Microcephaly
Microcephaly: Excerpt from The Diagnostic Approach to Symptoms and Signs in Pediatrics
Microcephaly is defined here as a head circumference >3standard deviations below mean for age and gender. It is a signof a small brain (micrencephaly), and affected children usuallyhave some degree of neurologic impairment.
Principal Causes of Microcephaly
- Normalvariation
- Primary microcephaly
- Microcephalyvera (genetic)
- Chromosomal disorders
- Dysmorphic syndromes with normal karyotype
- Structural defects in brain development
- Disordersof neuronal migration
- Polymicrogyria
- Lissencephaly
- Schizencephaly
- Holoprosencephaly
- Agenesis of the corpus callosum
- Macrogyria
- Secondary microcephaly
- Maternaldisorders and exposures
- Maternal phenylketonuria
- Drug or poison ingestion
- Fetalalcohol syndrome
- Fetal hydantoin syndrome
- Other ingestions
- Malnutrition (fetal and early infancy)
- Hypoxic-ischemic encephalopathy
- Infection
- Craniosynostosis
Clinical Features and Diagnosis
Normal Variation
Some normal infants have small head sizethat is proportional to their weight and height. This is often seenin preterm infants who never experience catch-up growth.
Primary Microcephaly
Small head size that occurs because brainis small and never developed properly due to genetic and embryologicdisturbances.
Microcephaly Vera (Genetic)
Mainly transmittedas autosomal-recessive trait. Gene locus has been mapped to chromosome8.Characteristic findings include foreheadthat slants backward, prominent ears and nose, small chin, spasticdiplegia, seizures, and mental deficiency.Brain imaging is normal. Chromosomal Disorders
Small headsize is frequent finding in trisomies 13 and 18, deletion syndromes(3p, 4p, 5p, 11q, 13q, 18p, 18 q), and duplication syndromes (4p,10q).In these disorders, children are dysmorphicand have other associated findings. Chromosomal karyotype confirmsdiagnosis.See Jones (1997) for description ofthese syndromes. Dysmorphic Syndromes with Normal Karyotype
Small headsize occurs in some syndromes in which gene loci have been mapped. Theseinclude Angelman, Bloom, Cockayne, Rubenstein-Taybi, Seckel, Smith-Lemli-Opitz,and Williams syndromes.Small head size also exists in othersyndromes in which the gene loci have not been mapped. These areCornelia de Lange, DeSanctis-Cacchione, and Dubowitz syndromes.See Jones (1997) for description ofthese syndromes. Structural Defects in Brain Development
Defects described in this section are associatedwith microcephaly and may be diagnosed by MRI.
Disorders of Neuronal Migration
Polymicrogyria
Characterizedby excessive number of abnormally developed gyri.Clinical findings depend on locationand extent of lesion.Involvement of entire cortex may producesmall head, severe psychomotor retardation, and seizures.Abnormal development and bilateralspasticity occur with bilateral focal lesions, whereas focal motorseizures and hemiparesis occur with unilateral focal involvement. Lissencephaly
Lissencephaly(smooth brain) is absence of gyri and sulci or reduced number ofgyri with normal or shallow sulci.Frequent findings include hypotonia,severe motor delay, and seizures. Schizencephaly
Malformationin which abnormal clefts extend from cortical surface of brain toventricles.Clinical findings depend on locationand extent of lesion; however, seizures and severe psychomotor retardationusually occur. Holoprosencephaly
Spectrumof malformations that result from defective cleavage of forebrainsuch that cerebral hemispheres fail to separate.In most severe form, microcephaly usuallyoccurs along with seizures, apneic episodes, severe psychomotorretardation, and occasionally, midline facial anomalies. Agenesis of Corpus Callosum
Absenceof commissural fibers that bridge cerebral hemispheres.Can be asymptomatic isolated findingor associated with other malformations of brain.Seizures and mental retardation arecommon associated findings. Macrogyria
Presenceof abnormally small number of large gyri.Besides microcephaly, seizures andmental retardation usually occur. Secondary Microcephaly
In this circumstance brain develops normally,until some process impairs its growth.
Maternal Disorders and Exposures
Maternal Phenylketonuria
Exposureof fetus to high phenylalanine levels from mother with phenylketonuria mayproduce certain abnormalities: microcephaly, round facies with prominentglabella, decreased weight and length, congenital heart defects,and mental retardation.Any woman with mental retardation ofunknown cause should be screened for phenylketonuria. Drug or Poison Ingestion
Fetal Alcohol Syndrome
Occurs inmany infants whose mothers are chronic alcoholics.Characteristic findings include microcephaly;facies with short palpebral fissures, maxillary hypoplasia, shortnose, and smooth philtrum with thin upper lip; cleft lip and palate;small distal phalanges with small fifth fingernail; congenital heartdisease (ventricular septal defect or atrial septal defect); prenataland postnatal growth deficiency; and mental retardation. Fetal Hydantoin Syndrome
Microcephalyis occasionally seen following maternal hydantoin intake duringpregnancy.Characteristic findings include facieswith wide nasal bridge, hypertelorism, short nose, and bowed upperlip; cleft lip and palate; wide anterior fontanelle; small distalphalanges with hypoplastic nails; poor growth that is usually prenatal;and mental retardation. Other Ingestions
Cocaine abuse and use of solvents (e.g.,toluene) also have been associated with microcephaly.
Malnutrition (Fetal and Early Infancy)
With severe malnutrition during pregnancyor early infancy, brain fails to grow normally and microcephalyoccurs.
Hypoxic-Ischemic Encephalopathy
May cause destruction of brain tissue withsubsequent atrophy and areas of cystic degeneration. CT or MRI isdiagnostic.
Infection
Microcephalymay occur with congenital infections (e.g., rubella, herpes simplex,cytomegalovirus, toxoplasmosis, and syphilis).Bacterial meningitis also may causebrain atrophy and small head size.See Chap.3, Alteration in Consciousness, and Chap. 36, Jaundice. Craniosynostosis
Primarydisorder of skull in which premature fusion of cranial sutures impairsbrain growth.Shape of head is abnormal.Skull radiographs and CT are diagnostic. Diagnostic Approach
Measurehead circumference, weight, and height at each well-child visitusing growth charts published by the CDC (2001).If these measurementsare proportional, child most likely is small in size and does not haveprimary abnormality in brain growth.Head circumference that is proportionallysmaller than weight and length usually signifies primary decreasein brain growth or secondary process (e.g., hypoxic-ischemic encephalopathyor infection). History of microcephaly in family memberssuggests genetic disorder, which is important for genetic counseling.History of maternal drug or alcoholingestion, maternal phenylketonuria, or other disorders should beclarified.Presence of dysmorphic features suggestschromosomal or malformation syndrome, and chromosomal karyotypeshould be determined.Skull radiographs help diagnose craniosynostosis.CT and MRI can diagnose structural defects in brain development.Presence of other findings helps diagnosevarious infections. References
- Aicardi J. Diseases of the nervous systemin childhood, 2nd ed. London: Mac Keith Press, 1998.
- Ball WS Jr, ed. Pediatric neuroradiology. Philadelphia:Lippincott-Raven, 1997.
- CDC Growth Charts: United States. U.S. Department ofHealth and Human Services. Centers for Disease Control and Prevention.National Center for Health Statistics. Division of Data Services. Hyattsville,MD, 2001. World Wide Web URL: http://www.cdc.gov/growthcharts.
- DeMyer W. Microcephaly, micrencephaly, megalocephaly,and megalencephaly. In: Swaiman KF, Ashwal S, eds. Pediatric neurology:principles and practice, 3rd ed. St. Louis: CV Mosby, 1999:301–311.
- Fenichel GM. Clinical pediatric neurology: a signsand symptoms approach, 4th ed. Philadelphia: WB Saunders, 2001.
- Hunter AGW. Brain. In: Stevenson RE, et al., eds. Humanmalformations and related anomalies. Oxford monographs on medicalgenetics. Vol. II, No. 27. New York: Oxford University Press, 1993:1–19.
- Jones KL. Smith's recognizable patterns ofhuman malformation, 5th ed. Philadelphia: WB Saunders, 1997.
- Online Mendelian Inheritance in Man (OMIM). McKusick-NathansInstitute for Genetic Medicine, Johns Hopkins University (Baltimore,MD) and National Center for Biotechnology Information, NationalLibrary of Medicine (Bethesda, MD), 2000. World Wide Web URL: http://www.ncbi.nlm.nih.gov/omim.
- Rudolph AM, ed. Rudolph's pediatrics, 20thed. Stamford, CT: Appleton & Lange, 1996.
Book Source Details
- Book Title: The Diagnostic Approach to Symptoms and Signs in Pediatrics
- Author(s): Paul S. Bellet
- Year of Publication: 2006
- Copyright Details: The Diagnostic Approach to Symptoms and Signs in Pediatrics, Copyright © 2006 Lippincott Williams & Wilkins.
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Copyright notice for book excerpts: Copyright © 2008 Lippincott Williams & Wilkins. All rights reserved.
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