Sir,

We read with interest the article ‘Untreated 24-h intraocular pressures measured with Goldmann applanation tonometry vs nighttime supine pressures with Perkins applanation tonometry’ by Quaranta et al.1 The study has methodological issues that are of interest and should be clarified.

One goal of this study was to compare the daytime sitting intraocular pressure (IOP) with the nighttime supine IOP in untreated patients with ocular hypertension or glaucoma. Ideally, such a comparison should be based on data using the same method of tonometry for all the measurements. As the Goldmann tonometer, the clinical gold standard, can only be used to obtain measurements in the sitting position, the handheld Perkins tonometer was used for the supine IOP measurements. Comparison of IOP measurements with these two different tonometers can be misleading, and would have been obviated by comparison of daytime sitting IOP measurements using the Perkins tonometer with those obtained using the Goldmann tonometry.

Using a Perkins tonometer to obtain both daytime and nighttime measurements, rather than the Goldmann tonometer for the daytime measurements and the Perkins tonometer for the nighttime measurements, would have been preferred. The authors lost an opportunity to maximize the statistical power by using two different tonometers. As mentioned by the authors, a Perkins tonometer may not provide the same IOP values as a Goldmann tonometer. A critical issue is the direction of possible measurement errors. It has been reported that the Perkins tonometer can underestimate IOP by 0.6–1.5 mm Hg compared with the Goldmann tonometer.2, 3, 4, 5 If an underestimation by the Perkins tonometer occurred in this study, IOP values at night may have been underestimated.

The authors used the Goldmann sitting IOP measured at 1000 hours as inclusion criterion for the study. Subjects who had IOP below 22 mm Hg at 1000 hours were excluded from the 24-h IOP evaluations. This may have created a bias toward inclusion of patients with higher IOPs in the morning, thus potentially helping to explain the relatively higher pressures obtained during the daytime period compared with the nighttime period. As the authors were interested in studying the general 24-h IOP profile in ocular hypertension and glaucoma, it seems logical that they should not have restricted their inclusion to only those patients with high pressures in the morning.

We also noticed the difference in daytime and nighttime definitions in this study compared with relevant publications in the literature. The authors performed IOP measurements every 4 h and divided the six readings in the 24-h period equally as daytime and nighttime readings. This definition is different from other publications that divided the 24-h day into a 16-h daytime/wake period and 8-h nighttime/sleep period to study IOP or aqueous flow.6, 7 The use of a similar definition would have allowed better comparison of the results of this study with those from earlier investigations.