What is the World Health Organization (WHO) standard for diagnosing microcephaly in terms of head circumference standard deviations?

WHO Standard for Diagnosing Microcephaly

According to the World Health Organization (WHO) standard, microcephaly is defined as a head circumference that is 2 or more standard deviations (SD) below the mean for age and sex. 1

Classification of Microcephaly

• Microcephaly is defined as an occipitofrontal circumference (OFC) below -2 SD (corresponding to the 3rd percentile) compared with age- and sex-matched controls 1
• Mild microcephaly is considered when OFC is between -2 and -3 SD (corresponding to the 0.3 percentile) 1
• Severe microcephaly is defined as an OFC that is less than three standard deviations below the average 2

Clinical Significance

• Microcephaly is the most common malformation of cortical development, present in approximately 15% of children referred for evaluation of developmental disabilities 1
• It is an important neurological sign and predictor of future disability, though clinical outcome cannot be predicted by head size alone 3
• The final brain size results from a complex process involving neural stem cell proliferation, migration, organization, synaptogenesis and apoptosis 1

Types of Microcephaly

• Primary (congenital) microcephaly: present at birth or detected in utero 1

  • Often characterized by a simplified gyral pattern 1
  • Most cases result from impairment of normal neurogenesis 1

• Secondary (postnatal) microcephaly: develops after birth 1, 4

  • Results from deceleration of brain growth due to infection, trauma, intoxication, metabolic disease, or CNS degenerative disease 4

Measurement Technique

• For accurate measurement, the infant should be held with the head upright 5
• The measuring tape should be positioned just above the supraorbital ridges anteriorly and around the occiput posteriorly 5
• Sufficient tension should be applied to compress the hair against the skull 5
• Measurements should be recorded to the nearest 0.1 cm 5

Important Considerations

• Serial measurements of head circumference are more valuable than a single measurement for assessing growth patterns and potential abnormalities 5
• 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

Population Variations

• Recent studies have shown that the WHO standards may not be universally applicable across all populations 6
• In many populations outside the WHO reference group, adopting WHO head circumference standards could overdiagnose macrocephaly and underdiagnose microcephaly 6
• Some researchers have suggested that local population-specific cut-offs or references may be more appropriate for many populations 6

Clinical Implications

• The distinction between -2 SD and -3 SD as a cutoff is clinically relevant, as the limit of -3 SD is more frequently associated with cognitive impairment 7
• Some researchers have suggested using the term "small head" rather than microcephaly for measurements between -2 and -3 SD without cognitive impairment, to avoid unnecessary distress for parents 7
• Evaluation of a child with microcephaly requires thorough analysis of history, clinical examination, and complementary studies, with MRI being the first step in etiologic research 3