Page 77 - May 2017

MIDDLE EAR INFECTIONS, THO, AND SKULL FRACTURES
Lynda Blythe, DVM, PhD, Professor of Neuroscience at Oregon State University (OSU), has done a lot of research on headshak- ing, particularly looking at one of the more serious causes, middle ear infections that can affect the joint between the stylohyoid bone and the skull. This is a situation that can become life threatening. She started working with headshaking horses 28 years ago when the OSU clinic was started. One of her first experiences with this problem that year was a headshaking horse brought to her by a horse owner from Washington, and Blythe discovered a disease that she subsequently named temporohyoid osteoarthropathy (THO).
That first year, Blythe saw three horses
with this disease, two of which had a history
of headshaking or ear rubbing. She found that they had a middle ear problem that caused an excess production of bone that was destroying the temporohyoid joint between the skull and the stylohyoid bone that comes up from the tongue and larynx. The joint fuses and when the horse moves its tongue, this can fracture the next weakest point of the skull in the area of
the inner and middle ear. Some of these horses have headshaking as the only clinical sign. Most of them have a history of headshaking prior to the development of acute neurological problems due to the fractured skull. Some owners never see any signs like headshaking. One day, they just find the horse down and it can’t get up. The facial muscles of the affected horse are twisted to one side and the head is held at a tilt.
The joint at the base of the skull moves freely in a normal horse. But if there’s inflam- mation of the bone, which can be caused
by a middle ear infection, the joint becomes arthritic. When the bones of the joint move,
it causes pain. This is the pain that can cause headshaking. Once the joint fuses, there’s no more pain and the horse stops the headshaking. But then when the horse moves its tongue or larynx, there is risk of fracturing the skull.
Blythe tells about a stallion that provided a classic example. When another stallion was brought into the barn, the resident stallion challenged the newcomer with a loud whinny, which involved moving the larynx forcefully. This fractured his skull and within a couple minutes he was down on the ground.
The fusion from the arthritis inflamma- tion becomes so strong that when it breaks, the fracture is adjacent to the fused portion, right through the inner ear. With the skull fractured at that location, there will be bleed- ing and a lot of irritation, and any bacteria present in the middle ear can go right into
Violent headshaking may be caused by a temporomandibular joint (TMJ) problem with joint damage.
the brain. The stallion Blythe examined was brought to her within 12 hours, but she was unable to save him. He died of a massive infection of the brain.
“I think horse owners today have become more aware of headshaking and more of them understand that it is actually a physical problem.”
This is one reason to get a complete diagnosis if a horse starts to head shake. Skull radiographs in the area of the middle ear can either help
rule out THO or show that this is a problem. If Blythe does a full examination, she anesthetizes the horse and radiographs the skull from below with the horse on its back and from each side, which gives a good look at the middle ear area. Sometimes, she also cultures fluid from the middle ear to see if the horse has an infection or inflammatory response bothering the horse.
If the horse has fused bones at the tempo- rohyoid joint below the middle ear, this may also be seen with an endoscope passed up into the guttural pouch, though it must have a fair amount of change, due to the arthritis, to be obvious and visible with the scope, according to Blythe. She feels that you can see changes
radiographically before you can see them
with the endoscope. With many headshaking horses, the changes have not all happened yet. A disease stemming from a middle ear infection may have not advanced far enough to visual- ize. She recommends taking more radiographs in six months to see if there’s a change because chronic inflammatory osteoarthritis would show a change in six months.
If it looks like the horse is going to fuse
that joint, she treats them surgically by taking
a piece of that bone out. This does not affect the horse’s ability to eat. A false joint can be created, so that tongue pressure won’t fracture the skull. Other veterinarians are now doing this procedure on horses that do fracture. By taking out part of the stylohyoid bone, pressure is taken off the fracture site and it is allowed to create a fibrous rather than a bony union, which heals quickly and with less nerve damage.
Blythe says this procedure is an attempt to resolve a possible problem and prevent the risk for fracture. Some of these horses might not fracture without the surgery, but if they do, they may die if a middle ear infection cannot be controlled. Headshaking can have many other causes, but this behavior can be the earliest clinical sign of something very serious. A middle ear infection can be treated with antibiotics, but bony involvement and arthritic changes can’t be reversed. When the affected joint fuses, the headshaking stops. Surgery
to prevent possible fracture might be the best answer for that horse.
Note: additional information about head- shaking in horses can be found on the website www.headshakerinfo.org.
SPEEDHORSE, May 2017 75
EQUINE HEALTH