Macrocephaly is defined as head circumference more than two standard deviations (SD) above the mean value for a given age and gender.  It has to be differentiated from megalencephaly, which is defined as increase in the size of the brain parenchyma. Head circumference is measured to monitor head growth in infants and children. It is also known as occipitofrontal circumference (OFC) and it denotes the size of the cranium. Macrocephaly can be the first manifestation of various congenital and acquired neurologic conditions or may be just a familial trait.  The images below show an infant with this condition.
Macrocephaly can result from enlargement of the skull bones or an increase in the volume of the intracranial structures like cerebrospinal fluid, blood, or the brain parenchyma itself (megalencephaly). It may be secondary to raised intracranial pressure or space-occupying lesions. It can also be a feature of various congenital syndromes and is then referred to as syndromic macrocephaly. The pathogenesis is therefore cause-dependent.
Macrocephaly is a relatively rare condition but has multiple etiologies, therefore the incidence and prevalence has not been documented by many studies. However, one community-based study in Israel quoted a prevalence of 1.4%.  There is no gender or racial predisposition.
The outcome and prognosis depends on the cause. The prognosis is quite favorable in children with benign familial macrocephaly.
The severity of neurologic dysfunction, cognitive decline, and physical handicap determine the degree of dependency of the child. Both parents and teachers have to be educated accordingly to address the problems of such children. All aspects of physical, mental, social, and behavioral caregiving should be taken care of.
The etiology of macrocephaly is diverse.  The most common cause is benign familial macrocephaly characterized by enlargement of the subarachnoid spaces and accounts for almost 50% of cases.
Other causes are described below.
Enlargement of skull bones
- Hyperostosis cranii - associated with disorders such as osteogenesis imperfecta, achondroplasia, and osteopetrosis
- Secondary enlargement due to bone marrow expansion - as seen in thalassemia major
Increase in volume of cerebrospinal fluid
- Choroid plexus papilloma
- Benign familial macrocephaly
- Leukodystrophies - Canavan disease, Alexander disease, megalencephalic leukoencephalopathy with subcortical cysts
- Lysosomal storage disorders - Tay-sachs, mucopolysaccharoidosis, gangliosidosis
- Neurocutaneous disroders - Tuberous sclerosis, Sturge-weber syndrome, neurofibromatosis, Gorlin syndrome
- Autism spectrum disorder
- Other syndromes - Fragile X syndrome, Cowden syndrome, Sotos syndrome
Increased intracranial pressure (ICP)
- CNS infections
- Pseudotumor cerebri
- Subdural collections including hygromas
Mass lesions and increase in volume of blood
- Intraventricular hemorrhage, subdural hematoma, arteriovenous malformation
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