Endothelial damage thresholds for retrocorneal Q-switched neodymium:YAG laser pulses in monkeys

Laser pulses were focused within 1 mm of the rhesus monkey corneal endothelium using the Coherent Model 9900 laser at energies of 3, 6, 9, and 12 mJ. Sixteen slightly off-axis pulses were applied with no contact lens on the eye. Corneal damage was studied clinically and by scanning electron microscopy. Q-switched pulses of 12 mJ or less are not likely to damage the cornea if focused more than 0.75 mm from the endothelium. The retrocorneal focal distance for a 50% incidence of endothelial damage for 6, 9, and 12 mJ pulses was found to be less than 0.5 mm. For 3 mJ pulses, it was less than 0.25 mm. The severe early effect of suprathreshold pulses is edema of endothelial cells in a 0.2- to 0.6-mm diameter circular zone surrounding a small central pit through Descemet's membrane. At two months, irregular enlargement of endothelial cells surrounding and partially covering the persistent pit exists at sites of severe damage. The extent of the longer-term change is proportional to the severity of the original insult, but in no case was there clinically significant, persistent damage in the healed corneas.