Is the superior cornea continuously swollen?

During waking hours, the upper eyelid typically covers the superior two millimetres of the cornea. This partial oxygen barrier is the presumed cause of the reversibly elevated epithelial oxygen uptake in this region of the cornea. We studied the thickness profile of the vertical corneal meridian before and after extended exposure to a uniform hypoxic stimulus to ascertain the extent of chronic superior corneal swelling that might be attributable to coverage by the upper lid.

Each of 16 male subjects, age 24 to 40 years, wore a uniformly thick polymacon lens (Dk/L = 5.7 x 10-9 cm ml O2/s ml mmHg) on one eye for four hours with the eyelid patched. Thickness was measured at five sites across the vertical corneal meridian prior to and immediately after patching. Baseline thickness and swelling were compared at the superior and inferior edges of selected central chord diameters.

At baseline, the superior cornea was, on average, 31 mm thicker (p = 0.0002) at the 7.1 mm chord and 9 mm thicker (not significant) at the 3.75 mm chord, compared to the inferior cornea at the corresponding chords. Mean corneal swelling ranged from 70.5 mm centrally to approximately 48 mm at the 7.1 mm chord diameter. The extent of swelling at the corresponding superior and inferior sites was virtually identical for both chord diameters.

Although the superior cornea is significantly thicker than the inferior cornea, this difference does not appear to be immediately related to the chronic differences in open eye oxygen tension between the superior and inferior regions.