The term laser is an acronym for light amplification by stimulated emission of radiation. A laser device produces a light beam that, unlike ordinary light, is coherent, monochromatic, unidirectional, and minimally divergent. Consequently, such a device can direct most of its radiant power over very small areas, even at great distances. Light radiation in the visible and near-IR part of the electromagnetic spectrum is gathered and focused by the refractive media of the eye to a retinal image about 5 to 30 µm in diameter (Azari, 2008, pp.95-104). This focusing increases the retinal irradiance (energy per time unit per unit area) by a factor of well over 10,000 above the irradiance incident at the cornea. With the use of binoculars or other magnifying optics, which further collect incoming light, the increase in irradiance may reach more than a million-fold. Since no other body organ focuses light radiation in this way, the retina is the tissue most vulnerable to laser injury (Chang, 2006, pp.41-9). Another consequence of this concentration of energy is that even small amounts of energy produced by relatively low-power laser devices can significantly damage the retina. Such lasers are commonly used in laboratories and the military, and they emit energy in the order of tens of millijoules (pulsed lasers) or hundreds of milliwatts (continuous-wave lasers). Since higher-energy lasers are less commonly used, accidental injury of external eye structures or skin is rare. For these reasons, this review will mainly address laser injuries of the retina (Maegawa, 2000, pp.427-437).
Many studies have documented the energy required to produce retinal damage (Table). Most of them report the minimal, or threshold energy required to produce an ophthalmoscopically detectable lesion, typically a small, gray-white spot (Fig. 1). The threshold value is usually recorded as the ED50 for the total energy entering the eye, i.e., the energy level that will produce the lesion in 50% of cases (Christensen, 2006, pp.235-8). These values, with additional safety factors and extrapolations, form the basis for laser safety practices and standards by specifying the “maximum permissible exposure” (MPE) levels to which a human can be exposed without incurring injury (Randolph, 2002, pp.473-490). Other studies have reported energy correlates for suprathreshold injuries that lead to retinal hemorrhage, and for the so-called subthreshold injuries observable only by methods that are more sensitive than ophthalmoscopy, such as retinal histopathology and fluorescein angiography (Bienstman, 2001, pp.327-341).
Fig. 1. A series of threshold argon laser lesions
Class IV: Laser Damage Mechanism
New laser clinics will certainly appear in this unregulated environment (many existing clinics will also cease to be regulated) and we are very concerned that potentially dangerous treatments that require skill, knowledge and experience to deliver safely will become open to untrained persons. There are three separate mechanisms that are known to cause damage in the retina when exposed to light, and two others are suspected: photochemical, thermal, microcavitation, photomechanical and laser induced breakdown. These mechanisms are principally associated with the exposure duration , i.e. damage from a 1-s ultraviolet exposure is caused by a different mechanism than damage ...