The bleb phenomenon—where some endothelial cells transiently lose their specular reflection after contact lens wear or goggle-induced hypoxia or hypercapnia—continues to puzzle both investigators and eye care practitioners alike. However, a study in Optometry and Vision Science found scleral lenses with apical clearance greater than 400μm may produce more blebs in the corneal endothelium, suggesting there may be reduced oxygen and increased carbon dioxide levels under scleral lenses that are fitted with excessive clearance.

The study, led by a team of Canadian researchers, also found blebs may occur more often in small cells.

The investigation enrolled 21 subjects who were fitted with two similar scleral lenses but with different targeted clearances of either 200μm or 400μm (the SL200 and SL400, respectively).

The patients wore each lens unilaterally for 25 minutes, with their other eye serving as the control. The researchers photographed the endothelium with specular microscopy before and after lens wear.

After lens wear, nine SL200 patients and 14 SL400 patients had at least one bleb. The investigators also found the bleb and cell areas were different (293 ± 28 blebs; 370 ± 32μm cells).

Clinicians should avoid a clearance of 400μm, since it limited oxygen delivery to the cornea by 30% more than in patients fitted with a clearance of 200μm, the researchers advised.

“Even if it is very likely that these blebs will disappear and not occur again once the wearer has adapted, the principle of non-maleficence requires avoidance of use of the higher clearance of 400μm whenever possible,” the researchers wrote in their paper. The investigators hope their study stimulates the industry to design improved tear exchange technology on sceral lenses and help guide practitioners to fit these lenses with the appropriate clearances.