By Margaret Evans

All horses will shake their heads to rid themselves of flies, dust, or a minor discomfort. It is a simple reflex action. But when it is persistent, intense, almost dangerous, and the horse is jerking his head up, rubbing, blowing, or constantly shaking as though a fly flew up his nose, it’s time to take a much closer look.

Headshaking syndrome in horses is similar to facial pain syndrome in people. It is a sensation of tingling, pricking, or unpleasant nerve pain known as trigeminal neuralgia. In horses, the condition can be triggered by overactivity of various branches of the trigeminal nerve that brings sensation to the face and muzzle.

“Usually, trigeminal-mediated headshaking is sudden in onset, although may be gradual, often [seen] between the ages of six and twelve, and may be more common in geldings,” says Dr. Veronica Roberts, European specialist in equine internal medicine at the University of Bristol. “Approximately one third of horses are seasonally affected, usually spring/summer. So far, the only fact about the pathogenesis of trigeminal-mediated headshaking of which we can be sure is that the trigeminal nerve of affected horses is sensitized, firing at too low a threshold.”

According to Roberts, the syndrome occurs due to neuropathic facial pain, perhaps from a nerve disease or a change in facial sensation due to a neuropathy of the trigeminal nerve, which is the main sensory nerve to the face.

“We do not know why this neuropathy occurs or indeed what it is that goes wrong in the nerve,” she said. “The nerve appears normal when it is examined under a microscope.”

In the past, the prognosis for the condition has been poor. Some 25 percent of horses will improve, 70 percent of them with the use of a nose net. Various medications, used to some degree of success with people, have been tried on horses but with mixed, short-term results. The costly drugs, though, come with side effects including drowsiness, which prohibits the horse from competing. 

“We were involved at the University of Liverpool in developing a surgical procedure, caudal compression of the infraorbital nerve. It carries a long term approximate success rate of 50 percent, but there is a significant risk of severe side effects which may require euthanasia and about a quarter of the cases relapsed. We therefore only recommend this procedure where euthanasia is the only other option.”

However, a new procedure has been developed and is still being monitored. Clinical trials of percutaneous electrical nerve stimulation (PENS neuromodulation) were pioneered at the University of Bristol in August 2013 and results were published in 2014.

The procedure involved the use of a probe that was inserted just beneath the skin and next to the nerve in question. Seven horses diagnosed with trigeminal-mediated headshaking were selected for the trials. They were sedated and the skin was desensitized with a local anesthetic during the probe’s insertion under ultrasonographic guidance. The nerve was then stimulated for 25 minutes following a protocol of alternating frequencies based on human clinical data. The probe was removed and the procedure repeated on the other side. Three treatments were done during the trials, each one repeated when signs of headshaking returned.

Photo (above): A new study has found that a treatment called percutaneous electrical nerve stimulation could reduce the symptoms of headshaking in horses. Image courtesy of University of Bristol

“All the horses tolerated the procedure extremely well under standing sedation, although there remains a risk to performing any procedure in a conscious horse,” says Roberts. “Side effects have been minimal with a few horses having a hematoma at the site of probe insertion, which resolved uneventfully without treatment and did not appear to cause any problems. Five horses returned to ridden work following their third procedure, with an average remission time of 15.5 weeks. One horse responded to the first procedure but not to later ones.”

Roberts says that, based on the trials, they have continued to offer the PENS treatment for horses that have not responded to a nose net. By August this year they had done the procedure 100 times with all the horses tolerating the procedure well and no significant side effects. However, there were some variables as to how each horse responded, some better than others. Three did not require repeat treatments and have been ridden since spring 2014.

The protocol guideline is that the second procedure is done five days after the first and the third procedure is done about ten days after the second. After that, two to three weeks is allowed to gauge the horse’s response.

“It certainly seems to be the case that horses can respond to the third treatment when they did not [respond] to earlier ones, so we should try three procedures before deciding if the horse will respond to treatment,” she says. “Some horses have a promising start but then fail to respond to later treatments. Some horses don’t respond at all, and some only for a short time. It is my early impression that where horses respond, but for insufficient time, length of remission may increase with repeated procedures. It is still very early days and we have a lot to learn and refine.”

Roberts says that trigeminal-mediated headshaking still carries a poor prognosis, even with the availability of neuromodulation. “I do think the future in treatment lies in regulating the nerve’s function back to normal and neuromodulation may be the way to do that for many individuals. We are still just using a human protocol stuck on to a horse so it needs refinement and development.”

All the horse owners were advised to lunge their horses each day to assess their behaviour following each treatment. If they appeared comfortable and were not headshaking, they could proceed to riding while watching for reactions.

Results of the trials were published in the Equine Veterinary Journal.