There is an opening in the skull of chihuahuas and many other toy breed of dogs that seems to be the center of much discussion. While there are many openings in the skull, this one in particular seems to draw much more attention. This opening is centered in the forehead and can be very tiny to very large. The edges can be round and smooth or jagged. We are going to discuss this opening in regard to chihuahuas but this can also apply to other breeds.
      In the chihuahua, this opening is called a "molera". While not all chihuahuas are born with this opening, it was once considered to be a "mark of being purebred". But there are many purebred and very well bred chihuahuas that have little or no molera. While this discussion about this opening could go on and on for a good little while, we think that if you are researching this subject it is because you have some concerns about the repercussions of an animal that has this condition. Unfortunately some Veterinarians have the misconception that a large open entrance in the forehead is also an indication of a more serious condition called "hydrocephalus". In layman's terms this means "water head" or water on the brain. You can also hear them use the term "open fontanel".  Actually, the fontanel is openings in the skull located about where you would look for eyebrows if this was a human child. Toy puppies and chihuahua puppies are all born with "open fontanels" that quickly close up and calcify to make a strong skull. Open fontanels allow the skull to give during birth to some degree so the puppy can pass through the birth canal. The skull also has openings at the point of ear placement and also the rear quadroon of the skull (the part behind the ears down to the base of the skull) has a crease that is open at birth from the molera down the back of the head to the spinal column. Most all of these openings should be closed on healthy puppies by the time they are 3 to 4 months old. Visible signs of a problem are usually found with enlargement (a swelling appearance) of the dome of the skull usually accompanied by the eyes appearing to rotate outward showing excess whites of the eyes at the point where the eyes join the nose or muzzle. Upon feeling the open points of the skull, you will find the fontanels are widening and the crack from the molera down the back of the skull is open. An x-ray is NOT a good indication of hydrocephalus. Instead it is common place to use either "ultra-sound" or an "MRI" for definite determination.

     The following excerpt was taken from a popular "Home Veterinary Handbook":     "During a physical examination, feel on top of the head for a soft spot. If present, the fontanel is open. This is not desirable. In Toy breeds a large dome, sunken eyes and open fontanel suggest hydrocephalus."

     This statement sums it up in a nutshell why so many Chihuahuas get misdiagnosed as being hydrocephalic. In a well bred chihuahua, a large dome is desirable and an open fontanel (or molera) is totally normal with no indication of a defect.

Hydrocephalus  is caused by an excessive accumulation of cerebrospinal fluid in the ventricles of the brain. This compresses the brain against the skull. Hydrocephalus can be caused by trauma, brain infections and tumors, or it can be congenital.
     Congenital hydrocephalus is most often found in Toy breeds and may be hereditary. The characteristic enlargement of the dome of the skull may not be noted until the puppy is several months old. With or without treatment some puppies do not survive.
     Treatment is directed at reducing the fluid buildup with drugs or surgery.