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Ranjay Chakraborty, Scott Read, Michael Collins; Monocular Defocus and Diurnal Variations in Axial Length and Intraocular Pressure of Human Eyes. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4452.
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To investigate the influence of monocular defocus upon the diurnal rhythms of axial length and intraocular pressure (IOP) in human subjects.
A series of diurnal axial length and IOP measurements (collected at ~3 hourly intervals, with the first measurement at ~9 am and the final measurement at ~9 pm on each day) were taken for 13 emmetropic young adults (mean age 26 ± 2 years) over three consecutive days. Axial length was measured using the Lenstar optical biometer, and IOP with the Ocular Response Analyser. Day 1 (baseline day, no defocus) examined the normal diurnal variations in axial length and IOP, day 2 (defocus day) investigated the influence of monocular myopic defocus on the diurnal variations in ocular parameters (subjects wore a +1.50 DS spectacle lens over the right eye, for the duration of the day), and day 3 (recovery day, no defocus) examined the recovery from any ocular changes induced by the defocus.
Both axial length and IOP underwent significant diurnal changes on each of the three days (p<0.0001). The magnitude and timing of the diurnal rhythms in axial length were significantly altered with the introduction of myopic defocus on day 2 (p<0.0001). With myopic defocus, the peak in axial length was typically observed at the fourth measurement session (mean time 06:23pm), whereas on days 1 and 3 with no defocus, the peak was typically observed at the second session (12:23pm). The mean amplitude (peak to trough) of change in axial length was significantly different between days (mean amplitude of change was 30 ± 12, 20 ± 10 and 33 ± 12 µm for the baseline, defocus and recovery days respectively, p=0.01). No significant difference was observed in the amplitude (mean amplitude of change of 4.72 ± 1.83 mmHg on each day) or timing of the diurnal rhythms in IOP between the three days, with no evidence of significant changes in IOP associated with defocus on day 2 (p>0.05).
The diurnal variations of axial length in human eyes appear to be significantly disrupted (in terms of both amplitude and phase) by the introduction of monocular myopic defocus, but rapidly return to normal the following day, upon removal of the defocus. The diurnal rhythms of IOP do not appear to be influenced by defocus.
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