What are the implications and necessary steps for an infant or young child with a large fontanelle (soft spot on the skull)?

Large Fontanelle: Clinical Evaluation and Management

Initial Assessment

A large anterior fontanelle requires systematic evaluation to identify underlying pathology, with the most common causes being achondroplasia, hypothyroidism, Down syndrome, increased intracranial pressure, and rickets. 1

Define "Large" Fontanelle

• The average anterior fontanelle size at birth is 2.1 cm 1
• Maximum size occurs at 1 month of age (2.2 cm average) 2
• Size progressively decreases: 1.0 cm at 12 months, 0.5 cm at 24 months 2
• Median closure time is 13.8 months, with 93% closed by 24 months 1, 2
• Delayed closure beyond 24 months warrants investigation 3

Systematic Diagnostic Approach

1. Screen for Common Metabolic and Genetic Causes

Hypothyroidism:

• Order thyroid function tests (TSH, free T4) immediately 3, 1
• This is a critical, treatable cause of delayed fontanelle closure 3

Down Syndrome:

• Assess for characteristic dysmorphic features 3, 1
• Consider karyotype if clinical suspicion exists 1

Rickets:

• Evaluate for vitamin D deficiency, calcium, phosphate, alkaline phosphatase 3, 1
• Look for bowing of long bones, rachitic rosary 1

Achondroplasia:

• Assess growth parameters and skeletal proportions 3, 1
• Consider skeletal survey if suspected 1

2. Evaluate for Increased Intracranial Pressure

Critical warning signs requiring urgent evaluation: 4

• Bulging fontanelle (when infant calm and upright)
• Altered mental status or lethargy
• Persistent vomiting
• Rapidly enlarging head circumference
• Scalp bruising or bogginess

If increased intracranial pressure suspected:

• Perform head ultrasound through the open fontanelle as first-line imaging 3, 4
• The anterior fontanelle serves as an excellent acoustic window for non-invasive brain ultrasonography 3
• Consider CT or MRI if ultrasound inadequate or fontanelle closing 3
• Evaluate for hydrocephalus, intracranial masses, or infection 4

3. Assess Fontanelle Characteristics

Normal fontanelle: 4

• Flat and soft when infant calm and upright
• Pulsations may be visible

Bulging fontanelle indicates: 4, 1

• Increased intracranial pressure from meningitis, encephalitis, or intracranial infection
• Hydrocephalus
• Intracranial or extracranial tumors
• Important caveat: In febrile infants, viral infections (enterovirus, RSV, influenza, COVID-19) can cause bulging fontanelle without bacterial meningitis 4, 5

Sunken fontanelle indicates: 4, 1

• Dehydration (though this has lower specificity than other dehydration markers)
• Must assess in context of dry mucous membranes, skin turgor, capillary refill, urine output 4

Special Considerations

Growth Hormone Therapy

• Monitor head circumference closely in children receiving growth hormone with open fontanelles 3
• Growth hormone can cause abnormal head growth when fontanelles remain open 3

Imaging After Fontanelle Closure

• Once fontanelles close, CT and MRI become the primary imaging modalities for intracranial assessment 3
• Ultrasound is no longer useful after closure 3

Common Pitfalls to Avoid

• Do not rely on fontanelle appearance alone for dehydration assessment - it has lower specificity than rapid deep breathing, prolonged skin retraction time, decreased perfusion, and acute weight loss 4
• Do not assume bulging fontanelle always means bacterial meningitis - viral infections are far more common (8.4% aseptic meningitis vs 0.4% bacterial meningitis in febrile infants under 90 days) 4
• Do not forget to measure head circumference serially - rapidly increasing head circumference with large fontanelle suggests hydrocephalus or increased intracranial pressure 4
• Do not overlook small-for-gestational-age infants - they have significantly larger fontanelles than term or preterm infants 6