The underlying cause of headshaking in horses remains cloaked in uncertainty after researchers in Britain compared it to a similar ailment in humans.
Headshaking in horses is a syndrome in which they spontaneously and repetitively move their head and neck, usually up and down. These movements are often accompanied by signs of nasal irritation.
The condition is usually worse at exercise but may be seen at rest.
No physical cause is identified in 98% of cases, even after extensive clinical investigation by vets.
University of Bristol researcher Veronica Roberts and her colleagues, writing in the journal Frontiers in Veterinary Science, said it was widely accepted that most cases were due to disease or dysfunction of unknown cause affecting the trigeminal nerve. The accepted term for the condition is trigeminal-mediated headshaking.
The trigeminal nerve in headshakers has been shown to be abnormally sensitized.
In a horse affected seasonally, the activation threshold was normal when the horse was tested out of season and therefore not showing clinical signs. This suggested a reversible disease process.
The study team said there was evidence that trigeminal-mediated headshakers suffered facial pain, as signs can be temporarily abolished by use of local anesthetic.
Trigeminal neuralgia is a human neuropathic facial pain syndrome. It is usually caused by demyelination – that’s deterioration or damage to the protective layer – around the trigeminal sensory fibers within either the nerve root or, less commonly, the brainstem.
Causes can involve compression by an overlying artery or vein multiple, multiple sclerosis and, probably, compressive space-occupying masses in the posterior fossa (a small space in the skull found near the brainstem and cerebellum).
In humans, determining the underlying cause has resulted in the development of targeted treatments.
The study team set out to investigate the possibility that the neuropathological cause of headshaking in horses was similar to that of trigeminal neuralgia in humans.
For the study, samples were collected from the heads of six horses euthanized as a result of their trigeminal-mediated headshaking after failing to respond to treatment. Three had undergone “coil compression” surgery, which had failed to alleviate the problem.
The control samples were collected from four horses at an abattoir. They has been deemed fit for slaughter and were assumed to be free of the condition.
The trigeminal nerves, nerve roots, ganglia, infraorbital, and caudal nasal nerves were subjected to microscopic examination by a veterinary pathologist and a neuropathologist with particular experience of trigeminal neuralgia histology.
No histological (microscopic) differences were detected between samples from horses with headshaking and those from normal horses.
In particular, there was no evidence to suggest demyelination, as often observed in people with trigeminal neuralgia.
“These results suggest that, while trigeminal-mediated headshaking in horses has apparent clinical similarity to trigeminal neuralgia in human, it may have a different underlying pathological substrate,” the study team reported.
Discussing their findings, the researchers said both headshaking in horses and trigeminal neuralgia in humans appeared to be acquired, although in people it usually occurred in an older equivalent population than in horses.
There were, they noted, a number of other clinical differences. In trigeminal neuralgia, there is a slight female predominance, whereas trigeminal-mediated headshaking may be more common in neutered males. Trigeminal neuralgia is typically unilateral, whereas in horses the trigeminal nerve is affected bilaterally.
Symptoms of the human condition were usually triggered by stimuli, such as touching, chewing, talking, or tooth brushing, that affected “trigger zones”.
Signs of trigeminal-mediated headshaking may be evident at rest but were often sparked by exercise, although in some cases signs manifest only when exercise is performed outdoors and not indoors. In some cases, signs may occur only during the spring and summer months and may be worsened in bright sunlight and wind.
“This suggests a complex role of the environment in the development of the condition.”
People may suffer post-herpetic neuralgia, but trigeminal-mediated headshaking has been shown to not be associated with latent herpes virus infection.
There were also differences at an electrophysiological level, they said.
“There appears to be no structural abnormality in the trigeminal nerve of horses affected by trigeminal-mediated headshaking.
“A functional, non-structural abnormality may reflect central pathology or membrane instability and raise the possibility that the condition is potentially reversible, as occurs naturally in seasonally affected horses,” they said.
“Further work should be directed at determining the cause of the functional abnormality and the factors which induce it, as this knowledge would allow the development of targeted treatments.
The full study team comprised Roberts and Debra Fews, from the School of Veterinary Clinical Sciences at the University of Bristol; Jennifer McNamara, from the university’s Centre for Comparative and Clinical Anatomy; and Seth Love, from the Department of Neuropathology at the University of Bristol’s Institute of Clinical Neurosciences at Southmead Hospital.
Trigeminal Nerve Root Demyelination Not Seen in Six Horses Diagnosed with Trigeminal-Mediated Headshaking
Veronica L. Roberts, Debra Fews, Jennifer M. McNamara and Seth Love.
Front. Vet. Sci., 15 May 2017 https://doi.org/10.3389/fvets.2017.00072