New generation of dynamic tonometers compared with traditional static models

This article is best viewed in a PDF Format.

View PDF

In recent years, extensive research has been conducted on the sensitivity and specificity of using the current 'gold standard' Goldmann applanation tonometer (GAT), as a reliable screening tool for ocular hypertension. Its accuracy has been questioned, with many concluding that biomechanical features such as thickness, rigidity and hydration are indeed variable and can have a significant effect on IOP readings.

Goldmann designed his applanation tonometer to measure IOP for an average CCT of 520µm, an assumption which does not hold true given thickness can vary significantly in both normal and glaucomatous populations. A thinner cornea leads to a measured IOP that is less than the actual value and vice versa. So it appears there is no universally agreed correction factor that can be applied to give a true reading. Some researchers advise a nominal reduction in IOP for every 100µm reduction below the average central thickness, but these suggested values fluctuate widely between 2.0 and 7.5mmHg.1 Furthermore, the alternative theory of applying a linear corrective algorithm is also unfounded as indicated by findings of the ocular hypertension treatment study (OHTS),2 in which CCT was shown to have a weak association with IOP. In fact, this 2002 US study suggested that IOP alteration may be in the wrong direction. This is evident in patients with corneal dystrophies, such as Fuchs' endothelial dystrophy where CCT is generally high while corresponding GAT measurements tend to be lower than average.3 It is apparent that CCT alone cannot account for these variations in IOP.