Head circumference naturally varies widely across the population, but a noticeable difference in size can cause parental concern. The medical community uses objective measurement standards to determine when a head size is significantly larger than average. This distinction is important because a large head, known as macrocephaly, can be a simple inherited trait or a sign of an underlying medical issue requiring attention.
Defining Macrocephaly: The Measurement Standard
The medical term for an unusually large head size is macrocephaly. Clinicians diagnose macrocephaly when the head circumference (HC) falls outside the statistical range typical for a child’s age and sex. Specifically, a measurement is considered macrocephalic if it is greater than two standard deviations (SD) above the mean.
This threshold typically corresponds to the 97th percentile on standardized growth charts. Falling into the 97th percentile means the individual’s head circumference is larger than that of 97 out of 100 children of the same age and sex. Head circumference is measured across the widest part of the head, known as the occipitofrontal circumference (OFC). The measurement only indicates that the head size is statistically large and warrants further evaluation, not the underlying cause.
The Role of Growth Charts in Monitoring
A child’s growth trajectory over time is often more informative than a single measurement above the 97th percentile. Healthcare providers use standardized growth charts, such as those from the World Health Organization (WHO) or Centers for Disease Control and Prevention (CDC), to track head circumference across multiple well-child visits. These charts allow for the serial plotting of measurements to establish a pattern of growth.
A child consistently tracking along a high percentile line, such as the 99th, is usually less concerning if their head growth rate remains stable and parallel to the curve. Concern arises when the growth rate is rapid, indicated by the head circumference “crossing percentile lines.” A sudden, upward shift through two or more major percentile lines suggests an accelerated rate of growth. This rapid increase, often referred to as progressive macrocephaly, may signal a developing problem, such as a buildup of fluid or an expanding mass inside the skull.
Primary (Benign) vs. Secondary (Pathological) Causes
The causes of macrocephaly are broadly divided into primary (benign) and secondary (pathological).
Primary (Benign) Causes
Primary macrocephaly, most commonly known as benign familial macrocephaly, is the most frequent cause and is generally harmless. This condition is genetic, meaning it is inherited from a parent who also has a larger-than-average head circumference. Children with benign familial macrocephaly typically have an enlarged brain, known as megalencephaly, but exhibit normal neurological development. Their head size is simply part of their genetic makeup. The head growth rate will often slow to a normal pace after the first six months of life. Another benign cause is benign external hydrocephalus, where cerebrospinal fluid accumulates in the subarachnoid space and usually resolves without intervention.
Secondary (Pathological) Causes
In contrast, secondary or pathological macrocephaly is caused by an underlying medical condition that requires investigation and possible treatment. A prominent cause is hydrocephalus, where cerebrospinal fluid accumulates within the brain’s ventricles, leading to increased pressure and head expansion. Other structural issues include intracranial bleeding, such as a subdural hematoma, or an expanding mass like a brain tumor. Macrocephaly can also be a feature of various genetic syndromes and metabolic disorders. Examples include overgrowth syndromes like Sotos syndrome, or neurocutaneous disorders such as Neurofibromatosis type 1 (NF1). Certain metabolic conditions, including leukodystrophies, can also manifest with macrocephaly due to abnormal accumulation of substances in the brain tissue.
Associated Symptoms Requiring Medical Review
When a large head circumference is a sign of a serious underlying condition, it is typically accompanied by specific symptoms reflecting increased intracranial pressure or impaired neurological function. These signs warrant immediate medical investigation, often involving brain imaging. In infants, a tense or bulging fontanelle (the soft spot on the top of the head) is a significant indicator of elevated pressure inside the skull.
Other concerning symptoms include persistent or projectile vomiting, marked irritability or lethargy, and changes in eye movement, such as a downward gaze. Older infants and children may show signs of developmental delay or exhibit seizures. These accompanying neurological symptoms distinguish pathological macrocephaly from the benign forms, guiding the need for prompt diagnosis and intervention.