It is important to understand the concept of power density delivery of the coagulating beam applied to retinal tissue. Spot size, power settings, and exposure times are determined prior to treatment but can be, and often are, adjusted during the treatment. Different protocols have been established for different applications and interactions; however, these are only general guidelines. Flexibility is important and is based on training and experience with different settings and parameters.

The relationship between spot size, exposure time, and power, which are used to produce the correct power density, need to be closely monitored. If the spot size does not change, the power and exposure intervals should be kept constant until a good power density has been found. Any decrease or increase in spot size should be accompanied by a decrease or increase in input power until an appropriate response is obtained. Careful titration of the laser response is warranted. In practice, there are many factors that affect the size of the spot and power density, such as wavelength, media opacity, and absorption quality of the tissue to be treated.

Extremely high irradiance delivered with a very short exposure in a small spot will result in electrons being energized from molecules in target tissue, producing a collection of free electrons and ions called plasma. Rapid expansion of the plasma creates acoustic and shock waves that, combined with latent tissue stress, incise the target tissue, producing photodisruption independent of tissue pigmentation or laser absorption. Transparent tissue can be incised in this way, obviating the need for conventional surgery. The most common laser source used is the Nd:YAG operating at the 1064-nm wavelength used in the anterior segment of the eye for discission of the posterior lens capsule, peripheral iridectomy, and synechotomy. In the posterior segment, it has been used successfully in the cutting of vitreous membranes and vitreoretinal bands. Because of the high amount of energy delivered at the focal point of disruption, it is important that the focus of energy be as far away from the retina as possible and at least 6 mm from the crystalline lens.⁴ Although there is intense heat at the point of plasma formation, the spot is so small that the heat dissipates rapidly.

A chemical reaction can be made to occur by intense laser energy if the energy of the photons is high enough. Energizing of photons increases with shorter wavelengths; therefore, ultraviolet light is more effective in producing photochemical reaction than is visible or infrared light. The photochemical effect of laser radiation has been used experimentally in photoradiation by red (630 nm) wavelength to produce cytotoxic free radicals in a tumor previously sensitized by a hematoporphyrin-derived uptake.⁵

The most common photochemical effect in use today is PDT, an example of laser treatment with a photosensitizing agent. This new treatment modality combines low-power diode lasers (689 nm) with an infusion of verteporfin to ablate subretinal neovascular membranes. The absorbed energy of the laser activates a series of photochemical events, beginning with the verteporfin and ending with the occlusion of vascular tissue in the subretinal membrane.⁶ The advantage of this therapy over conventional photocoagulation is that the lower energy levels result in much less damage to adjacent normal tissue.

Photoablation is produced by a photochemical reaction using an intense excimer (ultraviolet) laser. The most common ophthalmologic use of this therapy is in excimer lasers used in refractive surgery. At present, there are no uses for this therapy in the treatment of retinal diseases.