From the Guidelines
The most accurate method to measure intraocular pressure (IOP) is through the contour-matching Pascal technique, also known as dynamic contour tonometry, as it calculates an IOP independent of corneal properties. This technique utilizes a piezoresistive sensor embedded into the tonometer tip to digitally sample IOP 100 times per second, with a concave tip shape that causes a relaxation of the cornea to conform to the dynamic contour tonometer tip, minimizing any influence of corneal properties on IOP measurements 1. The device requires 6 seconds or six ocular pulse cycles to determine the IOP and requires topical anesthesia.
Key Features of Dynamic Contour Tonometry
- Calculates IOP independent of corneal properties
- Utilizes a piezoresistive sensor to digitally sample IOP 100 times per second
- Requires 6 seconds or six ocular pulse cycles to determine the IOP
- Requires topical anesthesia
Other methods, such as applanation tonometry, rebound tonometry, and the Mackay-Marg tonometer, may also be used to measure IOP, but they may be more influenced by corneal properties compared to dynamic contour tonometry 1. It is essential to use the same valid technique consistently, from visit to visit, to detect clinically significant and meaningful IOP elevations. Early detection of elevated IOP will allow timely initiation of IOP-lowering therapy before irreversible optic nerve damage occurs.
Comparison with Other Methods
- Applanation tonometry: more influenced by corneal properties, but may provide additional information in specific cases
- Rebound tonometry: does not require topical anesthesia, but may be less accurate in certain cases
- Mackay-Marg tonometer: combines mechanisms of both applanation and indentation, but may be less accurate than dynamic contour tonometry
In clinical practice, the choice of method for measuring IOP should be based on the individual patient's needs and the presence of any corneal abnormalities. However, dynamic contour tonometry remains the most accurate method for measuring IOP, particularly in cases where corneal properties may affect the measurement 1.
From the Research
Intraocular Pressure Measurement Methods
The most accurate method to measure intraocular pressure (IOP) is a topic of ongoing research. Several studies have compared different methods, including Goldmann applanation tonometry (GAT), dynamic contour tonometry (DCT), Tono-Pen XL, iCare, and ocular response analyzer (ORA).
Comparison of IOP Measurement Methods
- A study published in 2006 2 compared DCT and GAT, finding a clear correlation between the two methods, but with DCT resulting in higher IOP measurements.
- A 2013 study 3 explored the relationship between true IOP and tonometric IOP, external curvature radius, and central corneal thickness (CCT) using computational simulations and found that GAT measures are linearly dependent on the modulus of elasticity of the cornea.
- Another 2013 study 4 investigated the influence of corneal edema on IOP measurements using GAT, Tono-Pen XL, iCare, and ORA, finding that Tono-Pen XL and iCare yielded the most accurate ICP values across all adjusted ICP values.
- A 2012 study 5 analyzed the correlation between IOP, as measured by two types of applanation tonometers and one contour tonometer, and central corneal thickness, finding a very weak correlation between central corneal thickness and IOP for all three tonometers.
- A 2024 study 6 compared IOP measurements using GAT, Corvis ST, and iCare following penetrating keratoplasty, finding good agreement between the methods, with the comparison between GAT-IOP and bIOP resulting in the narrowest 95% limits of agreement.
Factors Influencing IOP Measurements
- Central corneal thickness (CCT) has been shown to affect IOP measurements, with some studies finding a significant influence of CCT on GAT measurements 2, 3, while others found a weak correlation between CCT and IOP 5, 6.
- Corneal edema has also been shown to impact IOP measurements, with Tono-Pen XL and iCare found to be more accurate in patients with corneal edema 4.
- The number of sutures at the graft-host interface has been found to influence the difference between GAT-IOP and iCare readings 6.