A sunken fontanel occurs when the soft spot on a baby’s skull becomes more deep set than usual. One major cause is dehydration.

The human skull is made up from several bones that are connected by tough fibrous tissue called sutures. These sutures give the skull some flexibility, allowing the head to pass through the birth canal.

Where several sutures meet, they create a fontanel. Fontanels make the skull flexible enough for the brain to grow. When a child is born, it will have several fontanels on the skull, but the ones on the back (posterior) and top (anterior) of its head are the most well-known.

For many babies, only the larger anterior fontanel is open after birth. People refer to this fontanel as the soft spot.

Sometimes, the smaller posterior fontanel is also open and may be felt at birth but soon closes.

Over time, this flexibility becomes less necessary, so the fontanels close up, and sutures harden.

The anterior fontanel should curve slightly inwards but remain relatively firm. It is not always visible, particularly under hair. In some cases, the dip may be very pronounced and noticeable. This is a sunken fontanel, which requires medical attention.

Sunken anterior fontanel. Image credit: Nojhan, 2005.Share on Pinterest
A baby’s skull can have a soft spot, which is called a fontanel.
Image credit: Nojhan, 2005.

Dehydration is the primary cause of a sunken fontanel.

Dehydration occurs when an infant does not have sufficient fluid in their body to maintain normal functioning. This can happen for a many reasons, including:

  • vomiting
  • not drinking enough fluids
  • diarrhea
  • fever
  • urinating too often

Dehydration in children can range from mild to severe. More severe cases will require immediate medical attention to avoid complications.

A sunken fontanel can be a sign of dehydration. Other symptoms will usually be present and can include:

  • dry tongue and lips
  • dry, cool skin
  • rapid heart rate and breathing
  • infrequent urination
  • no tears when crying
  • sunken eyes

It is also possible, though uncommon, for a sunken fontanel to be a sign of malnutrition. Malnutrition can result from a calorie deficit or an underlying condition, such as malabsorption syndrome.

Infants with malnutrition will almost always display other signs, including:

  • being underweight
  • dry, inelastic skin
  • dry hair that falls out easily
  • fatigue or lethargy
Share on Pinterest
A doctor may visually examine the baby to help diagnose the cause of a sunken fontanel.

A doctor will visually examine the fontanel and gently feel the area to help determine whether the structure of the fontanel is abnormal. An abnormal structure could indicate an underlying problem.

A doctor will also check for signs of dehydration or malnutrition. This could include monitoring the baby’s heart rate and breathing or feeling the skin for dryness and reduced elasticity.

The doctor will ask when a person first noticed the sunken fontanel. They will also ask questions relating to the infant’s overall health, such as whether they have had a fever or bouts of vomiting or diarrhea.

They will want to know about the baby’s feeding habits and whether they have been urinating more or less than usual.

The diagnosis may require blood or urine samples.

If the sunken fontanel is due to dehydration, immediate medical attention may be necessary, sometimes in the hospital.

In a hospital, a doctor will give the infant fluids orally to prompt rehydration. If the infant has been vomiting, doctors may administer fluids into the bloodstream through an intravenous (IV) line.

This treatment will continue until fluid levels reach an acceptable level.

Parents and carers may be able to treat mild cases of dehydration at home by encouraging the infant to drink or feed if they breastfeed. However, people should only try to treat dehydration at home if a doctor has examined the baby and instructed them to do so.

If the cause of the sunken fontanel is malnutrition, treatment will vary depending on the underlying cause. It will typically involve increasing overall calorie intake or a specific nutrient.

In some cases, it may be necessary to deliver nutrients through an IV line or a tube.

Share on Pinterest
Keeping a baby hydrated and well-fed can help prevent a sunken fontanel from occurring.

It is possible to treat most cases of dehydration quickly and easily.

Malnutrition can be challenging to treat, depending on the cause.

However, the longer an infant goes without treatment, the greater their chances of lasting damage become.

The human body needs to stay hydrated to function. A prolonged period of dehydration can lead to brain damage and even death.

Prolonged malnutrition can lead to developmental delays and disability.

Fontanels occur naturally in infants. It is normal for a fontanel to form an inward curve in infants while their skull is still hardening. But in some cases, it may become sunken, and the cause may need medical treatment.

A sunken fontanel, when accompanied by other symptoms, can be a sign of dehydration or malnutrition.

Some baby’s fontanels always appear slightly sunken even when they are well. Sometimes, a baby is dehydrated but does not have a sunken fontanel. It is vital to know the signs of illness and dehydration in a baby and seek medical help, regardless of the appearance of their fontanel.

Delaying treatment of dehydration or malnutrition can have severe and lasting consequences for the infant’s development. Anyone concerned about a sunken fontanel should speak to a doctor as soon as possible.