The tremendous scientific and medical advancements made possible by laser technology have come with some risks. Just as laser energy has been harnessed for therapeutic purposes, so can it seriously damage the human eye. “The eye is particularly vulnerable to lasers because the radiation is emitted in a well-collimated beam,” said Bruce E. Stuck, director of the U.S. Army Medical Research Detachment of the Walter Reed Army Institute of Research at Brooks Air Force Base in Texas. “When the beam intersects the eye, the light is transmitted through the clear ocular media and imaged onto a very small spot on the sensory retina.” In a mere moment, tissue necrosis and resulting visual impairment can occur.
Opportunity for exposure. Lasers are seen in three specific settings: community, industrial and military.
Community-acquired injuries typically are the least severe because commercially available lasers are not usually potent enough to cause serious or long-term damage. “Although different people have varying susceptibilities, the majority of these exposures tend to be to an eye-safe laser for a very short period of time and an injury does not result. The event itself can be quite disconcerting to the patient, though,” said David K. Scales, MD, former chief of vitreoretinal surgery for the Air Force and now in private practice in San Antonio.
Conversely, industrial personnel tend to sustain the most severe injuries. These are due to high-density exposures, often resulting from a failure to wear the required protective eyewear.
Laser trauma incurred in an industrial setting is probably underreported, Mr. Stuck said. People tend to suppress exposure incidents for a variety of reasons. A worker in a facility where protective eyewear is required might not report an incident because his failure to comply with regulations may be an actionable offense, Mr. Stuck added. “Sometimes we do not hear about these incidents and accidents because they are not life-threatening.”
Mechanism of Trauma
For lasers emitting wavelengths in the “retinal hazard spectral region,” approximately 400 to 1,400 nm, damage is characteristically confined to the retina. In such cases, radiation is transmitted through the cornea, aqueous humor, lens and vitreous humor and is imaged or focused to a small area on the retina. The severity of the injury is dependent on the exposure dose. In contrast, shorter ultraviolet laser radiation and longer infrared laser radiation are both absorbed by the cornea. And in some of the transitional spectral regions, the lens and the iris are at risk, said Mr. Stuck.
A spectrum of damage. “Injuries are usually encountered by an ophthalmologist in one of two extreme conditions—either shortly after a known high-density exposure, or considerably later, as artifacts of an exposure, which generally prove much less conclusive. We can still determine whether damage exists, but calculating when the damage occurred can be challenging,” Dr. Scales said. He added that laser trauma contrasts with thermal burns or mechanical injury, which disperse energy in a shockwave formation and create visible tears in ocular structures.
Many people exposed to laser energy only experience temporary problems, like short-term flash blinding, or dazzle. Others may take some time to realize that they were exposed or need prompting to recall the exposure. “An ophthalmologist can conduct an examination and observe pigmentary disturbances in the fovea, but the patient may not connect an exposure to the visual disturbance. When queried about a laser exposure, some will occasionally remember, ‘Oh yeah, I was working with lasers or a high-intensity light,’” said Dr. Scales. These mild forms of exposure usually cause headaches and photophobia.
Colonel Henry D. Hacker, MD, chief of ophthalmology research at the Walter Reed Army Institute of Research, has encountered a variety of cases that range from minor dazzle to more serious injuries in which a “spot” in the patient’s vision develops and persists. In more severe cases, the exposed person may experience sudden vision loss followed by improvement over several weeks. Serious complications may still occur during this time so patients with a confirmed exposure should be closely monitored, Dr. Hacker said.
Because a person may focus each eye independently when intersecting a collimated light bundle, injuries can be either bilateral or unilateral. “It all depends on how the person interacts with the source,” said Dr. Hacker. “Sometimes people are taught, if they work in dark environments, to keep one eye closed, intentionally keeping it dark-adapted. In these cases, only one eye is exposed. Similarly, just because a person incurs a bilateral injury, it does not always mean that the injuries are equal. Some people have stronger vision in their left or right eye. It also depends on how they are oriented when exposed. Even a couple of fractions of a degree in fixation seem to make a difference. Exposure duration also plays a critical role.”
For making a concise diagnosis, there are few informational resources available outside military medicine, which has devoted considerable time to the understanding of lasers and their mechanisms of injury. “It is very difficult to tease out the signs and symptoms of laser exposure,” explained Dr. Scales. “If a patient has clearly had an exposure, such as in an industrial setting, there may be some defined parameters to work with, for example, laser type and intensity. For others, who may have been briefly flashed by a low-powered laser, it can be more difficult to determine the exposure distance or power density.”
Often, there is no injury at all—there has merely been a laser exposure. Indeed, according to Mr. Stuck, “It is common for people to see a bright flash of light and think that they are injured when they really are not. The ophthalmologist has to be somewhat leery of what caused the injury. Was it caused by a laser? Or are you observing a visual anomaly that has been there all along? I recommend referring these patients to an ophthalmologist who has experience with this type of injury.”
Complicating matters further are the differences between community-acquired and battlefield injuries. “When a patient receives a laser injury on the battlefield, it looks nothing like what we encounter in the clinic. The exposures are typically irregular and scattered in a haphazard fashion. Because of the normal fixation reflex, injuries frequently appear in the fovea, but they are sometimes quite a bit off center. This can make it difficult to determine the exact cause of the injury,” said Dr. Hacker.
Factors that affect the extent of laser eye injury include:
- Pupil size
- RPE and choroidal pigmentation
- Poor retinal or choroidal circulation
- Presence of cataract
- Power and density of laser
- Duration, dose and distance from source of exposure
- Use of protective eyewear
- Enhancement by reflective media, such as mirrored surfaces
Evaluating the damage. Several tests can be used for diagnosing laser injuries, said Dr. Hacker. “If there is an indication that the patient is having trouble seeing an Amsler grid, we know that there is a more serious underlying problem. This can be as slight as an arc in the retina, which is sometimes only visible with instruments like OCT or fluorescein angiography.”
Even these may not be sensitive enough for diagnosing patients with minimal damage, Dr. Scales said. “These macro-tests only indicate damage incurred above a certain threshold. As these technologies evolve, we will be better equipped to differentiate these variables and make a more accurate diagnosis.”
Tailoring treatment. Evidence indicates some benefit to treating the injury as soon as possible after exposure, but determining the best treatment is not so straightforward. Dr. Scales primarily uses oral or IV steroids for severe or acute laser injuries, particularly YAG injuries. “There is no standard approach for treatment. Some studies indicate that steroids might be counterproductive and nonsteroidal anti-inflammatory drugs may be the best alternative,” he said.
Patients who develop scarring or hemorrhaging may respond to surgery to remove scar tissue or blood. “They may develop neovascular membrane complexes and therefore some of the current treatments for neovascularization may be helpful—for example, VEGF inhibitors and photodynamic therapy,” said Dr. Hacker. “In the future, tissue regeneration may be possible.”
The Ethics of Lasers in Armed Conflicts
Lasers are ubiquitous in military applications, serving in targeting and guidance systems, fire-control devices, access-denial systems and communications security. Although the use of lasers as a weapon is a violation of the Geneva Convention, the potential for nefarious use continues to attract international concern. “In military applications, just a few microjoules of laser through the pupils in a 10 to 30 nanosecond pulse can produce a visible lesion. At 150 to 300 microjoules, a small retinal hemorrhage can occur,” said Mr. Stuck. This type of damage can have a devastating effect on a pilot’s vision.
In 1995, Mr. Stuck was involved in an international conventional weapons conference that resulted in the “Protocol on Blinding Laser Weapons.” Under this protocol, to which the United States is a signatory, four articles delineate the parameters for the use of lasers in military maneuvers and war. The employment of lasers solely to cause permanent blindness—or a resulting visual acuity of 20/200—is strictly prohibited in the protocol.