A sunken soft spot on a baby’s head, medically known as a sunken fontanelle, understandably causes significant concern for parents. The fontanelle is a unique feature of the infant skull. While it should normally appear flat or curve inward slightly, a noticeable depression can signal a health issue. This change reflects a shift in the delicate balance of fluids and pressure within the baby’s body. Understanding the structure of the soft spot and the reasons behind its depressed appearance is the first step in determining whether a pediatrician should be contacted.
What the Soft Spot Is and Why Babies Have It
The soft spot is the common term for the fontanelle, a gap between the skull bones covered by a tough membrane. A newborn has several fontanelles, but the largest and most easily felt is the anterior fontanelle, located on the top of the head. The purpose of these fibrous gaps is twofold, facilitating both birth and subsequent development.
These flexible areas allow the bony plates of the skull to overlap slightly, which is necessary for the baby’s head to pass through the birth canal during delivery. After birth, the fontanelles accommodate the rapid growth of the brain. The healthy anterior fontanelle should feel relatively firm and flat, or possess a very slight inward curve when a baby is calm and upright. The smaller posterior fontanelle, located toward the back of the head, typically closes within the first two months of life, while the anterior fontanelle remains open until sometime between 7 and 19 months of age.
The Main Reason for Sunken Appearance: Dehydration
The most frequent and medically significant cause of a noticeable sunken fontanelle is moderate to severe dehydration. This condition occurs when a baby loses more fluid than is consumed, disrupting the body’s overall fluid balance. The mechanism behind the depressed appearance is directly related to the drop in fluid volume throughout the body, including the cerebrospinal fluid that surrounds the brain.
When the body lacks sufficient hydration, the reduction in fluid and blood volume causes the pressure inside the skull to decrease, pulling the fontanelle membrane inward. This change in contour is a sensitive indicator of a systemic fluid deficit. Dehydration in infants is commonly caused by excessive fluid loss from illnesses like vomiting and diarrhea, or from increased output due to fever or overheating.
A sunken fontanelle is rarely the only sign of dehydration, and parents should look for a cluster of accompanying symptoms. These include a significant reduction in wet diapers, often fewer than six in a 24-hour period, and urine that appears darker or smells stronger than usual. Other physical signs are a dry mouth and lips, crying without producing tears, and a general change in behavior such as unusual sleepiness or lethargy. If a baby is refusing to feed or seems excessively thirsty, these are further signs that the fluid imbalance is becoming medically concerning.
Less Common Causes of Depression
While dehydration is the primary concern, the soft spot may appear sunken for other, less acute reasons. Sometimes, the appearance of a slightly depressed fontanelle is simply a variation of normal when the baby is in certain positions, such as sitting upright. A temporary, mild dip may also be observed after the baby has been crying vigorously, which does not indicate a health problem if the baby is otherwise well-hydrated and acting normally.
Severe malnutrition, particularly a lack of protein known as Kwashiorkor, can also lead to a sunken fontanelle, although this is uncommon in developed countries. This cause is generally accompanied by other signs of insufficient growth and overall poor health. Rapid weight loss from any cause can sometimes contribute to a more sunken appearance. Rare underlying conditions, such as Diabetes Insipidus, which impairs the kidneys’ ability to conserve water, may also present with a persistent sunken fontanelle.
When to Call the Doctor Immediately
A sunken fontanelle accompanied by certain symptoms requires immediate medical evaluation, as infant dehydration can progress rapidly. Parents should contact their pediatrician or seek urgent care if the depression is significantly noticeable and the baby is also experiencing persistent vomiting or diarrhea. These symptoms indicate ongoing and substantial fluid loss that the baby cannot compensate for through normal feeding.
Other signs that elevate the situation to an emergency include extreme lethargy, where the baby is unusually difficult to wake or unresponsive, or a refusal to take any fluids. The presence of a high fever alongside the sunken spot suggests a serious underlying infection that is contributing to dehydration. Any combination of a sunken fontanelle and signs of shock, such as cold or mottled hands and feet, should prompt an immediate trip to the emergency room. Waiting to see if the symptoms improve can be unsafe, so clear and rapid communication with a healthcare provider is the most appropriate action.