Danielle S. Ross, PhD, MSc
The findings and conclusions in this report are those of the author and do not necessarily represent the views of the Centers for Disease Control and Prevention.
Hearing loss is the most common congenital condition, affecting 1 to 3 per 1,000 live births (Finitzo et al., 1998; Van Naarden et al., 1999). When left undetected, hearing loss of any degree, including mild bilateral (in both ears) and unilateral (in one ear), has been shown to adversely affect speech, language, and academic and psychosocial development (Bess et al., 1998; Bess and Tharpe, 1986 and 1988; Blair et al., 1985; Bovo et al., 1988; Brookhouser, et al., 1991; Culbetson and Gilbert, 1986; Davis et al., 2001; Davis et al., 1986; Klee and Davis-Dansky, 1986; Lieu, 2004; Moeller, 2000; Oyler et al., 1987; Yoshinaga-Itano et al., 1998). The definition of mild bilateral and unilateral hearing loss varies considerably from country to country, state to state, and even from practice to practice. It varies by the audiometric frequencies that are measured, which tests are performed (e.g. air conduction, bone conduction, and auditory brainstem response), and the threshold criteria that are used to determine the presence or absence of hearing loss. Therefore, a widely agreed upon (standard) definition of mild bilateral and unilateral hearing loss has not been established. Typically, a mild bilateral hearing loss is defined as a pure tone average (PTA) of three or four frequencies from 500, 1,000, 2,000, 3,000, 4,000 hertz (Hz) of between 15 or 20 and 35 to 45 decibels (dB) hearing level (HL) in the better ear or a high frequency loss at two to three frequencies above 4,000 Hz. A unilateral hearing loss is defined as a loss in one ear of any degree (mild to profound). Permanent conductive, sensorineural, and mixed losses are included in these definitions.
Incidence estimates in newborns range from 0.36 to 1.30 (per 1,000) for mild bilateral hearing loss and 0.8 to 2.7 (per 1,000) for unilateral hearing loss (Dalzell, et al., 2000; Johnson et al., 2005; Watkin and Baldwin, 1999; White et al., 1994). Prevalence estimates in school-aged children range from 10 to 15 (per 1,000) for mild bilateral hearing loss and 30 to 56 (per 1,000) for unilateral hearing loss (Bess et al., 1998; Niskar et al., 1998). Incidence estimates in newborns vary due to several factors, including study population (e.g., age of subjects and sample size), method of sampling (e.g., population based or clinical samples, estimates based on medical records, interviews, or audiological tests), test methods (e.g., behavioral and electrophysiologic), variations in the definition of hearing loss (e.g., frequencies included and threshold criteria), and whether infants were missed by hospitals or lost to follow up in universal newborn hearing screening (UNHS) programs.
Prevalence estimates in school-aged children likely range widely due to factors similar to those in the newborn population, as well as the inclusion of children with progressive or late-onset hearing loss. Causes of progressive or late-onset hearing loss include hereditary and environmental factors, infections or illnesses (e.g., otitis media with effusion, and cytomegalovirus), trauma, and noise-induced hearing loss. Clearly, however, the prevalence rates increase from the newborn period to school age. A variety of issues, including an emphasis on lowering referral rates in UNHS programs, technological constraints, a lack of awareness among some professionals and parents, loss to follow up, and the difficulty in following up at-risk infants could contribute to the lower prevalence of hearing loss reported in infants.
Several studies have suggested that a subset of school-aged children with mild degrees of bilateral hearing loss or unilateral hearing loss have academic, social, and behavioral difficulties (Bess et al., 1998; Bess and Tharpe, 1986, 1988; Blair et al., 1985; Bovo et al., 1988; Brookhouser, et al., 1991; Culbertson and Gilbert, 1986; Davis et al., 2001; Davis et al., 1986; Klee and Davis-Dansky, 1986; Lieu, 2004; Oyler et al., 1987). At least some infants and preschoolers with these types of hearing loss already experience delays in language development (Stredler-Brown, 2005). Some evidence suggests that children with any type and degree of hearing loss are at increased risk for developmental delays, particularly when the hearing loss is identified and treated after approximately 6 months of age (Moeller, 2000; Yoshinaga-Itano et al., 1998).
To decrease the age at which children are identified with hearing loss in the United States, early hearing detection and intervention (EHDI) programs have been implemented in all states and supported by federal agencies, such as the Centers for Disease Control and Prevention (CDC) and the Health Resources and Services Administration (HRSA). It is currently estimated that 90% of infants born in the United States are being screened for hearing loss, with the majority tested before discharge from the hospital (Directors of Speech and Hearing Programs in State Health and Welfare Agencies, 2003). As a result, significantly more infants with hearing loss are now being identified much earlier than in the past. However, for the most part, only infants with a PTA of greater than 35–40 dB HL are being identified in the newborn period. That is, the majority of newborns with congenital and neonatal onset hearing loss of moderate and greater degrees are being identified in the newborn period (Norton et al., 2000), while a substantial number with lesser degrees of hearing loss are not being identified as often as would be expected based on existing prevalence estimates (Bess et al., 1998; Dalzell et al., 2000; Johnson et al., 2005; Niskar et al., 1998; Watkin and Baldwin, 1999; White et al., 1994). In addition, some school-based hearing screening efforts, which are common in most states, also fail to identify milder forms of hearing loss (Bess et al., 1998). Evidence suggests that permanent mild bilateral and unilateral hearing loss generally remain undetected unless concerted efforts at identification are undertaken in infancy and at school age (Bess et al., 1998; Johnson et al., 2005).
One of the main concerns regarding diagnosis of hearing loss in infants and young children is an apparent lack of pediatric audiologists with the appropriate knowledge, training, and skills needed for this population. This can lead to limited access of appropriate pediatric audiological services for families whose children do not pass a hearing screening (during the newborn or school-age periods). This lack of access has been documented since for the past two decades (Oyler et al., 1987; Oyler and Gross, 2000). Another issue affecting access to appropriate care can be the varying eligibility criteria for Part C and Part B intervention services of the Individuals with Disabilities Education Act (IDEA). Mild bilateral and unilateral hearing loss might or might not be included in a particular state's list of conditions that qualify a child for services (i.e., diagnosed physical or mental conditions, including threshold criteria of severity, and/or at-risk infants and toddlers with disabilities) (National Center for Hearing Assessment and Management, 2002). If states exclude milder degrees of hearing loss from their eligibility lists, it will be difficult for these children to receive early intervention services until they demonstrate a measurable developmental delay.
Most classrooms have high levels of background noise and reverberation that interfere with speech perception, especially for children with mild hearing loss (Bess, 2000; Crandell and Smaldino, 2000). Several studies have indicated frequency-modulated (FM) systems used in the classroom can be beneficial for children with mild bilateral hearing loss (Anderson and Goldstein, 2004; Flexer, 1990; Smaldino and Crandell, 2000; Tharpe et al., 2003). Even though some studies suggest FM systems can be beneficial in the classroom setting for children with a unilateral hearing loss as well (Kenworthy et al., 1990; Kopun et al., 1992; McKay, 2002; Updike, 1994), more research is needed to support a strong recommendation for the use of hearing aids and FM systems in these children.
An important consideration for practicing clinicians is that studies indicate that a subset of children with mild bilateral or unilateral hearing loss has good outcomes and a subset does not. It is not clearly understood which factors predict success or failure (Bess et al., 1998). More research is needed on the characteristics of children with such losses who are most at risk (Bess et al., 1998).
To address some of these issues, CDC EHDI has been engaged in several activities. In collaboration with the Marion Downs Hearing Center, CDC EHDI convened the National Workshop on Mild and Unilateral Hearing Loss July 26–27, 2005, in Breckenridge, Colorado. The workshop brought together more than 50 national and international experts with the goal of reviewing and discussing information to facilitate development of short-term and long-term recommendations. Participants included researchers, clinical practitioners, early interventionists, representatives from parent and national organizations, and representatives from state and federal agencies. The proceedings from this workshop can be found at the CDC Web site. Future activities and updates to this web site are planned; therefore, frequent visits to the website can provide professionals and families with ongoing information on this topic. For example, a frequently updated literature review can be found at this website. This resource is available to help professionals familiarize themselves and remain up-to-date with the large body of research on mild and unilateral hearing loss in children in an easily accessible format.
In closing, the goal for audiologists is to remain informed about mild bilateral and unilateral hearing loss and provide information to the families they serve and other professionals. Audiologists can work to provide information about the risks of mild bilateral and unilateral hearing loss; encourage and support ongoing monitoring and follow up without unnecessary stigmatization; and, finally, address the needs of each child with a mild bilateral hearing loss or a unilateral hearing loss on an individual and continuing basis.
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