Chapter 88Counterirritation
Counterirritation techniques have long been a part of veterinary therapy, particularly in horses. Major veterinary texts, such as those by Percivall,1 Wooldridge,2 and even Stashak3 as late as 1987, devoted considerable attention to indications and techniques. To some this now seems misguided and even cruel, but one has to read only a few of the older texts to admire the authors’ inquiring minds and their thorough observations on matters medical and particularly orthopedic. They were not far from the truth, but they did not have the benefit of modern anesthetics or imaging equipment to learn and apply more direct and rational therapies. Techniques currently used include hot or cold firing and blisters, but use of those therapies is much reduced because of changes in horse ownership and veterinary attitudes. This chapter considers both British (DRE) and North American (SPD) perspectives.
Limited availability of ingredients and ignorance of preparation and safe use has restricted blistering of horses in the United Kingdom. Although veterinary surgeons or equine wholesalers are the source, the horse owner is usually the user. More severe blisters, such as those containing cantharides or croton oil, are used less commonly than the working blisters that are iodine or mercuric iodide based and mainly used on splints, sore shins, or curbs. The technique involves rubbing or brushing the liquid preparation onto the clipped skin once daily until a scale forms. The horse is kept in light exercise, such as walking, trotting, or hack cantering, and this phase usually is undertaken after the initial rest and antiinflammatory treatment have removed heat, swelling, and pain (i.e., 7 to 10 days after initial clinical signs). When the heat and soreness from counterirritation has settled, which takes 2 to 3 weeks, the horse resumes work.
Irritation from working blisters is minimal, and protective measures such as antiinflammatory treatment, keeping the horse crosstied, or fitting a bib or cradle are not necessary.
Strong blister was made mostly from cantharides (Spanish fly) or croton oil, but nowadays these ingredients are difficult to obtain and, because they are not licensed drugs in the United Kingdom, are probably illegal for a veterinary surgeon to import without a permit. As a consequence the strong blister that is used most commonly is made as a cream using red mercuric iodide. The skin is clipped and the blister rubbed into the area for 5 minutes. Excess is removed and petroleum jelly is applied to flexor surfaces distal to the blistered areas. The horse is kept shod for blistering, in case it paws the ground, and a neck cradle is applied until the initial inflammatory phase has subsided. It is essential that the horse continues walking exercise, usually 30 minutes twice daily. A substantial risk of laminitis or lymphangitis exists if blistered horses do not have this regular exercise. When blisters form and burst, they are left alone and sprayed with antibiotic powder. Rarely is the reaction severe enough to warrant antiinflammatory treatment. After 10 days the blistered areas of the legs are smeared with petroleum jelly, which is left on overnight and then washed off with warm water and soft soap. This softens and lifts the scabs free, without causing bleeding or further irritation. Any blistered areas that are still moist are dressed with antibiotic wound powder, and walking is continued. Scabs that persist after 7 to 10 days are removed by repeating this procedure. After 6 weeks the horse can be turned out to pasture or start light ridden exercise. The program for a return to training depends on the nature and progress of the original injury.
Areas treated most commonly with strong blister are the digital flexor tendon region of forelimbs and the fetlock joints. Indications are tendonitis, tenosynovitis, suspensory desmitis, and chronic effusion of fetlock joints.
In North America, curbs frequently are treated by blistering (see Chapter 78), and both hindlimbs are usually treated. The area is clipped and rubbed aggressively for 3 minutes with medical-grade turpentine and then with kosher salt for 3 minutes, alternating for a total of 15 minutes. This is repeated once daily for 3 days. The horse is exercised immediately after each treatment for at least 20 minutes. The skin excoriates and remains thickened and inflamed for some time, but therapeutic effects are almost immediate.
Blistering also is used to manage suspensory desmitis, tendonitis, sesamoiditis, and other fetlock problems. The severity of the injury determines which blister is used and the degree of exercise modification. If horses with suspensory desmitis or tendonitis are being treated, the metacarpal or metatarsal region usually is wrapped (bandaged) with a gauze layer, a leg quilt, and a bandage to control swelling. Moving areas such as the carpus or tarsus are not wrapped.
The blistering process is allowed to continue for 5 days, and then a firing paint is applied to the area for another 7 days, with the bandages reset every day if the blistering process involves the metacarpal (metatarsal) or fetlock region. After the firing paint application period ends, usually an iodine glycerin-based product is applied every day to soften the blister scabs and help lift the outer skin layers that have separated. Once a horse begins to be painted with the firing paint, it can be exercised on a walker or in a small paddock. This reduces inflammation and begins to reestablish normal venous and lymphatic flow.
Internal blisters have more widespread use in North America compared with Europe. A 2% iodine mixture, in almond oil or a lighter base, can be injected into tissues. One author (SPD) treats muscle soreness by injection of 1 mL of internal blister per site using an 18-gauge needle of variable length, depending on the depth of the injection. The mechanism of action is unknown, but trainers usually report that the gait improves (see Chapter 47). Internal blisters also are used to treat horses with suspensory desmitis, curbs, and splints. Varied results may reflect the inconsistency of the available products.
Cautery or firing is performed less frequently in the United Kingdom since the Royal College of Veterinary Surgeons deplored the practice in 1991. Cautery or firing is used for horses with injuries to soft and hard tissues, and some clinicians and clients are still enthusiastic about its value. Pin or needle firing is the most common technique.
Cold firing is used mainly for horses with persistent sore shins or splints, and the thermal injury is made with liquid nitrogen (see Chapter 89). The procedure is done soon after the initial inflammatory phase has subsided and is preferred for horses with intractable injuries that have not responded to antiinflammatory measures or blisters. The area is clipped, the horse is sedated, and local anesthesia is administered. An ordinary firing iron can be cooled by leaving it in the liquid nitrogen, or a special “gun” point freezes a small area of skin. Spots of lubricant gel on the areas to be fired improve the contact. The pattern should not allow the frozen spots to be closer than 3 cm apart. No systemic or local treatment is needed, and the horse is kept in walking exercise for 6 weeks. Ultimately the skin develops persistent white point scars. Results have been good enough to ensure its continuing use, but the rationale is difficult to ascertain. Cold firing probably causes local analgesia of some duration within periosteal nerves and may necrose small areas of the periosteum and underlying cortical bone.
Acid firing enjoyed a vogue several years ago but has deservedly fallen from use. Concentrated nitric acid was soaked into a small cork, which was then pressed onto the clipped and desensitized skin. The inflammatory reaction was substantial, but, like line firing, acid firing appeared to affect only the skin and not the underlying tendon.
Line or bar firing was used for horses with tendonitis or curbs. For the lines to be made evenly, the area was clipped the day before firing and covered with a mustard plaster overnight, which caused subcutaneous edema to smooth out the grooves between the structures before firing. Curved, blade-shaped irons were used, and management was the same as for pin firing. One author (SPD) has minimal experience with this technique but agrees with Silver and colleagues4 that it would not be “a useful therapeutic exercise.”
Horses are still pin fired regularly in the United Kingdom. The procedure is used for horses with superficial digital flexor tendonitis, suspensory desmitis, and sesamoiditis and has value for those with persistently painful splints. Pin firing is no longer used for distal hock joint pain or carpitis. The horse is fired after the initial inflammatory response has subsided and when the area is no longer warm or painful, 4 to 6 weeks after the injury. The area is clipped and anesthetized before being dressed with surgical spirit. Firing is started 10 to 15 minutes later and patterned no closer than 3 cm apart over the structure being treated. It is important to use a fine point so that minimal burning of the adjacent skin occurs. The point penetrates the underlying tendon or periosteum. An iodine-based ointment is then applied, and petroleum jelly is smeared distally, particularly in the flexural regions of joints. The horse is crosstied, or fitted with a cradle, and walked in hand twice daily for 30 minutes. As for blistered horses, walking is essential. It is wise to keep front shoes on, and only rarely is analgesia indicated. The area is powdered with antibiotic daily and after 10 days is smeared with petroleum jelly, which is left on overnight and washed off the following morning, as previously described. Walking exercise continues for at least 6 weeks, and then the horse can start to spend increasing periods of time turned out in a paddock. The horse should not return to training for about 1 year after a soft tissue injury. Those horses that have had bony tissue fired, such as with splints, can return gradually to training after 6 weeks of walking.
In North America, pin firing is used mostly to treat horses with splints, curbs, tendon and ligament injuries, and fetlock and surrounding soft tissue injuries. A similar pattern is used for pin firing curbs and splints (SPD). Marks are made down the center of the lesion, 1 cm apart. Alternating rows, medially and laterally, 0.5 cm between the original marks, are then made. When firing tendons or ligaments, a similar pattern is used, but the point never goes through the deep layer of epidermis. However, areas of maximum damage are refired to ensure deeper penetration to focus the healing process. The horse may require sedation and analgesia for 24 hours after treatment to control pain. The area is painted with firing paint for 10 days; it is covered with gauze, a quilt, and bandage. Another 30 days of rest are required before reevaluation of the horse. The total convalescent period depends on the initial injury. In sports horses, use of cryotherapy to treat splints and some soft tissue injuries is more common. Although cryotherapy is less invasive, the procedure is often less effective (see Chapter 89).
Current thoughts on thermal injury to tissues still emphasize the gulf between veterinary science and veterinary practice. Although much excellent science has illuminated the injury and healing of strained tendons, such injuries are still the most substantial injuries a flat racehorse can sustain and are a long-term threat to the horse’s continuing career. Horses that race over fences (steeplechasers) or hurdles seem to manage better than those that race at a faster pace on the flat. The expectation that counterirritation would produce a fiber pattern with crimp and better-quality collagen may have been ill-founded, but the successful results that veterinarians have experienced over the years need to be explained. Perhaps the mechanical side effect of pin fire scars, evening out the differing strength and elasticity along the length of the injured tendon and in the contralateral limb, has kept pin firing in equine orthopedics. Beyond enforced rest and low-grade exercise, seeing benefit accruing from blisters used in horses with tendon strain is difficult.
Aggressive antiinflammatory treatment in the early postinjury phase is important, but one author (DRE) is uncertain if subcutaneous corticosteroids are essential for a good long-term result.
Treatment regimens such as intralesional glycosaminoglycans or β-aminopropionitrile fumarate have disappointed, and the results of surgical splitting or desmotomy of the accessory ligament of the superficial digital flexor tendon have not always justified the expense in the United Kingdom. Prolonged rest and controlled exercise regimens seem as successful as any treatments.
The choice of treatment still emerges from consensus among the experiences of the veterinary surgeon, the trainer, and the owner. All equine veterinary surgeons share the fervent hope that successful and rational therapy for horses with strained flexor tendons will emerge soon.
Restrictions on using firing by the Royal College of Veterinary Surgeons were initiated by a question in the Houses of Parliament about the firing of horses. The Horserace Betting Levy Board then funded a research program by the Bristol group, led by Professor Ian Silver. This work was published in a supplement to the Equine Veterinary Journal.4 Extensive discussion followed, but in 1991 the Royal College of Veterinary Surgeons tried to outlaw the technique of firing tendons by declaring that they deplored it and would bring disciplinary action against any member performing it. This was not well received by a substantial number of respected equine veterinary surgeons, and the policy was one that had been poorly thought through, did not recognize hard tissue firing, and proved to be unenforceable. A compromise was later reached, whereby veterinary surgeons had to show that other treatments had failed and to justify their choice of firing a particular horse. The technique continues to be used, but since the introduction of ultrasonography and the retirement of veterinarians who preferred it, the use of firing has diminished considerably. One author (DRE) believes that firing would have gradually disappeared without legislation.
Techniques currently used in equine orthopedics have become more conservative since the introduction of ultrasonography and the development of finer sensitivity among horse owners. The regular monitoring of these horses has enabled more rational advice to be given regarding healing of the lesions and programming of exercise. Thus the implementation of direct therapy has taken second place among certain groups of clinicians. They remain cynical about therapy while maintaining client confidence through regular observation of the horse. Splitting or needle decompression of a tendon core lesion soon after injury is a rational first step, followed by regular monitoring and controlled, graded exercise throughout a long convalescence. The old adage of “the longer the rest, the better” still seems valid.