Navicular disease is one of the most frequently diagnosed, but misunderstood and difficult-to-treat lameness problems. Much of the confusion stems from other conditions being called navicular disease and from a tendency for veterinarians to label any horse with pain in the heel area a navicular horse and treat them all the same way.
What Is It'
Before you can even begin to talk about proper treatment or prognosis for navicular disease, it’s necessary to make sure everyone is on the same page with regard to exactly what navicular disease is. Unfortunately, that’s not easy.
Probably the most rational definition of navicular disease is the one used by the pathologist and noted author Dr. James Rooney of Maryland. He defines navicular disease as pathological changes in the navicular bursa, the deep flexor tendon and the cartilage of the navicular bone itself.
These changes are the result of inflammation that Rooney believes is caused by vibration and friction. Using some detailed mechanical models of the stress on the navicular area during movement, Rooney explains that toe-first landing, as occurs with jumping, changes the mechanics of joint movement to result in extreme compression of the navicular bone by the deep flexor tendon.
He also suggests horses that work on hard surfaces (like Amish buggy horses) experience similar vibration forces in their feet, as would a horse that is landing toe first for any reason. Interestingly, Rooney reports never seeing navicular-disease changes on postmortem examination of racing Standardbreds or Thoroughbreds, but only when these breeds are put to other uses that cause the vibration/friction in the navicular area.
The other major theory of navicular disease is that it’s actually a vascular problem, caused by poor blood flow to the navicular area. The basis of this theory is that researchers have found thrombosed arteries in the feet of horses diagnosed with navicular disease. Experimentally occluding part of the vascular supply to the navicular bone also produces changes in the bone and the vascular supply to it that resemble navicular disease.
Thermographic patterns of horses with a clinical diagnosis of navicular disease only add further to the confusion. (Thermography involves detecting and measuring areas of warmer and cooler patches on the skin surface.) Some studies report “hot spots” in the navicular bursa, exactly what you would expect from the friction theory. Others report that horses with navicular disease are actually cooler than normal on thermographic pictures and fail to show the normal increase in temperature after they are exercised, which would support the vascular theory.
These two theories aren’t necessarily mutually exclusive. Inflammatory changes may well produce local vascular clotting, while poor blood supply would cause pain and remodeling of the navicular bone. However, while both would produce pain in the heel area, it’s important to differentiate between what Rooney describes as “true navicular disease” and other problems, because the prognosis in “true” navicular disease is extremely poor, while other causes of heel pain are often reversible.
Sorting through causes of navicular might be confusing enough, but making the diagnosis is worse. Step one is noticing a lameness and localizing it to the foot.
Hoof testers are often used next, but a painful response to hoof testers applied across the heels isn’t specific for navicular disease, and many horses that do have navicular disease may not respond to the hoof testers.
With blocking, if the lameness is stopped when only the back part of the foot is anesthetized, navicular disease is on the list of possible causes but is far from being the only cause possible.
The next step is radiographs. A variety of changes in the navicular bone have been described, but a diagnosis of navicular disease cannot be made from radiographs.Changes noted in the navicular bone on radiographs may very well be abnormal, but they don’t always correlate well with lameness and can sometimes be found in horses that are perfectly sound. Bone remodeling changes from any cause don’t necessarily mean the navicular bursa and flexor tendon are involved, so the horse would not have “true” navicular disease. This may sound like splitting hairs, but it’s not because treatment and the odds of the horse returning to soundness are different.
Fortunately, thermography is becoming more readily available as a diagnostic tool, and it may help somewhat in determining if there is inflammation vs. a circulatory problem. If a circulatory problem is found, treatment with anticoagulants might be indicated. Anticoagulants (blood thinners) were used in the past but were largely abandoned both because they can cause bleeding complications and because not all “navicular” horses responded to them. However, in light of the growing amount of evidence to show that not all horses with heel-area pain have the same causes, a blanket condemnation may not be fair.
Veterinarians today tend to use the drug isoxsuprine, a vasodilator that has some blood-thinning properties as well. Another drug is pentoxifylline, which inhibits clot formation, makes blood flow smoother and may have mild anti-inflammatory effects.
While some veterinarians swear by these treatments, their use is debated because formal studies have found they are poorly absorbed when given orally, and effects on circulation have not been found, at least not in normal horses.
If the thermogram shows a pattern of inflammation, at least you know poor circulation isn’t the problem (an area becomes hot and inflamed because the blood responds to an injury/illness and increases its flow). However, inflammation still doesn’t tell you if the horse has navicular disease, navicular bursitis or something else entirely different. Further potentially useful diagnostics are:
• Bone scan: An inflamed and actively remodeling bone will “light up” on bone scan. Examination of the soft tissues in the first few minutes after the radioactive material is injected (“soft tissue phase”) might also pick up inflammation in the bursa, deep flexor tendon, or other structures
• Ultrasound or MRI: Both these diagnostics are used to better evaluate the soft tissues. MRI shows more detail, including detail of bone, but it isn’t widely available. Ultrasound, on the other hand, is easier to obtain, although you’ll need to find a clinic familiar with using it on the feet.
Normal-looking X-rays don’t mean the horse doesn’t have navicular area pain, and abnormal ones don’t mean he does. You need to rely on other findings:
• History: Slow onset of a lameness that is worse on uneven surfaces. History of foot becoming narrowed/contracted in the heel/frog.
• Gait: Short and choppy, tendency for toe first landing, lameness exaggerated by circling (inside foot worse), worst at a trot.
• Risk factors: Foot small compared to body size. Trimming errors of long toe/low heel and/or lateral imbalance.
• Hoof testers: Painful response to pressure over the middle third of the frog.
• Block tests: Pain is worse if the horse is made to stand with just the toe elevated on a small block of wood then jogged off, or if a narrow piece of wood is positioned under the middle third of the frog before jogging the horse off.
• Nerve blocks: With the recent finding that local anesthetic injected into the coffin joint can also block out causes of navicular area pain, and that posterior digital “heel” nerve blocks (PDB), may at least partially desensitize the coffin joint, differentiating between coffin joint and navicular pain is more of a challenge:
1) Pain relieved quickly by PDB, using low volume of anesthetic, is probably navicular area pain.
2) Pain not relieved by PDB, using low volume of anesthetic, some relief with high volume of anesthetic, definite relief with coffin joint block, means it’s likely coffin-joint pain.
3) Pain relieved by coffin-joint block could be either coffin joint or navicular.
Accurate diagnosis can be difficult, but a widening array of choices for diagnostics is making differentiating true navicular disease from other causes easier. The key is to get as accurate a diagnosis?''and that means the actual cause for the pain?''as you can achieve.