Lameness specialists and researchers of navicular disease have replaced the term “disease” with the term “syndrome”, writes Dr Robert N. Oglesby DVM.
A syndrome is a commonly recurring group of symptoms of unknown cause. We are not sure why navicular syndrome (NS) occurs. The diagnosis of NS involves trying to rule out some of these better understood causes of lameness. The proximity of these structures and that they lie under a thick horn makes direct examination difficult, but diagnosis is still possible.
The navicular region is a complicated mix of joints, bones, ligaments, tendons, and synovial bursas. All of these structures can succumb to the known causes of lameness referable to this area. Some of the more common causes of lameness in this area are
- subsolar bruising in the frog region
- synovitis of the coffin/navicular joint area
- degeneration of the navicular bone and associated deep digital flexor tendinitis
- fractured navicular bone
This list however can usually be accurately diagnosed with a good exam and radiography. Occasionally you get to the bottom of your exam and radiographs and all you can find is two lame front feet that refers to the navicular region. These are the horses with NS.
One current theory focuses on ischemia as a cause of NS. Ischemia means low oxygen levels in the tissues. The navicular bone (above, blue) is a very metabolically active bone. The bone is constantly remodeling to adjust to changing work loads. This remodeling requires many nutrients, including oxygen. If the bone cannot get enough oxygen, ischemia results. One of the results of ischemic tissues is increased sensitivity to painful stimuli. The navicular bone hurts because it cannot get enough oxygen. It is hypothesized, that if this goes on long enough, degenerative changes occur, resulting in permanently lame horses.
The results of recent work using special dyes and tagging techniques have yielded a second possible mechanism. The deep digital flexor tendon (yellow) and the navicular impar ligament (orange) insert together (red) on the back of the coffin bone. At maximal dorsoflexion of the foot during locomotion, just prior to the foot coming off the ground, shear stress is placed at this intersection of the tendon and ligament.
Improper conditioning, certain anatomic features, and excessive weight may result in damage to the area. In this area there is a rich bed of vessels and nerves along with some of the peptides associated with pain (substance p). In a small group of horses with navicular syndrome each had changes at this intersection consistent with this theory.
The diagnosis rests on history and physical signs. Since there is no proven cause nor diagnostic radiographic changes, a diagnosis is based on:
- verifying the navicular region as the area that is painful
- and ruling out known causes of lameness there.
A history of a slow onset of front limb lameness that initially comes and goes is a frequent finding. As the disease worsens the horse develops a short, choppy gait that may have no or only a very mild head bob at the trot.
Physical Exam Findings
The horse is much worse over hard, irregular footing like gravel. Frequently, in the turns at a trot, the horse will develop a head bob associated with the inside foot: He is lame on one foot going one way, and the other foot going the other way. Lameness of both forefeet is an important diagnostic feature. Occasionally, one limb is significantly more lame than the other and nerve blocks are required to show the bilateral nature of NS.
Diagnostic tests for NS center around stressing the navicular region and seeing if this aggravates the lameness. Hoof testers over the center third of the frog frequently elicits pain. Elevating the toe by having the horse stand with his toe on the handle of a hoof knife and the heel on the ground, creates stress in the deep digital flexor, navicular bursa, and navicular bone and should exacerbate the lameness. Still another test done is by having a horse stand on the handle of a hoof knife arranged to where the knife puts pressure over the central and back portion of the frog. The opposite foot is lifted up to increase pressure on the frog. Again, temporary increase in lameness is supportive of this disease. Many authors say that lack of response (exacerbated lameness) to these stress tests does not mean NS is not present. I think lack of response to the hoof testers over the frog makes it very risky to say that the lameness is coming from this area without further evidence. If there is no response to hoof testers then nerve and articular blocks become critical to the localization.
Nerve and Articular Blocks
A critical test for NS is the posterior digital nerve block. All of the nerves that run to the navicular region branch off the posterior digital nerve (see graphic below). Blocking the nerve at the level of the lateral cartilages should eradicate lameness if from the navicular area. Note well that all of the structures in the navicular region and the heel will be blocked. This makes a thorough exam, including hoof testers, before the block extremely important. Not only does the PDN block help in localizing the lameness, but another important piece of information is obtained: with the obviously lame foot blocked, it allows you to see if lameness is present in the opposite foot.
It should be investigated in the same manner to assure that the soreness originates from the navicular region.Recently it has been verified that the nerve supply to the navicular bone is derived from the proximal suspensory ligament of the n. bone and the distal impar ligament. The significance of this is that lameness caused by navicular disease would be alleviated by intra-articular analgesia of the distal interphalangeal joint (coffin joint). This makes further characterization of this disease possible by combining posterior digital nerve blocks(PDNb) and coffin joint blocks:
- Sound on PDN but not coffin joint block … foot pain that is not coffin joint or navicular related
- Sound on PDN and coffin joint block … foot pain referable to the coffin joint and/or navicular area
- No or little response to low PDN but sound on coffin joint block … coffin joint pain. (Caution: The PDN block may diffuse up the nerve somewhat resulting in significant loss of sensation in the coffin joint. Interpret in light of the location, amount required, and rapidness of response.)
- No response to PDN or coffin jt block … pain is higher than the hoof.
Horses that are not bilaterally lame with soreness and do not completely block out with a PDN block should not be diagnosed with NS.
Traditionally, radiographs were the cornerstone of diagnosing NS. In 1994 at the American Association of Equine Practitioners it was stated by lameness researcher Tracy Turner, DVM, that there is little correlation between radiograph changes of the navicular bone and NS.
This has been the belief in Europe for years and confirms my personal experience. I have seen horses with radiographically terrible navicular bones with no history of lameness and several cases that fit all the signs discussed above but have perfect navicular bones on radiographs. Radiographs are important, not to diagnose NS, but to rule out other problems of the foot. addendum: In 1995 a study by Dr. David Ramey, DVM, of 85 horses showed no correlation between radiographs and navicular syndrome.
Treatment and Prognosis
There is no treatment that is always successful. Generally a combination of proper shoeing, phenylbutazone, and isoxsuprine will allow approximately 2/3 of the horses to return to use if the disease is treated early. Another important finding is the localization of pain mediating neuropeptides. Not only do these chemicals mediate pain, promote vasodilation, and even the inflammatory process. That is these chemicals may cause the pain and inflammation. It would seem possible that if we can develop ways to reduce or block these peptides we might be able to effectively treat navicular syndrome.
Trimming and Shoeing: Many arguments surround what is good trimming for this problem. As a rule it makes the most sense to correct any imbalances in the foot and trim the foot to properly align the three bones making up the toe and pastern, then shorten the toe as much as possible. Toe and quarter rockers make sense also.
Many navicular horses will respond to good shoeing. Some stay usably sound following the farriers visit for four weeks, only to have the gait begin to shorten until retrimmed and shod. If this fails, try elevating the heels 3 degrees, rocker and roll the toe, and a set of egg bar shoes that fit full particularly in the heel. Another option is to try pads and bar shoes that protect the middle third of the frog.Isoxsuprine: In one study isoxsuprine was shown to be helpful in the treatment of early NS. This has not been my experience and more recent work is having trouble finding a pharmacological response at published doses. On the other hand the medication is safe and low cost. Along with proper shoeing, isoxsuprine has an overall success rate of over 60%. Proper shoeing alone has a success rate of only 30%. This drug works by increasing the circulation to the bone, so that it may repair and remodel its shape to adapt to changing stress. Dosage should start at 500 mg./1000 lbs. twice daily. The dosage is then adjusted every two weeks depending on response. If sound, the horse is reduced to once daily dosing and then weaned off the drug over the next 45 days. Remember that isoxsuprine is a forbidden substance by the American Horse Shows Association.Neurectomy: Though the last resort of horses with NS is neurectomy, neurectomy is very effective and has a low complication rate when done in a hospital setting under general anesthesia . The nerves (yellow) to the navicular region are severed at the level of the heavy black line.As large a segment as is possible is removed as are all obvious branches. Though complication rate is low some are serious and often the nerves regrow in two or three years.
Recent improvements in surgical procedures may increase the time before regrowth. It has been recommended that two incisions be made, above and below the extensor ligament (orange). The nerve is transected in both locations, heavy and light line, and a section removed.
This article reprinted with permission from Horseadvice.com, an internet information resource for the equestrian and horse industry since 1994. On the WWW at www.horseadvice.com we have tens of thousands of documents on the web about horse care, diseases, and training. © 2005
Originally published on Horsetalk in 2005