Definition:
- head circumference more than 2 standard deviations below the mean for sex and gestational age.
- < 3rd percentile: using this definition approximately 2% of the general population would be considered microcephalic even though many of these individuals are simply at the low end of the population distribution.
General
- Aka
- Microcrania
- Microcephalus
- Not a single entity.
Numbers
- Incidence is 2–12 babies per 10,000 live births in the U.S.
Risk factors
- There may be a genetic component in some cases. Since the main stimulus for growth of the skull is growth of the brain, risk factors for microcephaly include maladies that impede brain growth which include:
- infections during pregnancy: rubella, toxoplasmosis, cytomegalovirus, Zika virus
- severe malnutrition
- maternal exposure to certain drugs during pregnancy: cocaine, alcohol
- interruption of the blood supply to the brain during pregnancy
Aetiology
- Multiple classifications:
- primary microcephaly is present at birth VS secondary develops postnatally;
- genetic vs. environments,
- isolated vs. syndromic,
- symmetric vs. asymmetric.
Cause Type | Description |
Genetic - isolated | Present at birth and uncomplicated by other anomalies; genetic failure to produce enough neurons, relatively normal brain anatomy, no neurological signs but some degree of learning difficulty |
Genetic - syndromic | Down syndrome, Trisomy 18, Trisomy 13, Fetal alcohol syndrome, and many others |
Genetic or other - CNS | Neural Tube Defect, holoprosencephaly, lissencephaly, schizencephaly, polymicrogyria, pachygyria, hydranencephaly, fetal brain disruption sequence |
Metabolic | Aminoaciduria (e.g. phenylketonuria), storage disorders, and others |
Environmental | Antenatal, perinatal, postnatal infection (e.g. TORCH), in utero drug/toxin exposure, hypoxic-ischemic insult, IVH/stroke, malnutrition |
Mechanism
- Head growth is driven by brain growth, microcephaly usually implies microencephaly (small brain size)
- except in cases of generalized craniosynostosis in which skull growth is restricted,
- Microencephaly may be present in children with normal OFC.
Clinical features
- seizures
- developmental delay
- intellectual disability
- hearing problems
- vision problems
- feeding problems: including difficulty swallowing
Associated conditions
- Many of the conditions in holoprosencephaly may be associated with microcephaly.
- Differentiate the small skull of microcephaly from that due to craniosynostosis in which surgical treatment may provide the opportunity for improved cerebral development.
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