Data on the incidence of hearing loss in preterm infants are conflicting (Table 11-4). A number of studies of children born preterm with birth weights of less than 1,000 grams reported a 9 to 14 percent incidence of hearing impairment, whereas nine other studies reported an incidence similar to that in controls who had been born full term (1 to 2 percent) (AHRQ, 2002). A Swedish study of 18- to 19-year-olds born between 1973 and 1975 with birth weights of less than 1,500 grams found impaired hearing in 7 percent (Ericson and Kallen, 1998). An Australian study of 14-year-olds who had been born with birth weights of less than 1,000 grams reported that 5 percent required hearing aides (Doyle et al., 2001). In the majority of studies of children born in the 1990s, severe hearing impairment occurred in 2 to 4 percent of children who had been born at less than 25 weeks gestation and 1.5 to 3 percent of children who had weighed less than 1,000 grams at birth (Doyle and Anderson, 2005; Hansen and Greisen, 2004; Hintz et al., 2005; Vohr et al., 2005). A British study of 6-year-old children born in 1995 at gestational ages of less than 26 weeks reported that 3 percent had profound sensorineural hearing impairment that could not be corrected with hearing aides, 3 percent had sensorineural hearing impairment that could be corrected with hearing aides, and 4 percent had mild hearing impairment (Marlow et al., 2005). Among the comparison group of children who had been born full term, 1 percent had hearing impairment that could be corrected with hearing aides and 1 percent had mild hearing impairment.
Some data suggest that preterm infants have difficulty with auditory processing and auditory discrimination, which involve multiple neural pathways to the cortex of the brain. A study of preterm infants born between 24 and 32 weeks of gestation who had normal cranial ultrasounds assessed event-related potentials, which are neurophysiological recordings that evaluate learning and memory (Therien et al., 2004). Preterm infants had patterns of speech sound discrimination significantly different from those for the full-term infants. The preterm infants were unable to discriminate between their mother and a stranger. They demonstrated deficits in discriminating simple speech sounds and auditory recognition memory. Because speech discrimination is necessary for speech recognition, these types of deficits can compromise the acquisition of language skills.
There is universal agreement regarding the importance hearing for speech and language skills, and of the need to identify hearing impairment as early as possible. For this reason, most states in the United States are implementing plans to screen the hearing of all newborns (White, 2003). Because of their risk of hearing impairment and language processing problems, the hearing of all preterm infants should be screened before they are discharged from the hospital and later at the first sign of language delay or recurrent ear infections. Many infants with hearing impairment respond favorably to hearing aides, and a number of strategies can be used to teach language even when amplification is ineffective (Gabbard and Schryer, 2003; Gravel and O'Gara, 2003; Yoshinaga-Itano, 2003). Recent evidence also suggests that cochlear implants are the most successful when they are implanted early in infancy (Niparko and Blankenhorn, 2003).
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