Fontanelle

Anatomy

Fontanelles are soft spots on a baby's head which, during birth, enable the bony plates of the skull to flex, allowing the child's head to pass through the birth canal. The ossification of the bones of the skull cause the fontanelles to close over by a child's second birthday. The closures eventually form the sutures of the neurocranium. Although there are the two anterior and posterior fontanelles, there are two more fontanelles of interest, the mastoid fontanelle, and the sphenoidal fontanelle.

The skull of a newborn consists of five main bones: two frontal bones, two parietal bones, and one occipital bone. These are joined by fibrous sutures, which allow movement that facilitates childbirth and brain growth.

• At birth, the skull features a small posterior fontanelle, an open area covered by a tough membrane, where the two parietal bones adjoin the occipital bone (at the lambda). This fontanelle usually closes during the first several months of an infant's life. This is called intramembranous ossification. The mesenchymal connective tissue turns into bone tissue.

• The much larger, diamond-shaped anterior fontanelle where the two frontal and two parietal bones join generally remains open until the child is about two years of age, however, in cleidocranial dysostosis it is often late in closing or may never close. The anterior fontanelle is useful clinically. Examination of an infant includes palpating the anterior fontanelle. A sunken fontanelle indicates dehydration, whereas a very tense or bulging anterior fontanelle indicates raised intracranial pressure.

• Two smaller fontanelles are located on each side of the head, more anteriorly the sphenoidal (between the sphenoid, parietal, temporal, and frontal bones) and more posteriorly the mastoid (between the temporal, occipital, and parietal bones).

Clinical significance

Parents may worry that their infant may be more prone to injury at the fontanelles. In fact, although they may colloquially be called "soft-spots", the membrane covering the fontanelles is extremely tough and difficult to penetrate. However, the fontanelles allow the infant brain to be imaged using ultrasonography. Once they are closed, most of the brain is inaccessible to ultrasound imaging, because the bony skull presents an acoustic barrier.

In cleidocranial dysostosis the skull fontanelle may be enlarged, and may be slow to close or may never close.

n dogs

Human beings are not the only mammals to struggle with birth defects. One of the more serious problems that can affect canines is known as an "open fontanelle," 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 fontanelle 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.

Not all open fontanelles 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 fontanelle 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 fontanelles 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 fontanelles and hydrocephalus can occur in any dog, but are most prevalent in the toy breeds, including Chihuahuas, Maltese, Pomeranian's and Yorkshire Terriers