What is the appropriate evaluation and management for a 6-month-old infant with a large fontanelle?

Evaluation and Management of Large Fontanelle in a 6-Month-Old Infant

A large anterior fontanelle at 6 months requires assessment for underlying pathology including hypothyroidism, Down syndrome, increased intracranial pressure, achondroplasia, and rickets, as these are the most common causes of delayed fontanelle closure or enlargement. 1, 2

Understanding Normal Fontanelle Development

• The average anterior fontanelle size at birth is 2.1 cm, with maximum size occurring at 1 month of age (2.2 cm) 2, 3
• By 6 months, only 3% of normal infants have closed fontanelles, so an open fontanelle at this age is expected 3
• A fontanelle measuring above the 95th percentile for age warrants close monitoring and investigation for associated diseases 4
• The median closure time is 13.8 months, with 26.5% closed by 12 months and 93% by 24 months 2, 3

Key Clinical Assessment Points

Measure the Fontanelle Accurately

• Measure the diagonal (oblique) diameters of the fontanelle, as this method is as accurate as traditional measurements and clinically practical 4, 3
• Document whether the fontanelle is simply large versus abnormally enlarged for age

Assess Fontanelle Appearance and Tension

• A flat, soft fontanelle when the infant is calm and upright is normal 5
• A bulging fontanelle indicates increased intracranial pressure from meningitis, intracranial infection, or hydrocephalus 5, 1
• A sunken fontanelle primarily indicates dehydration (though this has low specificity) 5

Screen for Associated Conditions

Hypothyroidism 1, 2:

• Check thyroid function tests (TSH, free T4)
• Look for other signs: constipation, prolonged jaundice, poor feeding, hypotonia, large tongue

Down Syndrome 1, 2:

• Assess for characteristic facial features, hypotonia, single palmar crease
• Consider karyotype if not previously diagnosed

Increased Intracranial Pressure 1, 2:

• Evaluate for altered mental status, vomiting, lethargy, scalp bruising or bogginess 5
• Monitor head circumference growth—rapidly enlarging head circumference is concerning 5, 1
• Consider hydrocephalus if head circumference crosses percentiles upward 5

Rickets 1, 2:

• Assess vitamin D intake and sun exposure
• Check for bowing of legs, rachitic rosary, wrist widening
• Consider serum calcium, phosphorus, alkaline phosphatase, 25-OH vitamin D levels

Achondroplasia 1, 2:

• Evaluate growth pattern (short stature with disproportionately short limbs)
• Assess for characteristic facial features and skeletal findings

Imaging Considerations

• The open anterior fontanelle provides an excellent acoustic window for brain ultrasonography, making it the preferred initial imaging modality 1
• Brain ultrasound is non-invasive and can assess for hydrocephalus, structural abnormalities, and intracranial pathology 1
• CT or MRI become necessary after fontanelle closure or when ultrasound is inadequate 1
• Plain skull films have limited utility in modern practice 2

Important Clinical Caveats

• Early fontanelle closure is NOT associated with microcephaly, so a large fontanelle should not be dismissed as compensatory 4
• There are no gender differences in fontanelle size 4, 3
• Fontanelle size does not correlate with weight, length, or head circumference development 4, 3
• If the infant is receiving growth hormone therapy, monitor closely as this can cause abnormal head growth, especially with open fontanelles 1

Management Algorithm

1. Measure fontanelle size using oblique diameters and compare to age-appropriate percentiles 4, 3
2. Assess fontanelle tension (flat vs. bulging vs. sunken) with infant calm and upright 5
3. Measure and plot head circumference on growth chart 5, 1
4. Screen for the five most common causes: hypothyroidism (TSH/T4), Down syndrome (clinical features), increased ICP (symptoms/signs), rickets (vitamin D status), achondroplasia (skeletal features) 1, 2
5. Obtain brain ultrasound through the open fontanelle if any concerning features or if fontanelle measures above 95th percentile 1, 4
6. Monitor closely with serial measurements if fontanelle is large but infant is otherwise well 4