Head tremors in dogs are usually a type of seizure known as focal seizures or partial seizures. These can get worse over time, so a dog exhibiting head tremors needs to be taken to the vet. But there are other reasons for a dog to suddenly begin uncontrollable repetitive movements of the head.
According to Dog Owner's Home Veterinary Handbook (Debra M. Eldredge, DVM, et al; 2007) purebred small or toy white dogs such as poodles, Maltese, Bichon Frises or West Highland white terriers are susceptible to white dog shaker syndrome, a mysterious genetic disease that causes head tremors. The dog will not lose consciousness, but the tremors get worse over time.
Dogs have a tendency to not only explore the world with their mouths, but to swallow anything that resembles food. Head tremors or any loss of coordination can be symptoms that the dog has swallowed some kind of poison. This could be drinking antifreeze, eating poisonous plants, eating chocolate, swallowing illegal or prescription medications or ingesting other chemical poisons. Other symptoms of poisoning include vomiting, diarrhoea, excessive drooling or sudden bleeding from the nose, mouth or anus.
Some neurological diseases which affect coordination or brain function can also cause uncontrollable head tremors in dogs. These diseases include hydrocephalus or "water on the brain" (which makes the dog or puppy's head swell up), Lyme disease (caused by tick bites) and granulomatous meningoencephalits (GME). According to Mar Vista Animal medical Center, although GME can be caused by another illness such as rabies, sometimes the causes are mysterious.
Dog Owner's Home Veterinary Handbook notes that dogs with focal seizures often have a severe problem with the brain, kidney or liver. Brain problems could be injury, brain abscess or brain tumour. Kidney or liver failure can be the result of disease or poison. Diagnostic tests like blood tests, X-rays or CAT scans can show if the seizures are caused by some sort of damage to the internal organs.
Although not as dramatic as a grand mal seizure, focal seizures can also be the symptom of epilepsy. A dog can get epilepsy in three ways---from genetics, from a head injury that produced a lot of scar tissue in the brain or for an unknown reason. Dog breeds most prone to canine epilepsy include beagles, cocker spaniels, wire fox terriers, poodles, Irish setters, golden retrievers, Labrador retrievers, miniature schnauzers and German shepherd dogs.