In discussing the prevalence of permanent congenital hearing loss (PCHL), it is important to consider definitional issues related to severity and type of hearing loss, the age of onset of that hearing loss, and whether the hearing loss is unilateral or bilateral loss (see slide #12). Consider first the results of 11 different population-based studies designed to determine the number of children with bilateral PCHL in population-based cohorts ranging in size from 10,000 to over 4 million children. Not surprisingly, as shown in slide #13, the prevalence of bilateral PCHL is substantially higher when more lenient definitions are used. When children are required to have a 50 dB bilateral loss before they are considered to be hearing impaired, the prevalence is about one per thousand. However, when children with 30 dB bilateral losses are included, the prevalence increases to about 2.5 per thousand.
Each of these studies only included children with bilateral hearing losses. If children with unilateral losses had been included, the prevalence would increase by one-third to one-half as shown in slide #14. Based on these data, we should expect to find 3 to 4 children per thousand with PCHL from well-run, hospital-based, UNHS programs. As shown in slide #15, this is exactly what is being reported. Hospitals with UNHS programs in Rhode Island, Colorado, New York, Utah, Hawaii, and New Jersey, using a variety of different screening techniques and protocols, are reporting between 1.65 and 4.15 infants per thousand identified with PCHL.
It is important to note that the numbers reported from the retrospective studies shown in slide #13 could have included children with late onset or acquired hearing losses. Many people assume that there are about as many young children with acquired losses as with congenital losses. However, states with long-established UNHS programs, where tens of thousands of infants have now been screened, are finding very few children with acquired losses. For example, in Rhode Island, Hawaii, Colorado, and Utah, hundreds of children have now been identified with PCHL. Many of those programs have been operating long enough that there are thousands of 7, 8, and 9 year olds who were screened as infants. If acquired hearing loss were as frequent as is often assumed, we should now be discovering dozens of children with late onset losses as they enter school. In fact, there are only a couple of such children who have been identified. The emerging hypothesis (see slide #16) is that many of the children, who were previously thought to have late onset losses, were really children with congenital mild or moderate progressive PCHL. As infants, they had enough hearing that they began to babble, acquire some speech, startle, or turn to loud noises. By the time they were 3, 4, or 5 years old, however, it was clear that their speech was substantially delayed, and as a result, hearing loss was diagnosed. Because they were babbling and turning to noises as an infant, it was assumed that it was an acquired loss, when in fact many of these could have been mild or moderate progressive congenital losses.
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