Therapy for the Sore Performance Horse

by Tracy A. Turner, DVM, MS, Diplomate ACVS

Physical therapy for horses may be divided into therapeutic techniques and therapeutic modalities. Therapeutic techniques involve the use of acupuncture, chiropracty, and massage. Therapeutic modalities would involve the use of heat and cold, sweats, poultices and braces, laser therapy, magnetic therapy, therapeutic ultrasound, and electrical stimulation. This article will focus on therapeutic modalities.

Physical therapy may be defined as the healing of tissues through the stimulation of physiologic processes. Therapeutic modalities utilize mechanical means to achieve this goal. Among the more commonly used types of therapy are cold, heat, magnetic therapy, therapeutic ultrasound, electrical stimulation, and cold laser.

The application of cold is used to retard swelling and prevent hematoma formation by causing capillary constriction and reducing blood flow. Cold also decreases tissue metabolism and is thought to have some analgesic affect. However, if cold is applied long enough to decrease sympathetic nerve transmission then vasodilation may occur. This phenomenon is known as the Hunting Reaction. Cold is most often applied to acute injuries, within the first 24 to 48 hours after trauma and is used to decrease the acute inflammatory process. There are numerous methods to apply the cold to the limb. These methods vary from cold water hosing, standing in ice buckets, frozen gel wraps, and as elaborate as a refrigeration apparatus. When applied to the lower limb of a horse the most profound and sustained cooling effect is on the skin overlying the dorsal metacarpus (metatarsus). This "cooling" effect has been shown to be sustained for 3 hours. The duration of this effect is longer than some authors have previously speculated. The reflex vasodilation that has been described has not been shown to occur in the lower leg of the horse. Because of the prolonged cold effect on lower limb temperature, the immediacy of application of a compression bandage is reduced. In fact, the bandage's insulating effect may tend to counteract the cold effect. A secondary effect of cold therapy is analgesia, although the duration of this effect has not been studied, one might expect the effect to persist as long as the "cooling" effect.

Heat can be applied in a variety of ways but the effects are the same. The main effect is to increase tissue temperature which causes vasodilation. The increase in circulation causes the number of phagocytes to increase and provides a higher level of oxygen at the site. In addition, the local metabolic rate is increased as is the lymph flow. A disadvantage is that there is an increase in capillary permeability which can cause increased edema or may lead to increased toxin absorption. Reflex vasodilation of areas distal to the site of heat application has been noted. Heat is used in the post acute phase of injury (48 hours past) to cause resorption of swelling and increase healing. The duration of action of heat on the limb is unknown but in one study 30 minutes of heat application caused heating of the limb which lasted for more than 4 hours. Heat, like cold, can be applied in a variety of ways. Mild to moderate edema was the most common adverse effect associated with heat application to the lower leg of a horse. An interesting effect that has been noted during the application of heat is a secondary vasodilation that occurs 30-45 minutes after the removal of the heat source. The cause of this effect is unknown but this is much like the reflex vasodilation that has been hypothesized to occur after the application of cold. It may be due to sufficient relaxation of the muscular coat of the arteries and arterioles that the vessels cannot constrict. Because of this effect, it would seem prudent to apply a compressive wrap to a heat treated leg. The wrap would help prevent edema and because of the insulating effects would help prevent loss of heat from the treated area.

Magnetic therapy has been used to stimulate healing of soft tissue and osseous injuries. Pulsating electromagnetic fields will change metabolic processes. The magnets are made of a variety of different materials. The magnetism produces eddy currents in blood vessels to produce localized warming and dilation of blood vessels which increases the circulation to the injured area. These devices have been recommended to reduce recuperation time for lower leg injuries in the horse such as tendonitis. Research results regarding the effects of magnets on the horse have been mixed. Only one report supports the use of magnets. In this study, scintigraphy was used to evaluate changes in circulation. The investigators did show an increase in the blood flow to the area treated but they failed to control for bandaging effects. In a similar study where thermography was used to evaluate blood flow the investigators showed there was no difference between magnet treated and the untreated group when the effects of bandaging were controlled. The increased heat noted in the magnet and placebo treated limbs was thought to be due to the insulating effects of a wrap. Heat loss from convection, conduction, and radiation cannot readily occur while the limb is covered. In the latter study, after wrap removal the temperature increased probably due to lack of impedance of blood flow. The temperature then decreases as the heat loss mechanisms previously mentioned begin to exert their effects.

Ultrasound is sound waves at a frequency (250-8000 Hz) beyond the range of human hearing. The compression and rarefaction of tissues, produced by the alternating pressure of the vibrating disc, causes the individual cells to act like oscillating particles. The vibration of these cells produces a local deep heating effect. This leads to vasodilation with a subsequent increase in the rate of nutrient transfer, an increase in the numbers of fibroblasts and an increase in the removal of undesirable products. In addition, therapeutic ultrasound alters connective tissue extensibility, causes fibroblast activation, stimulates protein synthesis and decreases neuronal conductivity. Therapeutic ultrasound has been reported to be beneficial in increasing the rate of healing of skin, muscle, ligaments, tendons and bones. In one study, therapeutic ultrasound caused an increase in skin temperature at all wattages and the effect was sustained for more than 1 hour. This study supports the concept that ultrasound has a tissue heating effect which stimulates vasodilation. In the aforementioned study, increasing the intensity of the ultrasound increased the heating effect. Treatment at 0.5 watts/cm2 had the least profound effect but it was sustained for over 2 hours. Treatment with 1.0 watts/cm2 had the most sustained effect lasting more than 4 hours. The use of high intensity (1.5 watts/cm2) increased skin temperature 0.8oC but the tissues cooled below the untreated skin temperature within 2 hours. This drop in temperature may be due an increase loss of temperature through convection, conduction and radiation. In order to prevent this effect, and enhance the effects of the ultrasound, it may be beneficial to wrap the leg after treatment.

Electrical stimulation has been shown to increase the blood flow within muscles as well as blood to non-contractile tissues. When applied to muscles the stimulation causes rhythmic muscle contractions causing a pumping action. Also electrical stimulation causes an increase in capillary density. When used over a 14 to 28 day period increases in capillary density from 50 to 100 percent have occurred. In non-contractile tissues the electrodes should be placed over large arteries and stimulation between 8 to 32 hertz will cause maximal increases in blood flow.

The use of therapeutic or cold lasers is highly controversial. There are two types of therapeutic lasers, the gallium arsenide and the helium-neon. They differ in the light emitted and the reputed depth of penetration. Most support for this therapy is anecdotal, most research has not been able to confirm any of the therapeutic claims. One area that laser might be effective is in stimulating acupuncture points. At least one study showed that therapeutic laser light did have an effect on acupuncture points.

Extracorporeal shock wave therapy (ESWT) is increasingly being used by veterinarians to treat several musculoskeletal conditions in horses, including suspensory ligament desmitis and osteoarthritis of the joints. ESWT is also used to treat dorsal metacarpal disease or bucked shins, which occurs commonly during training of Thoroughbred racehorses. Despite the amount of information supporting the efficacy of ESWT in clinical trials, there is not a complete understanding of the mechanism of action of this new treatment on equine bone and its surrounding tissues. Drawn from results seen in laboratory animals, it has been suggested that the clinical improvement seen in horses treated with ESWT may be due to an increase in bone remodeling. However, we have shown that ESWT will not change the bone. Other theories elate o ESWT’s effect on mediators and nerve conduction. To date, no one knows how ESWT works. Therefore, it is difficult to know which cases are best to use it on.

The use of these therapeutic modalities increases the effectiveness of any therapeutic regime. As a result, these modalities can be very useful in the treatment and rehabilitation of the horse for many musculoskeletal problems.