for some patient populations. A 2023 review describes applanation, indentation, rebound, and dynamic methods among the tonometry modalities used to measure IOP, an important treatable risk factor in glaucoma management.
Decoding the Study: Misalignment and Your IOP
Researchers at Aston University used a precise alignment rig to measure IOP in healthy volunteers. They compared IOP readings taken with the iCare tonometer at the central corneal apex (the ideal location) to readings taken with slight misalignments—both laterally (2mm nasally and temporally) and angularly (5° and 10° nasally and temporally). Goldmann applanation tonometry (GAT), the clinical gold standard, was also used as a benchmark.
- iCare Overestimation: The iCare tonometer tended to read higher than the GAT, confirming previous findings.
- Lateral Shifts: Small lateral misalignments (2mm) didn't significantly alter IOP readings.
- Angular Deviations: Only a 10° nasal deviation of the probe resulted in a statistically significant (but clinically small) underestimation of IOP.
Position and Measurement Context Matter
Studies highlight that tonometer position and measurement context can affect IOP readings. In human patients, 10° of angulation has been reported to produce a statistically significant underestimation, although the cited animal work did not find the same effect in rats measured with the Tonolab rebound tonometer. A study of measurements over positive soft contact lenses notes that corneal measurement location may matter, while a 2026 report cautions that post-CST rebound readings may be overestimated in highly myopic eyes and underestimated in low myopes.
Rebound Readings Can Differ from Goldmann
In a study of 36 healthy subjects, central iCare rebound readings averaged 17.4 ± 3.6 mmHg, compared with 14.7 ± 2.5 mmHg measured by GAT. The rebound readings had a positive correlation with GAT readings, but averaged 2.7 ± 2.8 mmHg higher, with the bias increasing at higher IOPs. These results show that rebound readings and GAT readings may differ, even when they are correlated.
What This Means for Your Eye Health
While the study indicates that minor misalignments during rebound tonometry may not significantly impact IOP readings in healthy eyes, it's always best to ensure the measurement is taken as accurately as possible. This is particularly important if you have risk factors for glaucoma or other eye conditions.
Repeatability and Patient Measurement
A study of 53 glaucoma patients examined agreement between patient-measured IOP using rebound tonometry and ophthalmologist-measured IOP using GAT. A separate 2016 study evaluated repeatability and variation associated with probe placement on the cornea. In three prospective studies, iCare showed significantly lower test-retest variability than Goldmann tonometry, with good interoperator and interdevice reproducibility, supporting its potential for monitoring IOP changes over time.
Here are some points to consider:
Ask your eye care provider about their technique for IOP measurement and what steps they take to ensure accuracy. If you have any concerns about the readings or the testing process, don't hesitate to voice them. Remember, you are a partner in your eye health.