Is it concerning if a 6-month-old infant's head circumference increases from above the 50th percentile to above the 97th percentile, with corresponding increases in length above the 97th percentile and weight between the 85th and 97th percentiles?

Evaluation of Rapid Head Circumference Growth in a Six-Month-Old Infant

A six-month-old infant crossing two percentile lines for head circumference (from slightly above 50th to slightly above 97th percentile), with length above 97th percentile and weight between 85th-97th percentile, requires prompt neurological evaluation to rule out hydrocephalus and other pathological causes.

Assessment of Head Circumference Growth

When evaluating head circumference growth in infants, it's important to understand normal growth patterns and concerning deviations:

• According to CDC and WHO growth charts, head circumference should follow a relatively consistent percentile channel during infancy 1
• Crossing two major percentile lines (e.g., from 50th to 97th) is considered clinically significant and warrants further evaluation 1
• The timing of this rapid head growth (occurring before 7 months of age) is particularly concerning, as this is when most cases of pathological head growth are detected 2

Differential Diagnosis

Several conditions can cause accelerated head growth in infants:

1. Benign External Hydrocephalus

   • Most common cause of rapid head growth in infants
   • Typically affects males (86.4%)
   • Usually detected when head circumference crosses percentiles before 7 months of age 2
   • Often associated with enlarged subarachnoid spaces and sometimes dilated ventricles

2. Hydrocephalus

   • 89% of cases are detected in the first year of life 3
   • Requires prompt neuroimaging and neurosurgical evaluation

3. Familial Macrocephaly

   • Important to compare child's head size with same-sex parent 4
   • Usually follows a consistent growth curve rather than crossing percentiles

4. Normal Variant (with accelerated growth in all parameters)

   • When all growth parameters are increasing proportionally
   • However, head circumference crossing two major percentile lines is rarely normal

Evaluation Algorithm

1. Detailed Neurological Examination

   • Assess for signs of increased intracranial pressure:
     • Bulging fontanelle
     • Splitting sutures
     • Setting-sun sign
     • Irritability or lethargy
     • Vomiting
   • Evaluate developmental milestones

2. Measure Parents' Head Circumference

   • Compare with same-sex parent to assess for familial macrocephaly 4
   • Even with familial large head size, crossing two percentile lines warrants investigation

3. Neuroimaging

   • Brain ultrasound (if fontanelle is open)
   • MRI or CT scan to evaluate for:
     • Enlarged subarachnoid spaces
     • Ventricular size
     • Other intracranial abnormalities

4. Neurosurgical Consultation

   • Required if imaging shows hydrocephalus or other abnormalities

Important Considerations

• The combination of rapidly increasing head circumference with accelerated length and weight growth is somewhat reassuring but does not eliminate the need for evaluation
• Head circumference measurement is a critical screening tool for neurological conditions in the first year of life 1
• Continued monitoring of head growth is essential, as some conditions like benign external hydrocephalus may normalize after 12 months 2

Common Pitfalls to Avoid

1. Dismissing rapid head growth as proportional to overall growth

   • Even with accelerated growth in other parameters, crossing two major percentile lines for head circumference requires investigation

2. Failing to compare with parental head size

   • Parental head circumference should be measured before extensive workup 4

3. Delaying evaluation

   • Early detection of conditions like hydrocephalus leads to better outcomes
   • Most pathological causes are detected in the first year of life 3

4. Stopping head circumference measurements at 36 months

   • While most pathological causes are detected in infancy, head circumference should be monitored throughout childhood when concerns exist 4

In this case, the infant's rapid head growth crossing two major percentile lines, even with proportional growth in other parameters, requires prompt neurological evaluation to rule out pathological causes.