Definition of Wide-Opened Anterior Fontanelle
A wide-opened anterior fontanelle is defined as an anterior fontanelle that measures larger than the normal range (average 2.1-2.6 cm at birth, maximum around 2.2 cm at 1 month of age), persists beyond the expected closure timeline (median closure at 13.8 months, with 93% closed by 24 months), or represents a pathologically enlarged soft spot requiring evaluation for underlying metabolic, genetic, or endocrine disorders. 1, 2, 3
Normal Anterior Fontanelle Parameters
Understanding what constitutes "wide-opened" requires knowledge of normal developmental ranges:
- At birth: The average anterior fontanelle measures 2.1 cm, with a normal range of 0.3-2.5 cm 1, 2
- At 1 month: The fontanelle reaches its maximum size of approximately 2.2 cm 2
- At 12 months: The size decreases to approximately 1.0 cm (range 0.3-2.0 cm) 2
- At 24 months: The size further decreases to 0.5 cm (range 0.3-0.7 cm), with 93% of fontanelles closed by this age 2
- Median closure time: 13.8 months 1
Male newborns have statistically significantly larger fontanelles than females, with a mean difference of 0.15 cm 3
Clinical Significance of Abnormally Large or Delayed Closure
The American Academy of Pediatrics identifies the most common causes of a large anterior fontanelle or delayed fontanelle closure as achondroplasia, hypothyroidism, Down syndrome, increased intracranial pressure, and rickets. 4, 1
Additional conditions associated with delayed closure beyond 24 months include:
Assessment Approach
When evaluating a potentially wide-opened anterior fontanelle:
- Immediate measurement of head circumference and plotting on growth charts to assess for microcephaly or abnormal head growth 5
- Complete neurological examination to assess for signs of increased intracranial pressure, including altered mental status, vomiting, lethargy, scalp bruising or bogginess, and rapidly enlarging head circumference 5
- Developmental assessment to identify delays that may accompany underlying conditions 5
Diagnostic Imaging Considerations
- Ultrasound examination is often sufficient for suspected anomalies of the cranial sutures and anterior fontanelle 5
- The anterior fontanelle serves as an acoustic window for brain ultrasonography, allowing non-invasive imaging 4
- CT imaging is indicated only for treatment planning in selected cases, not as routine diagnostic tool 5
- MRI consideration is recommended if there are concerns about brain development or other neurological symptoms 5
Important Clinical Caveats
The fontanelle dimensions should be represented by oblique diameters (anterior-posterior and transverse measurements) in clinical pediatrics 2. There is wide variation in both size and age of closure among normal infants, so clinical context—including head circumference trends, neurological examination, and developmental milestones—is essential for determining pathological significance 1, 6.