Macrocephaly, meaning “large head,” often concerns parents who wonder if the condition will resolve over time. Resolution or persistence depends entirely on the specific underlying cause, which ranges from a harmless, inherited trait to a serious medical condition. Clinicians must distinguish between benign and pathological etiologies, as this distinction guides whether the child requires intervention or simply observation. Determining the cause is the only way to establish the prognosis and the need for ongoing medical management.
Defining Macrocephaly: Measurements and Classification
Macrocephaly is objectively defined by a measurement of the head circumference, known as the occipitofrontal circumference (OFC), that is significantly larger than population averages. Healthcare providers measure the OFC and plot it on standardized growth charts, such as those from the World Health Organization (WHO) or the Centers for Disease Control and Prevention (CDC). A diagnosis is typically made when the head circumference falls above the 97th or 98th percentile for the child’s age, sex, and gestational age. This threshold indicates the child’s head is larger than 97 to 98 percent of children in the general population.
The clinical classification distinguishes between the overall size of the head and the size of the brain itself. Macrocephaly refers broadly to the large OFC, which can be due to excess fluid, blood, or bone thickness. Megalencephaly is a more specific term meaning an unusually large brain, which is one potential cause. Providers also look for disproportionate growth, where the head size accelerates out of proportion to the child’s height and weight, which may suggest a neurological issue.
Categorizing the Underlying Causes
The diverse causes of macrocephaly fall into two major categories: benign and pathological, which determines the long-term outlook. Benign causes are the most common and generally carry a favorable prognosis, often requiring only monitoring.
Benign Causes
Benign Familial Macrocephaly (BFM) is a hereditary form where a large head size is simply a family trait, often passed down in an autosomal dominant pattern. These children are generally developmentally normal, and the head circumference typically follows a stable growth curve parallel to the 98th percentile after an initial increase.
Another common benign cause is Benign External Hydrocephalus (BEH), also known as Benign Enlargement of the Subarachnoid Space (BESS). This condition involves an accumulation of cerebrospinal fluid (CSF) in the subarachnoid spaces, particularly over the front of the brain. Infants with BEH often experience a rapid increase in head circumference during the first six months of life, but this is not associated with increased pressure on the brain. The large head size is an isolated finding, and the child’s neurological examination and development remain age-appropriate.
Pathological Causes
Pathological causes are more serious and require close medical or surgical management. The most recognized pathological cause is hydrocephalus, involving an increase in CSF within the ventricles. This can be due to a blockage (obstructive hydrocephalus) or an issue with CSF absorption (communicating hydrocephalus). Other serious etiologies include intracranial mass lesions, such as tumors or cysts, and subdural collections, like chronic hematomas. Pathological macrocephaly is also a feature of numerous genetic and metabolic syndromes.
Prognosis: When Does Macrocephaly Resolve or Persist?
For macrocephaly to “go away,” the underlying cause must either be self-limiting or successfully treated, which is often the case with the benign forms.
Benign Forms
Benign Familial Macrocephaly (BFM) does not resolve by shrinking the head, but the condition stabilizes. The large head size persists, but the rapid growth rate slows to a normal pace, and the head circumference growth curve continues along a higher, stable percentile. Since BFM is not associated with neurological complications, the long-term prognosis is favorable, and the child’s development is expected to be normal.
Benign External Hydrocephalus (BEH) is considered a self-limiting condition that resolves spontaneously, typically by the age of two or three years. The initial rapid head growth slows down, and the excess fluid in the subarachnoid space gradually diminishes or is reabsorbed over time. While some infants with BEH may exhibit temporary gross motor delays, these delays often resolve without intervention as the child develops. The condition is generally not treated with surgery, and the goal of management is careful observation to ensure the child continues to meet developmental milestones.
Pathological Forms
Pathological macrocephaly does not resolve without targeted medical or surgical intervention. Conditions like progressive hydrocephalus require ongoing management, which often involves the surgical placement of a shunt to divert the excess CSF and relieve pressure on the brain. If left untreated, the continuous increase in intracranial pressure can lead to serious complications, including developmental disabilities and neurological deficits. For macrocephaly caused by a structural lesion, such as a tumor, the prognosis depends entirely on the successful removal or treatment of the mass.
When to Seek Immediate Medical Evaluation
Parents should be aware of “red flags” that indicate the macrocephaly may be pathological and require immediate medical attention, even if a previous diagnosis was benign. The most concerning signs are those related to increased intracranial pressure, which occurs when the volume of fluid, blood, or tissue inside the skull exceeds the available space. A rapidly accelerating head circumference, where the growth curve suddenly crosses several percentile lines, is a significant warning sign that requires urgent evaluation.
Urgent symptoms in infants include:
- A bulging or tense soft spot (fontanelle) on the top of the head.
- Separation of the cranial sutures.
- Behavioral changes, such as extreme irritability, a high-pitched cry, or increased sleepiness.
- Persistent vomiting or difficulty feeding.
- The “sunset eyes” phenomenon, where the eyes appear driven downward.
These symptoms suggest an urgent need for neuroimaging and consultation with a specialist, such as a pediatric neurosurgeon.