Effect of Axial Length on Laser Spot Size During Photodynamic Therapy: An Experimental Study in Monkeys

Abstract

Purpose: : To investigate the effect of the shorter axial length of the monkey eye on the size and energy distribution of the laser spot during photodynamic therapy (PDT).

Methods: : PDT was performed on the normal retina of four rhesus monkeys whose ocular axial lengths are shorter (19.55 to 20.25 mm) than that of humans (24–25 mm). PDT was performed under the same conditions as those for patients. Verteporfin (6 mg/m² body surface area) was administrated, and a laser at 689 nm and intensity of 600 mW/ cm² with an irradiation time of 83 seconds was used to irradiate the retina. The spot sizes were set at 5000, 4000, and 3000 µm. Fundus photographs and fluorescein angiograms were recorded one week after PDT.

Results: : One week after PDT, an intense retinal whitening was seen coinciding with the site of the laser spot. To examine the cause of this strong effect of PDT, the eyes were enucleated and the diameter of the irradiated laser spot was measured using a microcaliper. The area of laser spot on the retina was only 0.56 to 0.61 times of the planned area, indicating that the laser energy/area was 1.64 to1.78 times more intense than initially planned.

Conclusions: : These results are the first in vivo demonstration that the diameter of PDT laser spot is smaller for eyes with shorter axial lengths. The results also suggest that when the PDT is performed on patients with short axial lengths, the effects of PDT will be stronger.

Keywords: age-related macular degeneration • photodynamic therapy