Macrocephaly refers to an overly large head in infants. Not all cases of macrocephaly are cause for alarm. However, it is often a sign of other complications or conditions in the brain.

There is a general standard used for defining macrocephaly: when the circumference of a person’s head is more than two standard deviations above average for their age, or larger than the 98th percentile.

Macrocephaly is not a condition in itself. It’s a symptom of other conditions. Benign familial macrocephaly is an inherited condition where a family is predisposed to having a larger head circumference.

Sometimes there is a problem with the brain, such as hydrocephalus or excess fluid. These underlying conditions will require treatment.

Benign extra-axial collection is a condition where there is a small amount of fluid in the brain. The amount of fluid is so minor that it does not require treatment.

Other conditions that can cause macrocephaly include:

  • brain tumors
  • intracranial bleeding
  • chronic hematomas and other lesions
  • Morquio syndrome
  • Hurler syndrome

Some children will have benign macrocephaly. Many of these children will experience no other symptoms aside from a larger head circumference.

In other cases, children may experience developmental delays. These include delays in reaching learning milestones. Other symptoms include:

  • mental disabilities or delays
  • rapid head growth
  • slowed growth of the rest of the body
  • comorbidity with other conditions, including autism or epilepsy

There are certain factors that increase the likelihood of macrocephaly. Genetics is one factor thought to play a part. Familial macrocephaly is an inherited condition. It’s also thought that infants with autism have a higher likelihood of macrocephaly. One study estimates that macrocephaly will be evident in 15 to 35 percent of children with autism.

Other risk factors include:

  • hydrocephalus
  • Alexander disease
  • Canavan disease
  • neurofibromatosis
  • a mother being infected with Zika virus while pregnant

There is no evidence that macrocephaly affects children of any particular gender, nationality, or race more often.

A pediatrician can diagnose macrocephaly with an examination of the infant’s head measurements from birth to present. They may also perform neurological tests. These can include a CT scan, ultrasound, or MRI to get a better look at the head and brain.

Because macrocephaly can be a symptom, your doctor will evaluate the infant’s head to determine whether there is an increase in pressure. Signs of increased pressure include vomiting, irritability, and headaches. The doctor will also look for bulging veins and eye problems. These symptoms will warrant further neurological evaluation to determine the underlying problem and its severity.

Be sure to tell your doctor of any family history of larger-than-average head size.

Treatment for macrocephaly will depend on the diagnosis.

If tests indicate no problems and brain scans come back normal, the infant’s head will continue to be monitored. During the monitoring phase, parents are advised to watch for:

  • a bulging soft spot
  • vomiting
  • lack of interest in food
  • abnormal movements in the eyes
  • excessive sleeping
  • irritability

Studies on macrocephaly in adults are limited. This is partly because head measurements are often only taken during a baby’s development. Macrocephaly in adults is defined as an occipitofrontal (head) circumference up to three standard deviations over the average. It can also be defined as a brain weighing more than 1,800 grams due to expansion of cerebral tissue. Most adults with macrocephaly will not have continual growth through adulthood.

Complications rarely occur with benign macrocephaly. But they can occur in all types of macrocephaly. People with brain overgrowth may experience brainstem compression. This requires a surgical procedure to decompress the brain stem.

Hydrocephalus, or an abnormally high accumulation of cerebrospinal fluid in the brain, is commonly seen in people with macrocephaly.

Other complications include:

  • seizures or epilepsy
  • perinatal risk factors
  • neurologic comorbidity, or the coexistence of two conditions (this can lead to other complications and health problems)

Infants with benign familial macrocephaly usually grow up to live normal lives with no major complications. In other cases, the outlook for macrocephaly depends on what caused the condition and its severity.