Contributed by Lisa Packer, staff writer, Healthy Hearing | Thursday, August 25th, 2016
If you think the Zika virus has been dominating the news lately, you are correct. In February of 2016, the World Health Organization declared the Zika virus outbreak to be a public emergency. But there is another aspect of Zika that is just beginning to be discussed and explored: its effect on hearing.
The Zika virus is a mosquito-transmitted infection similar to the West Nile virus and dengue fever, but with more serious implications for infected pregnant women and their unborn babies. People of all ages can be affected, but those with weaker immune systems such as the very young and the very old are especially susceptible. For its adult victims, 4 out of 5 are asymptomatic. Those who do exhibit symptoms, which can include fever, rash, headaches, joint pain, conjunctivitis and sensitivity to light, usually recover in a week. The bad news is that 1 out of 5 adults will have lingering effects, and many babies exposed to Zika in utero will have brain damage and birth defects includingsensorineural hearing loss.
The connection to hearing loss
Research on the virus is becoming a top priority in areas where it is more commonly transmitted. A researcher in Brazil, for example, is studying a group of adult patients who have tested positive for the Zika virus and have a different set of symptoms that are causing concern: tinnitus, vertigo and hearing loss. Dr. Viviane Boaventura, an ear, nose and throat physician (ENT) at the epicenter of the outbreak in Brazil, noticed that some of her patients who had contracted Zika were still exhibiting the hearing-related symptoms several months after the other symptoms had abated. Although the majority of her patients seem to have now recovered fully, it remains to be seen what the long term effects will be. We are just starting,” Boaventura said. “And we don’t have a lot of patients to tell you if [the damage] will be reversible or it will be permanent.”
And the American Academy of Audiology is focusing on the younger Zika virus victims: newborns whose mothers exhibited symptoms of the virus early in pregnancy. Hearing loss is a risk for these youngest victims of the Zika virus, as the infants born to infected mothers have a high risk of developing microcephaly, which is characterized by an abnormally small cranium.
Smaller head size itself isn’t the issue, however; it is the reason for the small head size that is causing concern among health officials. As a fetus grows, its skull gets larger to accommodate the growth of the brain. Thus, “If the brain doesn’t grow, the skull doesn’t grow,” said Dr. Sumit Parikh, a pediatric neurologist at Cleveland Clinic Children’s Hospital.
And therein lies the problem. Though not all microcephaly is accompanied by brain damage, the Zika virus can cause a very severe form of microcephaly, in which the brain not only stops growing, it becomes smooth instead of developing the normal ridges and folds. The absence of those ridges and folds means there is less surface area for neurons. Most severe are the cases in which the mother exhibited symptoms between the second and fourth month of a pregnancy, when most neurons are formed. “This is the critical period for neurogenesis. You cannot ‘catch up’ if not enough neurons are made during this period,” said Dr. Ganeshwaran Mochida, a pediatric neurologist at Boston Children’s Hospital.
With the first cases of Zika in the Western hemisphere not being reported until 2013, research is still in the beginning stages and scientists can only theorize as to the cause of hearing loss in infants. Some evidence points to calcifications on the brains of the infants, while other evidence points to insufficient neurons or to cochlear nerve damage. No matter the reason, the American Academy of Audiology warns that infants who contract the virus through infected mothers might have “absent or poorly functioning” hearing at birth, or that hearing loss could occur later. That means that for at-risk infants, newborn hearing screenings are essential and should be followed up with preschool and elementary hearing screenings in subsequent years.