Written by Heather Smith Thomas in the June 2002 issue of Barrel Horse News
Head shaking syndrome in horses is a sign of disease, rather than a disease in itself.
There are a number of causes, and several veterinarian shave done studies to explore these causes and find ways to treat them. Linda Blythe, DVM, PhD (Oregon State University) has done much of the work on one of the more serious causes of head shaking – middle ear infections that can affect the joint between the stylohyoid bone and the skull, a situation that can become life threatening. In her 22-year career, most of her research has been in horses with neurologic problems.
“We started working with head-shaking horses 20 years ago when we opened this clinic,” she said. “One of our first cases was a head-shaking horse with neurologic signs, and it seemed like it might have a middle ear problem. The owner was going to put the horse down, but instead, she trailered it down from Washington to Oregon State. This horse was one of the first patients in our hospital, and we found a disease that we subsequently named temporohyoid osteoarthropathy.
“That first year, we saw three horses with this disease, two of which had a history of head shaking or ear rubbing. What we found was a middle ear problem that causes an excess production of bone, which destroys the temporohyoid joint with the stylohyoid bone that comes up from the tongue and the larynx. When we discovered this disease process, we found that this joint fuses. Then, when the horse moves its tongue, it can fracture the next weakest point in the skull, in the area of the inner and middle ear.
“Subsequently, we’ve seen about 50 cases. Not all horses with this ear problem exhibit head shaking, but many do. Some of them have head shaking as the only clinical sign. More have a history of head shaking prior to the development of acute neurological deficits, and some owners never notice that the horses are having a problem; they just find the horse down one day, and it can’t get up. The muscles of the horse’s face are twisted to one side, and the head is held at a tilt; both are clinical signs of temporohyoid osteoarthropathy.”
These horses often have a corneal ulcer, due to inability to shut the eye or spread the tears over it (the eye becomes dry and damaged). Often, this is the first clinical sign noticed by the horse owner.
“So that’s how we got into working on one of the causes of head shaking,” Blythe said. “There are multiple causes – everything from something in the external ear canal, to dental problems. There was a study in England (authored by Tim Mair) that looked at 100 cases of head shaking. This study was written up in the Equine Veterinary Journal in 1987. They found reasons for the head shaking in only 11 of the cases and the others they could never find a definitive diagnosis for the head shaking. However, no radiographs were taken to rule out temporohyoid osteoarthropathy.
“Some of the things they found in the 11 were inner ear infections, dental problems, cranial nerves dysfunctioning (something wrong with the brain), guttural pouch problems, fungal infections, injuries to the neck, and eye disease. These comprised the reasons for the head shaking in the 11 for
which they found causes. They postulated that because you see more cases in spring, summer and fall (a seasonal pattern), there could be allergy involved. In a later publication, Mair described a larval form of a parasite associated with head shaking.”
Another thought on the subject has been put forth by Dr. W.R Cook. Blythe said, “Cook has researched and written papers on head shaking and listed 58 diseases in which head shaking could be a clinical sign. In his later review articles, he said he thought that when a horse is a roarer (the airway obstructed by a partial paralysis of the larynx, which droops down to block the air passage – a condition which causes the horse to breathe noisily), and the horse is working with head tucked, it cannot get enough air. He says this can be a factor in highly collected performance horses, such as the dressage horse. So they shake their heads, due to not being able to get enough air, and wanting to straighten out their head and neck.”
Blythe said that one of the newest discovered reasons for head shaking has been described by Dr. John Madigan (UC Davis), as a photophobic reaction to sunlight, similar to humans who sneeze in bright sunshine.
“When these horses are in bright light, it irritates the trigeminal nerve and they start head shaking,” Blythe said. “You can diagric>se those, because if you put them where there’s no sunlight, like working in an indoor arena, the horse won’t head-shake. Or you can use a face mask or gray lenses on a horse working out in the sun, and see if he stops doing it.
“Other possibilities you have to think about are horses with EPM (Equine Protozoal Myeloencephalitis). This has recently been shown to be a possible cause of head shaking. Three horses that had head shaking as the only sign were shown to be EPM positive. They were treated for EPM and head shaking stopped.
“That’s not proof that EPM was the reason for the head shaking; it might have been coincidence it stopped. Or the horses could have been accidentally EPM positive. We know there are some of those.
“So, with a head-shaker, there are many things you have to rule out. And there may even be some horses in which it’s a learned behavior. They’ve discovered that when they head shake, the rider will take the saddle off and put them away; they learn that this behavior gets them out of doing what they don’t want to do.”
This is like a horse that learns to fake lameness while being ridden. Blythe tells of a case in which a foot-stomping horse was thought to have a brain tumor. It was a show horse that got bored when later used by children. She stomped her front feet a few times, and the riding instructors immediately ran and took the child off and took the mare out of the ring. The mare apparently quickly learned that this behavior got her out of work.
“The owner was just sure the mare had a brain tumor – there had to be something very wrong – because she’d known the horse all her life,” she said. “But actually, it was just a very smart horse.”
She said the behavioral head-shakers are a tough challenge, “because you have to rule everything else out that you could possibly treat.”
She has videotapes of horses where it’s not just a jerk of the head; the horse is obviously in pain, “as in the case with a horse suffering from middle ear infection that affects the bone – technically called an osteitis. There’s a joint at the base of the skull that normally moves freely. But when you start to get inflammation of the bone, then it becomes very arthritic, and when the bones comprising the joint move, there is pain. This is the pain related to middle ear disease or temporohyoid osteoarthropathy. Once the joint fuses, there’s no more pain, and the horse doesn’t head-shake anymore. But then, if the horse moves its tongue or larynx, there’s risk of fracturing the skull.”
She tells about one stallion that showed a classic example of this.
“When another stallion was brought into the barn, he challenged the newcomer with a loud whinny, and within a couple minutes was down on the ground; he had fractured his skull,” she said. “The fusion gets so strong that when it breaks, it breaks adjacent to the fused part, right through the inner ear, actually fracturing the skull itself So any bacteria present in the middle ear will go right into the brain. Just the problem of the fracture causes bleeding and irritation.
“That stallion I mentioned was brought to us within 12 hours, but we could not keep him alive. He died of a massive infection of the brain; what was in his middle ear gained entry into the brain.”
She explains that the CSF (cerebrospinal fluid) coats and buffers the brain, acting as a cushion against bumps and blows. An infection from the ear can get into this fluid when the skull fractures, causing an encephalitis. One way to diagnose a middle ear problem, according to Blythe, is to press with your three middle fingers at the base of the cartilage of the ear.
“Sometimes you get a painful response,” she said. “This is one test. It isn’t always painful, but if it is, this can help with the rule-outs and diagnosis. Most horses we treat for temperohyoid osteoarthropathy recover, but we’ve done necropsy on some that were put to sleep or that died, showing where the arthritic changes are located.”
The examination process she goes through with a head-shaker (when the owner wants it diagnosed and treated) is quite involved.
“A complete history as to the onset and frequency of the problem is important,” she said. “A complete physical and neurological examination should also be done. Next, we try to see if it could possibly be a photophobic problem. We have the owner work the horse out of the sun, usually in an indoor arena. A horse that head shakes in full sun, but does not do it in a shaded or indoor arena is a suspect for the photophobic cause. That’s one problem that’s easier to rule out than the rest of them.
“Then we check the external ear for parasites or foreign bodies. If we end up doing a full workup, we anesthetize the horse and radiograph the skull from the ventral dorsal position (on their backs), then we roll the head up on each side and take radiographs of side (oblique) views. All these radiographs allow us to look at the middle ear temporohyoid osteoarthropathy. At the same time, we will also take a CSF tap (cerebrospinal fluid analysis) to see if the horse is EPM positive.
“On occasion, such as with the latest head-shaker we had in here (a donated horse), we checked and cultured the middle ear. This horse had a low-grade infection in both middle ears, with exudate and inflammatory response. It was a chronic infection that bothered the horse. We were able to get a sample, by flushing the middle ear with sterile saline, a procedure called a transtympanic tap, as you do with children with chronic ear problems. With the horse under anesthesia, we take a 4-inch needle
and run it through an otoscope (an instrument for inspecting the ear), and puncture it through the eardrum. You have a preloaded syringe with .5 cc of sterile saline solution, and you inject that into the middle ear, count to 10, and suck it back out. That fluid then goes to a laboratory for culture, and to be examined cytologically.”
She explains that the middle ear should be an air-filled chamber.
“The injected fluid will pick up anything extra in there – any cellular response to inflammation,” she said.
Diagnosis of head shaking is a difficult detective job, and sometimes you come up with everything being normal, and you’ve only found out what things it is not.
“You may be back to looking at it as a behavioral problem,” she said. “But this is really about the only way to approach these cases; you really have to rule out the serious problems first. And that can be a real challenge.”
Treatment varies, of course, according to the cause. One famous neurologist believes that some of these cases are due to an inflammation of the major nerves to the face (trigeminal neuritis), said Blythe, “and one of his ways of diagnosing it is to put some local anesthetic right where this nerve exits the upper part of the horse’s nose, creating a nerve block to see if this stops the head shaking. All of these problems can be causes, so it is a very complicated subject.”
If a person has a dressage horse that might be a roarer, and it’s shaking its head when working with head tucked, Blythe said the horse can be scoped to see if it’s a roarer, to rule that out.
“So, it’s a matter of having a good list of differentials, figuring out what might or might not be causing the problem,” she said. “This gives the client a package, saying here’s what it could be, here’s what we do to rule out each of these. We’ll start with the most benign. We’ll look in the ear, then go on down the line. Once you put the horse under anesthesia, you want to do as many of the tests as possible that must be done with anesthesia, like the transtympanic membrane tap, radiographs of the head, CSF taps, etc.”
Another diagnostic test that helps when a horse has fused bones at the temporohyoid joint below the middle ear is an endoscopic visualization of the joint inside the guttural pouch.
“When the horse is standing, you put a small scope up into what you would call his eustachian tube (his guttural pouch), and you can visualize this joint,” she said. “It has to have a fair amount of change (from the arthritis) to be obvious, and visualized with the scope.”
A paper written on this, from research done at Washington State University, stated that a person could see all the changes earlier with an endoscope than can be seen radiographically, but Blythe disagrees.
“I think you can see changes radiographically before you can see them with the endoscope,” she said. “That’s just my opinion about horses that I have seen and scoped and then radiographed. A lot of the time with a head-shaking horse, the changes haven’t all happened yet; the disease hasn’t advanced far enough to see them. Some of these things can turn up later. So in six months, you should take more radiographs to see if there’s a change. If there’s a chronic inflammatory osteitis (inflammation of the bone), you should see a change in six months.”
As for treatment, if horses are going to fuse that joint, she said, “We know we can go in and take a piece of that bone out, and this does not affect their ability to eat. We can create a false joint, so that the pressure of moving their tongue won’t fracture their skull. We have done experiments with this, to first see if we could do it, and secondly to make sure the horses can do well afterward.”
Veterinarians are now doing this procedure clinically on horses that do fracture.
“This wasn’t really what we designed the surgery for,” Blythe said, “but they found – primarily in California trials – that by going in and doing this surgical procedure to take out part of the stylohyoid
bone, they take the pressure off the fracture site and allow it to have a fibrous union, healing quickly and with less nerve damage. The animals clinically responded quicker. So that’s now being used throughout the U.S. on these kinds of cases.”
She said that this procedure is an attempt to make the best of a possible problem.
“You can’t say for sure that they’re going to fracture,” she said. “Some may never fracture. But if they do, they may die. We have lost three out of about 50 horses – when they fractured we could not control the infection. The others, if you get on them right away, and get antibiotics and anti-inflammatory medication in them, and DMSO to reduce the swelling in the brain, you can save most of them, and they will return to normal function.”
Occasionally, some facial paralysis or scarring of the cornea of the eye may be a residual effect, she says. The head shaking could be the earliest clinical sign of something very serious.
“When we went back and did histories (the AAEP reported on 26 cases) we separated them into three groups – the horses that had head shaking signs only, the horses that had head shaking prior to developing neurologic deficits (acute onsets) and those that just showed up with neurologic deficits,” she said. “That paper was published with a graph of all the clinical signs they showed.”
Head shaking caused by temporohyoid osteoarthropathy has a guarded prognosis, depending upon whether or not the clinical signs cease after the horse is treated. Although the ear infection itself can be treated with antibiotics, the bony involvement and athritic changes can’t be reversed, Blythe said. When the affected joint fuses,.the head shaking ceases.
“The horses with acute or subacute onset of neurologic signs have a guarded to good prognosis, in most cases,” she said. “At OSU, only three of 35 horses that have experienced temporohyoid joint fusion and subsequent skull fracture have died.”
She also said there may be some residual facial nerve paralysis when the nerves are affected, and some eye damage due to lack of corneal lubrication and inability to blink. If a horse starts head shaking, the owner should not just assume it is a behavioral problem; it is actually more often the horse’s reaction to a physical problem that needs to be corrected.