A term used to describe a condition where a horse continuously shakes its head, usually but not only when being ridden. This condition can make a horse unrideable and result in a complete loss of use. The cause of this condition is still subject of intense study. Conditions of the eyes, ears, nose, teeth, cervical spine or nerves are all thought to play a role. The involvement of allergies such as to pollen or dust is likely, due to the seasonal nature of some horse's symptoms. On occasions, sunlight also leads to an aggravation of symptoms.