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.

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, 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)

• alopecia

• regenerative support

• CNS injury and degeneration

• peripheral nerve injury

• neuroprotection

• internal organ dysfunction

• laser acupuncture applications

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.

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.

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.

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.

• Also see pet health content regarding complementary and alternative therapies for pain.