Pain is a complex, highly evolved experience that acts as a warning system to prevent further tissue damage.
Management of pain is a careful but critical part of recovery from illness or injury. Excessive analgesia in the recovery process can increase the risk of injury. Alternatively, pain during the rehabilitation process may cause anxiety and panic, leading to sudden and unbalanced movement, further harming the rehabbing injury.
This can cause delays in recovery. There are many types of pain management strategies, and it is critical to understand that there is no “one-size-fits-all” for the equine athlete in a rehabilitation program. The thorough approach includes both medications and physical modalities. Judicious use of analgesic medications allows horses to recover more quickly without “masking” pain. Likewise, physical rehabilitation modalities improve function as well as offer analgesia. By combining these, we can see improvements while returning our equine patients to a healthy state.
ASSESSMENT OF PAIN
The diagnosis of pain comes from the measurement of parameters (heart rate, respiratory rate) and the identification of a catalog of behaviors (ethogram) associated with discomfort and pain in horses. Some discomfort behaviors are so obvious to us that we make the connection immediately, such as weight shifting or toe-touching when ambulating. Others are more subtle and are only readily seen when we train ourselves to look for them. It is important to be familiar with the various established equine ethograms for determining pain. This enables the practitioner to both identify pain in the equine patient and evaluate the success of pain management strategies. Several recent publications by Sue Dyson have highlighted the development of a ridden-horse pain ethogram, taking the pain scores a step further to delineate the subtle signs of pain that can be observed with low-grade lameness.
MEDICATIONS NON-STEROIDAL ANTI-INFLAMMATORIES (NSAIDS)
Inflammation is part of damage to soft tissues as well as osseous structures; therefore, it is only logical that anti-inflammatories are used in the rehabilitation plan. NSAIDs by far are the most common therapies used for analgesia in horses, and just how much and what type is a highly debated subject. Typical doses for phenylbutazone range between 2.2 mg/kg and 4.4 mg/ kg administered once or twice a day, and doses for flunixin meglumine range from 0.25 mg/kg IV every eight hours to 1.1 mg/kg IV every 12 hours.
Both NSAIDs are COX-1 and COX- 2 inhibitors and therefore have the possibilities of resulting in side effects often associated with COX-1 inhibition, including gastrointestinal ulceration and renal papillary necrosis, particularly when higher doses are used. The use of a COX-2- specific NSAID, firocoxib, would suggest that its use would have increased benefits over the traditional NSAIDs particularly in longer term use by having decreased gastrointestinal side effects while still being able to limit the up-regulation of COX-2.
Reported dosages are 0.1 mg/kg orally every 24 hours or 0.09 mg/kg IV every 24 hours. It has also been suggested by Merial that the first initial dosage be three times the recommended dose for the route of administration selected. There is no current evidence to support the intentional combining or “stacking” of NSAIDs in the management of pain in horses, and the adverse effects are well documented.
ALPHA-2 AGONISTS
Alpha-2 agonists decrease neuronal excitation by activating presynaptic and postsynaptic alpha-2 receptors in the descending inhibitory pain pathway centrally and in joints peripherally. Clinically used most for sedation, alpha-2 adrenoceptor agonists are also good analgesics, and in acute conditions supplement a multimodal approach to analgesia. Sole or chronic use is limited by sedative effects and ataxia.
GABAPENTIN
Gabapentin is an anticonvulsant drug that has shown promise in humans and small-animal patients as a treatment for neuropathic pain. Gabapentin is rapidly absorbed and has an elimination half-life of 3.4 hours in horses. Although the collective experience with this drug in horses it is limited, reports indicate there may be possible benefits.
PHYSICAL MODALITIES OF PAIN MANAGEMENT MASSAGE THERAPY
Although there are limited case-controlled data to support clinical observations of improvement in pain states following massage, studies have demonstrated an increased mechanical nociceptive threshold in the thoracolumbar region and improved stride lengths in walk and trot after massage. Tissue manipulation affects changes in neurologic signaling related to pain processing and motor control and upregulates signaling within large-diameter nerve fibers to provide inhibition of ascending nociceptive signals. Soft-tissue mobilization improves blood flow, improving tissue viability and so reduces pain associated with tissue damage. Release of endorphins and serotonin through these mechanisms may also modulate pain perception.
ACUPUNCTURE
Acupuncture is becoming a respected and widely used form of therapy, especially for pain management in both human and veterinary medicine. In conjunction with electrical stimuli (electroacupuncture), it produces analgesia through nonopioid means as well as through release of beta-endorphin into the cerebrospinal fluid. The effects are varied, and studies support endorphin release because the effects can be reversed with naloxone.
Differences in response between sexes have been shown. Navicular pain has resolved, and laminitis has been treated successfully.
The International Veterinary Acupuncture Society (IVAS) and the Chi Institute are two well-respected veterinary training organizations for those interested in learning more about this treatment modality.
CRYOTHERAPY
Ice packs, ice boots, ice-water circulating boots, cold hosing, and products such as the Game Ready (Game Ready, Concord, California), can deliver cold therapy, making cryotherapy more accessible. Cold therapy is most beneficial when applied immediately after injury and reapplied every two to four hours. Cold therapy provides analgesia to an area of injury by initiating local vasoconstriction, decreasing vascular permeability, reducing the influx of inflammatory markers to the area, depressing neuronal conductivity, and decreasing the metabolic rate, thereby reducing cell hypoxia and death. Cryotherapy has been demonstrated to reduce the activity of enzymatic mediators of acute laminitis in the horse.
THERAPEUTIC EXERCISE
Exercises such as hand walking, walking over poles, backing, and hill work, aim to return the soft tissues and bones to normal physical capacity. Sensory, neuromotor, and mechanical abnormalities that occur as a consequence of injury may be alleviated by the analgesic effects of therapeutic exercise. However, alternative pain modulation may also be necessary to ensure that therapeutic exercises are executed optimally.
ELECTROPHYSICAL MODALITIES PULSED ELECTROMAGNETIC FIELD (PEMF) THERAPY
PEMF is a lower frequency modality and is derived from heat-generating therapy. Short wave diathermy can be adjusted to a low frequency (less than 600 pps) and short phase duration (65 seconds). This magnetic field results in currents within the tissues but no heating inside the tissues. In the equine world, there are several therapeutic options available for PEMF therapy. There are blankets and wraps with coils and energy-generating battery units built into them. There are small coil systems and large coil systems, which produce magnetic fields in different sizes and strengths. The treatment protocols vary significantly in literature and seem to be specific to the type and manufacturer of the machine used. They are based on the frequency of the pulses and the treatment time.
THERAPEUTIC ULTRASOUND
Unlike diagnostic ultrasound, therapeutic ultrasound is designed specifically to have a biologic effect on the tissues. The ultrasound uses cyclic vibration frequencies of 1 to 3 MHz, and the mechanical energy produced generates a wave of acoustic energy, which is inaudible to the human ear. This energy travels through tissues and is absorbed by the deep tissues via molecular vibration, without altering the temperature of the skin surface.
Therapeutic ultrasound frequencies are well absorbed by high-protein tissues and minimally absorbed by tissues high in water content. Cartilage and bone, although high in protein, reflect the ultrasound waves, so this therapy has no effect on these tissues. There are also nonthermal effects of therapeutic ultrasound. These occur via cavitation, which occurs from mechanical vibration energy that forms tiny gas bubbles that improve the acoustic streaming, thereby altering cellular diffusion and permeability. Sodium and calcium ion transport channels are most affected, altering the membrane potential and cellular secretions. Therapeutic ultrasound is used in two different modes: pulsed and continuous. The pulsed mode provides nonthermal effects, such as cavitation and mechanical effects, whereas the continuous mode provides the thermal effects on the tissue.
Treatment with therapeutic ultrasound is typically performed for 10 to 14 days, one to two times daily. If used to increase joint passive range of motion (for example, in cases of capsulitis), it can be helpful to apply it prior to stretching exercises. Therapeutic ultrasound must be used with coupling gel to be effective; it also is recommended to clip the region being treated at least once weekly. The depth of penetration depends on the frequency of transducer: a 1 MHz transducer penetrates 3 to 5 cm, and a 3 MHz transducer penetrates 1 to 2 cm.
In terms of therapeutic dose, in humans, a setting of 0.1 W/cm2 for five minutes is used to help resolve inflammation and promote healing. Experimental data for heating equine flexor tendons exist. Therapeutic ultrasound, when applied at 1W/cm2 for 10 minutes, increased the superficial digital flexor tendon (SDFT) temperature by 3.5 degrees Celsius and the deep digital flexor tendon (DDFT) temperature by 2.5 degrees Celsius. When applied for 10 minutes at 1.5W/cm2, the SDFT temperature increased 5.2 degrees Celsius and the DDFT temperature increased three degrees Celsius from baseline temperatures.
EXTRACORPOREAL SHOCK WAVE THERAPY (ESWT)
Extracorporeal shock wave therapy was first introduced as lithotripsy to treat kidney stones and is still used for that purpose. Following this application in humans, it was used to treat soft-tissue injuries as well as osseous lesions such as heel spurs. In veterinary medicine, ESWT is most often used to treat osseous or ligamentous injuries. It uses pressure waves that increase as they travel through the tissues. The pressure changes lead to cavitation, and the formation and collapse of tiny gas bubbles, which leads to microtrauma of the tissues and increased blood flow. It is thought that shockwaves also promote the alignment of collagen fibers in tendons and ligaments.
ESWT is most used in equine practice to treat tendons, ligaments, navicular syndrome, osteoarthritis, and sesamoiditis. Studies evaluating ESWT on navicular pain and hock osteoarthritis have shown reduced lameness. Immediate pain relief is thought to be from a direct analgesic effect rather than a modulation of disease processes.
LASER THERAPY
The word laser stands for light amplification by stimulated emission of radiation. Laser light is created when energy from a power source excites a lasing medium, and energy is released as photons. Usually, a lasing medium creates light with one wavelength.
PROGNOSIS AND MANAGEMENT
Different wavelengths are claimed to have different effects on living tissues, based on their absorption peaks and penetration depths. The use of lasers in small-and large-animal veterinary medicine has increased dramatically in recent years, with several laser units available commercially.
Lasers are classified based on their potential for causing injury — especially eye damage, since the eye is most susceptible to excess laser light. There are four main classes for visible-beam lasers: Class 2, Class 3R, Class 3B, and Class 4. The first two are relatively safe for eye exposure; the last two are hazardous. This chart shows that the eye injury hazard increases as the laser’s power increases. The lasers commercially available for therapeutic use are Class 3B and Class 4 lasers.
Despite frequent use in equine practice, there is a lack of information on the exact mechanism of action. The current thought, although not proven, is that photon energy from laser light is converted to chemical energy in the cell. This in turn causes changes in cell membrane permeability and improvement of cellular function, leading to analgesia and healing.
Therapeutic protocols vary by wavelength, power, pulse rate, area treated, exposure time, and treatment intervals. On commercially available units, some of these variables are preset, and some can be changed depending on the protocol used. The depth of penetration of a laser is significantly limited by hair and skin pigmentation. Controlled trials using lasers in therapeutic applications are few and report varying degrees of success. Laser therapy causes an increase in nitric oxide production, causing vasodilation and increased circulation. It decreases pain via a temporary reduction in the conduction of A-delta and C nerve fibers and can cause endorphin release. Other physiologic effects of laser therapy include decreasing inflammation, including IL1, and reducing edema.
Laser therapy, depending on the settings, can be used as a form of pain management. In our practice, it is frequently employed to stimulate soft-tissue healing. For example, the high intensity laser is a frequent component in the rehabilitation of suspensory branch and proximal suspensory ligament injuries. One of the most well documented uses of laser therapy is for wound management, and for that reason, it can be a valuable tool to help with post-operative incision healing. Additionally, it is often used in cases of epaxial muscle pain. A recent study reported that laser, in combination with chiropractic treatment, provides more back pain relief than chiropractic treatment alone.
REGULATION OF MEDICATION IN EQUINE SPORT
It is of the utmost importance that medications administered to the rehabilitating athlete follow the guidelines for drugs and medications as outlined in the governing body of the athlete’s respective discipline. Many equine athletes perform while under a rehabilitation program, and there is no room for error in these instances.
REFERENCES
- Chamberlain, GA; Colborne, GR (2016). “A review of the cellular and molecular effects of extracorporeal shockwave therapy”. Veterinary and comparative orthopaedics and traumatology : V.C.O.T. 29 (2): 99–107.
- Dyson, S. (2022) The Ridden Horse Pain Ethogram. Equine Veterinary Education. Equine Vet. Educ. 34 (7) 372-3802022.
- Dyson, S. and Ellis, A. (2022) Application of a Ridden Horse Pain Ethogram to horses competing at 5-star three-day-events:Comparison with performance. Equine Vet. Educ. 34, 306-315. https://doi.org/10.1111/eve.13415.
- Haussler, K. Laser therapy. In: Proceedings, 8th Int’l Symp on Vet Rehab and PT 2014, Corvallis, OR.
- Hinchcliff KW, Kaneps A, et al. (2013). Equine Sports Medicine and Surgery 2nd edition. Saunders ltd.
- Imamura M, Furlan AD, Dryden T, et al. Evidence-informed management of chronic low back pain with massage. Spine J 2008;8(1):121–33.
- Keegan KG, Messer NT, Reed SK, et al. Effectiveness of administration of phenylbutazone alone or concurrent administration of phenylbutazone and flunixin meglumine to alleviate lameness in horses. Am J Vet Res 2008;69(2): 167–73.
- Montgomery L, Elliot SB, Adair S. Muscle and tendon heating rates with therapeutic ultrasound in horses. Vet Surg 2013;42:243-249.
About the Author
Laura Riggs, DVM, PhD, DACVS, DACVSMR
Dr. Laura Riggs is an associate professor and the service chief of equine surgery in the Department of Veterinary Clinical Sciences at Louisiana State University. She is a 2001 graduate of the University of Tennessee, College of Veterinary Medicine. Following graduation, she completed a large animal rotating internship and surgery residency at the University Of Georgia College Of Veterinary Medicine. In 2007, she became a Diplomate of the American College of Veterinary Surgeons. The same year she completed a PhD in veterinary physiology at the University of Georgia. In 2016, she became a Diplomate of the American College of Veterinary Sports Medicine and Rehabilitation. Since 2008 Dr. Riggs has been a member of the clinical faculty in the Veterinary Teaching Hospital at Louisiana State University where she is actively involved with clinical, teaching and research activities in the Equine Health and Sports Performance program.