Exposure to nicotine before and after birth causes hearing difficulties in children

Smoking has been linked to a variety of health risks and birth complications. New research suggests that exposure to nicotine, before and after birth, can cause hearing problems for the child.

The use of tobacco and nicotine has been shown to affect the healthy development of the fetus, as well as causing premature birth or even infant death. Birth defects, sudden infant death syndrome, and low birth weight have all been associated with tobacco, electronic cigarettes (e-cigarettes), and nicotine replacement therapy.

Despite the well-known harmful effects of smoking, the Centers for Disease Control and Prevention (CDC) report that 10 percent of women still smoke during the final 3 months of pregnancy.

E-cigarettes are perceived to be safer, as they do not contain tobacco and contain fewer harmful chemicals than regular cigarettes. However, the CDC warn that e-cigarettes are also harmful for the baby, as they still contain nicotine - which can affect the development of an infant's brain and lungs.

New research - led by Ursula Koch, a professor at the Freie Universität Berlin in Germany - investigates further effects that nicotine may have on the developing fetus as well as on the newborn baby. Specifically, the research looks at the development of the auditory brainstem in mice.

The findings were published in The Journal of Physiology.

Analyzing the effect of nicotine in pregnant mice

Koch and team added nicotine to the drinking water of pregnant mice to a level equivalent to that of heavy smoking in humans. After they gave birth, their offspring continued to be exposed to nicotine through their mother's milk until they reached 3 weeks of age - roughly equivalent to the age of primary school children.

The researchers analyzed the young mice's brain activity - namely, the signaling intensity between neurons. The team also compared these results with an age-equivalent control group of mice that had not been exposed to nicotine.

Koch and colleagues found that in nicotine-exposed mice, neurons that normally receive sensory input from the cochlea (the sensory hearing organ) did not send signals to the other neurons in the auditory brainstem as accurately and effectively as neurons in the control mice.

Usually, the auditory nerve carries an electrical signal to the brain, which then encodes it into sound.

In the case of nicotine-exposed mice, however, the neural signaling was less precise, which damages the coding of sound patterns.

This is likely to cause the difficulties in auditory processing typical of children whose mothers smoked heavily during pregnancy.

The lead author of the study comments on the findings:

"We do not know how many other parts of the auditory system are affected by nicotine exposure. More research is needed about the cumulative effect of nicotine exposure and the molecular mechanisms of how nicotine influences the development of neurons in the auditory brainstem. If mothers smoke during pregnancy and their children show learning difficulties at school, they should be tested for auditory processing deficits."

Prof. Ursula Koch

To the authors' knowledge, this is the first time that a study has shown the auditory brainstem - the area responsible for analyzing sound patterns - to develop abnormally in offspring that have been exposed to nicotine perinatally.

Hearing impairment in children due to a dysfunctional auditory brainstem tends to cause language and learning problems. This can, in turn, lead to poor expressive communication skills, poor academic achievement, and social isolation.

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