Sir,

We enjoyed reading the article by Chew et al.1 There are several issues that we think the authors may like to address.

It would be helpful for the authors to clarify whether they employed the median of several intraocular pressure (IOP) readings for analysis. All measurements of IOP are subject to random errors. Single measurement is suboptimal in reflecting the true IOP. Taking the median of several readings is a standard way to approximate the true IOP values for most tonometry. Comparison of single measurement may introduce more error into the mean difference.

The authors did not describe the visual field status of their subjects. Theoretically, a proper perception of pressure phosphene requires the presence of functioning bipolar cells, rods, and cones in the retina.2 If the recruited subjects were having advanced glaucoma or significant retinal disease such that there was a significant bipolar cells and visual field loss, the perception of phosphene may prove difficult. However, this does not necessarily negate the potential use of the pressure phosphene tonometer (PPT) in those with early or preperimetric glaucoma.

The authors talked of testing for reliability of PPT in their aim of study, and concluded that PPT cannot be a reliable instrument. However, the authors have only tested for accuracy of PPT vs Goldmann tonometer (GT), not reliability, as they did not present data such as coefficients of variations, which is a proper way to assess reliability.

It is uncertain whether suboptimal hand–eye coordination, intelligence, and patient understanding will have significant influence on the accuracy in using PPT. The recruited subjects in this study consisted of an elderly population (median age=73 years), which might have been suboptimal with regard to the factors listed above. The authors may like to give an analysis on the group with younger age, to see whether PPT might be more useful.