Fontanelle Concerns in a 6-Month-Old Infant
When evaluating fontanelle concerns in a 6-month-old, first determine if the fontanelle is bulging, sunken, abnormally large, abnormally small, or prematurely closed, as each finding has distinct clinical implications and requires different management approaches.
Initial Assessment
The anterior fontanelle examination should focus on:
- Size measurement: Use the oblique diameter method (measuring the diagonal distance across the fontanelle). Normal size at 6 months ranges from approximately 1.0-2.0 cm, with significant variation considered normal 1, 2.
- Tension assessment: Palpate with the infant in an upright, calm position to accurately assess for bulging or depression 1.
- Associated symptoms: Fever, vomiting, altered consciousness, developmental delays, or growth abnormalities 1, 3.
Bulging Fontanelle
A bulging fontanelle represents elevated intracranial pressure until proven otherwise and requires urgent evaluation 1, 3.
Immediate Actions:
- Rule out meningitis: Check for fever, irritability, lethargy, and neck stiffness (though neck stiffness may be absent in infants) 4.
- Assess hydration and vital signs: Evaluate for signs of sepsis or severe illness 4.
- Consider imaging: CT head is appropriate if bacterial meningitis or other intracranial pathology is suspected, though lumbar puncture findings often guide management 3.
Important Caveat:
Recent evidence shows that bulging fontanelle has low specificity for serious intracranial pathology, particularly in viral illnesses including COVID-19 3. However, bacterial meningitis must be excluded before attributing the finding to a benign cause.
Common Causes:
- Increased intracranial pressure from meningitis, encephalitis, or intracranial hemorrhage 1
- Intracranial or extracranial tumors 1
- Viral infections (may present with bulging fontanelle without serious pathology) 3
Sunken Fontanelle
A sunken fontanelle is typically a sign of dehydration and requires assessment of hydration status and fluid resuscitation if indicated 1.
- Evaluate for signs of dehydration: decreased urine output, dry mucous membranes, poor skin turgor 1
- Initiate appropriate fluid replacement based on degree of dehydration 1
Large or Delayed Closure
If the fontanelle measures >2.5 cm at 6 months or appears disproportionately large, evaluate for underlying systemic conditions 1.
Most Common Causes:
Workup:
- Thyroid function tests if hypothyroidism suspected 1
- Skeletal survey or vitamin D levels if rickets suspected 1
- Genetic evaluation if dysmorphic features present 1
Early Closure Concerns
At 6 months of age, only 3% of normal infants have closed anterior fontanelles, so closure at this age warrants evaluation for craniosynostosis 2, 5.
- Physical examination: Assess head shape for plagiocephaly, brachycephaly, scaphocephaly, or trigonocephaly 5
- Palpate sutures: Feel for ridging along suture lines 5
- Refer to craniofacial specialist: If craniosynostosis suspected, refer promptly as minimally invasive surgical options are available for infants <6 months 5
Key Point:
Pattern recognition through physical examination is more valuable than routine imaging for suspected craniosynostosis, though imaging may be needed for surgical planning 5.
Special Considerations for Imaging
- Ultrasound through open fontanelle has limited sensitivity for CNS pathology and should not be relied upon as the primary imaging modality when serious intracranial pathology is suspected 4.
- Brain MRI is preferred over ultrasound when CNS imaging is indicated in infants with open fontanelles, particularly for suspected tumors or structural abnormalities 4.
- CT is appropriate for acute presentations requiring rapid assessment (suspected hemorrhage, acute hydrocephalus, or when MRI unavailable) 4.
Normal Variation
Most fontanelle concerns represent normal variation 1, 5, 6, 2: