Birth defects are common in canines with one of the most serious of these being a defect known as an "open fontanel”. This defect is often found in conjunction with hydrocephalus and occurs when the bones on the top of the skull fail to join together and close. Hydrocephalus is an excess of fluid within or around the brain which places pressure on the brain and tissues and will often appear as dome-shaped and create a "soft spot" on the top of the dog's head, or the “open fontanel”.

Not all open fontanels are connected with hydrocephalus and generally skull bones in young dogs that are not fused at birth will close over a few months. Occasionally these bones fail to close, but the dog is healthy and owners need to pay special attention to preventing injury or bumping of the animals head which can cause brain damage.

Open skull bones or “open fontanels” as well as hydrocephalus may be seen in any dog breed, but are more commonly seen in toy breeds such as Yorkies, Chihuahuas, Maltese Poodles and Pomeranians.

The outlook for dogs that suffer from open fontanels in conjunction with hydrocephalus is grim and the dogs will more often be blind, have seizures, have a misshapen head and/or an unusual walk and may also have brain damage. Treatment can be in the form of surgery to drain the excess fluid or fluid-reducing medications, but are very expensive and have a low success rate.