What are the implications of a 17-inch head circumference in a 2-month-old patient and what is the recommended course of action?

Assessment of 17-Inch Head Circumference in a 2-Month-Old

A 17-inch (43.2 cm) head circumference in a 2-month-old is above the 97th percentile and requires immediate evaluation to determine if this represents benign familial megalencephaly or a pathologic process requiring intervention. 1, 2

Immediate Clinical Assessment

Accurate Measurement Confirmation

• Remeasure head circumference using proper technique: place a firm, non-stretchable measuring tape just above the supraorbital ridges anteriorly and around the occiput posteriorly to obtain maximum circumference, with sufficient tension to compress hair against the skull 3, 2
• Record measurement to the nearest 0.1 cm and plot on WHO growth charts, as these are recommended for children under 24 months 3, 2
• Review all previous head circumference measurements from birth to determine growth trajectory—serial measurements are far more valuable than a single data point for identifying abnormal patterns 1, 2

Critical History Elements to Obtain

• Parental head circumferences: Measure both parents' head circumferences, particularly the same-sex parent, as benign familial megalencephaly is the most common cause of isolated macrocephaly 1, 4
• Symptoms of increased intracranial pressure: Assess for rapidly enlarging head circumference, increased splaying of cranial sutures, full/tense fontanel, worsening apnea and bradycardia, lethargy, feeding intolerance, or vomiting 1
• Developmental concerns: Screen for motor delays, regression of previously acquired skills, or asymmetric motor development 1
• Seizure history: Evaluate for any nonfebrile seizures, which may indicate underlying structural brain abnormalities 1

Physical Examination Priorities

• Fontanel assessment: Check for full, tense, or bulging anterior fontanel, which suggests elevated intracranial pressure requiring urgent evaluation 1
• Neurologic examination: Assess for altered level of consciousness, abnormal tone, persistence of primitive reflexes, absent protective reflexes, cranial nerve abnormalities, or focal neurologic deficits 1
• Ophthalmologic examination: Perform fundoscopic examination to evaluate for papilledema, which indicates increased intracranial pressure 1
• Growth parameters: Measure and plot weight and length to identify disproportionate growth patterns—preservation of length with excessive head growth suggests a primary cranial process 1, 2

Decision Algorithm for Neuroimaging

Proceed with MRI (preferred over CT) if ANY of the following are present:

• Signs of symptomatic hydrocephalus (bulging fontanel, splayed sutures, vomiting, lethargy) 1
• Abnormal neurologic examination findings 1
• Papilledema or visual changes on fundoscopic examination 1
• Disproportionate growth (head circumference crossing percentiles upward while weight/length remain stable) 1
• Rapidly accelerating head growth trajectory on serial measurements 1
• Developmental delays or regression 1

Neuroimaging is NOT routinely indicated if:

• Both parents have large head circumferences (suggesting benign familial megalencephaly) 1, 4
• Stable growth trajectory parallel to but above the 97th percentile 1
• Completely normal neurologic examination 1
• No symptoms of increased intracranial pressure 1

Follow-Up Protocol

For Infants WITHOUT Concerning Features

• Serial measurements every 2-4 weeks to monitor growth trajectory and ensure the head circumference curve remains parallel to growth charts rather than crossing percentiles 1
• Developmental surveillance at each visit to identify emerging concerns, as intellectual delay is the most common disability associated with abnormal head growth 1, 5
• Reassess immediately if clinical status changes, including new symptoms, developmental regression, or accelerating head growth 1

Referral Indications

• Consider genetics or neurology referral if syndromic features are present, developmental delays emerge, or growth pattern remains concerning despite benign initial workup 1
• An interdisciplinary approach is warranted if microcephaly or macrocephaly is associated with other anomalies, including epilepsy, cerebral palsy, language delay, strabismus, ophthalmologic disorders, or cardiac/renal/skeletal anomalies 5

Key Clinical Pitfalls to Avoid

• Do not dismiss macrocephaly without measuring parental head circumferences—failure to compare with same-sex parent head size is a common error that leads to unnecessary imaging 4
• Do not rely on a single measurement—growth trajectory over time is more diagnostically valuable than any isolated measurement 1, 2
• Do not assume head growth stops at 36 months—the cranium continues growing through adolescence, and ongoing monitoring beyond infancy may be clinically indicated 4
• Do not order CT when MRI is available—MRI is preferred for evaluating structural brain abnormalities without radiation exposure 1