In this month’s equine vet feature, head shaking is discussed by Dr Sarah Davidson, BVMS MRCVS of the Sussex Equine Hospital at Ashington.
Dr. Sarah. Davidson, BVMS, MRCVS of Sussex Equine Hospital. Ashington.
Head shaking in horses is a mysterious condition that can be incredibly frustrating to diagnose, manage and treat. There is a wide spectrum of severity, from a mild, occasional ‘tic’ to a persistent and sometimes violent shaking of the head.
Most commonly, head shaking occurs in a vertical direction, but can also manifest itself from side to side. Head shaking is involuntary and, while we do not understand a huge amount about the condition, we know it is a painful condition that can justify euthanasia in its most severe form.
Clinical presentation of head shaking is most often a downward jerking of the nose followed by tossing up of the head. Also sometimes apparent is rubbing of the nose and/ or face that suggests the horse is in pain.
This pain is believed to stem from trigeminal nerve hypersensitivity, although we commonly do not see any structural change to the nerve or surrounding tissue during investigation.
The onset of trigeminal-mediated, sometimes called idiopathic head shaking, is most often sudden, but has been known to come on gradually and appears to affect horses between the ages of six and 12. Head shaking is usually more prevalent when the horse is working and less pronounced at rest, although some horses show no signs at all of the syndrome at rest.
It has also been noted that changes in season affect some horses with typical head shaking behaviour seen more frequently in spring and summer and it appears that insects, wind, pollen, dust and rain can all be triggers for the condition.
Diagnosing head shaking is based on a number of factors. Observing the horse at rest and while being ridden is a key part of the investigation, it may also be deemed necessary to observe the horse in lots of different conditions: wet, dry, sunny, overcast, indoor, outdoor and so on.
This can help to highlight whether there is a trigger for the condition. Ruling out other conditions that may affect the head is also very important. Eye irritation, inner and outer ear disease, dental pain and guttural pouch or sinus abnormalities may all cause your horse to display behaviour that could be perceived as head shaking.
These investigations take the form of radiography, endoscopy, detailed ear, eye and dental examinations and. if these fail to highlight a cause. your horse may be referred for a CT or MRI scan. If you suspect your horse is a head shaker and you have the opportunity to video the behavior, this can help greatly with the investigation as, in mild cases, your vet may be unlucky enough to examine your horse on a ‘good’ day.
Following a thorough investigation, if no abnormalities have been detected, a diagnosis of trigeminal-mediated head shaking will often be made. This can be categorised further based on seasonality, intermittency and severity (a grade one being mild up to a grade five when the horse is distressed and uncontrollable).
As with many things in veterinary medicine that have a variety of treatment options, it usually means that no one treatment is reliably successful. Finding the trigger, which often centres around some form of nasal stimulation, can be the most important part of treatment in a lot of horses.
This is where nose nets and face masks come into play. It is thought that a nose net or face mask alters the airflow pattern, temperature of the air or particle content meeting the nasal passages and helps to control symptoms.
Other treatment options include tinted contact lenses which combat head shaking in horses that are light sensitive, antihistamine drugs such as hydroxyzine and cyproheptadine (although side-effects of lethargy and anorexia occasionally outweigh the benefits), gabapentin to treat generalised neuropathic pain and dexamethasone to reduce inflammation.
Supplementing magnesium is also discussed as a treatment option and is thought to work by reducing the threshold for nerve firing and hence decrease the hypersensitivity to external triggers. A treatment called PENS (percutaneous electrical nerve stimulation) has recently been developed by the University of Bristol and this involves repeated nerve stimulation under standing sedation, which results in periods of remission of increasing duration and/or resolution of clinical signs. In 39 per cent of cases, it has been documented that horses have been able to return to previous levels of work following PENS treatment.
Response to treatment is very variable and notably not always successful. Treatment may involve multiple trials of different options and in some severe cases more than one treatment at any one time with each being withdrawn individually to determine which is improving clinical signs.
Head shaking is a condition that is often managed rather than cured and has been known to be progressive in some cases. Unfortunately, in up to half of cases, it is not possible to find a treatment or effective management protocol and it is important to remember that unlike in previous years when we suspected it was a behavioural condition, we now know it is attributed to pain.
If a horse has severe, or persistent head shaking and no treatment improves clinical signs, welfare of that horse is paramount and euthanasia should be strongly considered.