Many babies, including humans and puppies, are born with a soft spot in their skulls called the fontanels. This area where the plates of the skull haven't yet closed allows easier passage through the birth canal. After birth, the plates begin hardening. In a puppy, the fontanel usually closes by the age of 4 to 6 weeks. In a puppy whose fontanel doesn't completely close, problems can occur.
A puppy whose fontanel doesn't completely close has an open area at the top of his skull. Depending on how much bony plate growth occurred, the spot may range in size from that a penny to that of half-dollar coin, according to WebMD. But if the fontanel hasn't closed by the time a puppy is weaned, that doesn't mean it won't happen. If the hole is still there by the dog's first birthday, it's not likely to close further.
While the fontanel can fail to close in any dog, toy breeds are most often affected. Dog breeds predisposed to open fontanels include the Chihuahua, Yorkshire terrier, shih tzu, Maltese and Pomeranian. It's particularly prevalent in so-called teacup dogs, the extremely tiny versions of the littlest breeds. Because the condition is hereditary, affected dogs shouldn't be bred.
Because your dog has that vulnerable area in his skull, you must take extra precautions to keep him safe from trauma. A mild bump on the head that wouldn't bother the average dog can have serious consequences if it involves a soft spot in the skull that can't protect the brain. It's probably not a good idea for a dog with an open fontanel to live with young children or active dogs. Most dogs with open fontanels are fairly normal otherwise, so you must protect them from rough play. Inform your groomer about the fontanel so she is extra careful about dealing with the dog's head.
Hydrocephalus, also known as water on the brain, sometimes develops in puppies with open fontanels. Hydrocephalus occurs when cerebrospinal fluid in the brain can't drain properly, leading to a domed shape in the head from fluid collection. Puppies with hydrocephalus usually show neurological symptoms by the age of 3 to 4 months. These include seizures, blindness and downcast eyes. Puppies that don't suffer from obvious symptoms might just be a lot "slower" and smaller than their littermates. In mildly affected dogs, medication can relieve some of the fluids. Severely affected dogs require surgery to install shunts in their brains that drain fluid to elsewhere in the body.
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