In order for a child’s speech and language skills to develop properly, he must be able to hear what’s going on in the world around him. If she can’t hear her parents’ voices, she isn’t going to learn to speak.
Quite a few factors can cause a baby to be born with a hearing impairment, or even completely deaf. Hearing loss that’s present at birth is called congenital hearing loss. It may be caused by genetic factors, or something that happened during the mother’s pregnancy or during the birth process.
More than half the cases of congenital hearing loss are caused by genetic factors. There are three different kinds of genetic factors that affect hearing. Two of them are autosomal, which simply means that they’re carried on a “regular” chromosome, not the one that determines the sex of the child. The third type of genetic factor is sex-linked.
In autosomal dominant congenital hearing loss, one parent carries a dominant gene for hearing loss, and also has a hearing loss of his or her own. If only one parent carries this gene, the child has a 50 percent chance of inheriting it. However, if both parents have autosomal dominant congenital hearing loss and carry the dominant gene, there’s a higher probability that the child will inherit it. The risk goes up even more if both grandparents on one side have genetic hearing loss.
In autosomal recessive congenital hearing loss, both parents may have normal hearing and show no hearing loss of their own. In order for the child to inherit this type of congenital hearing loss, both parents must carry the recessive gene, and the child stands only a 25% chance of having genetic hearing loss. In other words, the child must receive the recessive gene from both parents in order for the hearing loss to express itself. This type of congenital hearing loss may come as a complete surprise, because other family members all may have normal hearing.
In X-linked hearing loss, the recessive gene for hearing loss is carried on the X chromosome of the mother. In this case, her daughters may inherit the gene but show no hearing loss, but her sons will have hearing loss.
Congenital hearing loss also is associated with several genetic syndromes.
Down syndrome results from either an abnormality on a gene, or an extra copy of chromosome 21. The hearing loss in these children may be the result of multiple ear infections, so frequent hearing tests are advised.
Usher syndrome affects both vision and hearing. This is an autosomal recessive syndrome; only about 4 children in 100,000 in the US are born with it.
Crouzon syndrome is autosomal dominant. In this syndrome, the bones of the skull fuse too early, which alters the shape of the head and face.
Treacher Collins syndrome also is an autosomal dominant syndrome. It’s caused by a defective protein. There’s some evidence that this particular syndrome often results from a genetic mutation, because it may pop up in families with no history of the syndrome.
Alport syndrome is X-linked, so only boys have it. It’s an inherited type of kidney inflammation, and one of its characteristics is very early hearing loss.
Not every case of congenital hearing loss is genetic; there are a number of other factors that can cause babies to be born with reduced hearing or even deaf. Some infections such as herpes simplex, German measles (rubella) and other diseases can damage the hearing of the fetus. Premature babies also may have hearing loss. Babies born to mothers with diabetes are at risk. Some antibiotics also can cause hearing loss if a mother takes them when she’s pregnant.