Navicular Syndrome (NS) is a cause of incurable lameness in mainly the front feet of horses that are predisposed and subjected to extreme exercise and burdensome work. Middle aged to older animals are diagnosed more frequently with NS than younger horses. Changes in gait due to heel pain are the initial symptoms, due to inflammation and degenerative changes in the navicular bursa, a fluid-filled cushion between navicular bone and deep digital flexor tendon of the horse’s foot. The bone and tendon will also become degenerative as the disease progresses. This problem usually affects both front feet, but the symptoms may alternate from side to side as the horse favors the most painful foot of the moment. Rest will help alleviate pain initially, but symptoms quickly return upon activity.
When observing the horse with NS, a few classical symptoms may be noted: The animal may constantly fidget, shifting weight from side to side in order to alleviate pressure on the heels. It may also point the front feet, and alternate which foot is pointed. Abnormal wear may be seen on the toe of the hoof or shoe due to heel avoidance during walking. And, one or both hoofs may contract in width at the heel due to poor circulation.
The mechanism that causes navicular degeneration is not completely understood, but there are several theories. They all result in the de-vitalization of the navicular bone, compromising its function and structural integrity, and causing persistent heel pain due to one or more of the following: decreased blood circulation and tissue necrosis, impact damage and excessive bone remodeling, and / or degenerative joint disease – a breakdown of synovial fluid and cartilage (lubricant and cushioning in the joints).
A diagnosis of NS is based upon a number of supporting indicators: typical clinical signs, a history of intermittency and heavy work load, localization of pain within the heel, and navicular bone lesions if present on an x-ray or bone scan (scintigraphy). A hoof tester and “wedge tests” may be used to confirm and isolate pain at the frog (soft part of the heel) and in the navicular region. Local anesthetics can also be helpful in diagnosing NS by ruling out another source of the pain. When a palmar digital nerve block is placed, the horse will experience great improvement in the anesthetized heel. When both heels are blocked, the horse may appear practically normal.
There are numerous protocols that have been developed for treating Navicular Syndrome, all of which attempt to achieve the same goals: to alleviate pain and to slow the progression of the disease. Treatment may include hoof trimming, corrective shoeing, and rest in young animals with mild symptoms. As pain becomes more chronic, non-steroidal anti-inflammatory drugs (NSAIDs) may be administered. These steps will be a foundation upon which further treatment options may be added.
Bar shoes and rubber frog pads can be used to protect and cushion the heels from further impact damage. Intra-articular injections can also be considered for advancing cases. Cortisone, injectable NSAIDs, vasodilators, and restorative nutraceuticals like hyaluronic acid and polysulfated glycosaminoglycans may be injected alone or in combinations into the navicular region of the foot in an effort to control symptoms.
At some point, surgical options may be investigated. The risks / benefits of these procedures are weighed when a poor response to the aforementioned therapies is seen. There are two surgeries commonly performed to provide pain relief and improve mobility. A palmar digital neurectomy severs the nerve that is responsible for communicating sensations from the palmar surfaces of the foot, eliminating pain entirely if its source is indeed originating from navicular bone degeneration. Side effects are fairly rare, but this surgery performed on a horse without NS can increase the risk of injuries due to lack of sensation. The nerve may also re-grow, defeating the procedure, and occasionally a painful neuroma may form. A navicular suspensory desmotomy is preferred in moderately affected horses that may benefit from reduced compression on the navicular bone before completely denervating the palmar aspect of the foot. In either case, infection is a serious possible complication of surgery, and the horse must be strictly rested for at least 6 weeks post-operatively while it recovers.
The prevention of NS is difficult since there is no organism responsible for the disease that could be combated by vaccination strategies, testing, or quarantine efforts. It is impossible to predict how much exercise a susceptible horse may tolerate before developing degenerative problems of the navicular regions. Aside from exercise avoidance, which could lead to other serious problems, NS can only be prevented by early recognition of animals that may be predisposed. Horses with hoof confirmation problems, i.e. narrow hoofs, and those exposed to extreme abuses of the feet (racing, cutting, and roping horses), should be monitored for the development of NS. Early detection is key to slowing the progression of the degenerative effects that lead to permanent lameness. Proper and preventive shoeing is also vital to avoiding the development of NS.
Once horses are obviously affected by NS, the prognosis is guarded to poor, depending upon the diligence and aggressiveness of therapy. If the underlying causes are caught early, i.e. inappropriate shoeing or exorbitant stresses on the feet, the disease can be reversed or slowed for a period of time, significantly extending the quality of life for the animal.