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Navicular Syndrome

By Carla Huston, BES
Navicular syndrome is so called because a number of conditions may cause the pathological changes and clinical signs, and many structures may be involved. The navicular bone lies on the back of the coffin bone, with the navicular bursa, a small fluid-filled sac, between them. The small boat-shaped bone serves as a pulley for the deep flexor tendon that glides along its underside. Navicular syndrome is a degenerative process that can affect the bone, bursa or tendon.
This condition occurs almost exclusively in the front feet, and usually affects both feet. The first signs of navicular are a shortening of the stride and slight unevenness on turns. Because both forefeet are affected simultaneously, obvious lameness is not commonly observed. Instead, the horse appears reluctant to go forward properly or lengthen his stride. Pointing is another common sign; the horse will stand at rest with one leg extended, the weight resting on the toe. When the foot is pressure tested, the horse will usually indicate pain at the heels. X-rays do not always detect early signs of navicular since the soft tissues are usually the first problem areas. The cartilage on the underside of the navicular bone and the deep flexor tendon become inflamed first.
The causes of navicular, unfortunately, are usually manufactured. First, horses with small feet (too small for their weight) are predisposed to this condition. The support structures in the hoof are designed by nature to absorb shock and carry the horse; the smaller the hoof in relation to the horse's size, the greater the shock to the navicular bone. Breeders often select for small feet, believing them more "attractive." A second cause is using the horse in stressful athletic activities for which they are not suited. Circulation trouble is the first step in the breakdown. The reduced blood flow weakens the structure and leads to the deterioration. Horses that undergo a fair amount of percussion on the forefeet are inclined to experience navicular syndrome.
No single treatment is uniformly successful in treating navicular. These therapies treat the symptoms, but the syndrome is irreversible. The possibilities range from drugs to corrective farriery work and surgery to retirement. Bute and other anti-inflammatory drugs will alleviate pain. The administration of warfarin in the feed may help some horses. This drug prolongs the blood clotting time, but is potentially dangerous, and permanent medication is usually necessary. Why the drug works is not known. Also occasionally successful is the administration of isoxuprine, also delivered through the feed or in a paste. The peripheral blood vessels are dilated, improving circulation. Isoxuprine is usually only successful in the early stages of the syndrome. The foot must be properly trimmed, with the toes kept short and the foot balanced. Therapeutic shoeing may include an egg bar shoe with wedge pads, or the newly developed Tennessee Navicular shoe. Neurectomy is a surgical option for the navicular horse. This procedure involves removing a small piece of the two nerves that innervate the area. The nerves may regenerate, however, and daily care of the foot must be observed to recognize any injuries to the foot. Finally, for the advanced navicular syndrome, retirement may be the only recourse. This should include daily turnout to maintain regular bloodflow and promote healthy hoof growth.