Acute ultrastructural changes of cornea after excimer laser ablation

Corneal ultrastructural changes induced by an argon fluoride excimer laser using different parameters were investigated. Twenty-eight rabbit corneas were ablated at light doses per pulse and repetition rates ranging from 25-800 mJ/cm2 and 1-100 Hz, respectively, at four different total light doses (25-150 J/cm2). Transmission electron microscopy showed that corneal ablations done at subthreshold light doses per pulse with repetition rates higher than 30 Hz and with an exposure more than 100 sec caused significant surface coagulation and an increase in pseudomembrane thickness. These changes were not observed in ablations done above threshold light doses per pulse, regardless of repetition rate and exposure time. However, repetition rates as high as 80 Hz caused damage to the endothelium and Descemet's membrane at the same ablation depth that did not cause such damage using repetition rates under 40 Hz. It appears that high repetition rates used during excimer laser corneal surgery may cause irreversible damage to the cornea.