Navicular Syndrome is a condition of the feet that results in lameness. It most commonly presents as a forelimb lameness in one leg, but on investigation is often found to be present in both front feet. Although uncommon, it can also occur in the hind feet. The lameness that is seen varies from slight to moderate; only rarely is severe lameness seen. Working in a circle often exacerbates the lameness, usually on the inside leg.
Navicular Syndrome is more often a disease of horses than ponies, and is vastly more common in horses with the typical thoroughbred-like foot conformation i.e. low, collapsed heels and long toes. Certain types of work may also be a risk factor; horses that do a lot of jumping or road work are more often found to be suffering.
What is Navicular Syndrome?
Twenty years ago vets were sure that navicular disease was a degeneration of the navicular bone in the foot, however we now recognise that this is but a small part of the condition. This “classic” disease of the navicular bone is now known as navicular bone disease, and forms one part of navicular syndrome. The other part of navicular syndrome is degeneration or damage to other structures in the foot. Overall, the potential structures that can be involved in navicular syndrome are:
- Navicular bone
- Navicular bursa
- Suspensory ligaments of the navicular bone
- Impar Ligament (Distal Sesamoidean Impar Ligament = DSIL)
- Deep digital flexor tendon (DDFT)
- Coffin Joint (Distal InterPhalangeal Joint = DIPJ)
- Collateral ligaments of the coffin joint
In a particular horse, any combination of the above structures may be involved.
How Is Navicular Diagnosed?
Horses with navicular syndrome often have mild forelimb lameness, which can be intermittent and may appear to be in different legs at different times. Basic treatment with rest and anti-inflammatories is often a short term success, but in the long term the lameness will recur. It is usually at this stage that we embark on a “lameness workup” to identify the cause of the lameness.
The horse will be examined at walk and trot in both a straight line and on the lunge. We are fortunate to have a hard surface suitable for lunging, as well as a soft school; different horses are more or less lame on different footing. If necessary then we will also watch the horse being ridden. The level of lameness will be graded out of ten in all situations.
The next step is to perform nerve blocks to identify the region of the horse’s leg that is causing the lameness. It is vital to perform these, so that any imaging performed later is of the correct region. A small amount of local anaesthetic is injected under the skin around the nerves that supply the foot so they can no longer carry pain signals. If the pain is coming from below the level of the nerve block, then the horse’s lameness will improve. A positive response to a nerve block is usually defined as more than a 50% improvement in the grade of lameness. Due to the fact that both fore feet are commonly involved, desensitisation of the initially lame foot will often result in the opposite foot showing lameness after the block.
After performance of the first nerve block, most horses with navicular will show a positive response. Occasionally a second nerve block is needed, a little higher up the leg. This is most often the case with lesions in the Deep Digital Flexor Tendon, but certainly not exclusively.
Once the lameness has been localised to the foot, a selective desensitisation of the navicular bursa is usually performed to confirm the diagnosis. This is usually done the following day as the first nerve blocks must first wear off.
Imaging The Foot
After the confirming that the horse’s lameness is coming from the foot/feet, it is now necessary to perform some imaging studies to assess what changes are occurring. X-rays are taken of both front feet which allow examination of the navicular bone, pedal bone and of the coffin joint. This allows us to evaluate the severity of changes that are present, and also to eliminate other problems that can present in the same way, such as coffin joint arthritis and pedal bone fractures.
Ultrasonography (ultrasound) is also a useful tool we have at our disposal. As mentioned earlier, the bony changes are a single part of navicular syndrome and we must also try to image the soft tissues. Ultrasound enables examination of the collateral ligaments of the coffin joint, and depending on the horse’s conformation also parts of the deep digital flexor tendon.
In recent years, there has been an interest in MRI (Magnetic Resonance Imaging) of horse’s feet for navicular syndrome. There is no doubt that this is by far the most detailed and accurate way to “see inside” a horse’s foot, but there are significant downsides, mainly related to its expense. An MRI scan of both front feet costs (at present) around £1200, and this is a cost that is not covered at all in many insurance policies, and covered only 50% in others. It is able to give an exact diagnosis of the areas affected and to what degree, which then allows an accurate prognosis to be given. However, it is unlikely to dramatically change what treatment is performed. The other disadvantage is the need to refer the horse to another veterinary hospital (we usually refer to The University of Liverpool at Leahurst), with further delays and travelling expenses. We currently think that MRI does not offer significant advantages that match its significant cost, and typically recommend that available finance is used for treatment instead. Every case is individual however, and the option is always available
Treating Navicular Syndrome
There are many treatment options available for navicular syndrome, and many treatments that have been used in the past that are now known to be of little use.
Rest and Controlled Exercise
Although it is not always desirable, rest from work followed by a strict controlled exercise program is essential to ensure the best chance of recovery. Your vet will discuss the details with you, depending on the particulars of each case.
Many horses with navicular syndrome have a low heel-long toe foot conformation, and working with your farrier we will aim to correct this as far as possible. The general aims are usually a more upright foot, with greater weight-bearing toward the heel. Egg bar, heart bar and full round shoes have all been used, in conjunction with heel wedges when required. Sole packing with flexible silicon can also be useful. Assessment of foot balance, both externally and based on the x-rays taken previously will be used to guide the farriery process.
Tildren (tiludronate) is a drug that has come into common usage over the past 5-10 years. Its primary purpose is in treating the navicular bone disease part of navicular syndrome. In order to understand its action, we must first look at normal bones. Old bone is being constantly reabsorbed (by cells called osteoclasts), with replacement bone being produced (by cells called osteoblasts). Navicular bone disease occurs when the action of the osteoclasts (that reabsorb old bone) increases, so that new bone production cannot keep up. This results in a weaker, less dense bone. Tildren acts to slow down the osteoclastic bone resorption, allowing new bone production to catch up and produce a denser more robust bone.
The drug is given intravenously, and must be given very slowly; for this reason it is diluted into a litre of saline, and given as a drip over 30 minutes. The only side effect is the potential to cause mild temporary colic signs, however when given slowly, with either mild pain relief or sedation we see this very rarely. Tildren is usually given twice, 6-8 weeks apart with a 3rd treatment 6 months later if required.
Tildren is a good treatment option for the bony changes associated with navicular syndrome, however, we must also treat any soft tissue injuries that are present. Adequan and Cartrophen are drugs known as PSGAGs (Polysulphylate GlycosAminoGlycans) that are used for this purpose. They contain the “building blocks” of ligaments and tendons, to ensure adequate supply for the best possible repair of these injuries. They also have some anti-inflammatory effect.
They are given as a series of seven intra-muscular injections (just like vaccination), each 5 days apart.
Steroids injected into the navicular bursa are superb anti-inflammatories, and can have an extremely strong effect on lameness. They are often included when performing a navicular bursa block to avoid repeated injection of the same area. Whenever steroids are injected into a horse, there is a small risk of causing laminitis. This risk is very small, but it can happen so we will always discuss the potential risks and benefits with you before going ahead.
There is a huge variety of oral supplements available for joint, bone and ligament disease. They usually contain glucosamine, chondroitin or MSM (methyl-sulphonyl-methane), along with various other ingredients. The evidence for their effect is not strong, but they can be helpful for long-term management in some cases. It is important to buy a reputable brand, as cheaper alternatives have been known to not contain what they claim. Currently we feel the Simon Constable ASU is a most suitable product.
There is one potential surgical option for horse with navicular, however, it is seldom used within our practice. Known as a Palmar Digital Neurectomy it involves cutting the nerves that supply to the foot. Unfortunately this means that serious issues can go undetected in the future, as the horse will show no lameness. The biggest problem with the surgery is that they nerves will often regrow with 2-3 years, with a much worse lameness present when sensation returns.
Navicular syndrome is a lifelong condition, however, many horse can return to athletic function and soundness for long periods of time. Once the initial lameness is resolved, then corrective farriery can keep a horse from having repeated flare ups. Certain types of the syndrome have a worse prognosis – horses with DDFT lesions are less likely to recover and be able to perform athletically.