Early hearing loss and brain development.

Early hearing loss and brain development

Severe damage to an infant's or fetus'sinner ear can trigger damage to certain areas of the brain and impede brain development, according to studies with chicks and chick embryos by researchers in Seattle. Exposing adult chicks to the the same type of ear damage--roughly equivalent to that induced by extremely loud noise--results in no such brain damage, reports Edwin W. Rubel, professor of otolaryngology at the University of Washington School of Medicine.

While he says it is premature to directlyextrapolate these findings to humans, Rubel nevertheless suggests that "human fetuses and infants also may be hypersensitive to certain types of noises and that this sensitivity changes during the course of early development.' He reported his results recently in Chicago at the annual meeting of the American Association for the Advancement of Science.

In a series of experiments, Rubel andhis colleagues surgically destroyed inner-ear cells in chick embryos and in baby chicks up to 6 weeks of age. (The same type of destruction could be triggered by "high-intensity' sound, he says, equivalent to that found in some industrial settings or "on a jet runway.') Left intact were neurons that projected from the inner ear into the brain.

As little as two days later, the researchersdiscovered "dramatic cell loss' in the cochlear nucleus of the brainstem, Rubel reports. In addition, the brain cells that did remain in the affected regions had atrophied. "We found fewer and smaller neurons in areas of the brain corresponding to the areas where inner ear cells had been destroyed,' he says.

In addition, he and his colleaguesfound that the affected brain areas displayed no protein synthesis and had retarded levels of certain enzyme metabolism. These changes were not seen in chicks older than 6 weeks.

"This tells us that early in life, [braincells] not only receive information from the periphery [the ear] but are metabolically dependent on stimulation from the periphery,' Rubel said in an interview. "At some point in life--at least in the chicken--there is a metabolic uncoupling, although the information coupling remains the same.'

Rubel's latest work is based on anumber of previous studies, including his own, suggesting that the inner ear's cochlea codes for sound differently in the infant than in the adult. Those studies found that whereas in the adult, the base of the cochlea responds to high-frequency sounds and the apex of the cochlea to low-frequency sounds, the opposite is true in infants and embryos.

Whether or not these shifts in sensitivityare involved in protecting the adult from ear-damage-induced brain damage is not known.

"The question is,' Rubel says, "can wefind out how adults are protected and can we provide this protection for young children?' Although there are conflicting views regarding fetal hearing, Rubel says, "we do know that a lot of low-frequency sound gets into the uterus from the external environment and that the baby is hearing.' And though he cautions against jumping to conclusions from his chick studies, he adds, "If it were my wife, I certainly wouldn't let her use a jackhammer.'