What are the causes and risk factors of microcephaly?

Unfortunately we don’t know what causes microcephaly. Some babies have it because of changes in their genes. Conditions like Zika virus, rubella, toxoplasmosis and cytomegalovirus may cause it. There also seems to be a possible link with severe malnutrition, but we haven’t nailed down exactly what that link is.

Microcephaly, a disorder in which the brain is much smaller than normal, has been tied to at least two genes: microcepahin and ASPM.

When mutations occur in these genes, the size of the brain is affected.

ASPM evolved faster in apes that it did in mice, perhaps providing a clue as to how our brains evolved.

A study conducted in 2004 that compared ASPM in humans to the ASPM in other primates found that the sequence of the gene was somewhat similar, which may suggest that ASPM was not responsible, on its own, for differentiating human beings from chimps.

However, ASPM could have played a role in causing our brains to dramatically expand.

A year later, Dr. Bruce Lahn of the University of Chicago led a study that continued tracking the presence of ASPM, as well as that of microcephalin, in human populations. Lahn noticed that these genes were changing slightly. These alternative forms of a gene are known as alleles. The researcher's group tracked the alleles in the DNA of several populations, including those of individuals from Africa, Europe, East Asia and the Middle East, to ensure diversity.

The allele associated with microcephalin is thought to have developed around 37,000 years ago. About 70 percent of the East Asia and European populations exhibited this allele.

Lahn's team deemed the variations were common enough to suggest that their presence provided evidence of natural selection as opposed to an accidental mutation. This finding suggests that the brain may still be evolving.

Microcephaly may be caused by certain viruses. In particular, it has been linked to the Zika virus, a tropical virus that began spreading throughout the Americas and the Caribbean in 2015. The virus was first reported in Brazil in May 2015, and spread very quickly in the months afterward; around the same general time, the number of cases of microcephaly in Brazil increased dramatically. The same has happened in other countries. Some studies found that a large percentage of these babies tested positive for Zika, or their mothers showed possible symptoms of it during pregnancy. That cause-and-effect link hasn't yet been proven definitively, but experts are concerned. Other viruses, including rubella and cytomegalovirus, were previously known to be a risk factor for microcephaly.

Microcephaly has other causes, of course, included genetic disorders, toxic exposures and substance abuse by the mother.

Continue Learning about Brain & Nervous System Disorders

What You Need to Know About Amyotrophic Lateral Sclerosis
What You Need to Know About Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that attacks the nerve cells controlling voluntary muscle movement in the brain and...
Read More
What is High Altitude Cerebral Edema (HACE)?
Discovery HealthDiscovery Health
High Altitude Cerebral Edema (HACE) occurs when higher altitudes -- and the resulting lower air pres...
More Answers
Are loss of balance and neuropathy related?
Darshan N. Shah, MDDarshan N. Shah, MD
Loss of balance and neuropathy are related.  The nerves in our feet are important for being able to ...
More Answers
Can You Die from Parkinson's Disease?
Can You Die from Parkinson's Disease?

Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.