(From the researchers at Kentucky Equine Research)
Navicular syndrome (or navicular disease, or caudal heel pain syndrome) is a degenerative condition of structures in the horse's heel. The navicular bone lies at the back of the heel, and the deep digital flexor tendon runs down the leg and wraps under the navicular bone before anchoring to the coffin bone. Pain results from changes in the bones, bursa (fluid-filled joint structures designed to absorb shock and reduce friction), tendons, and ligaments in this area.
What signs does an affected horse show?
Lameness is the classic sign of navicular syndrome. This can appear suddenly, but a more common pattern is mild lameness that becomes progressively worse over time. A horse with navicular syndrome feels pain in the heels of the front feet, and its movements reflect attempts to keep pressure off this area. At rest, the more painful foot is often "pointed," or held slightly in front of the other forefoot, thus bearing little or no weight. Because the horse tries to impact the ground flat-footed or toe-first instead of the more normal heel-first pattern, the gaits are short-strided and stiff. A horse with navicular syndrome has difficulty turning sharply, going downhill, and moving on rocky or hard ground. Picking up one front foot for trimming or shoeing is painful because weight is concentrated on the other foreleg, and affected animals may become quite uncooperative during farrier visits.
Does navicular syndrome affect all types of horses?
While there's no guarantee that a particular horse will or will not develop navicular trouble, the problem is most common in stock-type horses (Quarter Horses, Paints, Appaloosas). There is a fairly high incidence in Thoroughbreds and some warmblood breeds. Arabians, on the other hand, are rarely affected. Lameness from navicular syndrome is most often diagnosed in horses between the ages of seven and fourteen.
What causes this condition?
No one knows precisely what causes navicular syndrome. Like some other lamenesses, a combination of factors is probably to blame. Conformation seems to be important, with more cases occurring in horses with heavy bodies, upright pasterns, and small hooves. A large number of affected horses have a history of work involving front-leg impact (jumping, cutting, roping, and reining) or increased concussion (parade work or other use involving hard or rocky surfaces). Suspicion has also been directed at irregular farrier care, unbalanced hooves, and shoeing practices that reduce contact between the frog and the ground. A common thread seems to be the combination of increased stress and limited oxygenation of structures in the heel area, but the exact cause of tissue damage and inflammation has not yet been determined.
How is the diagnosis made?
Heel pain is not always caused by navicular disease: temporary lameness from bone fractures, muscle strains, and trauma to tendons or ligaments can mimic the syndrome. A vet may need to use a combination of flexion tests, hoof and frog pressure tests, nerve blocks, X-rays, scintigraphy, thermography, ultrasound, venograms, and magnetic resonance imaging (MRI) to determine the cause of the horse's discomfort. Diagnosis is made after a consideration of the horse's history, use, conformation, and test results.
At one time the appearance of lollipop-shaped invaginations (holes) in the navicular bone on X-rays was considered to be definitive evidence of navicular syndrome. However, it has been found that not all horses with such invaginations exhibit clinical signs of lameness, and some acutely lame horses fail to show any trace of bone abnormality. Interestingly, research at the Idaho Museum of Natural History found lollipops in 17% of navicular bones recovered from equine skeletons up to three million years old, showing that the phenomenon is not of strictly modern origin. In the "history repeats itself" category, bones from large-bodied horses were more likely to show invaginations, and animals living in areas of hard and rocky terrain were also more often affected than grassland-dwelling animals.
Can navicular syndrome be cured?
Consultations with a veterinarian and a farrier are the first steps in combating navicular syndrome. While there is no cure, a prompt diagnosis allows treatment-farrier, medical, or surgical-to begin early in the course of the disease. Proper trimming and therapeutic shoeing can provide pain relief for many horses. Farrier care is aimed at correcting broken-back or broken-forward pastern angles and normalizing underrun or contracted heels. Generally a shortened toe, either through trimming or shoe design, is a goal. Heel support afforded by egg bar or wide-web shoes reduces pain in some horses, and a shock-absorbing polyurethane shoe from Switzerland is being tested as a possible therapeutic aid. Overall, proper trimming and shoeing can relieve discomfort in about 30% of horses with navicular syndrome.
Polysulfated glycosaminoglycans and hyaluronic acid sometimes lead to improvement, possibly by inhibiting enzymes involved in tissue breakdown. Anti-inflammatory medications can be injected into the heel area or given orally for pain relief. Medical treatment combined with therapeutic shoeing can help about 60% of affected horses.
Palmar digital neurectomy, a surgical procedure to sever the nerves to the painful area, is a last resort in treating navicular syndrome. This option eliminates sensation in the rear third of the foot, thus ending pain and lameness, but it is not a permanent cure. Degenerative changes continue to occur within the hoof, and about one-third of treated horses are lame again within two years.
Can feeding practices help to prevent or treat navicular syndrome?
Navicular syndrome is not directly caused by feeding practices, but as with any condition affecting the legs and feet, an overweight horse puts excessive strain on its musculoskeletal system. With the strong correlation between heavy-bodied, small-footed horses and navicular syndrome, common sense should warn owners not to allow their horses to become too fat.
Pasture intake can be limited by muzzling or dry-lotting horses, and easy-keeping animals can be fed a low-calorie supplement pellet such as All-Phase, thus providing essential vitamins and minerals without an overabundance of calories. Increasing exercise is also beneficial for keeping horses in trim condition. Although turnout and light exercise are preferred to stall rest for navicular horses, heavy work is usually ruled out as a conditioning measure for these animals. Keeping a horse at a mid-range body weight may delay the onset of navicular syndrome in susceptible individuals, and can also help to keep affected animals more comfortable.