Diagnosis and Classification of Microcephaly

Definition and Classification

• The World Health Organization (WHO) defines microcephaly as a head circumference that is 2 or more standard deviations (SD) below the mean for age and sex 1, 2, 3
• Microcephaly is defined as an occipitofrontal circumference (OFC) below -2 SD (corresponding to the 3rd percentile) compared with age- and sex-matched controls 1, 2
• Mild microcephaly is considered when OFC is between -2 and -3 SD (corresponding to the 0.3 percentile) 1

Clinical Significance

• Microcephaly is the most common malformation of cortical development, present in approximately 15% of children referred for evaluation of developmental disabilities 2
• The final brain size results from a complex process involving neural stem cell proliferation, migration, organization, synaptogenesis and apoptosis 2

Types of Microcephaly

• Primary (congenital) microcephaly: present at birth or detected in utero, often characterized by a simplified gyral pattern and most cases result from impairment of normal neurogenesis 1, 2

Important Considerations

• While microcephaly refers to a small head circumference, micrencephaly indicates a small volume of brain parenchyma 1
• In most cases, microcephaly implies micrencephaly, with the terms often used interchangeably 1
• An exception is craniosynostosis, where small head circumference may be present despite normal brain size due to early fusion of sutures 1