Diurnal variation of ocular hysteresis in normal subjects: Relevance in clinical context

Background: This study was conducted to assess the diurnal variation in ocular hysteresis, as measured by the Ocular Response Analyser to establish a relationship between diurnal hysteresis variation and diurnal intraocular pressure (IOP) variation. Methods: Forty-two normal eyes of 21 colleagues and staff in a teaching hospital in Birmingham, UK, were recruited. The IOP and hysteresis were measured by the Ocular Response Analyser. The central corneal thickness (CCT) was measured using a hand-held ultrasonic pachymeter in the mid-pupillary axis. Results: The mean ocular hysteresis at 8 AM was 12.7 ± 2.3 mmHg, at 11 AM was 12.2 ± 2.0 mmHg, at 2 PM was 12.7 ± 2.1 mmHg and at 5 PM was 12.7 ± 1.7 mmHg; the difference between the values at any time of measurement was not statistically significant (P > 0.9, repeated measures). IOP as measured by non-contact tonometry was 18.4 ± 2.8 mmHg, 17.9 ± 3.3 mmHg, 16.9 ± 3.1 mmHg and 16.8 ± 3.2 mmHg, respectively, for the same time period; the difference between the values in the morning and afternoon was statistically significant (P < 0.0001, repeated measures). The CCT was 548.8 ± 29.5 μ 547.0 ± 31.4 μm, 548.2 ± 29.6 μm and 548.6 ± 29.4 μm, respectively; the difference between the values was not statistically significant at any time points. Multiple regression analysis showed the relationship between IOP and hysteresis was not statistically significant (P = 0.9). Conclusion: The ocular hysteresis reading was almost constant throughout the day, whereas the IOP readings showed highest values in the morning with a reducing trend being lowest in the afternoon. The CCT values were almost stable throughout the day. IOP appears to vary independently of a variation in hysteresis or CCT.