Headshaking is, as the name suggests, a condition where the horse involuntarily shakes his head to varying degrees.
In the past, it was thought to be a behavioural problem, but new research has revealed there are endless possible causes, from a sensitivity to pollen to extreme pain across the face.
This pain can occur as the result of a physical problem, such as tooth abscess or trigeminal neuropathy (a severe chronic pain disorder that affects the trigeminal nerve in the face, termed trigeminal-mediated headshaking).
Headshaking commonly affects horses between the ages of five and 10, and is more common in geldings.
It can occur when the horse is at rest or during exercise, where it's referred to as 'glass wall syndrome' as the horse reacts so violently he looks as though he's just hit an invisible wall.
As exercise increases, symptoms tend to worsen.
Although this condition can be seen all year round, one-third of horses are only affected in spring and summer and are known as seasonal headshakers.