Macrocephaly refers to an overly large head. It’s often a symptom of complications or conditions in the brain.

There’s a standard used to define macrocephaly: The circumference of a person’s head is more than two standard deviations above average for their age. Or, their head is larger than the 98th percentile.

Macrocephaly is usually a symptom of other conditions. Benign familial macrocephaly is an inherited condition. It happens in families predisposed to having larger heads.

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

Benign extra-axial collection is a condition where there’s fluid in the brain. But this condition doesn’t require treatment because the amount of fluid is minor.

Other conditions that can cause macrocephaly include:

Some children will have benign macrocephaly. And they won’t experience symptoms other than having a larger head circumference.

In other cases, children may experience developmental delays, like 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 factors that increase the likelihood of macrocephaly, like genetics. Familial macrocephaly is an inherited condition. It’s also thought that children with autism have a higher incidence of macrocephaly. One study estimates 15 to 35 percent of children with autism will have macrocephaly.

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

A pediatrician can diagnose macrocephaly. They will track an infant’s head measurements over time. Your doctor will 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, you doctor will check your infant’s head for an increase in pressure. Symptoms of increased pressure include:

Your doctor will also look for bulging veins and eye problems. These symptoms require neurological evaluation to find the underlying problem and its severity.

Be sure to tell your doctor if you have a family history of larger-than-average head size.

Treatment for macrocephaly will depend on the diagnosis.

If tests don’t indicate problems, and brain scans are normal, your doctor will continue to monitor you infant’s head. Parents are also 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 an occipitofrontal (head) circumference up to three standard deviations over the average. It can also be a brain weighing more than 1,800 grams. This is due to expansion of cerebral tissue. Most adults with macrocephaly don’t continue to grow through adulthood.

Complications are rare in benign macrocephaly. But they can occur. People with brain overgrowth may experience brainstem compression. This requires surgery to decompress the brain stem.

People with macrocephaly often have hydrocephalus. This is a condition where an abnormally high amount of cerebrospinal fluid collects in the brain.

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 with no major complications. In other cases, the outlook for macrocephaly depends on the underlying condition and its severity.