Photocoagulation uses light energy to coagulate tissue. After light energy is applied to the target tissue, it converts into thermal energy, and the tissue temperature rises above 65°C, causing denaturation of tissue proteins and coagulative necrosis.
Current posterior segment laser delivery systems span the visible light spectrum of 400–700 nm (green, yellow, red) and venture into the infrared wavelengths (>700 nm). Delivery systems may employ a transpupillary approach with slit-lamp or indirect ophthalmoscopic delivery, endophotocoagulation during vitrectomy, or transscleral application with a contact probe.
The effectiveness of photocoagulation depends on the transmission of light through ocular media and the absorption of that light by pigment in the target tissue. Light is absorbed principally by ocular tissues that contain melanin, xanthophyll, or hemoglobin. Figure 19-1 illustrates the absorption spectra of the key pigments found in ocular tissues:
Choice of Laser Wavelength
Laser wavelength selection depends on the specific goals of treatment and the degree to which the photocoagulation must be targeted to the particular tissue while sparing adjacent healthy tissue. The area (depth and diameter) of effective coagulation is directly related to the intensity and duration of irradiation. For a specific set of laser parameters (spot size, duration, and power), the intensity of the burn obtained depends on the clarity of the ocular media and the degree of pigmentation.
The green laser produces light that is absorbed well by melanin and hemoglobin and less completely by xanthophyll. Because of these characteristics and the absence of undesirable short (blue) wavelengths, the green laser has replaced the blue-green laser for treatment of retinal vascular abnormalities and choroidal neovascularization (CNV).
Figure 19-1 Absorption spectra of xanthophyll, hemoglobin, and melanin.
(From Folk JC, Pulido JS. Laser Photocoagulation of the Retina and Choroid. Ophthalmology Monograph 11. San Francisco: American Academy of Ophthalmology; 1997:9.)
The red laser penetrates through nuclear sclerotic cataracts and moderate vitreous hemorrhages better than lasers with other wavelengths do. In addition, it is minimally absorbed by xanthophyll and thus may be useful in treatments near the fovea. The red laser causes deeper burns with a higher rate of patient discomfort and inhomogeneous absorption at the level of the choroid. The infrared laser has characteristics similar to those of the red laser with even deeper tissue penetration.
The yellow laser has, among its advantages, minimal scatter through nuclear sclerotic lenses, low xanthophyll absorption, and little potential for photochemical damage. It appears to be useful for destroying vascular structures while causing minimal damage to adjacent pigmented tissue; thus, it may be valuable for treating retinal vascular and choroidal neovascular lesions. For typical laser wavelengths of specific lasers, see BCSC Section 3, Clinical Optics, Chapter 2.
Laser effects on posterior segment tissues include photochemical and thermal effects and vaporization. Photochemical reactions can be induced by ultraviolet or visible light that is absorbed by tissue molecules or by molecules of a photosensitizing medication (eg, verteporfin), producing cytotoxic reactive oxygen species (eg, free radicals). Absorption of laser energy by pigment results in a temperature rise by tens of degrees and subsequent protein denaturation; the exact temperature rise depends on laser wavelength, laser power, duration of laser application, and spot size. Vaporization is generated by the rise in water temperature above the boiling point, which causes microexplosions, as can occur in overly intense burns. For further discussion of laser light characteristics and light–tissue interactions, see BCSC Section 3, Clinical Optics, Chapter 2.
Atebara NH, Thall EH. Principles of lasers. In: Yanoff M, Duker JS. Ophthalmology. 4th ed. Philadelphia: Elsevier/Saunders; 2014:32–37.
Palanker D, Blumenkranz MS. Retinal laser therapy: biophysical basis and applications. In: Schachat AP, Wilkinson CP, Hinton DR, Sadda SR, Wiedemann P, eds. Ryan’s Retina. Vol 1. 6th ed. Philadelphia: Elsevier/Saunders; 2018:chap 41.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.