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An open fontanelle in dogs in simple terms consists of a "hole" in the dog's head.

 To be more exact, that 'hole," is the result of an incomplete ossification of the bones in the center of the puppy's skull, right in between the frontal and parietal bones.

Technically speaking, this soft spot is known as "bregmatic fontanelle." It's basically, the equivalent of the  anterior fontanelle that is also found in human babies.

Open fontanelle are commonly found in several small, toy-breed dogs with domed heads including Chihuahuas, Pomeranians, miniature Dachshunds, Maltese, Pekingese and Lhasa Apsos. 

When Do Open Fontanelle Close in Dogs?

Open fontanelle in dogs will typically close by the time puppies reach 9 to 12 weeks of age. 

As the affected puppies mature, the growth plates found in their skull will gradually fuse so that the open fontanelle will shrink until the hole no longer exists. 

However, although a puppy's fontanelles closes with age, some pups are born with a permanent open fontanelle. 

Open fontanelle that don't close by the time the puppy is 3 to 4 months of age are diagnosed as persistent open fontanelles, also  technically referred to as "permanent bregmatic fontanelle." 

Picture of open fontanelle 

Picture of open fontanelle 

My Dog Has a 1 CM Fontanelle

An open fontanelle of 1 cm is generally the cutoff size for a fontanel that will close. It may be a normal size for the pup's size and breed. In general, open fontanelles that are larger than 1 cm will be unlikely to fuse closed.   

If the fontanelle does not close by the time the puppy is 3 to 4 months of age, it will likely stay permanently open. 

In the Chihuahua dog breed, the open fontanel is referred to as a "molera" and is listed as part of the breed standard.

Problems With Permanently Opened Fontanelles in Dogs 

These dogs require extreme care. Even a light blow to the head can kill them. If you have children, they should be instructed to never try to stick a finger in or press down on the fontanelle as this may lead to brain damage.

In addition, affected dogs are sensitive to cold, and may shiver uncontrollably.

Dog owners should consult a veterinarian if they notice an open fontanelle in their puppy. 

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The underlying cause of this condition is unknown. The condition can be genetic or acquired. An ultrasound of the brain is helpful to rule out any abnormalities

Did you know? According to a Finnish study, persistent fontanelles in Chihuahuas were associated with small body size, syringomyelia, ventriculomegaly and craniocervical junction abnormalities. 

Dangers of Hydrocephalus

Because some dogs with moleras are prone to congenital hydrocephalus, there's a  risk of this condition. The build-up of cerebrospinal fluid in the skull and brain causes pressure on the brain and spinal cord, causing chronic pain and loss of function.

 The good news is that there's a high recovery rate for this condition. 

If the puppy has hydrocephalus, it's important to seek help as early as possible. In some cases, shunting is the only viable option, but it does carry significant risks. 

The shunt is a tube implanted into the brain's ventricle. This tube drains excess cerebrospinal fluid. This procedure may require additional surgeries as the puppy grows, but the overall success rate is around 80 percent.

Treatment for hydrocephalus in dogs ultimately aims to control intracranial pressure and reduce the production of cerebrospinal fluid to alleviate the symptoms. In addition to corticosteroids and diuretics, hydrocephalus in dogs may require surgery to drain the fluid from the brain

Regardless of the type of treatment, the dog should undergo regular veterinary follow-ups to monitor progress and monitor any recurring problems.

Although open fontanels can lead to a long and healthy life, it requires special handling and monitoring to prevent complications. 

While the presence of a molera in a dog's head is not a sure sign of hydrocephalus, there is recent research that has found connections between persistent fontanelle and health issues.

What Recent Studies Reveal 

A recent study conducted in 2021 found some possible connections between persistent fontanelle with other structural abnormalities. 

Furthermore, an association was found between lower body weight in Chihuahuas and a higher number of and larger persistent fontanelles. 

Both these findings question the ethics of selective breeding of Chihuahuas that have very low body weights. More studies are needed on this. 

References:

 Kiviranta, A‐M, Rusbridge, C, Lappalainen, AK, Junnila, JJT, Jokinen, TS. Persistent fontanelles in Chihuahuas. Part II: Association with craniocervical junction abnormalities, syringomyelia, and ventricular volume. J Vet Intern Med. 2021

Rivers WJ, Walter PA Hydrocephalus in the Dog: Utility of Ultrasonography as an Alternate Diagnostic Imaging Technique, Journal of the American Animal Hospital Association, 1992,

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