Difficult Diagnosis
Navicular: It is well accepted now that response to a heel nerve block does not confirm a diagnosis of navicular disease. Radiographs have always been used, but radiologists are still arguing over what types of changes really diagnose navicular disease. Enlargements in the synovial fluid channels along the back edge of the navicular bone, where it contacts the bursa and flexor tendon, as seen in the so-called "skyline view," are generally accepted as at least indicating inflammation. The formation of large "holes" in the body of the navicular bone itself, believed to be enlarged vascular channels, is considered diagnostic by some, but others feel it is unreliable and can be seen in older horses that are perfectly sound.
Injection of local anesthetic directly into the navicular bursa is a bit more specific than a heel nerve block but could end up anesthetizing nearby structures, such as the flexor tendon and impar ligament, so it's not really 100% reliable. Similarly, injection of anesthetic into the coffin joint often improves navicular-related lameness but would improve coffin joint arthritis pain as well.
By far the best diagnostic technique is MRI imaging of the foot. MRI can detect a variety of changes that x-rays, and even bone scans, cannot. This includes increased fluid in the navicular bursa and adhesions between the bursa and the tendon. Considered most diagnostic of all is increased fluid within the body of the navicular bone itself.
Other causes of heel pain: Ultrasound is of some use in determining additional causes of pain. It can pick up damage to the collateral ligaments of the coffin joint, find some lesions in the deep flexor tendon if not located too deeply inside the foot, and may be able to see a very enlarged navicular bursa.
However, again, MRI is the premier diagnostic tool. All of the other mentioned causes of heel pain can be easily diagnosed by MRI.
Unfortunately, MRI isn't widely available yet and is expensive. But if you have a horse plagued by chronic foot pain that isn't responding to therapies or shoeing changes, you may want an MRI examination to find out exactly what you are dealing with so that a reasonable treatment plan can be made.
Treatment
Pain medications such as phenylbutazone are almost always part of the treatment approach to navicular, but they don't fix or cure anything. The cornerstone of traditional treatment for navicular/heel pain has been shoeing changes. The usual approach is a rocker-toed shoe, usually padding, and heel elevation either by shoes that are thicker in the heels or wedges. This relieves some stretch on the deep flexor tendon, which can bring relief in horses with both true navicular syndrome or lesions in the deep flexor tendon.
It is critically important to realize that even before considering a shoeing change, you must make sure the foot is properly trimmed and balanced. Slapping a shoe onto a foot in which the toe is too long, the heels are underrun, and/or there is an imbalance side to side will do you little, if any, good. The hoof itself must be properly trimmed first. For many horses, this alone provides considerable relief. If you don't take that step, everything else you might do is doomed from the start.
The shoe adjustments we described earlier often provide at least temporary relief and improvements. A major drawback is that over time, these shoeing arrangements can take the natural shock-absorbing structures of the foot-the frog and digital cushion-out of the picture. Soft packing under a sole pad helps provide stimulation to those areas the way dirt does in a barefoot horse. But if the heels are continually raised, as is often done if the horse begins to show increased discomfort, the frog and digital cushion will eventually wither and the navicular bone will become more locked in place. Some feel this is detrimental to the horse's circulation. These long-term complications of navicular shoeing may be at least part of the reason why navicular is considered to be incurable.
Is it incurable? The world of navicular disease research and the prognosis for it has only recently been turned on its head by the knowledge being gained from MRI imaging. We still don't know the answer to the curable-or-not question, and it's going to take several years of following horses with various navicular area changes to see how they respond.
In the final analysis, it may well turn out that horses with the classical picture of big, heavy bodies and tiny feet are doomed to navicular disease and lameness no matter what we do. Researchers have already determined that horses with adhesions between the navicular bursa and the flexor tendon also have a very poor outlook for soundness. Beyond that, the prognosis for horses with fluid in their navicular bone or excess fluid in the navicular bursa is largely unknown.
Precise diagnosis of the problem may improve the outcome for horses with heel pain syndrome by changing the way they are managed. For example, while a horse with a diagnosis of presumed navicular disease might not be worked as hard as usual, actual rest usually isn't part of the treatment plan. However, for some soft tissue lamenesses in the foot, rest could be critical. One study found that horses with damage to the collateral ligaments of the coffin joint responded very well to rest and casting of the feet. Tearing of the flexor tendon at its attachment to the coffin bone might also respond well to a similar approach. And with any ligament damage, strictly controlled exercise is always important to healing-as is providing a lot of time for the tissue to heal and following its progress with periodic imaging exams.
Bottom Line
While navicular disease/syndrome is a very real thing, recent advances in diagnosis have revealed other causes of heel pain. Furthermore, different problems may call for different treatment approaches. Researchers are following horses with various changes in their feet to determine the best treatment options. In the meantime, all this new knowledge is leaving us even more confused about what to do. However, it will ultimately result in finding better ways to help horses be pain free.
