The term "laser" originated as an acronym that describes its process, ie, light amplification of stimulated emission of radiation. Like acupuncture and massage, laser therapy may reduce pain, relax muscles, and improve circulation. It accomplishes this by local cellular and tissue effects by means of light (photons) versus an acupuncture needle or manual pressure. Therefore, treatment effectiveness and the types of responses seen depend heavily on if and how light enters the body.
Mechanisms of Action of Photomedicine in Veterinary Patients
For tissue to absorb light and alter its physiology, a photochemical or photobiologic event must occur. Ideally, this event would occur within the target tissue(s), whether it be skin, muscle, fascia, nerves, vessels, bones, joints or some combination thereof. A "photoacceptor" molecule, also known as a "chromophore," responds to light by initiating a series of physiologic responses that reportedly engender healing and improved tissue homeostasis. When a chromophore (such as cytochrome c oxidase in the mitochondrial respiratory chain) absorbs a photon from laser-treated tissue, this produces an excitatory response in electrons within the chromophore. This increased energy provides the impetus for cellular activities directed toward growth and repair.
The effects of laser on mitochondria, cells, and tissue is called "photobiomodulation." This collective process encompasses not only the effects of lasers but also those of light-emitting diodes (LEDs) and other light sources. Photobiomodulation entails changes at the subcellular, cellular, and tissular levels. Within the mitochondria, activated photons purportedly engender increases in production of ATP, modulation of reactive oxygen species, and induction of transcription factors. These factors apparently encourage cell proliferation and migration, normalized cytokine concentration, enhanced production of growth factors, modulated levels of inflammatory mediators, and improved oxygenation of tissue.
Light therapy also may cause vasodilation by relaxing endothelial smooth muscle, potentially via effects on nitric oxides. Vasodilation improves tissue oxygenation and supports the migration of immune cells into tissue, further aiding recovery.
Treatment Parameters of Photomedicine in Veterinary Patients
Many factors impact how light influences tissue, including its power, wavelength, frequency or pulsing, tissue contact, and the diameter of the beam.
The table Photomedicine Parameters offers an overview of some of the main considerations that are considered when devising treatment protocols for laser therapy only, because many LED therapy (LEDT) units do not have the same degree of customizability.
Photomedicine Parameters
Photomedicine Parameter | Description | Clinical Relevance |
|---|---|---|
Device power (watts) | The term "low level laser therapy" refers to the use of light at much lower levels than those used for tissue ablation or photocoagulation. Newer, high-powered therapy devices that deliver power similar to surgical lasers (but with a less concentrated beam) no longer constitute low level laser therapy. Class IV devices, for example, routinely heat tissue, rendering the term "cold laser" inaccurate. | Photomedicine units that produce more intense beams reduce the time required to deliver treatment. That said, ideal "dosing" of light for specific clinical conditions (described below) remains controversial, considering that a wide range of photomedicine treatment applications have demonstrated benefit. In general, the advantage of more intense beam strength relates to stronger analgesia, whereas less intense light with a longer period of application offers better tissue healing results. |
Spot size (cm2) | A laser's power, combined with spot size, determines power density (see below). The spot size amounts to the area to which the photons affect the target. A small spot size cones down the light to a concentrated area, whereas a larger spot size spreads it out, reducing the intensity per cm2 As such, two devices or applicator heads that emit light with dramatically different spot sizes can change treatment characteristics and temperature elevation even if the power is identical. | Typically, the spot size of an applicator head does not change, meaning that the user must be cognizant of the clinical significance of a small or large spot size in terms of how it affects the delivery of light. That is, if treating a broad expanse of tissue, a device with a larger spot size (eg, 1 cm2) will accomplish the desired outcome more efficiently than a device with a much smaller spot size. Furthermore, delivering a large amount of light to a tiny area increases the risk of excessive heating and tissue burn. |
Power density (W/cm2), or "dose" (J/cm2) | Power density describes the amount of photons directed to a site, which in turn provides information about the energy and heat delivered to a treatment target. Power densities usually range from 10–100 mW/cm2, and energy densities range from 4–50 J/cm2. Some practitioners use higher power and energy densities to attempt to treat deeper joint, spinal, or brain problems. | The specific dose(s) of laser required to heal tissue and treat pain remains unclear. Calculating actual joules of energy delivered is considerably complex and, ultimately, uncertain. Fortunately, a wide range of doses has shown benefit for human patients and experimental animals, despite the wide variety in size, color, and hair coat. |
Wavelength (nm) | Therapeutic photomedicine devices typically use red (630–680 nm) or near infrared (700–1100 nm) light, although lasers and LED units that emit visible purple, blue, and green light, with shorter wavelengths, have also become available. Some laser therapy units emit 2 or more beams to target multiple tissues and varying depths. Laser beams differ from other types of light therapy, including LEDs, as they are monochromatic (existing within a narrow band of wavelengths), coherent (tightly aligned), and collimated (photons travel in parallel). The more light scatters within tissue, the less intense the effect, which may or may not be the desired outcome. However, debate continues about the relative value and differences between laser light and LEDs. | The types and depth of tissue that respond to light therapy depend on the wavelength delivered. Certain molecules, such as melanin and hemoglobin, preferentially absorb light in the 630—670 nm interval. To reach deeper tissues, wavelengths (810 nm, 980 nm) in which photons proceed through superficial layers unabsorbed, reach deeper sites such as bone, nervous tissue, and internal organs. The ideal wavelength for photobiomodulation of nervous system tissue reportedly ranges from 810 nm to 830 nm. |
Pulsation (Hz) and pulse width | Photomedicine devices have frequency settings that can provide a "pulsed" treatment versus continuous wave. Pulsation frequency provides information about the on/off sequencing of light, whereas pulse width indicates how long the light is on versus off. Pulsing reduces the number of photons delivered per unit time and allows tissue to cool during the "off" periods. | The clinical benefits of both regular "pulsed" light delivery and of "super-pulsed" remain unclear. "Super-pulsed" photomedicine devices deliver high-powered light (25 W or greater) in nanosecond bursts. This may allow the number of photons to reach a clinically significant level, deep in the tissue, while offsetting the risk of thermal injury with the brief exposure, allowing cells to cool between bursts. Laser device manufacturers are increasingly introducing units with super-pulsing options. |
Indications for Photomedicine in Veterinary Patients
Photomedicine, whether with laser therapy or LEDT, has shown value for the following conditions:
musculoskeletal pain (post-surgical, sports injury, arthritis, fracture)
athletic performance issues
inflammation and edema (eg, mastitis, otitis)
arthritis, degenerative joint disease
wound healing (postoperative, acral lick dermatitis, fungal and bacterial infections, snake envenomation, burns, abscesses, skin graft)
tissue repair (many types, including mucosal, dermal, muscular, neural, dental, connective tissue, and osseous)
regenerative support
CNS injury and degeneration
peripheral nerve injury
neuroprotection
internal organ dysfunction
laser acupuncture applications
Contraindications for Photomedicine in Veterinary Patients
Laser therapy must avoid areas of neoplasia/malignancy, a hyperactive thyroid gland, areas of active hemorrhage, the retina, and a pregnant uterus. Proper eye protection guidelines should be followed at all times. Laser therapy is also contraindicated for patients with lymphoma or on immunosuppressant medications. In young patients, higher powered laser therapy devices may stimulate premature closure of epiphyses. Thus, caution is warranted over long bones in animals < 1 year old. For patients on photosensitizing pharmaceuticals or botanicals, treatment intensity should be lowered.
Adverse Effects of Photomedicine in Veterinary Patients
The main risks of laser therapy involve retinal damage and thermal burns when improperly applied. Laser light can damage the retina, whether reflected off shiny surfaces or shown directly into the eye. Laser goggles protect against indirect, but not direct, exposure, and the operator should never look into the applicator of a laser therapy device. Tattoos, when lasered, can cause intense pain due to the high amount of light absorption by deposited pigment. Questions remain about the ability of laser therapy to stimulate neoplastic growth and, if so, at what wavelength(s) and power(s).
Laser light longer than 760 nm is invisible to the human eye. As such, unless a treatment applicator produces a visible finder beam or audible signal, the practitioner will not know when the laser is emitting light. This could cause inadvertent eye exposure and retinal damage. It also could cause confusion about where the beam is pointing. For this reason, infrared laser devices without a finder beam pose potential safety hazards.
Drug Interactions of Photomedicine in Veterinary Patients
The immunostimulatory effects of laser therapy may counteract immunosuppressive actions of certain medications. In addition, photosensitizing agents such as hypericin in St. John's wort may augment the dermatologic impact of laser light.
Controversies of Photomedicine in Veterinary Patients
The main controversies surrounding laser therapy involve questions and unproven claims related to pulsed therapy, the advantage of low- as opposed to high-powered units, and the longterm safety of high-powered treatment, especially for patients with a history of cancer.
For More Information
Also see pet health content regarding complementary and alternative therapies for pain.
