Handbook of Genetic Counseling/Microcephaly

Microcephaly

Contracting

• Acknowledge prior phone contact
• Review main concerns of family
• What do they hope to gain from the session?
• Overview of today's session, explaining what we will do to try and figure out the cause of ____'s small head.

Family and medical history

• Obtain information about maternal and neonatal infections, pregnancy exposures to drugs, birth complications, head circumference at birth, head sizes of other family members, seizures, MR, LD, birth defects, unusual physical features, chronic health problems, anyone with a genetic condition, what type of tests have been done

Overview

• Definitions vary somewhat
  • A congenital anomaly of the central nervous system (CNS) where the head circumference is > 2 or 3 standard deviations below the mean for age and sex
• most often occurs as a result failure of normal brain growth but may also be due to a poorly growing skull (i.e., craniosynostosis)
• may be present at birth or occur in first 2 years of life
• if insult to brain occurs beyond 2 years of age less likely to produce severe microcephaly
• Clinically and genetically heterogeneous condition
• Genetic and environmental causes
• Generally associated with MR, but not in all cases
• Cognitive effects can vary significantly (normal development or mild developmental delay to severe MR)
• May be associated with seizures
• May also occur with other symptoms as in the case of a genetic syndrome

Genetic Causes

• Autosomal Recessive (Familial) RR 25%
• Autosomal Dominant RR 50%
• Syndromes (have other characteristics and often have some degree of MR or learning difficulties)
  • Gross chromosomal abnormalities
    • Trisomy 18
    • Trisomy 13
    • Wolf-Hirschhorn syndrome
    • Cri du Chat syndrome
    • Partial deletion of long arm of 13
  • Contigous gene syndromes
    • Miller-Dieker syndrome
    • Langer-Giedion syndrome
    • Prader-Willi syndrome
    • Aniridia-Wilms tumour syndrome
  • Autosomal recessive disorder
    • Johanson-Blizzard syndrome
    • Seckel syndrome
    • Smith-Lemli-OPitz syndrome
    • Coffin-Siris syndrome (?)

Nongenetic causes

• Congenital CNS Anomalies
  • Agenesis of the Cerebellar Vermis
  • Agenesis of the Corpus Callosum
  • Craniosynostosis
  • Encephalocele
  • Holoprosencephaly
  • Lissencephaly
  • Macrogyria
  • Porencephaly
  • Schizencephaly
• Congenital Infections
  • CMV
  • Rubella
  • Toxoplasmosis
  • Varicella
• Drugs
  • Alcohol
  • Aminopterin
  • Dilantin
  • Methyl Mercury
• Infections
  • Encephalitis
  • Meningitis
• Metabolic
  • Maternal Diabetes Mellitus
  • Maternal PKU
  • Inborn Errors of Metabolism
    • Neuronal Ceroid Lipofuscinoses

Testing

• Imaging Studies

CT/MRI -- for structural anomalies: cerebral/cerebellar atrophy, hydrocephaly, calcification, craniosynostosis

• Serum
  • karyotype
  • plasma for amino acids
  • serum ammonia
  • TORCH screen (toxo, rubella, CMV, HSV)
  • maternal blood for PKU
• Urine
  • amino acids and organic acids
  • CMV

References and resources

• OMIM
• Mochida GH, Walsh CA. Molecular genetics of human microcephaly. Curr Opin Neurol 2001 Apr;14(2):151-6
• PedBase.org: http://pedbase.org
• PedBase: http://pedbase.org/m/microcephaly
• There is a microcephaly online support group, but I don't know how good it is http://groups.yahoo.com/group/microcephaly/
• Definition and types of microcephaly: http://www.ayles.demon.co.uk/microcep.htm
• Teaching Files:Microcephaly
• Division of Neonatology, Cedars-Sinai Medical Center, Los Angeles, California.

Notes

The information in this outline was last updated in 2003.