Ocuton-S self tonometry vs. Goldmann tonometry; a diurnal comparison study

PURPOSE: To compare 24-hour diurnal intraocular pressure (IOP) measurements obtained using the Ocuton-S applanation self tonometer and the Goldmann tonometer. METHODS: 24-hour diurnal IOP curves were obtained on 14 eyes of 7 trained patients suffering from medically controlled primary open angle glaucoma. IOP was measured every third hour starting at 9 a.m. with a calibrated Goldmann tonometer; one week later, a similar set of measurements was obtained with Ocuton-S self tonometry by the patients. One week later still, ultrasound corneal pachymetry was performed at the same hours. RESULTS: Overall IOP (24-hour mean) did not differ significantly between the different measuring techniques (ANOVA, p = 0.74), but the IOP differed in a statistically significant manner around the clock (ANOVA, p = 0.00006). The mean Goldmann tonometric readings were up to 2.8 mmHg lower than the Ocuton-S values during the daytime (9 a.m. to 9 p.m.), however, during the night (12 midnight to 6 a.m.) mean IOP measured with Goldmann tonometry was 2.2 to 3.3 mmHg higher than the corresponding average of the self tonometry readings. There was a statistically significant interaction between the type of tonometry and the time of the measurement (p = 0.0007). Central corneal thickness (CCT) showed a significant change during the 24-hour period (p = 0.000001). CONCLUSION: IOP shows a different diurnal curve when measured with the Goldmann tonometer and with the Ocuton-S applanation self tonometer. The instruments' readings might be influenced in different ways by the diurnal changes of the corneal thickness. Since Ocuton-S self tonometry underestimates the IOP in the early morning period, a careful evaluation is necessary when nocturnal and early morning IOP elevation is investigated with this technique.