Measuring true IOP: factors that confuse GAT readings

Key Points

By now nearly everyone recognizes that the current gold standard for measuring intraocular pressure (IOP), the Goldmann tonometer (GAT), has considerable flaws. Dr Goldmann designed his tonometer to provide accurate measurements in eyes with average corneas, but we now know that many corneas vary significantly from the "average".

GAT is based on application of the Imbert-Finck principle to the cornea, which states that the force of flattening (F) equals the pressure (P) multiplied by the surface area flattened (A), or F=PA. However, this principle assumes that we are examining a dry, perfectly flexible, infinitely thin spherical surface - in other words, a cornea that does not exist. The GAT (and tonometers calibrated to "read" like GAT) are affected by corneal properties including rigidity, thickness, hydration, curvature, and perhaps also by other factors not yet identified.

In theory, GAT measurements of thicker corneas overstate IOP values while measurements of thinner corneas understate IOP values. Many attempts have been made to establish a central corneal thickness (CCT)-based correction algorithm to adjust the GAT-obtained IOP values. However, the relevance of the CCT/IOP relationship identified by various studies is inconsistent, resulting in the creation of numerous algorithms that differ significantly from one another.

Evidence of the effect of CCT on IOP continues to mount in the literature, however, causing experts to question whether the GAT is sensitive and specific enough to be used for the critical purpose of measuring IOP in the diagnosis and management of glaucoma.

Refractive surgery, perhaps more than anything else, has exposed just how seriously the GAT is compromised by corneal properties.

It is well documented that there is an apparent, but artificial, reduction in post-LASIK GAT-measured IOP. However, this anomaly cannot be explained by, or corrected for, using CCT data or any currently known parameter. This is because the LASIK procedure not only thins the cornea but also induces complex biomechanical changes, which were not really measurable by any previously available device.

The importance of CCT

A report in 2002 from the United States Ocular Hypertension Treatment Study (OHTS) underlined the importance of CCT in IOP measurements. The OHTS followed 1636 patients and identified a number of risk factors for conversion from ocular hypertension (OHT) to glaucoma.

Multivariable analysis identified GAT as a predictor for conversion to primary open angle glaucoma (POAG), with subjects with thin corneas being more likely to convert, although precisely why this occurs is not known. One possibility is through IOP measurement error: eyes with thin corneas will have a lower applanated IOP than actual IOP, while thicker corneas will have higher pressures. Thus, thin cornea patients might be diagnosed with POAG incorrectly, simply because of an underestimation of the true IOP.