Diurnal variation of corneal shape and thickness

To investigate associations between the diurnal variation in a range of corneal parameters, including anterior and posterior corneal topography, and regional corneal thickness.

Fifteen subjects had their corneas measured using a rotating Scheimpflug camera (Pentacam) every 3 to 7 h over a 24-h period. Anterior and posterior corneal axial curvature, pachymetry and anterior chamber depth were analyzed. The best fitting corneal sphero-cylinder from the axial curvature, and the average corneal thickness for a series of different corneal regions were calculated. Measures of intraocular pressure and axial length were also taken at each measurement session.

Significant diurnal variation was found to occur in both the anterior and posterior corneal curvature and in the regional corneal thickness. Flattening of the anterior corneal best sphere was observed at the early morning measurement upon waking (p < 0.0001). The posterior cornea also underwent a significant steepening (p < 0.0001) and change in astigmatism 90/180 degrees at this time. A significant swelling of the cornea (p < 0.0001) was also evident upon waking. Highly significant associations were found between the diurnal variation in corneal thickness and the changes in corneal curvature.

Small but significant diurnal variation occurs in the regional thickness and the shape of the anterior and posterior cornea. The largest changes in the cornea are typically evident upon waking. The regional corneal swelling observed at this time was not uniform, being slightly greater in peripheral corneal regions. The pattern of corneal swelling evident on waking resulted in a steepening of the posterior, and a flattening of the anterior corneal surface.