The Equine Foot: The Relationship Between Disease,
Clinical Signs, Diagnosis And Treatment.
Andrew H. Parks, MA, Vet MB, MRCVS, Diplomate ACVS DIAGNOSIS AND CLINICAL SIGNS
College of Veterinary Medicine
University of Georgia, Athens Georgia
Reprinted with permission from the North American Veterinary Conference.
Original printed in the 2001 NAVC conference proceedings
The presenting symptom is one manifestation of abnormal form or function, usually lameness or abnormal appearance. This initial symptom directs the course of clinical investigation for more subtle symptoms to better define the disease process to obtain the most accurate diagnosis possible. The presenting symptom may obviously identify the horse's foot as the site of disease, However, the source of the problem is not obvious in many horses with lameness originating from the foot. Therefore the breadth of the examination may be determined by the specificity or vagueness of the presenting symptom(s).
The basic principles or physical examination of the foot follow those for the rest of the musculoskeletal system whenever possible: visual observation, palpation for pain and heat, flexion and extension. Emphasis is given to identifying the cardinal signs of acute or chronic inflammation - the commonest causes of disease in the musculoskeletal system.
Again, it is the structure of the integument of the foot that makes it different from the rest of the limb. The hoof offers clues that are not present elsewhere , yet the rigid hoof capsule inhibits basic palpation of the structures within the foot. Therefore it is important to develop an ability to "read" the hoof capsule (which seems to be a life long process). Visual examination of the hoof capsule in conjunction with the phalangeal axis from the lateral, medial, dorsal, palmar and solar aspects to compare the balance and conformation of the distal limb with an "ideal balance and conformation" is straightforward. Closer observation reveals more subtle signs. Examination of the coronary band should show any local areas of proximal or distal displacement. Examination of the growth rings below the coronary band should show local variations in the spacing of the growth rings. By sweeping ones hand on the outside of the hoof from the coronary band to the weightbearing surface, convexities or concavities in a proximal to distal direction can be identified. Similarly, by sweeping an outstretched hand around the hoof from the medial to lateral heel, deviations from the normal smooth curvature can be identified as increased or decreased convexity and even local concavity can be identified. Palpation of the coronary band in a similar manner may reveal lipping when the coronary band extends abaxial to the wall. Taking any one of these more subtle symptoms alone can be misleading, but taken together they can provide supporting evidence. For example, local coronary band displaced proximally at the junction of the toe and the quarter may be accompanied by growth rings that are closer together and an exaggerated convexity in the hoof capsule at that point in the horizontal plane. Taken together, these findings are very suggestive that there is excessive compressive vertical stresses within the hoof wall at that point. But if the coronary band is displaced without the difference in spacing of the growth rings or convexity, more caution is needed before this assertion can be made.
Having examined the outside of the foot, is important to maximize the information we can learn about the inside of the foot. Heat within the foot can often be readily appreciated. However, because pain within the hoof is rarely appreciable on digital palpation, compression of the hoof with hoof testers or percussion with a hammer is necessary to localize pain. Cleaning the weightbearing surface of the foot by lightly removing the surface layer of the sole, frog and distal wall indicates any potential entry sides for infection, usually seen as small dirt filled cavities. Also visible may be speckled red discoloration of the horn indicating hemorrhage into the hoof capsule from underlying trauma. Interestingly, hemorrhage within the hoof maintains its blood red color rather than turning black and blue as bruises do elsewhere. Hemorrhage within the wall represents a past event and not necessarily an ongoing process, though of course, it may still be.
Various ancillary diagnostic aids are used to gain further information about the disease process: exploration, regional and intra-articular anesthesia, radiography, ultrasound, nuclear scintigraphy and thermography.
Removal of exfoliating hoof and careful exploration of tracts is indicated if infection is suspected.
Local anesthesia is very useful to localize lameness to the distal limb. Unfortunately, comparing the results of the different perineural blocks and intrasynovial anesthesia has not proven to be as useful a discriminator in determining the affected tissues as clinicians would like it to be. Recent advances in our knowledge of the affects of these techniques have, if anything, introduced ambiguities in interpreting the results obtained with local anesthesia.
Radiography is a time honored technique that has proven useful in identifying osseous disease, though ambiguity persists in interpretation of specific findings, for example navicular disease or pedal osteitis. Only recently has the examination of the soft tissues received more attention, both in their own right and in the relation between the soft tissues and the osseous structures. Some diseases of the foot require radiographs to make a diagnosis, in other diseases radiographs are a useful adjunct to confirm a diagnosis or help determine the severity of a disease process, while in other diseases, radiographs are of limited benefit except in ruling out other concurrent conditions.
Ultrasound is a well established modality for examination of soft tissue structures of the limb, but it is only recently that it has been used for structures within the foot, either by examining the deeper structures from above the coronary band or through the frog. Further development can be expected in this area.
Nuclear scintigraphy has proven to be a valuable aid in guiding the interpretation of equivocal radiographic changes, identifying pathological processes before radiographic changes are evident and in identifying some processes for which radiographic changes cannot be visualized.
Thermography has been less enthusiastically embraced that ultrasound and nuclear scintigraphy because interpretation of the results at this juncture is more ambiguous. But it is a technique that deserves more attention, at least until its interpretation has been fine tuned enough so that more precise benefits of the technique can be identified.
In summary, examination of the equine foot offers many challenges. While there are a finite number of tissues within the foot and a limited number of pathological processes that can affect these tissues, this knowledge does us no good unless we can put the two together. With the advent of ultrasound and scintigraphy, we can now make diagnoses that were impossible 20 years ago; for example, insertional tenopathy of the deep digital flexor tendon. With the development and application of newer technology, the obvious examples being computed tomography and magnetic resonance imaging, we will be able to make diagnoses in the future that are currently impossible; for example, sprains of the many small ligaments within the foot.
Just as the foot shares many disease processes and symptoms with the musculoskeletal system, so it shares treatments. Hence the focus of the ensuing discussion will focus on principles of general musculoskeletal diseases and specific treatments limited to diseases of the foot.
Treatment may be directed at a symptom(s), a pathogenic process, or a specific diagnosis. Symptomatic treatment is likely to have the broadest applicability, but may not address the cause. Treatment based on the type of pathogenic process is more likely to be effective against the cause but requires more specific diagnostic information to implement. Treatment directed at a specific disease, i.e. a specific pathogenic process in a specific tissue is obviously optimal, but a specific treatment may be unknown, or a specific treatment may exist that appears to be effective empirically without a rational basis for its use. Only treatment that can be discussed in generalities is covered below.
Treatment based on symptoms
Rest is employed to prevent exacerbation of a disease or injury, and allow tissues to heal.
Pain is frequently the most prominent symptom. Pain is obviously controlled for humanitarian reasons. Pain is also controlled to improve function, either to make the horse sound enough for a specific purpose, or to prevent complications secondary to prolonged excessive weightbearing by unaffected limbs. Pain may be controlled or reduced by pharmacological means or by biomechanical manipulation of the distal limb. Phenylbutazone is by far the most commonly used analgesic, but other non-steroidal anti-inflammatory drugs and DMSO are used especially when systemic endotoxemia is also suspected as part of the underlying pathogenesis of the disease.
Correcting poor balance may correct some causes of lameness. For other diseases restoring normal balance may minimize discomfort and/or slow progression. For example, trimming and shoeing may reduce the moment about the distal interphalangeal joint, which decreases the stress in structures involved in flexing this joint during weightbearing and breakover. However, be cautious about changing the balance in sound horses! While poor conformation cannot be readily corrected, it may be compensated for by shoeing and trimming, though this often involves a compromise.
Providing stability may be necessary for several reasons: within the hoof capsule itself, either following a crack or avulsion; between the hoof capsule and the distal phalanx in laminitis; or between the skeletal elements themselves. As well as providing an optimal environment for healing, stabilizing the digit often provides pain relief. There are frequently several options, the choice of which will depend on the severity of the instability. Hoof defects or cracks may require bridging, reconstructing or casting. Instability between the hoof capsule and the distal phalanx in laminitic horses usually involves shoeing to minimize further rotational forces. Instability about the distal interphalangeal joint in either the dorsopalmar plane, e.g. post deep digital flexor tendon rupture, or the mediolateral plane, e.g. post trauma to the collateral ligament may be shod to decrease distracting or rotational forces, or cast.
As wounds heal of the hoof heal in very much the same manner as elsewhere on the body, the basic principles should be followed just as rigorously. The proximity of the foot to the ground predisposes the foot to contamination and further injury, and the role of the hoof in weightbearing may necessitate additional precautions. Injuries to the hoof can seldom be closed by primary intention except those limited to the coronary band.
Treatment based on the pathological process
Infection may involve any of the structures of the foot. All benefit from either drainage, debridement or lavage. Tetanus prophylaxis is mandatory. Superficial infection of the epidermis is generally treated by debridement and exposure to air. Supplemental treatment with topical antibiotics or astringents is likely to decrease the recurrence for some of these diseases. All of them benefit from protection from excessive moisture and fecal contamination.
Dermal and subdermal infection proximal to the coronary band is treated as it is elsewhere on the limb. Below the coronary band exposure of infected structures involves a compromise between providing adequate drainage and maintaining stability of the hoof capsule. Broad spectrum antibiotics are routinely used for deep digital infections and are frequently supplemented by regional intravascular infusion and topically applied antibiotics.
Neoplasia is not common enough to make general recommendations except to say that keratomas require excision.
Ischemia is involved in the pathogenesis of laminitis and suspected in the pathogenesis of navicular disease. Typically it is treated with vasodilators, rheologic agents and anticoagulants.
The treatment of degerative processes within the joints and bursa are well documented elsewhere, but involve pain control, joint lubrication and chondroprotection with various systemic and intra-articular medications.
The symptomatic mainstays of traumatic injuries are rest, analgesia and anti-inflammatory drugs. If the injury is the result of repetitive trauma, the cause must be removed or recurrence is inevitable. This may involve improving balance, providing protection or the changing nature of athletic activity or the exercise surface. Most fractures of the distal phalanx and navicular bone are treated conservatively by minimizing distracting forces. Occasionally, fragment removal or internal fixation is required.