Assessment of reproducibility of measures of intraocular pressure and central corneal thickness in young white adults over a 16-h time period

Purpose: The aims of this study were to obtain clinical measures of intraocular pressure (IOP) and central corneal thickness (CCT) and during waking hours to further assess both the nature of diurnal changes, and the reproducibility of any possible association between the two measures. Methods: Fifteen white (Norwegian) healthy young adult subjects aged 20-29 years had IOP and CCT measures made, using a non-contact specular microscope and non-contact tonometry (NCT), every hour over a period of 16 h, starting at 06.30 hours. The experiment was repeated on two successive days. Results: The average NCT readings were 12.5 and 11.4 mmHg for days 1 and 2, while pachometry averages were 0.518 and 0.514 mm. The range of NCT values, for any set of readings, averaged 2 mmHg, while this range was 0.014 mm for CCT measures. The reproducibility of these measures, as estimated by the coefficient of variation (COV) was 7.2% for NCT and 1.0% for pachometry measures. The COV for tonometry was inversely proportional to the actual values of these measures. A significant time-related decline in tonometry values was seen over day 1 but not day 2. Pachometry values declined very rapidly over the first hour in both sessions and were essentially constant for the rest of the sessions. The strongest association between pachometry data and NCT measures was noted around midday (12.30 hours) on both sessions. Conclusions: These studies confirm that the reproducibility (repeatability) of NCT and pachometry are substantially different. It is suggested that this aspect of tonometry needs to be taken into account, especially where there are lower values, when considering the possible impact of corneal thickness on tonometry measures and their interpretation. These types of effects need to be further assessed in older patients with ocular hypertension, or other glaucoma suspects.