Researchers shine light on high-intensity laser therapy in horses

An example of a thermographic image of the side of a right non-pigmented hindlimb fetlock joint taken after high-intensity laser therapy. The rectangular area (R1) indicates the average surface temperature of 31.7 °C.
An example of a thermographic image of the side of a right non-pigmented hindlimb fetlock joint taken after high-intensity laser therapy. The rectangular area (R1) indicates the average surface temperature of 31.7 °C. Image: Zielińska et al.

Skin pigmentation should be considered when using high-intensity laser therapy (HILT) to treat horse injuries, study findings suggest.

In human medicine, HILT is successfully used in the rehabilitation of many orthopedic injuries and disorders. In equine veterinary medicine, it is a relatively new treatment.

Only a few studies have evaluated its impact in the treatment of orthopaedic disorders in horses, mainly focused on tendon injuries, Paulina Zielińska and her fellow researchers noted in the journal, Animals.

They said promising results have been reported in their previous randomized controlled non-blind study using HILT, which focused on 26 horses with tendon and ligament injuries.

The researchers noted that little is known about the differences in the impact of HILT performed on pigmented and non-pigmented skin.

They set out to assess differences in the influence of HILT on skin surface temperature and vein diameter in a group of healthy racehorses with pigmented and non-pigmented skin in the treatment area — the side of the fetlock joint.

The hypothesis was that HILT would cause a greater increase in skin surface temperature and vein diameter in horses with pigmented skin compared to non-pigmented skin.

Ten thoroughbreds with pigmented skin and 10 with non-pigmented skin in the treatment area received HILT.

Changes in the vein diameter and skin surface temperature of the irradiated area were measured before and after HILT through a thermographic examination using a high-resolution infrared camera. Ultrasound technology was used to assess the diameter of the digital palmar vein before and after treatment.

The study team found that the HILT treatment caused an increase in the pigmented skin surface temperature and a decrease in the non-pigmented skin surface temperature.

The vein diameter increased after HILT in horses in both groups, but the difference was not significant.

“In conclusion, melanin content in the epidermis plays an important role in light energy absorption and photothermal effects,” they said. “The vein diameter changes after HILT application indicated that the increase in vessel diameter may partly depend on photothermal mechanisms occurring in irradiated tissue.”

They said further research is necessary to describe the physiological and clinical effects of HILT performed on pigmented and non-pigmented skin to help clinicians choose appropriate HILT parameters.

Discussing their findings, the authors said the reaction of the skin to laser radiation depends on many physical and chemical factors. Among them are skin color and the amount of melanin present. Melanin absorbs the visible and near-infrared part of the optical spectrum, and the absorbed energy can be transformed into heat, resulting in photothermal effects.

They said the decrease in skin surface temperature in the non-pigmented skin group was probably caused by the skin cooling during the experiment after both the shaving of the treatment area and the use of coupling ultrasound gel.

“The cooling effect likely took place in both groups and continued for longer than the time it took us to perform HILT therapy and all measurements.”

They said it is important to predict the risk of thermal exposure to tissue when a treatment program is designed using thermal technology.

None of the horses from either group showed discomfort or pain behavior during therapy, experienced skin burns, or developed swelling in the treatment area.

“Therefore, HILT performed on healthy tissue was classified as a safe procedure, which was well tolerated by the horses. However, the present study lacks long-term follow-up data, which reduces the clinical application of our findings to the assessment of the short-term effects of HILT.

“It would be beneficial to measure the skin surface temperature and vein diameter changes for a longer time after HILT treatment.

“Furthermore, we tested only healthy horse tissue as opposed to that of patients. Future research should investigate the long-term effects and benefits of HILT in horses with musculoskeletal diseases and injuries.”

The study team comprised Zielińska, Maria Soroko Maria Godlewska and Weronika Hildebrand, with the Wroclaw University of Environmental and Life Sciences in Poland; Kevin Howell, with Microvascular Diagnostics at the Institute of Immunity and Transplantation at the Royal Free Hospital in London; and Krzysztof Dudek, with the Wroclaw University of Technology.

Zielińska, P.; Soroko, M.; Howell, K.; Godlewska, M.; Hildebrand, W.; Dudek, K. Comparison of the Effect of High-Intensity Laser Therapy (HILT) on Skin Surface Temperature and Vein Diameter in Pigmented and Non-Pigmented Skin in Healthy Racehorses. Animals 2021, 11, 1965.

The study, published under a Creative Commons License, can be read here

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