Posterior Fontanelle Closure in Infants
The posterior fontanelle typically closes by 2-3 months of age, much earlier than the anterior fontanelle which closes around 9-14 months. 1, 2, 3
Normal Fontanelle Development
Posterior Fontanelle
- Present at birth as one of six fontanelles
- Located at the junction of the occipital and parietal bones
- Much smaller than the anterior fontanelle
- Closes significantly earlier than other fontanelles
- May not be palpable in approximately 50% of neonates at birth 4
- Normal closure occurs by 2-3 months of age 2
Anterior Fontanelle (for comparison)
- Average size at birth is approximately 2.1 cm 2
- Mean closure time is 13.8 months 2
- In a recent prospective cohort study, mean closure time was 9.7 ± 5.0 months (slightly earlier in boys at 9.2 ± 5.1 months compared to girls at 10.3 ± 4.7 months) 3
Clinical Significance of Posterior Fontanelle Assessment
Diagnostic Value
- While less clinically significant than the anterior fontanelle, the posterior fontanelle can provide valuable information:
- Early closure may occasionally be associated with craniosynostosis
- Delayed closure may be seen in conditions affecting bone development
- Can serve as an acoustic window for cranial ultrasound in specific situations 5
Imaging Considerations
- The posterior fontanelle can be used as an alternative sonographic window when evaluating neonates for intraventricular hemorrhage
- Sonography via the posterior fontanelle improves visualization of the occipital horns of the lateral ventricles 5
- This approach has been shown to increase detection of intraventricular hemorrhage in neonates with normally sized ventricles 5
Variations in Fontanelle Size and Closure
Normal Variations
- Significant variability exists in fontanelle size among healthy infants
- Racial differences have been documented, with studies showing larger fontanelles in African infants compared to Caucasian infants 4
- In one Nigerian study, the posterior fontanelle ranged from 0.0 to 5.5 cm with a mean of 1.4 cm, and was not palpable in 49.5% of neonates at birth 4
Abnormal Findings
- Delayed fontanelle closure may be associated with:
- Increased intracranial pressure
- Hypothyroidism
- Down syndrome
- Rickets
- Achondroplasia 2
Clinical Assessment
- Palpation should assess:
- Size
- Tension
- Presence or absence of the fontanelle
- Measurement should be performed when the infant is calm and in an upright position
- Serial measurements provide more valuable information than a single assessment
- Fontanelle assessment should be part of routine well-child visits until closure
Remember that while the posterior fontanelle typically closes by 2-3 months, there is normal variation in closure timing, and clinical context should always be considered when evaluating fontanelle status.