Introduction

One of the most distressing moments for any parent is hearing that an abnormality has been found during a routine health check-up for their baby. One concern may be that the baby’s head circumference is larger than expected. The medical term for this condition is known as macrocephaly (or macrocrania). While the initial news can be distressing, an enlarged head is not always a reason for concern. In some cases, it may simply be a normal variation that runs in some families. However, in other cases, a larger head circumference may indicate an underlying condition that affects the brain’s development or structure.

When a child’s head size is larger than expected, it is important to distinguish between two related medical conditions: macrocephaly and megalencephaly. Although often used interchangeably, these two conditions are different and have distinct causes, clinical implications, and treatment approaches. While macrocephaly describes a head circumference that is significantly larger than average for the sex and age, megalencephaly specifically refers to an increase in brain size, rather than just an increase in skull size. Although megalencephaly can lead to macrocephaly, it is only one of several possible causes.

This article will explain the differences between macrocephaly and megalencephaly, including their causes, describe how they are diagnosed, and outline the management and prognosis for affected children.

Macrocephaly vs megalencephaly

What is macrocephaly?

Macrocephaly is a broad term used to describe a head circumference which is noticeably larger than the average for the child’s age or sex. It is observed in up to 5% of babies and can range from a harmless variation to a more serious condition that requires prompt evaluation and treatment. It can have many different causes, including problems relating to the brain, skull, fluid, or other tissue. Some of the most common causes include:¹

• Benign familial macrocephaly: This is a common and harmless cause of head enlargement that tends to run in families
• Megalencephaly: A condition in which the brain itself is larger than normal
• Hydrocephalus: It is characterised by a fluid build-up in or around the brain and spinal cord, which can put pressure on the brain and skull
• Skeletal dysplasia: A broad term used to describe conditions that cause abnormal bone and cartilage growth
• Space-occupying lesions:  Tumours or cysts can take up space in the skull and may cause the head to enlarge
• Other causes of increased pressure within the skull (known as intracranial hypertension), such as infection, inflammation or toxic abnormalities

What is megalencephaly?

Megalencephaly (MEG) is a condition characterised by an oversized or overweight brain. It can be classified regarding its origin:¹

• Congenital or primary megalencephaly develops during early pregnancy
• Postnatal or secondary megalencephaly develops later in babies and young children

Based on the underlying causes, megalencephaly can be grouped into three main categories that we will analyse next.  

Idiopathic or benign megalencephaly

It refers to children who have an enlarged brain but show no signs of neurological problems. The most common disorder in this group is known as benign familial megalencephaly. In this condition, the brain is larger than usual, but there are no related problems with brain function. Benign familial megalencephaly often runs in families and is passed down through genes.²  

Anatomic megalencephaly

It includes a wide range of disorders in which the brain cells are either larger than normal, more numerous than normal, or both. Notably, anatomic megalencephaly does not always lead to macrocephaly. For example, in some cases, brain enlargement may only occur on one side or one area.² 

Metabolic megalencephaly

This includes various disorders in which metabolic substances accumulate in brain cells. Examples of metabolic disorders include Canavan disease, Tay-Sachs disease, galactosaemia, Alexander disease, and mucopolysaccharidoses.²

Diagnosis

To correctly differentiate between macrocephaly and megalencephaly, a combination of physical exams, a review of the family’s medical history, imaging and genetics tests may be necessary.¹

Physical examination

The initial assessment starts with measuring the child’s head circumference. Since an enlarged head is often a sign of problems in several body systems, such as the brain, hormones, skeletal and genetic, further tests are usually recommended. These could include:^1,3

• Blood and urine tests are performed if a metabolic or hormonal disorder is suspected
• A lumbar puncture (spinal tap) to check the pressure of the cerebrospinal fluid or to rule out infections, bleeding or other neurological conditions
• X-rays for children with suspected skeletal dysplasia
• Echocardiogram (heart scan)
• Eye examinations

All children with signs of raised intracranial pressure should be referred immediately. Signs of raised intracranial pressure include:¹

• Tight or bulging fontanelles, which are the soft areas on a baby’s head where the skull bone formation is not yet complete
• Headaches
• Abnormal eye movements or position, such as crossed eyes, unequal-sized pupils, droopy eyelids, or when the eyes appear to be continuously looking downwards (known as sunsetting)
• Unsteadiness 
• An increasing head circumference
• Change in behaviour, such as sleepiness or irritability
• Seizures
• Loss of consciousness

Neurological and developmental assessment

Children with macrocephaly and megalencephaly are more likely to have intellectual disability or autism spectrum disorders. As part of the assessment, the healthcare team will look for signs of typical development, including whether the child is meeting expected milestones. They will also assess any recent changes in behaviour.^4,5

Medical history

Family history and parental head sizes should always be taken into account, as they can help identify inherited conditions or genetic syndromes linked to macrocephaly and megalencephaly.⁴

Imaging tests

Your healthcare provider may order an ultrasound, MRI and/or CT scans to evaluate brain structure and volume.³

• Ultrasound is usually the first step for children who have no neurological abnormalities, no signs of increased intracranial pressure and have a normal open fontanelle
• MRI or CT is recommended for children with neurological abnormalities, increased intracranial pressure or when the head size is enlarging on repeated head circumference measurement. MRI is one of the best ways to diagnose the cause of an enlarged head

Genetic testing

Genetic testing is recommended when developmental delays or other abnormalities are noted on physical exams.^1,4

Treatment

While there is no cure for macrocephaly or megalencephaly, keeping a close eye on head growth and a child’s development helps guide care and detect any concerns early. Treatment will depend on the underlying cause and could include:¹

• Brain surgery is required if hydrocephalus or brain tumours are present
• Antibiotic treatment for infections causing macrocephaly
• Therapies, such as occupational, vision, speech or physical therapy
• Genetic counselling for inherited conditions

Prognosis

Prognosis is variable and will depend on many factors, such as the severity of the condition at the time of diagnosis, as well as the underlying cause. In about 10 to 15% of cases, mild macrocephaly is a normal variation that runs in families.³ Megalencephaly usually has a less favourable outlook than macrocephaly and is more commonly associated with developmental delays and seizures.²

FAQs

When should a child’s head circumference be measured?  

Any child under the age of 4 with an abnormal head shape or size should have their head circumference measured.

How is the head circumference measured?

The head circumference is measured using a measuring tape. The tape is placed around the largest part of the head, which is just above the eyebrows at the front and around the most prominent part at the back of the head (the occiput). This measurement is the occipitofrontal circumference or OFC.

What is an abnormal head circumference?

The head circumference is measured in centimetres and plotted on a standard growth chart for the child’s age and gender. This is used to determine if the head size falls within the normal range. Macrocephaly is diagnosed if the head circumference is larger than about 97% of the average.

What are the common mistakes when measuring head circumference?

The tape may be misplaced, or the child’s hair may be too thick.

How often should a baby’s head circumference be measured? 

The American Academy of Paediatrics recommends the head circumference be measured routinely until two years of age. In addition, the head circumference should be measured in all children if there are signs of increased intracranial pressure, neurological impairment or neurodevelopmental delays.¹

Is macrocephaly linked to autism spectrum disorder?

Yes, studies have shown that approximately 15% of people with autism spectrum disorder have macrocephaly.⁵

At what age is the brain fully developed? 

At 25 years old.¹

How much does the brain weigh? 

The brain of a newborn baby weighs about 370g, while the adult brain weighs 1500g.²

Summary

Head size can vary from child to child, and in many cases, a larger head is simply a normal variation, especially when it runs in families. However, conditions such as macrocephaly and megalencephaly can sometimes point to underlying medical or developmental issues. Some causes of head enlargement are medical emergencies and require early intervention to prevent complications and protect the child’s health. This is why early recognition, careful monitoring, and appropriate testing are so important. With timely diagnosis and the right support, many children with these conditions can achieve good outcomes and lead healthy, fulfilling lives.