Is an open anterior fontanelle in a 21‑month‑old child normal, and what evaluation and management are indicated?

Open Anterior Fontanelle at 21 Months

An open anterior fontanelle at 21 months can be a normal variant, as the normal range for closure extends from 4 to 26 months, though the median closure time is 13.8 months. 1, 2

Initial Assessment Priority

Your first step is to measure head circumference and plot it on a growth chart to assess for microcephaly or abnormal head growth patterns. 3 This single measurement will guide your entire diagnostic approach.

Key Clinical Examination Points

Perform a focused neurological examination looking specifically for:

• Signs of increased intracranial pressure: altered mental status, vomiting, lethargy, scalp bruising or bogginess, and rapidly enlarging head circumference 3, 4
• Palpate the fontanelle: it should be flat and soft when the infant is calm and upright 4
• Developmental assessment: identify any delays that may accompany underlying pathology 3

Diagnostic Algorithm Based on Findings

If Head Growth is Normal AND No Neurological Signs

This is likely a benign normal variant. 2, 5 Research demonstrates that persistent open anterior fontanelle beyond accepted ranges can be a normal outlier when other disorders are ruled out. 2 One case report documented a healthy child with an open fontanelle persisting to 4 years of age without any underlying pathology. 5

Your management approach:

• Serial head circumference measurements at regular intervals 3
• Ongoing developmental surveillance 3
• Reassurance to parents that 7% of normal children still have an open fontanelle at 24 months 6
• No imaging is required 3

If Head Growth is Abnormal OR Neurological Signs Present

Obtain ultrasound examination first, as it is often sufficient for suspected anomalies of the cranial sutures and anterior fontanelle. 3 The American College of Radiology states that diagnosis of craniosynostosis and skull malformations is strictly clinical and does not require routine radiographic imaging. 3

Proceed to MRI if:

• Ultrasound is inconclusive or further anatomical detail is required 4
• Concerns exist about brain development or other neurological symptoms 3
• Posterior fossa pathology is suspected (ultrasound has limited visualization here) 4

CT imaging should only be used:

• For surgical planning in selected cases, not as routine diagnostic 3
• When ultrasound is unavailable and immediate imaging is clinically necessary 4

Common Pitfalls to Avoid

Do not assume early closure equals craniosynostosis. A comparative study found that early anterior fontanelle closure had only 36.1% sensitivity and 59% positive predictive value for craniosynostosis diagnosis. 7 Conversely, pediatricians should be aware that craniosynostosis can exist even with a widely open fontanelle. 7

Do not order imaging reflexively. The size of the anterior fontanelle shows no correlation with weight, length, or head circumference development in healthy children. 6 Imaging is indicated only when abnormal head growth or neurological signs are present. 3

Conditions to Consider if Abnormalities Detected

The most common causes of delayed fontanelle closure include:

• Achondroplasia 1
• Hypothyroidism 1, 2
• Down syndrome 1
• Increased intracranial pressure 1, 2
• Rickets 1
• Skeletal anomalies 2

However, these conditions will present with additional clinical findings beyond an isolated open fontanelle. 1, 2 History and physical examination will identify which diagnostic testing is appropriate. 1

Follow-Up Strategy

For isolated open fontanelle without other abnormalities, close monitoring of head growth and neurological development over time is recommended. 3 Early intervention services should be provided if developmental delays are identified. 3