Due to the special anatomy and special physiological relationship of the eyeball, the laser damage to the retina is closely related to the angle of the incident eye. The reason is that the eyeball itself is a concentrating lens system. When the injected laser beam enters the eye parallel to the visual axis, it focuses on a small spot in the fovea of the fundus macular area, and its energy density is 3-4 times higher than that of the cornea. . The fovea of the macula is the most sensitive and important area of visual function of the eye. Once the damaged visual function changes, it will change to varying degrees. Serious people will be blind for life. Because the vision is damaged by the laser photon, the photoreceptor cells are coagulated and denatured, causing irreversible damage.
During the day, the color vision of the human eye is completely obtained by the photosensitivity of the macula. The area of the macula is only a small part of the total area of the retina, and the diameter of the fovea is only about 0.5 mm. However, the reflected field of vision (the total area that the eye can see clearly in front of it) accounts for a large proportion. In the physiological structure, there are 20,000 to 30,000 long and thin cone photoreceptor cells in the fovea of the macula. The photoreceptor cells have a high density of distribution and are mainly responsible for visual function. The visual function of the white ochre is lost after being damaged. Furthermore, there is no blood vessel and nerve distribution in the central recess of the macula of the retina, so the heat diffusion function of this part is very poor. Once the injury is made, the hope of re-repair is very blind. The fovea is the weakest part of the retina and is more susceptible to damage than other parts of the retina after being exposed to laser light, so direct laser exposure is very dangerous.
When the laser is incident on the eye slightly away from the visual axis angle, the focused spot does not fall on the macular area and falls on the retina on its periphery. Therefore, the angle of incidence is different, the damage is different, even if the energy entering the eye is exactly the same as when it is in direct sunlight, the damage caused is much lighter. The reason is that the photoreceptor cells outside the macula are much less densely distributed than the macular area, and the retina outside the macula is thicker and receives the same energy per unit area, and the temperature rises to a much lesser extent. In addition, microvessels are densely packed in the retina outside the macula, and part of the heat can be removed from the blood circulation, reducing the possibility of temperature rise. The higher the temperature, the heavier the damage; conversely, the smaller the temperature rise, the less likely the damage is. The main role of laser damage to the retina is caused by thermal effects.
The incident angle of the laser is not synchronized with the visual axis. The larger the deviation angle, the lighter the damage of the retina, and the iris can block the deviated laser without entering the fundus. Since the fovea in the macula is extremely important in the visual function, and this part is most vulnerable to damage, the degree of danger of directing the laser beam is much greater than the angle of the eye from the visual axis. avoid.
Fourth, the fundus pigment content and damage relationship
There is a specific relationship between the amount of pigment in the fundus and the degree of laser damage. Pigment tissue is very easy to absorb laser energy, so the amount of pigment directly affects the laser damage to the retina. The literature reports that the skin color of the body is positively correlated with the pigmentation of the fundus. The skin is black and heavy, and the amount of pigment contained in the fundus is also large. The skin color is white, and the amount of pigment in the fundus is relatively small. Therefore, the more the pigment content, the greater the absorption of the laser, and the greater the degree of damage. Eye tissue that absorbs energy beyond its own damage threshold will be harmed. The more you exceed, the more damage you receive.