ALLPCB
Overview
Laser has been widely adopted across medicine, with the most extensive and intensive applications in ophthalmology. The eye itself is an optical system, so light can reach multiple tissue layers through refractive media. Because lasers provide monochromatic, directional beams, different wavelengths can be chosen to target specific ocular tissues accurately. As a result, laser technology was among the first to be applied in ophthalmology and has become a distinct subspecialty: laser ophthalmology.
Laser Treatment of Eye Diseases
1. Effects of Different Wavelengths on Ocular Tissues
Ocular tissues in different locations contain different pigments, so their absorption of laser energy varies with wavelength. When selecting a laser wavelength for treatment, it is important that the target tissue has high absorption while intervening refractive media and other tissues have minimal absorption. In general, melanin absorbs shorter wavelengths more strongly, although the differences are not large. Oxyhemoglobin absorbs blue, green, and yellow light strongly, but has minimal absorption in red and infrared. Lutein has high absorption in the blue region. Therefore, blue, green, and yellow wavelengths are commonly used for the iris, angle structures, retinal pigment epithelium, and neovascular membranes. Blue light should not be used in the macular area because lutein absorbs it strongly and can damage the retinal neural epithelium. Red and infrared wavelengths, although they mainly rely on melanin absorption, can penetrate thin hemorrhages to reach the choroid and retinal pigment epithelium; they are not absorbed by lutein and scatter less, so they are often used when the refractive media are not clear, when there is thin retinal hemorrhage, or for macular tissue. However, these wavelengths perform poorly in depigmented areas and, because of their high penetration, can more easily damage deep fundus structures. Ultraviolet light with wavelengths shorter than 295 nm is mostly absorbed by the cornea and cannot reach intraocular tissues, so it is currently used only for corneal surgery.
2. Mechanisms of Laser Treatment
When laser energy is absorbed by ocular tissues, a series of physical, chemical, and mechanical changes occur; these effects form the basis of laser therapy in ophthalmology.
1) Photothermal Effects
Photothermal effect refers to the conversion of absorbed laser energy into heat by biological tissues. It is the most common mechanism in laser treatment of eye disease. Depending on the local thermal response level, thermal effects include hyperthermia, coagulation, vaporization, perforation, and cutting. Factors influencing the tissue response include laser power density, the tissue's absorption at the given wavelength, and the duration of exposure. Photothermal effects can also induce secondary physicochemical reactions such as pressure changes and chemical alterations.
2) Photochemical Effects
Photochemical effects occur when absorbed laser energy is converted into chemical energy, leading to chemical reactions. Main types include photolysis, photo-oxidation, photopolymerization, and photosensitization. In ophthalmic practice, photolysis and photosensitization are commonly encountered. An example of photolysis is the 193 nm ArF excimer laser used as a "cold scalpel" to break molecular bonds for corneal photoablation. A typical example of photosensitization is photodynamic therapy used to treat retinoblastoma.
3) Electromagnetic Field Effects
Light is an oscillating electromagnetic wave. Biological effects caused by interactions between tissues and electromagnetic fields in the optical range are referred to as electromagnetic field effects of light, primarily related to strong electric fields. For ordinary light, power density is low and electric field bioeffects are negligible. However, laser energy can be highly concentrated spatially, and techniques such as Q-switching and mode-locking can also concentrate it temporally, producing significant electric field intensities and observable biological effects.
4) Photomechanical (Photopressure) Effects
Lasers at certain power densities can produce photomechanical or photopressure effects. These pressures can arise from multiple mechanisms, such as radiation pressure, recoil from thermally induced vaporization, thermal expansion, expansion-induced ultrasound, field-induced scattering, and field-induced strain. These photopressure effects can act on ocular structures to produce biological responses.
5) Vaporization, Cutting, and Perforation
When high-power-density continuous-wave lasers are absorbed by biological tissue, heating occurs. If temperature reaches around 100°C, tissues with 60%–80% water content begin to boil and generate steam pressure. Because the tissue surface can be sealed like a pressure cooker, continued absorption of laser energy rapidly increases internal temperature and pressure until steam ruptures the surface and ejects material, with tissue fragments carried out by the vapor stream.
In clinical terms, "vaporization" typically refers to surface ablation of lesions or proliferative tissue. Linear vaporization is considered cutting, while point vaporization is considered perforation. For tissues that absorb the applied energy, the depth of vaporization depends on the laser irradiation time and power density.
