What is the definition of microcephaly in newborns?

Definition of Microcephaly in Newborns

Microcephaly in newborns is defined as an occipitofrontal circumference (OFC) that is less than 2 standard deviations (SD) below the mean for age and sex when compared with appropriate population-based growth charts. 1, 2

Classification and Severity

• Mild microcephaly: OFC between -2 SD and -3 SD (corresponding to the 3rd to 0.3 percentile) 1, 2
• Severe microcephaly: OFC less than -3 SD (below the 0.3 percentile) 3

Measurement Technique

Accurate measurement is critical for proper diagnosis:

• Use a firm, non-stretchable measuring tape
• Position the infant with head upright
• Place tape just above the supraorbital ridges (eyebrows)
• Wrap around to the occiput (back of head)
• Apply sufficient tension to compress hair against skull
• Record measurement to nearest 0.1 cm 2

Clinical Significance

Microcephaly is not a diagnosis itself but a clinical finding that may indicate underlying conditions:

• Primary (congenital) microcephaly: Present at birth or detected in utero, often characterized by a simplified gyral pattern 1
• Secondary (postnatal) microcephaly: Normal head size at birth with subsequent deceleration of brain growth 1, 4

Important Considerations

1. Serial measurements are more informative than a single measurement 2
2. Population-specific growth charts should be used to prevent misclassification 2
3. Parental head circumference should be considered when evaluating borderline cases 4
4. Relative microcephaly (small head on a small child) may have better intellectual prognosis than absolute microcephaly 4

Associated Conditions

Children with microcephaly, particularly severe cases, may have:

• Developmental impairments (approximately 50%)
• Epilepsy (approximately 40%)
• Cerebral palsy (approximately 20%)
• Visual/hearing disorders (approximately 20-50%) 5

Diagnostic Approach

For newborns with confirmed microcephaly:

• Neuroimaging is recommended (yield ranges from 43% to 80%) 5
• Children with severe microcephaly (<-3 SD) have higher likelihood (approximately 80%) of imaging abnormalities 5
• Targeted genetic testing should be considered, especially with no evidence of acquired etiology 5

Common Pitfalls

1. Terminology confusion: Some experts suggest using "small head" rather than "microcephaly" for borderline cases (-2 to -3 SD) without cognitive impairment to avoid unnecessary parental distress 6

2. Measurement errors: Improper technique can lead to misdiagnosis; ensure consistent positioning and proper tape tension 2

3. Growth chart selection: Using inappropriate charts for ethnicity or gestational age can lead to misclassification 2

4. Single measurement interpretation: Relying on a single measurement rather than tracking growth trajectory over time 2

5. Failure to correct for parental head size: Not accounting for familial patterns of head circumference 4

By understanding the precise definition and proper measurement techniques, clinicians can accurately identify microcephaly in newborns and initiate appropriate evaluation for underlying causes.