Overview 

It’s quite normal for infants' heads to change and grow from the moment they’re born, but sometimes an infant’s head can grow beyond what is normal for their size, which can seem quite alarming to parents. This is a condition called macrocephaly (mack-roh-seff-uh-lee).¹ One cause may be a subdural hematoma (SH), which can occur after a brain injury. This is when excessive amounts of blood get trapped between the layers covering the brain. The extra blood starts to hold some weight and puts quite a bit of pressure on the brain, making the head appear larger.²

But subdural hematomas aren’t the only reason an infant might develop macrocephaly. Other possible causes include:

• Fluid buildup in the brain (hydrocephalus)
• An enlarged brain (megalencephaly). 
• Brain tumours (which are rare but serious).
• Infections or internal bleeding, which create sacs of fluid 
• Genetic disorders like Sotos Syndrome, Fragile X Syndrome, and Gorlin Syndrome

Healthcare professionals can measure the baby’s head circumference over time and often order imaging tests such as an ultrasound or MRI. If the size is concerning, treatments can range from close monitoring to surgery, depending on the cause. Fortunately, with early intervention and holistic care, infants born with macrocephaly can develop and grow healthily.¹

This article seeks to explore the causes behind macrocephaly, including subdural hematomas and other intracranial lesions, to better understand how to diagnose and therefore treat it. 

What is macrocephaly?

Macrocephaly is a neurological condition whereby an infant has an abnormally large head, with a circumference measured greater than 2 standard deviations (SD) above the 97th percentile for a given age and sex. This means the widest part of their infant's head is bigger than 97% of children of the same age and sex. Infants with mild macrocephaly, which is a circumference between two and three  SD, have normal development, and up to 60% of them who develop benign macrocephaly do so due to genetics: the macrocephaly runs in the family and is harmless. Contrastingly, infants with a circumference above 3 SD usually show traits of neurogenetic disorders like intellectual disability or autism spectrum disorders.⁴ 

It should be noted that macrocephaly is different from megalencephaly, which refers to an oversized and overweight brain.  This is only one of many potential causes of this condition; other causes can include hydrocephalus, excessive subdural fluid, genetic disorders, and brain lesions.^4,5

This article will focus more specifically on how subdural hematomas and intracranial lesions cause macrocephaly. 

Understanding subdural hematomas 

What is a subdural hematoma?

To understand subdural hematomas (SH), it is crucial to first understand the brain’s anatomy. Just as we have bubble wrap to protect important items from shattering, our brain has its own version, called the skull. The purpose of the skull is to protect the delicate brain and other important sensory organs, including the eyes and ears. For extra protection, the brain has three layers called the meninges. The first layer is called the dura, which covers the skull and surrounds the brain. The arachnoid is the second layer, and the final layer is called the pia, which is closest to the brain. A fluid called cerebrospinal fluid (CSF) helps shield the brain by filling the spaces inside the meninges.⁶

During a subdural hematoma, the blood seeps between the dura and arachnoid layers. This is often caused by a blood vessel that is injured or damaged and bleeds into the subdural space, also known as a traumatic brain injury (TBI)7. This excess of blood presses against the brain, creating intracranial pressure (ICP) and damaging the surrounding tissue, which can be life-threatening if not treated immediately.⁸

Anyone is at risk of a subdural hematoma, especially if one has a TBI. However, elderly people, even with a small injury, can develop a brain bleed. The cause for this is twofold: one, the veins surrounding the brain are more prone to tears and damage due to the brain shrinking as one ages (senile atrophy). Two: elderly patients are more prone to falls due to various factors, such as muscle weakness and slower reflexes. These falls may produce enough movement within the skull and tear these veins, resulting in SH.

In infants, SH often arises due to abusive head trauma, often called shaken baby syndrome. Other causes for the same include:

• Accidental injuries
• Birth trauma
• Non-traumatic conditions

How does this cause macrocephaly? 

SH can lead to macrocephaly in infants through one of two mechanisms: 

1. Increased intracranial pressure 
2. Benign enlargement of the subarachnoid spaces (BESS)¹¹ 

As we already know, when SH occurs, it creates pressure on the brain. In infants, however, their skull is not completely developed yet; therefore, the joints between the skull bones, called sutures, have not connected properly. This allowed the head to expand in response to the increased pool of blood and therefore pressure, which causes macrocephaly.¹²

The second mechanism is quite commonly found in infants diagnosed with macrocephaly. This condition (BESS) describes the spaces between the brain and skull, called the subarachnoid spaces, being larger than normal. SH can occur with BESS either due to the same underlying cause, like minor trauma or stretching of bridging veins or due to the veins being stretched over the enlarged spaces, which makes them more prone to tearing and bleeding. This leads to an SH, which expands the head circumference of an infant.¹¹

Clinical manifestation and management of a subdural hematoma 

Subdural hematomas can be categorised as acute (<3 days of bleeding), subacute (between 4 and 20 days of bleeding), and chronic (>20 days of bleeding). 

Symptoms can appear as: 

• Changes in patients' behaviour⁸ 
• Headache¹³ 
• Nausea and vomiting 
• Abnormal breathing patterns 
• Seizures 
• Lethargy or tiredness 
• Clear sign of Cephalohematoma in infants 
• Open fontanel, which is tense and bulging 

The diagnostic methods to check for subdural hematomas are first to take a detailed patient history and physical examination, and then the diagnosis can be ideally confirmed with CT or MRI imaging techniques. Both methods can help identify the size of the hematoma, the length of bleeding and if there was any midline shift ⁸.  Depending on the severity of the subdural hematoma, any mild cases and symptoms caused by the bleeding, such as seizures and headaches, can be treated with medication like prophylaxis against seizures and vitamin K therapy to reverse any use of blood-thinning medications. However, for severe cases, whereby the hematoma is >10 mm and the patient is deteriorating, either one of the two surgical interventions is brought in: 

1. Craniotomy: Cutting part of the skull open to remove excess blood
2. Burr Holes: Drilling multiple holes in the skull to drain out the excess blood and relieve pressure¹⁴

Other intracranial lesions causing macrocephaly

Subdural hematomas are not the only cause behind macrocephaly; there are other intracranial (brain) lesions that can also contribute towards an abnormally large head. This includes: 

1. Hydrocephalus: This is caused by a disproportion between CSF production and absorption. As there may be a blockage in the flow of CSF, this causes a pool of fluid to build up within the brain's ventricles, causing increased intracranial pressure on the brain. In infants, their open sutures allow for the skull to enlarge to compensate for this extra fluid, which shows the signs of macrocephaly¹⁵
2. Arachnoid cysts: these are filled with CSF that can be traced back to the surface between the brain and the arachnoid membrane. Whilst the etiology is not fully understood, the cyst can come from the failure of the arachnoid to properly develop during embryological development during pregnancy. This causes the CSG fluid to be trapped within a sac or a cyst. As this grows, it can block CSF flow, leading to hydrocephalus and then macrocephaly¹⁶
3. Porencephalic Cysts: These can develop from either ischemic or hemorrhagic injury during pregnancy or after birth and may involve either the ventricles or the subarachnoid space. After an injury, the fluid flows into the injured space, creating a CSF-filled cyst. Depending on size and vessels involved, these cysts can cause the skull to expand or abnormally large ventricles resembling hydrocephalus, leading to macrocephaly¹⁷

Diagnosis and management of macrocephaly 

To diagnose macrocephaly, a detailed history and physical examination of the patient is needed, which includes evaluation of head growth rate (i.e., documenting serial head circumferences) along with perinatal history and signs of ICP, which is vital to differentiate between the different diagnoses. Any prevalent symptoms and signs of ICP, depending on age, and increasing head circumference, hypertonia, bulging fontanelle in infants, impaired vision, split sutures, headaches, poor feeding patterns, vomiting, irritability, and fatigue can be some indications within the patient's history that could indicate macrocephaly. 

To confirm the diagnosis as well as identify the cause in order to decide how to treat the condition, ordering neuroimaging scans like CT, MRI, or ultrasound would be the first step. An MRI might be preferred for a more detailed evaluation of the brain structures and any brain injuries, whilst a CT might be used in case of emergency, like a suspected haemorrhage. 

As mentioned, treating macrocephaly depends entirely on what caused the head to expand, whether it’s benign hydrocephalus, which often just requires regular monitoring, or a subdural hematoma, which may need surgery. Lesions like cysts, tumours or subdural collections are handled based on how much they're affecting the patient and the severity of the symptoms ¹⁵. 

Final thoughts 

It can be quite scary as a new parent to see an infant with a growing head, but something to remember is that not all causes of macrocephaly are concerning. Some are quite harmless and just require close monitoring; sometimes it's just genetic and runs in families, whilst other times any bleeding in the brain or excessive fluid buildup does require medical attention. The key is to spot these signs quite early. With an MRI, we can find out what is going on and treat it appropriately. If at any point you can see your child showing signs or symptoms of macrocephaly, it is important to reach out to a healthcare professional immediately. 

Summary

Macrocephaly is a condition where an infant’s head is abnormally large, sometimes caused by subdural hematomas (bleeding between the brain’s protective layers) or other intracranial lesions. Subdural hematomas occur from trauma, birth injury, or medical conditions, leading to increased intracranial pressure or benign enlargement of subarachnoid spaces, which expand the skull. Other causes of macrocephaly include hydrocephalus (fluid buildup), arachnoid or porencephalic cysts, tumours, infections, and genetic syndromes. Diagnosis relies on tracking head growth, clinical signs (such as bulging fontanelle, headaches, seizures), and imaging tests like MRI or CT. Treatment depends on the underlying cause, ranging from simple monitoring in benign cases to surgical intervention for severe hematomas or lesions. Early detection and intervention are essential for healthy development.