2.4. Laser retinal photocoagulation

Clinical effects of laser photocoagulation are obtained by thermal reaction. When the temperature in the tissue reaches a critical level, proteins are denatured and coagulation occurs. The factors that influence the results of coagulation for a certain power are: the energy of a specified wavelength, the spot size and the exposure time. According to the degree of coagulation, ocular reaction manifests by cell proliferation, cell migration, and scar formation [5].

Tissue effects of lasers depend upon the interaction between the laser wavelength and ocular pigments: melanin, hemoglobin and xanthophyll.

Melanin is found in the RPE as melanosomes and in the choroid as granules. The maximum absorption of melanin is for wavelengths of 400–600 nm, followed by blue, green, red and infrared. Subsequently, shorter wavelengths are better absorbed by melanin as compared to longer ones. When it comes to the penetration through tissues of the laser radiation, longer wavelengths will manifest their effect deep in the choroid. Because the quantity of melanin varies between individuals and from one region to another of the fundus, the coagulation effect of longer wavelengths is unequal. Longer wavelengths require higher energies to obtain similar effects with the shorter ones.

Hemoglobin is more selective in terms of absorption. Its maximum absorption is for blue, green and yellow wavelengths, whereas for red and infrared, there is no absorption.

Xanthophyll pigment is located exclusively in the fovea, and it absorbs the blue and blue-green laser radiations. Because of the damaging effects on the central vision of these radiations, they are no longer used in the clinical practice [5].

Lasers that create thermal reactions have direct and indirect effects on the ocular tissues.

In ischemic retinopathies, such as ROP, the indirect effect is used to induce the regression of new vessels that appear due to retinal ischemia.

Several mechanisms of action have been described. It is postulated that tissue destruction by laser photocoagulation decreases the need for oxygen of the tissues, and in the same time, it lowers the stimulus for the production of angiogenic factor.

Photocoagulation also eliminates the photoreceptor cells, which are high oxygen consumers, allowing the use of available oxygen by the viable cells.

A tight adhesion between the retina and choriocapillaris is created following laser photocoagulation, increasing oxygen flow to the retina.

The pigmented cells destroyed by laser liberate a substance that inhibits angiogenesis [5].
