Researcher: Gurjeet Sammi
Supervisor: Professor Bernard Gilmartin, Dr James S. Wolffsohn
University: Aston University
Abstract:
Previous literature suggests that IOP reduces by 1.32±0.43 mmHg for a 4D accommodative stimulus. Our own preliminary data showed that change in IOP with accommodation varied substantially in 21 subjects. To date, no studies have accounted for fluctuations in IOP linked to cardiac and respiratory cycles. We report here on the change in IOP with accommodation when variance associated with these cycles is controlled.
Method:
A modified EasyEye Pulsair (Keeler) tonometer and a finger pulse transducer were linked to a Labview Acquisition Program (National Instruments) which displayed the cardiac pulse. The respiratory cycle was kept constant at 15 breathes/min using a metronome. Three IOP measures were taken on 50 subjects using the standard Pulsair (Keeler) and Goldmann contact tonometers. Three measures were obtained at the peak, middle and trough of the cardiac pulse (i.e. 9 measures) in a randomised order using the modified Pulsair. In addition, 3 IOP measures synchronized with the middle of the cardiac pulse (as this showed the least variance) were taken on 7 subjects while fixating a 0D and 4D accommodative stimulus for 3 minutes.
Results:
Measures taken with the standard Pulsair (r=0.56, p<0.001) and those taken at the peak (r=0.77, p<0.001), middle (r=0.76, p<0.001) and trough (r=0.75, p<0.001) of the cardiac pulse were correlated with Goldmann tonometry. The mean difference (mmHg) between measures taken with Goldmann tonometry and measures taken at the peak (0.28, t=0.937, p=0.353), middle (0.85, t=2.784, p=0.008) and trough (0.61, t=1.953, p=0.057) failed to reach statistical significance (criterion p<0.007 for multiple t-tests). Coefficients of variability at the peak, middle and trough of the cardiac pulse were 11.06, 8.41, and 16.73%, respectively. The IOP reduced in all subjects by a mean value of 0.86±0.45mmHg (p=0.002) for a mean accommodative response of 4.02±0.35D.
Conclusion:
Synchronization of the IOP measures with the middle of the cardiac cycle indicates that, consistent with previous studies, IOP reduces on accommodation. Future work will investigate further temporal aspects of IOP change with accommodation and convergence.
Acknowledgements:
College of Optometrist’s research scholarship to GS; Keeler Instruments Ltd for technical help and instrument supply; Alejandro Cervino for his assistance in collecting data. The authors do not have a proprietary or financial interest in the EasyEye Pulsair.