Matthew Petroll

Severing corneal nerves in one eye induces sympathetic loss of immune privilege and promotes rejection of future corneal allografts placed in either eye

Less than 10% of corneal allografts undergo rejection even though HLA matching is not performed. However, second corneal transplants experience a threefold increase in rejection, which is not due to prior sensitization to histocompatibility antigens shared by the first and second transplants since corneal grafts are selected at random without histocompatibility matching. Using a mouse model of penetrating keratoplasty, we found that 50% of the initial corneal transplants survived, yet 100% of the subsequent corneal allografts (unrelated to the first graft) placed in the opposite eye underwent rejection. The severing of corneal nerves that occurs during surgery induced substance P (SP) secretion in both eyes, which disabled T regulatory cells that are required for allograft survival. Administration of an SP antagonist restored immune privilege and promoted graft survival. Thus, corneal surgery produces a sympathetic response that permanently abolishes immune privilege of subsequent corneal allografts, even those placed in the opposite eye and expressing a completely different array of foreign histocompatibility antigens from the first corneal graft.

Surgically induced astigmatism after hyperopic and myopic photorefractive keratectomy

Purpose: To compare the axis and magnitude of surgically induced refractive astigmatism (SIA) after hyperopic and myopic photorefractive keratectomy (PRK). Setting: Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, Texas, USA. Methods: In this single‐center retrospective study, the VISX Star S2 excimer laser was used to create a peripheral annular ablation profile to correct spherical hyperopia in 23 eyes of 16 consecutive patients. Attempted corrections ranged from +0.50 diopter (D) to +4.25 D with 0 to 1.00 D of astigmatism. The same laser was used to create a central ablation profile to correct spherical myopia in 25 eyes of 17 consecutive patients. Attempted corrections ranged from –2.25 to –6.50 D with 0 to 1.00 D of astigmatism. The absolute change in refractive astigmatism was calculated by taking the difference in magnitudes of astigmatism before and after laser treatment without regard to axis. Axis and magnitude of SIA were analyzed by vector differences. Magnitudes were compared using the Student t test, and axial shifts were compared using the chi‐square test. All patients were followed for a minimum of 6 months. Results: The mean changes in absolute astigmatism were 0.29 ± 0.28 D at 3 months and 0.34 ± 0.29 D at 6 months after hyperopic PRK and 0.40 ± 0.35 D at 3 months and 0.39 ± 0.36 D at 6 months after myopic PRK. The mean vectoral magnitudes were 0.49 ± 0.29 at 3 months and 0.52 ± 0.25 at 6 months after hyperopic PRK and 0.48 ± 0.39 at 3 months and 0.44 ± 0.38 at 6 months after myopic PRK. The mean values for SIA (the centroid) were 0.10 ± 0.57 D × 113 degrees at 3 months and 0.15 ± 0.57 D × 131 degrees at 6 months after hyperopic PRK and 0.04 ± 0.63 D × 160 degrees at 3 months and 0.08 ± 0.58 D × 171 degrees at 6 months after myopic PRK. There was no statistically significant difference between the 2 groups in vectoral axis or magnitude of SIA. Conclusion: Surgically induced astigmatism after hyperopic PRK was comparable to astigmatism induced by myopic PRK. A peripheral annular ablation for hyperopic correction, similar to a central ablation in myopic PRK, did not appear to result in uneven corneal healing causing astigmatism.