Unfortunately, human beings aren't the only mammals to struggle with birth defects. One of the more serious problems that can affect canines is known as an "open fontanel," which occurs when the skull bones at the top of the head fail to close. The problem is often found in conjunction with hydrocephalus, which is a condition in which too much fluid is found within and around the brain, placing pressure on the brain and surrounding tissues. Often the head will appear dome-shaped, and the open fontanel is noticeable as a "soft spot" on the top of the dog's head. The fluid-filled spaces within the brain, known as ventricles, also become swollen. The increased pressure damages or prevents the development of brain tissue.

The good news is that not all open fontanels are connected with hydrocephalus. In many young dogs the skull bones are not fused at birth, but instead will close slowly over a three- to six-month period. Occasionally these bones fail to close, but the dog is still healthy. In these cases, however, the dog's owners need to be very careful, since any injury or bumps to the animal's head could cause significant brain damage, as well as conditions like epilepsy.

In many cases, though, an open fontanel is a symptom of hydrocephalus, commonly known as "water on the brain." The latter can appear as a congenital birth defect or can result from head injuries and trauma. In the congenital variety, the dog's body simply forms too much fluid, which is unable to drain.

Dogs with open fontanels and hydrocephalus often are blind, have seizures, a misshapen head and/or an unusual walk. Most also have brain damage that impairs their intelligence and leaves them permanently restless, yet with a limited ability to learn. Cases are usually diagnosed before the puppy is four months old, and the outlook for these animals is grim.

Surgery by a veterinary neurologist occasionally can drain the excess fluid and provide some relief. Fluid-reducing medications like prednisone and Lasix also may help. Treatment for hydrocephalus is expensive and very often unsuccessful; for this reason many pet owners opt to have the animal put down or else left untreated. Even with the best of care, most dogs with hydrocephalus do not live beyond two years. Whatever treatment is or isn't prescribed, dogs with this condition should not be bred.

Open fontanels and hydrocephalus can occur in any dog, but are most prevalent in the toy breeds, including Chihuahuas, Maltese, Pomeranians and Yorkshire Terriers.