Causes of Bulging Fontanelle in Well-Appearing Infants
In well-appearing infants with a bulging fontanelle, the most important non-meningitis causes to consider are viral infections (particularly COVID-19, measles, enterovirus, and HHV-6), transient intracranial hypertension, hydrocephalus, and dehydration assessment errors (fontanelle examined while infant is crying or supine). 1, 2, 3, 4, 5
Primary Non-Infectious Causes
Transient Intracranial Hypertension
- This benign condition mimics meningitis but resolves spontaneously within 24-48 hours and is thought to result from transient increases in cerebrospinal fluid circulation 5
- Presents with elevated CSF opening pressure but normal cell counts, glucose, and negative cultures 5
- Associated with viral infections including HHV-6, measles, and COVID-19 2, 5
Hydrocephalus
- Can cause bulging fontanelle due to increased intracranial pressure from impaired CSF drainage 1
- The American Association of Neurological Surgeons identifies this as a key structural cause requiring imaging evaluation 1
Viral Infections Without Meningitis
COVID-19 (Omicron and Other Variants)
- Recent case series demonstrate COVID-19 commonly presents with fever and bulging fontanelle in well-appearing infants under 12 months, with benign course and resolution within 2 days 3, 4
- All reported cases had negative CSF cultures and normal cell counts 3, 4
- The bulging fontanelle in COVID-19 has low specificity for serious pathology 3
Enterovirus
- Approximately 20% of febrile infants under 90 days have enterovirus, and roughly 50% of enterovirus-positive infants develop CSF pleocytosis without bacterial meningitis 6
- This represents aseptic meningitis (8.4% of cases) rather than bacterial disease (0.4% of cases) 6
Other Viral Illnesses
- Measles can cause transient intracranial hypertension with bulging fontanelle that resolves within 24 hours 2
- HHV-6 infection has been documented causing fever and bulging fontanelle with elevated CSF pressure but normal cell counts 5
Assessment Errors and Normal Variants
Position and State-Dependent Changes
- The American Academy of Pediatrics emphasizes that a normal fontanelle should be assessed when the infant is calm and upright 1
- Crying, lying supine, or agitation can cause physiologic fontanelle bulging that mimics pathology 1
Dehydration Misinterpretation
- While a sunken fontanelle indicates dehydration, assessment has limited specificity and should not be used in isolation 1
- More reliable dehydration markers include rapid deep breathing, prolonged skin retraction time, decreased perfusion, and acute weight loss 1
Clinical Approach Algorithm
When evaluating a well-appearing infant with bulging fontanelle:
First, ensure proper assessment technique: Examine with infant calm and upright 1
Check for viral illness indicators: Test 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) 6, 4
Assess hydration status comprehensively: Evaluate mucous membranes, skin turgor, capillary refill, urine output, and acute weight loss 1
Look for concerning features requiring immediate LP:
Consider imaging before LP if: Signs suggest hydrocephalus or structural abnormality 1, 8
Important Clinical Caveats
- The presence of a bulging fontanelle alone in a well-appearing infant has low specificity for bacterial meningitis, particularly in the context of documented viral illness 3, 4
- RSV-positive infants have 0% incidence of bacterial meningitis (0/251 cases) 6
- Influenza-positive infants similarly showed no cases of meningitis (0/119 cases) 6
- 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 6
Avoid excessive antibiotic treatment and invasive procedures when viral etiology is confirmed and infant remains well-appearing with normal vital signs and activity level 3