Is a flat Fontanelle (soft spot) normal in infants?

Is a Flat Fontanelle Normal in Infants?

Yes, a flat fontanelle is normal in healthy, well-hydrated infants and is the expected baseline finding on physical examination. 1, 2

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, 2
• Average size of 2.1 cm at birth, with median closure at 13.8 months 2
• Neither sunken (depressed) nor bulging above the level of the surrounding skull 1, 2

Clinical Significance of Fontanelle Appearance

Sunken (Depressed) Fontanelle

A sunken fontanelle is abnormal and primarily indicates dehydration, not a normal variant 1, 2:

• Mild dehydration (3-5% fluid deficit): Mildly depressed fontanelle with slightly dry mucous membranes 1
• Moderate dehydration (6-9% fluid deficit): Noticeably sunken fontanelle with loss of skin turgor and dry mucous membranes 1
• Severe dehydration (≥10% fluid deficit): Markedly sunken fontanelle with severe lethargy, prolonged skin tenting (>2 seconds), and decreased perfusion 1

Bulging Fontanelle

A bulging fontanelle is abnormal and suggests increased intracranial pressure from:

• Meningitis or intracranial infection 3, 2
• Intracranial or extracranial tumors 2
• Hydrocephalus 3

Important Clinical Caveats

The fontanelle assessment has limitations in specificity and should not be used in isolation 1:

• Fontanelle fullness shows limited reliability when assessed by practitioners of varied skill levels, though it can be reliable when used by trained practitioners to assess the same infant over time 3
• More reliable dehydration markers include: rapid deep breathing (acidosis), prolonged skin retraction time (>2 seconds), decreased perfusion, and acute weight loss 1
• The sunken fontanelle has lower specificity compared to other dehydration markers 1

Assessment Approach

When evaluating fontanelle appearance, always assess:

• Hydration status: Check for dry mucous membranes, skin turgor, capillary refill, and urine output 1
• Signs of increased intracranial pressure: Look for altered mental status, vomiting, lethargy, scalp bruising or bogginess, and large or rapidly enlarging head circumference (>95th percentile) 3
• Acute weight change: This is the most accurate assessment of fluid status, though premorbid weight is often unknown 1
• Context of illness: Consider sepsis, metabolic disorders, or serious bacterial infection in infants with poor feeding and abnormal fontanelle findings 1

In summary, a flat fontanelle is the normal baseline finding; any deviation (sunken or bulging) warrants further clinical evaluation.