Mind the Gap

IOP measurements from Goldmann applanation tonometry (GAT) assessments aren’t all that accurate; central corneal thickness (CCT) can impact pressure readings, with IOPGAT being underestimated on thin corneas and overestimated on thick ones (1). But although several GAT correction formulas exist, how accurate are they? And how might those inaccuracies impact glaucoma care?

A Zurich-based team of researchers decided to investigate (2). “Over years working with dynamic contour tonometry (DCT) we realized that the well-known relationship between CCT and over- and underestimation of IOPGAT is not valid in all cases. The question came up of what results correction formulas provide as they calculate mainly with CCT parameters,” says corresponding author Christoph Kniestedt of Talacker Eye Center, Zurich, Switzerland. In their prospective, cross-sectional clinical trial, they measured IOP in 112 patients with glaucoma using Pascal DCT and GAT. Comparing IOPDCT with conventional (uncorrected) IOPGAT (IOPDCT-IOPGAT), they found a mean discordance of 3.3 mm Hg (p<0.001). Comparing IOPGAT that had been corrected by five separate formulas, the discordance between DCT and GAT ranged between 2.7 to 5.4 mmHg (all p<0.001).

The group also identified a positive correlation between discordant IOP values and glaucoma severity (r_s=0.33, p<0.001). “We have shown that glaucoma is in a more advanced stage when the difference between IOPDCT and IOPGAT increases,” says Kniestedt. CCT was found to be negatively associated with discordant values (r_s=-0.22, p<0.02). Concluding that GAT values are significantly discordant with DCT measurements in eyes with thin corneas and advanced glaucoma, the authors advised: “[…] to not place reliance on GAT readings, and abandon any correction formula” (2).

Commenting on the results, Kniestedt says, “The question is whether we really need to know the accurate IOP, or if it is sufficient to be aware of an additional risk factor such as IOPGAT-DCT difference. I would say that the second is sufficient; if we had a device to measure the real IOP – and if the measurement was significantly different from the ‘gold standard’ GAT – then we would need to re-write our textbooks and guidelines.” Kniestedt explains that a patient who has an IOP of 18 mmHg and an IOPDCT-GAT difference of 5 mmHg might have a higher glaucoma risk than a patient who has a GAT value of 20 mmHg but an IOPDCT-GAT difference of 1 mmHg. “If we know the DCT-GAT difference and, as such, the additional risk factor in any individual patient, then we can use the old GAT value perfectly well.”