Fontanelle Assessment in Infants

Normal Fontanelle Characteristics

A normal anterior fontanelle should be flat and soft to palpation when the infant is calm and in an upright position 1

A sunken fontanelle is abnormal and primarily indicates dehydration, with mild dehydration (3-5% fluid deficit) presenting as a mildly depressed fontanelle with slightly dry mucous membranes 1
A bulging fontanelle is abnormal and suggests increased intracranial pressure from conditions such as meningitis or intracranial infection, as suggested by the American Academy of Pediatrics 2
Hydrocephalus can also cause a bulging fontanelle, as indicated by the American Association of Neurological Surgeons 3, 4

The fontanelle assessment has limitations in specificity and should not be used in isolation, with more reliable dehydration markers including rapid deep breathing, prolonged skin retraction time, decreased perfusion, and acute weight loss, as recommended by the American Academy of Pediatrics 1
The sunken fontanelle has lower specificity compared to other dehydration markers, highlighting the need for a comprehensive assessment, as noted by the American Academy of Pediatrics 1

When evaluating fontanelle appearance, always assess hydration status, including dry mucous membranes, skin turgor, capillary refill, and urine output, as advised by the American Academy of Pediatrics 1
Signs of increased intracranial pressure, such as altered mental status, vomiting, lethargy, scalp bruising or bogginess, and large or rapidly enlarging head circumference, should also be evaluated, as recommended by the American Academy of Pediatrics 2

Approximately 20% of febrile infants under 90 days have enterovirus, and roughly 50% of enterovirus-positive infants develop CSF pleocytosis without bacterial meningitis, representing aseptic meningitis (8.4% of cases) rather than bacterial disease (0.4% of cases) 5, 6, 7
RSV-positive infants have 0% incidence of bacterial meningitis (0/251 cases), and influenza-positive infants similarly showed no cases of meningitis (0/119 cases) 8, 9

When evaluating a well-appearing infant with bulging fontanelle, check for viral illness indicators, including testing for COVID-19, RSV, influenza, and consider enterovirus (especially during summer months when 17.4% of febrile infants have CSF pleocytosis vs 5% in non-summer months) 5, 8
Even with CSF pleocytosis, bacterial meningitis occurred in only 0.4% of febrile infants under 90 days, compared to 8.4% with aseptic (viral) meningitis 5, 7

Proceed to magnetic resonance imaging when the initial cranial ultrasound is inconclusive or when further anatomical detail is required (American College of Radiology guideline) 10.
Obtain MRI if posterior‑fossa pathology is suspected, because ultrasound provides limited visualization of this region (American College of Radiology guideline) 11.
Use MRI for detailed parenchymal assessment or to detect small intracranial hemorrhages that may be missed on ultrasound (American College of Radiology guideline) 12.
Select MRI when a vascular malformation is suspected and precise anatomic delineation is needed (American College of Radiology guideline) 13.

Computed tomography is not recommended as the first‑line study in infants because of radiation exposure concerns (American College of Radiology guideline) 14.
Consider CT only if ultrasound is unavailable and the clinical situation demands immediate imaging (American College of Radiology guideline) 15.
Employ CT in cases of suspected head trauma when rapid detection of skull fractures is required, acknowledging that high‑quality ultrasound can approach CT sensitivity (American College of Radiology guideline) 15.
Use CT to confirm intracranial calcifications when MRI susceptibility‑weighted imaging is insufficient (American College of Radiology guideline) 14.

Recognize that cranial ultrasound has reduced sensitivity for small subdural and extra‑axial hemorrhages, especially in the posterior fossa, compared with CT or MRI (American College of Radiology guideline) 11.
Optimize ultrasound examinations by employing dedicated neonatal convex and high‑frequency linear transducers through both the anterior and mastoid fontanelles to improve posterior‑fossa visualization (American College of Radiology guideline) 14.
Ensure operators are specifically trained in neonatal neurosonography, as image quality and diagnostic accuracy are highly operator‑dependent (American College of Radiology guideline) 14.