Microcephaly and Macrocephaly
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Direct answer
Microcephaly is an occipitofrontal head circumference more than two standard deviations below the mean for age and sex, while macrocephaly lies more than two standard deviations above it. Primary microcephaly is usually genetic, whereas secondary forms follow prenatal infection, toxins or hypoxic injury. Hydrocephalus is a leading serious cause of macrocephaly in infancy, and serial measurement is the first step in any evaluation.
What you must remember
- Measurement: occipitofrontal circumference is taken with a non-stretchable tape over the glabella and the most prominent part of the occiput.
- Microcephaly threshold: below minus 2 SD for age and sex; severe microcephaly is usually defined as below minus 3 SD.
- Macrocephaly threshold: above plus 2 SD for age and sex, with the growth curve tracked across visits.
- Primary microcephaly: a genetic cause with reduced brain size but normal architecture, typically inherited in an autosomal recessive pattern.
- Secondary microcephaly: congenital cytomegalovirus, a leading infective cause, along with Zika virus, maternal alcohol, radiation, hypoxic-ischaemic injury and postnatal meningitis.
- Benign familial macrocephaly: autosomal dominant, with normal development and a parent or sibling who has a large head.
- Hydrocephalus and subdural collections: cause rapid head growth, a tense fontanelle or split sutures, and need urgent imaging.
Approach to a child with an abnormal head size
A two-month-old infant has an occipitofrontal circumference that has fallen across centile lines on serial charts. The evaluation moves from measurement to history and examination, then to targeted investigation, as set out below.
- Measure and plot serially: use a non-stretchable tape and plot each value, because a single measurement is prone to error and the trend matters most.
- Compare with the parents: a child whose head matches a large- or small-headed parent and who develops normally is likely to have a familial variant.
- Take a focused history: ask about perinatal events, maternal infections, alcohol and drug exposure, radiation, consanguinity and family head size.
- Examine for clues: check fontanelle tension, sutures, sunset eyes, the fundus for papilloedema, the skin for neurocutaneous signs and any dysmorphic features.
- Investigate and refer: request neuroimaging first, then infective, metabolic and genetic tests guided by the findings, with multidisciplinary follow-up.
The first two steps often settle whether the problem is a familial variant or a true deviation. Imaging and metabolic work are reserved for children whose head size, fontanelle or development points to an intracranial or systemic cause.
Where the exam tries to catch you
Most head size questions turn on measurement trends and on which cause is most likely.
- Trap: labelling microcephaly from one low measurement. Correct: serial plotting, parental head size and development decide, because measurement error is common in clinic.
- Trap: assuming a large head always means hydrocephalus. Correct: benign familial macrocephaly with normal development is common, and the parents usually have large heads.
- Trap: assuming microcephaly is always genetic. Correct: congenital CMV, alcohol, hypoxia and Zika are acquired causes, and several of them are preventable.
- Trap: dismissing rapid head enlargement as normal growth. Correct: upward crossing of centiles with a tense fontanelle, split sutures or sunset eyes needs urgent neuroimaging.
Frequently asked questions
What is the threshold for microcephaly in children?
Microcephaly is an occipitofrontal circumference below minus 2 standard deviations for age and sex, while severe microcephaly is usually defined below minus 3 standard deviations. A single value is insufficient, so the diagnosis rests on serial plotting over time.
What is benign familial macrocephaly?
Benign familial macrocephaly is an autosomal dominant variant in which a child has a large head, usually with normal development, and one or both parents also have a large head. It is a diagnosis of exclusion, made only after raised intracranial pressure and other causes are excluded.
Which congenital infections cause microcephaly?
Congenital cytomegalovirus is the leading infective cause of microcephaly, and rubella, toxoplasmosis, herpes and Zika virus are also recognised causes. The TORCH group is therefore considered when microcephaly is detected at birth.
Why is a rapidly enlarging head an emergency?
Rapid enlargement suggests raised intracranial pressure from hydrocephalus or a subdural collection, which can damage the developing brain. A tense fontanelle, split sutures or sunset eyes call for urgent neuroimaging.
Does microcephaly always mean intellectual disability?
No. Outcome depends on the underlying cause and the degree of brain involvement, so development must be assessed directly rather than inferred from head size alone.