Because of its special optical properties, the human eye is considered to be most vulnerable to laser light. Apart from the oral mucosa, the only living tissue exposed to the environment is the cornea and conjunctiva. Without the comparative protective features of the stratum corneum of the skin, the eye is exposed to the harsh environment of the sun, wind, dust, ultraviolet radiation, and intense light. The eye has a natural protective mechanism in its aversion response, which limits the retinal exposure to very intense visible light by papillary constriction and eye movement or by the lid reflex and eye movement, which in turn limits exposures from intense infrared radiant energy that raise the temperature of the cornea. However, some laser-beam intensities are so great that injury can occur faster than the protective action of the aversion response, which occurs within 0.25 seconds [9].
Obviously, laser energy cannot damage the tissue, unless the light energy is able to penetrate and be absorbed in that structure. For this reason, rays in the visible and near infrared (visible and IR-A band), which can be transmitted through the clear ocular media and be absorbed in the retina, can, in sufficient intensity, damage the retina. The actual size of such a “point” at the retina is of the order of 10-20 pm (smaller than the diameter of a human hair). For this reason, lasers operating between 400 and 1400 nm are particularly dangerous to the retina. This spectral region is often referred to as the “retinal – hazard region”, since the increased concentration of light after entering the eye and focused on the retina is of the order of 100,000-fold! Hence, a collimated beam of 1 W/cm2 at the cornea will focus to a small spot with an irradiance of 100 kW/cm2. Although damage to such a small region of the retina may seem insignificant initially, it is important to realize that certain parts of the retina, for example, the central retina, the macula, and its fovea (center of the macula), are extremely small areas responsible for critically important high-acuity vision. If these areas are damaged by laser radiation, substantial loss of vision can result. The argon, krypton, KTP, helium-neon (He-Ne), ruby, alexandrite, diode and neodymium:YAG, all emit in the retinal-hazard spectral region (~400 to ~ 1400 nm). The concentration of energy at the retina, make these lasers particularly hazardous to the eye, as shown in Fig. 24.1.
The retinal image area alone may not be the only site of damage, but as a result of heat flow and mechanical (acoustic) transients, the tissue surrounding the image site may also be damaged, leading to more severe consequences on visual function. For example, it has not been uncommon for an individual to lose almost the total function in an eye exposed to a very small amount of energy (several hundred microjoules) when a short-pulse (Q-switched) laser that has been accidentally imaged on the fovea. Instead of a normal visual acuity of 6/6 (20/20 in the United States), the visual acuity in such accidental situations has often been recorded as 6/60 (20/200) following an accident. Fortunately, in most
accidents, only one eye is exposed to a collimated beam. However, after some recovery, any visual loss remaining after 60 days is generally permanent, since the neural tissue of the retinal has very little ability for repair [1,14,15].
At wavelengths outside of the retinal hazard region—in both the ultraviolet and far-infrared regions of the spectrum—injury to the anterior segment of the eye is possible. Certain spectral bands may injure the lens (notable at wavelengths between 295 and 320 nm in the ultraviolet region and wavelengths between 1 and 2 pm in the infrared region may actually pose a greater risk for permanent injury than the 10.6 pm CO2 laser wavelength), which does not penetrate the cornea [15]. See Fig. 24.2 for hazards by spectral region.
Excimer lasers operating in the ultraviolet spectral region are no longer unusual in some special applications. Certain excimer lasers pose a particular hazard to the cornea, and the 308-nm Xe-Cl excimer laser can be considered additionally dangerous, as it can produce an immediate cataract of the lens. By contrast, the 193-nm ArF excimer laser wavelength used in laser refractive surgery cannot ever penetrate deep into a single cell; hence the biological consequences of scattered radiation are not at all serious, even if the conservative MPE were to be exceeded by hundredfold at this extremely short wavelength; hazard zones are therefore only a few centimeter from the ablated cornea [12]. The surface cells (wing cells) of the cornea have an average lifetime of only 48 hours, and are quickly sloughed off after being damaged, thus leading to no sequellae [13]. The holmium: YAG, hydrogen – fluoride, carbon dioxide, and carbon monoxide lasers are all potentially hazardous to the
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Figure 24.2 Hazards summarized by CIE spectral region. The CIE is the International Commission on Illumination. |
cornea, but because wavelengths that cause corneal damage are not reconcentrated by the eyes as are wavelengths in the retinal hazard region, the thresholds for injury of the cornea are generally much higher than those that may injure the retina. Table 24.1 lists permissible occupational exposure limits for many of the commonly used dermatological lasers [2-8].