Corneal bulging is cause of increasing myopia after keratoconus graft

VIENNA, Austria — Keratectasia seems to be the major cause of increasing myopia after surgery for keratoconus, according to a study. Eyes that underwent penetrating keratoplasty for keratoconus tended to show a continued increase in myopia, with an associated increase in keratometric values, the study authors said.

Researchers here with the Vienna Medical School examined 30 eyes of 28 patients who had undergone keratoplasty for keratoconus with the Guided Trephine System (Polytech Ophthalmologie). Subjective refraction was evaluated preoperatively and at intervals up to 2 years postoperatively. Axial length was measured using applanation ultrasonography before surgery and at 2 years postop.

The mean spherical equivalent was +2.22 D at 1 month postop. By 2 years postop, a continuing myopic shift changed that value to a mean of -1.02 D. The myopic shift was associated with a significant increase in keratometric levels from +41.72 D at 1 month postop to +43.77 D at 2 years postop. Overall axial length did not change significantly, but the vitreous length showed a small but statistically significant increase.

The study authors noted that the follow-up time in this study was too short to determine whether corneal steepening after suture removal affected the myopic shift.

They also noted that eight eyes experienced a myopic shift despite a decrease in keratometry. “In these eyes, we found a significant growth of axial and vitreous length compared with the eyes with an increase in keratometry,” the authors reported in the November issue of American Journal of Ophthalmology. The authors suggested that growth of the globe may have been responsible for the longer axial length in these eyes, and the resulting axial myopia.