May 23, 2012
An increasing number of veterinary practices are using class IV therapy lasers to help patients with conditions ranging from ear infections to anal glands, and acute injuries to chronic hip dysplasia. Laser therapy is a scientifically based modality1 that helps reduce pain and inflammation while increasing blood flow2 and enhancing the rate and quality of tissue healing.3
Lasers are classified by the potential for hazard to the eye, with class IV being the highest. There are complicated definitions for the specifications of each laser class,4 but generally a laser with power greater than half a watt is a class IV laser. Several companies market class IV therapy lasers in the veterinary industry.
The therapy laser used to treat Tails the dachshund for intervertebral disc disease5 and the airborne military laser that shot down a ballistic missile6 are both class IV lasers, which has led to many misconceptions. Some report that the former can and will cause the same degree of thermal damage as a surgical or industrial laser.
Class IV therapy lasers deliver more energy per unit of time, but are they safe? This article addresses laser safety concerns for class IV therapy lasers in veterinary practice.
Laser safety guidelines must be followed due to the risk of eye injury and, secondarily, skin injury. Exposure to direct or reflected laser light can be focused by the lens of the eye, potentially causing damage to the retina and resulting in scotoma, a blind spot in the fovea. Light incident on the lens is magnified more than 100,000 times by the lens.
Visible light has wavelengths between 400 and 700nm (Figure 1), and class IV therapy lasers use wavelengths near infrared, between 800 and 980nm. Infrared light is invisible to the human eye but can be visualized using a digital camera (Figure 2). The beam from a surgical laser is tightly collimated, whereas the beam from a therapy laser is divergent. Wavelengths from 400 to 1,400nm are focused by the cornea and lens and absorbed by the retina.
Humans and animals have innate protection against exposure to bright light. The blink reflex is “lid closure associated with the involuntary upward movement of the eye, triggered by an external event such as a bright flash of light7” and the aversion response is “movement of the eye, eyelid or the head to avoid an exposure to a bright light.”8 It can occur in a quarter of a second, which includes the blink reflex time.
Because class IV therapy lasers use invisible infrared wavelengths, they do not trigger the blink reflex or aversion response. Therefore, class IV therapy laser companies must supply the appropriate eye protection, which clinics need to use appropriately.
The maximum permissible exposure (MPE) is “the level of laser radiation to which a person may be exposed without hazardous effects … in the eye or skin.”9 In the MPE calculation, the worst-case scenario is assumed, in which the eye lens focuses the light into the smallest possible spot size on the retina for the particular wavelength and the pupil is fully open. Exposure to direct or reflected laser light above the MPE can result in injury.
The nominal hazard zone (NHZ) is “the space within which the level of the direct, reflected or scattered radiation during normal operation exceeds the applicable MPE.”10 The NHZ for most class IV therapy lasers is about 20 feet. All people within the NHZ must wear appropriate eye protection when the laser is in operation.
Eye protection must be supplied by the laser company, as normal sunglasses do not provide adequate protection. Laser safety eyewear is marked with the optical density (OD), a measure of the ability to block out specific wavelengths of laser light to a safe level below the MPE.11 Laser manufacturers and distributors should provide information on the MPE, NHZ and OD in the safety manual supplied with the laser equipment.
Because pets are unlikely to cooperate by wearing safety glasses, a dark towel can shield the patient’s eyes. Some companies sell special goggles for dogs, but the specifications must be inspected to be sure they block infrared light. However, in most cases the animal is not able to look directly at the area of laser application and will not have direct exposure to the therapeutic laser light (Figure 3).
A small amount of infrared laser light does reflect back to the laser technician, but it does not have any harmful effects on the technician’s skin. Clothing will block reflected therapy laser light, so it is safe for a pregnant technician to apply class IV laser therapy treatments.
Class IV therapy lasers do not emit ionizing radiation. Dosimetry badges, such as those for exposure to X-rays, are not required.
Clinics must designate a laser safety officer (LSO), with training supplied by the laser therapy company. The LSO is responsible for monitoring and overseeing laser safety hazards.12 Most important are ensuring proper use of safety glasses and posting of the laser in-use sign.
To summarize, an increasing number of veterinary practices implement class IV laser therapy as an effective modality for pain management and injury healing. They can deliver a therapeutic dosage in less time, but also carry an increased risk of eye injury. These risks are orders of magnitude lower than for surgical lasers. Proper handling and use of laser safety eyewear will ensure safety for all.
Dr. Harrington is manager of education and clinical support for K-LaserUSA. He has a degree in physics from Iowa State University and is a Certified Medical Laser Safety Officer (CMLSO).
This article was underwritten by K-Laser USA of Franklin, Tenn. Readers with questions about K-Laser’s products are welcome to contact Dr. Harrington at 866-595-7749, ext. 104, or firstname.lastname@example.org
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