Hello and thanks for researching this very important question!
A number of things can cause a disturbance of the "vestibular" system in the brain, which is responsible for balance, among other things. These conditions include (but are not limited to): 1) ear infection: a dog's ear canal runs all the way down the side of the face, so there may be irritation deep down against the ear drum that is not visible from outside;2) cancer in the brain,;3) contamination of the brain with waste products usually handled by healthy kidneys and liver; OR
4) a stroke, or other kind of bleeding/blockage episode in the brain...pretty rare in dogs, but can be confirmed with an MRI scan. Most of these pets have other serious disabilities.
The chance of more severe problems increases with increased age of your dog. Symptoms of vestibular disturbance include rapid eye movement from side-to-side, uncoordinated movement or inability to stand or walk due to dizziness, nausea and/or vomit (with refusal or inability to eat), spastic head movements, and a tilt in the orientation of the head. Affected dogs prefer to lie on one side of the body only. Mild symptoms can worsen if there is a serious health problem at the root of the head tilt.
There is such a thing as "Idiopathic Vestibular Syndrome"...(idiopathic--pathology which doesn't make sense)...which does not have a cause that we can find. This is pretty common in middle-aged and older dogs. The most severe symptoms fade (or become adjusted to!) over the course of a few weeks, and the most prominent thing we might see long term is the head-tilt...looks weird, but is not a serious problem in itself.
There are medications available to treat the dizzy and nauseated symptoms themselves: you would ask your vet for a prescription of either diphenhydramine or meclizine hydrochloride to help with these problems.
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