Fontanelle

In humans, the sequence of fontanelle closure is as follows:[2][4]

1. The posterior fontanelle generally closes 2 to 3 months after birth;
2. The sphenoidal fontanelle is the next to close around 6 months after birth;
3. The mastoid fontanelle closes next from 6 to 18 months after birth; and
4. The anterior fontanelle is generally the last to close between 12 and 18 months.

A very tense or bulging anterior fontanelle indicates raised intracranial pressure. Increased cranial pressure in infants may cause the fontanelles to bulge or the head to begin to enlarge abnormally.[7] It can occur due to:[4]

• Craniosynostosis – premature fusion of the cranial sutures[8]
• Encephalitis – swelling (inflammation) of the brain, most often due to infections
• Hydrocephalus – a buildup of fluid inside the skull
• Meningitis – infection of the membranes covering the brain
• Shaken baby syndrome

A sunken (also called "depressed") fontanelle indicates dehydration or malnutrition.[9]

Anterior fontanelle of a 1-month-old infant

The fontanelles may be enlarged, may be slow to close, or may never close, most commonly due to causes like:[10]

• Down syndrome
• Hydrocephalus
• Intrauterine growth restriction (IUGR)
• Premature birth

Rarer causes include:[10]

• Achondroplasia
• Apert syndrome
• Cleidocranial dysostosis
• Congenital rubella
• Neonatal hypothyroidism
• Osteogenesis imperfecta
• Rickets

Sometimes there is a third bigger fontanelle other than posterior and anterior ones in a newborn. In one study, the frequency of third fontanelles in an unselected population of newborn infants was 6.3%. It is very common in Down syndrome and some congenital infections. If present, the physician should rule out serious conditions associated with the third fontanelle.[11]

In apes the fontanelles fuse soon after birth. In chimpanzees the anterior fontanelle is fully closed by 3 months of age.[2]

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.[12]

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.

An open fontanelle, known as a "molera", is a recognized feature of the Chihuahua breed. The American Kennel Club breed standard states that the skull of the Chihuahua should be domed, with or without the molera being present.[13] However, the Fédération Cynologique Internationale (FCI) standard for the Chihuahua lists an open fontanelle as a disqualification.[14]