Fontanelle Definition

Fontanelle, fontanel, or soft area on a baby’s cranium is an anatomical characteristic. In the places where some adult skull sutures are present, a newborn has six fontanelles, which are made up of membranous tissue. As fontanelles enable the skull to move via the birth canal, they also let the brain grow.

The Baby Fontanelle

From the time a baby is born until roughly the age of eighteen months, six newborn fontanelles close at various times. This phase may differ somewhat from one kid to the next.

Extremely thick membranous connective tissue safeguards the delicate structure of the brain’s subsurface, which is why the fontanelle should be protected against harm. Despite this, numerous young parents are too worried about this part of their infant’s anatomy.

The six fontanelles are the

• Anterior fontanelle x 1.
• Posterior fontanelle x 1
• Mastoid fontanelle x 2.
• Sphenoid or sphenoidal fontanelle x 2

Anterior Fontanelle

The anterior fontanelle is the biggest. Between the frontal and parietal bones lies a diamond-shaped soft area. At this age, the frontal bone is not a separate bone, but rather a pair of bones that must grow and fuse as the brain’s mass increases.

In a newborn, the now-fused fontanelle is located at the intersection of the frontal, coronal, and sagittal sutures of the skull, in children, and also at the connection of the frontal, coronal, and sagittal sutures of the skull in adults. The bregma—Greek for “top of the head”—is where these sutures meet. An anterior fontanel is a kind of bregma that develops early.

By the age of eighteen months, this soft area is normally closed.

Posterior Fontanelle

The posterior fontanelle has a triangular shape and shuts faster than the anterior fontanelle, which shuts during the seventh postnatal week. It’s situated where the sagittal and lambdoid sutures meet at the intersection of the parietal and occipital bones. The lambda is the point where these sutures meet. An early version of the lambda is the posterior fontanelle.

Mastoid Fontanelle

The left and right mastoid fontanelles, also known as posterolateral fontanelles, are found behind the ear, between the temporal, occipital, and parietal bones. The entrance of the auditory meatus may be seen behind the temporal bone’s zygomatic process in the image below.

The mastoid fontanel represents the junction of the parietomastoid, occipitomastoid, and lambdoid sutures on the side of the head. The Asterion is the name given to this place. An early version of the asterion is a mastoid soft spot.

Between the sixth and the eighteenth postnatal month, it shuts.

Sphenoidal Fontanelle

At the temple, the paired sphenoid fontanelle, the anterolateral fontanelle, also known as the anterior fontanelle, is located between the sphenoid, parietal, temporal, as well as frontal bones. At this point, the coronal, sphenofrontal, sphenoparietal, sphenosquamosal, as well as squamosal sutures converge. The pterion—the weakest region of the adult skull—is located here. A sphenoidal fontanelle is a kind of pterion that closes about six months after birth.

The Third Fontanelle

A third soft spot is discovered in a minority of instances. This is usually a sign of a condition, such as Down syndrome or congenital hypothyroidism (low thyroid hormone levels). In front of the posterior fontanel, there is an aberrant fontanelle.

Other Disorders

Soft spots heal at somewhat different rates in typical people. Nevertheless, very large soft spots may signal underlying problems.

Soft patches that are very big are linked to:

• Down syndrome, trisomy 18, and trisomy 13 are chromosomal abnormalities.
• Achondroplasia (dwarfism) and Beckwith-Wiedemann syndrome (unusual expansion) are two genetic illnesses. 
• Hypothyroidism
• Rickets (long-term vitamin D deficiency); malnutrition, including rickets.
• Problems during pregnancy, foetal hydantoin syndrome (occurs if a pregnant woman takes a certain kind of epilepsy medicine),
• The most frequent cause of increased pressure in the brain is hydrocephalus (water in the brain).

While the brain cannot discharge surplus cerebrospinal fluid or blood, fontanels may save the life of a newborn. Soft patches enable a newborn baby’s skull to grow, but an adult skull cannot alter its form once bonded. As a consequence, if the source of the brain swelling is not addressed, the skull may become deformed.

Very small fontanelles may be caused by hyperthyroidism (excess thyroid hormone synthesis), sickness, and hereditary illnesses that produce microcephaly (short head) with craniosynostosis. In craniosynostosis, the bones of the head fuse abnormally. If a single suture is compromised, the other sutures may stretch to accommodate the brain’s growth, resulting in a misshapen skull. When all the sutures are damaged, immediate surgery is required to relieve the resulting high intracranial pressure.

Fontanelle Closure

Fontanelle closure is generally foreseeable if the above-mentioned diseases are not present. 

• Closure of the anterior: 18 months 
• 8 weeks for posterior closure
• Mastoid closure time ranges from 6 to 18 months. 
• Closure of the sphenoid: 6 months

These parts of the skull are vulnerable to harm until the soft patches have vanished due to a process known as intramembranous ossification. In the event that the skin is broken, bacteria and/or viruses may gain access to the brain.

Normal Fontanelle vs Sunken Fontanelle

A fontanelle is typically horizontal or gently recessed. It may be touched readily with the fingers. Numerous new moms may worry if they feel it swelling when their baby screams (as seen below), but this is completely normal. The bulge vanishes as the infant stops crying.

Only thick membranes divide the blood circulation to the brain from the skin, therefore a normal fontanel may also pulse.

Dehydration is indicated by a sunken fontanelle with a noticeable dip. Dehydration may be caused by a variety of things, including diarrhoea, vomiting, fever, or hyperthermia.

A medical emergency exists if a newborn baby ceases to produce wet diapers or cries without tears. A sunken soft area will rebound to a more healthy state once fluid levels normalise.

Bulging Fontanelle

A fontanelle bulge may happen if a baby is sobbing or while he or she is irritated or furious. Yet, a persistent soft spot bulging after the weeping has ended might indicate a range of diseases.

Neurological issues, cardiovascular difficulties, infections, blood diseases, endocrine and metabolic abnormalities, poisoning, and trauma are all potential conditions. A bulge in any of the soft areas (not only while the infant is sobbing) must be checked by an expert doctor, ideally while the infant is calm and content.

When the fontanel is softly touched, a physician would hear the Macewan symbol, often known as the “cracked pot” symbol. The physician may use a stethoscope to listen for a bruit; this rushing tone may indicate an arteriovenous malformation, or AVM. An AVM is a convoluted network of arteries and veins that depletes the circulation of oxygen.

References

• Avery MB, Tausch HW, Ballard RA, Gleason CA. (2003). Avery’s Diseases of the Newborn, Eighth Edition. Philadelphia, Elsevier Saunders.
• Lipsett BJ, Reddy V, Steanson K. (Updated 2020). Anatomy, Head and Neck, Fontanelles. Treasure Island (FL), StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK542197/
• Kiesler J, Ricer R. (2003). The Abnormal Fontanel. Am Fam Physician. 2003 Jun 15;67(12):2547-52. PMID: 12825844.