Posterior Fontanelle Closure
The posterior fontanelle normally closes by 2 months of age in all infants, and if it remains open beyond 6 months, evaluation for underlying metabolic, genetic, or skeletal disorders is warranted 1, 2.
Normal Closure Timeline
The posterior fontanelle follows a predictable closure pattern:
- At birth: Mean size approximately 0.8 cm 1
- By 2 months: Closed in all healthy infants 1
- This contrasts sharply with the anterior fontanelle, which remains open until 9-15 months on average
Clinical Significance of Delayed Closure
When the posterior fontanelle remains open beyond the expected 2-month window, this represents a significant deviation from normal development. Persistence beyond 6 months should prompt immediate investigation 2.
Key Differential Diagnoses to Consider
The most common pathological causes of delayed posterior fontanelle closure include:
- Hypothyroidism - Among the most frequent metabolic causes 2
- Skeletal dysplasias - Particularly achondroplasia 2
- Chromosomal abnormalities - Down syndrome is commonly associated 2
- Increased intracranial pressure - Though more commonly affects anterior fontanelle 2
- Rickets and metabolic bone disease - Impaired ossification 2
- Cleidocranial dysplasia - Generalized defect in membranous bone formation with persistent fontanelles 3
Recommended Evaluation Approach
For a posterior fontanelle remaining open after 6 months:
Initial assessment should include:
- Measurement of head circumference and plotting on growth charts
- Palpation of all cranial sutures and the anterior fontanelle
- Assessment for dysmorphic features (hypertelorism, midface hypoplasia, skeletal abnormalities)
- Examination of clavicles for hypoplasia or aplasia 3
- Developmental milestone assessment
Laboratory workup:
- Thyroid function tests (TSH, free T4) - to exclude hypothyroidism 2
- Serum calcium, phosphorus, alkaline phosphatase - to evaluate for rickets 2
- Consider chromosomal analysis if dysmorphic features present 2
Imaging considerations:
- Plain skull radiographs can demonstrate delayed ossification patterns and identify wormian bones 2
- Ultrasound through the open fontanelle is useful for evaluating brain parenchyma and ventricular size in infants, requires no sedation, and can be performed portably 4
- CT or MRI reserved for cases with suspected intracranial pathology or when ultrasound is inadequate 2
Important Clinical Pitfalls
Do not assume delayed closure is benign variation - While some population-specific variations exist 5, persistent posterior fontanelle opening beyond 2 months warrants investigation, and beyond 6 months mandates thorough evaluation.
Distinguish from anterior fontanelle - The posterior fontanelle is much smaller and closes far earlier than the anterior fontanelle. Confusion between the two can lead to inappropriate reassurance.
Consider cleidocranial dysplasia specifically - This autosomal dominant condition presents with persistently open fontanelles, delayed suture closure, and clavicular abnormalities that are easily missed without specific examination 3. The midfrontal fontanelle area may remain open indefinitely in these patients.