Diurnal variation of anterior scleral and conjunctival thickness

Purpose: To examine whether anterior scleral and conjunctival thickness undergoes significant diurnal variation over a 24-h period. Methods: Nineteen healthy young adults (mean age 22 +/- 2 years) with minimal refractive error (mean spherical equivalent refraction -0.08 +/- 0.39 D), had measures of anterior scleral and conjunctival thickness collected using anterior segment optical coherence tomography (AS-OCT) at seven measurement sessions over a 24-h period. The thickness of the temporal anterior sclera and conjunctiva were determined at six locations (each separated by 0.5 mm) at varying distances from the scleral spur (SS) for each subject at each measurement session. Results: Both the anterior sclera and conjunctiva were found to undergo significant diurnal variations in thickness over a 24-h period (both p < 0.01). The sclera and conjunctiva exhibited a similar pattern of diurnal change, with a small magnitude thinning observed close to midday, and a larger magnitude thickening observed in the early morning immediately after waking. The amplitude of diurnal thickness change was larger in the conjunctiva (mean amplitude 69 +/- 29 mu m) compared to the sclera (21 +/- 8 mu m). The conjunctiva exhibited its smallest magnitude of change at the SS location (mean amplitude 56 +/- 17 mu m) whereas the sclera exhibited its largest magnitude of change at this location (52 +/- 21 mu m). Conclusions: This study provides the first evidence of diurnal variations occurring in the thickness of the anterior sclera and conjunctiva. Studies requiring precise measures of these anatomical layers should therefore take time of day into consideration. The majority of the observed changes occurred in the early morning immediately after waking and were of larger magnitude in the conjunctiva compared to the sclera. Thickness changes at other times of the day were of smaller magnitude and generally not statistically significant.