Microcephaly is diagnosed when a baby's head is much smaller than typical. There are varying criteria used by doctors. The most common is defined as a baby with a head circumference smaller than 2 standard deviations from normal (smaller than the 3rd percentile).
In about 15% of cases, babies with microcephaly have no other problems - they simply have small heads. For many babies though, microcephaly is the most easily observed sign that brain development has not gone normally. Babies with microcephaly are much more likely to have epilepsy, cerebral palsy, genetic issues, and mental retardation. These co-conditions are more common with more severe cases of microcephaly. Some experts advocate for using 3 standard deviations from the norm for diagnosis because they believe this more accurately predicts which babies will have serious problems.
Sources: American Academy of Neurology's paper, "Practice Parameter: Evaluation of a child with microcephaly", New York Times' "Short Answers to Hard Questions About Zika Virus"
How common is microcephaly?
In the U.S., about 0.56% of babies are diagnosed with microcephaly (using the 2 standard deviation definition, see note * below). this is about 25,000 babies annually. Far fewer babies, only 0.14%, have severe microcephaly (greater than 3 standard deviations from normal). These numbers are prior from prior to 2009, so these are pre-Zika rates.
Sources: American Academy of Neurology's paper, "Practice Parameter: Evaluation of a child with microcephaly"
Aside from Zika, what causes microcephaly?
There are many things linked to microcephaly.
- Infections of the fetus during pregnancy:
- Toxoplasmosis - This is caused by a parasite in under cooked meat and the reason doctors advise pregnant women to avoid cat litter.
- Cytomegalovirus - A common virus that you probably already have.
- Rubella, also called German measles.
- Varicella, also called chicken pox.
- Malnutrition
- Chromosomal abnormalities
- Exposure to drugs, certain toxic chemicals, and alcohol.
- Chromosomal abnormalities
Severe malnutrition, drugs, and toxic chemicals are all worth avoiding regardless of whether you are pregnant or not, and I hope these are not an issue for anyone reading this!
You will notice that the infections listed are common and hard to avoid. Even worse, all of them lurk in our bodies after the initial infection, reemerging when our immune systems are weak as things like shingles. The good news is that they only pose a risk to a baby during active infection.
Sources: Mayo Clinic "Microcephaly: Causes"
* For the statisticians in the audience - It's awfully strange that only 0.56% of babies have heads smaller than 2 standard deviations of the norm, when, by definition, we'd expect 2.3%! Baby heads are not normally distributed even though the diagnosis scheme assumes they are.
Sources: American Academy of Neurology's paper, "Practice Parameter: Evaluation of a child with microcephaly"
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