What Causes Hearing Loss? [STUDY]
Epidemiology of Permanent Hearing Loss in Children
Michel Picard, Ph.D.
This article reviews current epidemiological studies estimating the prevalence and incidence of permanent hearing loss in children. In particular, it addresses trends in ages of onset of hearing loss and causes. Studies estimating the number of children with hearing loss and additional special needs in the various countries are presented, as well as some of the well-known causes of hearing loss which have co-occurring associated disabilities. The case of linguistically diverse children is presented as a new group of children with hearing loss who may function with multiple disabilities.
The global prevalence of permanent (or sensorineural) hearing loss in the newborn population is currently estimated by Mencher (2000) at 1.368 in every 1,000 live births in the world. This is comparable to the 1 to 2 incidents of hearing loss per every 1,000 births as reported by Downs and Sterritt in 1964. In fact, there is a wide variety in current estimates of hearing losses worldwide, with ratios ranging between the low of 0.43 per 1,000 (Germany) and the high of four (Sierra Leone and Thailand). The current estimate for the United States has declined to 0.71 (Mencher, 2000), which is also the rate for Belgium. Data from the 1980s places Canada at the lower end with ratio of 1.046. This is close to the prevalence of 1.06 for hearing loss ≥ 40 dB in the United Kingdom (Fortnum, Summerfield, Marshall, Davis, & Bamford, 2001). On average, European countries are showing lower prevalence estimates: France at 0.54 / 1,000 live births (Parving, 1999; Baille et al., 1996); Italy at 0.9 in the early 1980s (Mencher, 2000) rising to 2.3 with the inclusion of older children (Parving, 1999); and, Germany with a reported ratio of 0.43 (Streppell, Richling, Roth, Walger, von Wedel, & Eckel, 1998; Streppell et al. 2000). Even a small country like Cyprus is exemplary in this particular respect, with reported estimates of 0.4 to 0.6 / 1,000 in the mid-1990s (Hadjikadou & Bamford, 2000). One notable exception to this trend is the former Soviet Republic, Estonia, which declared a ratio of 1.52 between 1985 and 1990 (Uus & Davis, 2000).
In comparison, there are consistent indications of the high prevalence of newborn hearing loss in Asian children (Fortnum & Davis, 1997; Naeem & Newton, 1996), and in Pakistani groups (Morton, Sharma, Nicholson, Broderick, & Poyser, 2002) as derived from cohorts living in the United Kingdom. The prevalence of hearing loss in Asian children may be two to three times that of the non-Asian population. The same would be true of Hispanic groups living in the United States (Lee, Gomez-Marin, & Lee, 1996). Sociodemographic and educational variables (poverty and lower educational attainment by parents) may be contributing factors (Lee, Gomez-Marin, & Lee 1997). Similarly, limited access to quality medical assistance during pregnancy and the early post-natal years may be an additional factor.
While there is a reported decline in hearing loss in newborns and young children in more developed countries, there appears to be an almost twofold increase in the numbers of children with acquired hearing loss between the ages of 3 and 9 years. In a population-based study using the case-ascertainment method, Fortnum et al. (2001) derived a ratio of 1.06 / 1,000 children ages birth to 3 years with an average permanent bilateral hearing loss of ≥ 40 dB (after adjustment for under-ascertainment). Between the ages of 3 and 9, estimates linearly increased to 2.05. Three main explanations were offered for the overall twofold increase: lack of early identification; progressive and acquired hearing loss; and, increased survival rates of “at risk” infants.
1) Lack of Early Identification: The lack of early identification was believed to be primarily responsible for the increase of reported cases of hearing loss, given the limitations of current audiometric methods. Some of the additional cases were comprised of children with congenital disabilities who either failed neonatal screening or passed it despite having a hearing loss, resulting in delayed confirmation. Losses of a milder degree would be critical in this particular respect. Reporting on the German Registry for hearing loss in children for the period 1996 to 2000 Finckh-Kramer, Spormann-Lagodzinski, and Gross (2000) determined that hearing loss was diagnosed on average at 6.2 years for mild hearing loss, 4.4 years for moderate hearing loss, 2.5 years for severe hearing loss, and 1.9 years for profound hearing loss. However, with greater accessibility to newborn screening and enhanced test performance, late identification should dramatically decline. This would result in the age of diagnosis and hearing aid fitting at less than 6 months of age (Vohr, 2003a) and preferably before 2 months of age (Vohr, 2003b).
2) Progressive and Acquired Hearing Loss: Another reason for increase in numbers with age may be due to delayed identification. Some children with genetic hearing loss present with late-onset or progressive involvement. Children with acquired hearing loss as a result of childhood illness, such as meningitis, also increase these numbers.
3) Increased Survival Rates of “at risk” Infants: Improved perinatal medical care has resulted in more surviving infants, but with complications that may include hearing loss (Streppel et al., 1998). In particular, reduced mortality rates for premature babies with very low birth weight and/or gestational age is responsible for more infants with hearing loss.
Based on the way prevalence ratios of permanent hearing loss are reported worldwide (especially in children under the age of 10), these findings suggest a differentiation between countries, based on the level of industrial development. More developed countries show lower ratios. There is a small cluster of countries with exceptionally low prevalence ratios (that is, less than 1 incident of hearing loss per every 1,000 live births). These are mostly European countries, which offer the additional benefit of socialized medicine.
The general overview of more than 40 countries and regions worldwide suggests a high degree of consistency of data in spite of large differences in methods. These differences include, but are not limited to the age of the child at the time of investigation, the inclusion criteria, and the nature and degree of hearing loss. Consequently, Mencher’s estimate (2000) of 1.368 /1,000 would represent the single most credible indication of prevalence of hearing loss in the newborn population currently available. Furthermore, contemporary findings by Fortnum et al. (2001; Fortnum & Davis, 1997) indicate that in more developed countries like the United Kingdom, national estimates as reported by Mencher (2000) would generalize to bilateral sensorineural hearing loss in excess of 40 dB averaged over .5, 1, 2, and 4 kHz in the ‘better’ ear - that is, the one with the most residual hearing. Given that such a liberal definition of permanent hearing loss in the newborn population may produce as low a yield as 0.5-0.7 / 1,000 live births in more developed countries, prevalence values reported by Fortnum et al. (2001) as well as by Streppell et al. (1998; 2000) and by Hadjikadou and Bamford (2000) are clear indications of significant progress in the prevention of childhood deafness over the last 10 to 15 years. By contrast, however, there is an increased prevalence of hearing loss between ages 3 and 9 years.
Changes in the profile of children who are deaf or hard of hearing due to different etiologies and age of manifestation also bring consequences in terms of the severity of hearing losses. The general tendency seems to be a change towards lesser degrees of hearing loss (Nelson, 2001). GRI surveys have been tracking this particular shift for several years as seen in Table I (Nelson, 2001; GRI 2001, 2002, 2003a, b). It reveals a clear reversal in trends from 1986 to 2003, with significant reductions of severe-to-profound degrees of hearing loss over the study period and an increased number of students with near-normal hearing. More specifically, numbers of children with severe and profound hearing losses decreased from 63% of respondents in 1986 to 46.6 % in 2003. Numbers of children with a near-normal range of hearing (<27 dB loss) increased from 7% of respondents in 1986 to 16.3% in 2003. In terms of educational management, this finding may be of paramount importance because new cohorts of children may have significantly more residual hearing.
Causes of Hearing Loss
The main reason for the current decline in permanent hearing loss in newborns and infants in more developed countries is attributed primarily to more aggressive medical treatment of pregnant women, babies, and young children (Northern & Downs, 2002). In the United States in particular, widespread vaccine programs have done much to decrease common childhood infectious diseases such as mumps, measles, and meningitis, all conditions that are well known for their deleterious effects on children’s hearing. Similarly, vaccination programs have practically eradicated maternal rubella infection. This particular interpretation of current epidemiological data is supported by Morzaria, Westerberg, & Kozak (2002) in their review of contemporary causes of hearing loss. Data by Streppel et al. (1998) are also indicative of a similar decline in maternal rubella infection in Germany, as are the data reported by Fortnum and Davis (1997) in the United Kingdom.
This favorable outcome of contemporary medical practice can be traced to the mid-1980s using the GRI annual data of children and youth who are deaf or hard of hearing (Holt, Hotto, & Colt, 1994). Table II, adapted from this study, incorporates data from the 1999-2000, 2000-01, 2001-02, and 2002-03 GRI annual surveys to analyze more closely the changing causes of hearing loss in the last three decades.
As expected, examination of Table II shows a marked reduction in number of cases attributed to maternal rubella infection. Decreased instances of hearing loss due to mumps, measles, and Rh incompatibility is particularly evident between 1982 and 1987. Indications of a reduction in adverse effects of meningitis on hearing remain uncertain and, at best, may have become noticeable at the turn of the millennium. These figures on causes of permanent hearing loss in children are quite similar to data reported in Europe by Streppel et al. (1998) and by Fortnum and Davis (1997).
Other salient findings of GRI analyses are noteworthy. The large number of unknown causes of deafness remains a puzzle and it is compelling evidence of our limited understanding of the reasons for permanent hearing loss in children. One reasonable, although speculative, explanation is that a large number (at least half of all cases of permanent hearing loss in children according to Parker, Fortnum, Young, Davis & Mueller, 2000) may have a genetic cause. The growing importance of genetic, hereditary, and/or familial origins shown in recent years by GRI analyses can also find an explanation in progress in genetic medicine specifying genes of deafness (Bitner-Glindzicz, 2002) or reduction in reluctance of families to acknowledge the presence of a hearing loss (less of a stigma factor). The apparent trend for more hearing losses attributable to cytomegalovirus is also noteworthy. This could be due to more accurate reporting in recent years or to a genuine increase in this infection in pregnant women. The possibility for children with this particular etiology to have a progressive hearing loss and motor or cognitive deficits makes determination of this cause of deafness important to parents and educators (Northern & Downs, 2002). Hearing loss due to complications of pregnancy may have momentarily dipped in the early 1990s, but currently show data comparable to what they were in the early 1980s. In comparison, trauma at birth has been declining since the mid-1990s. By contrast, the turn of the century saw a marked increase in the number of reported cases due to chronic otitis media – a medically preventable situation.
Finally, the case of premature infants of very low birth weight/gestational age (<1500g/ <25 weeks) is a growing concern. The risk factor for hearing loss associated with babies of less than 25 weeks gestation is currently estimated at 2.1 times that of less premature babies (data adapted from Cone-Wesson, Vohr , Sininger, Widen, Folsom, Gorga, & Norton, 20002). This is probably the reason why Cone-Wesson et al. (2000) have ranked extreme prematurity as the fifth most prevalent cause of hearing loss in infants.
Estimates of Children with Hearing Loss and Multiple Disabilities
Current understanding of epidemiology of hearing loss in children with associated developmental problems or disabilities is far from being as comprehensive as estimates of children with hearing loss in general. For instance, the Center for Disease Control and Prevention (CDCP) in Atlanta reported that 30% of newborns who are deaf or hard of hearing had another neurodevelopmental condition, most frequently mental retardation (Van Naarden, Decoufle, & Caldwell, 1999). This is consistent with reports by Fortnum and Davis (1997), who found 38.7% of children who were deaf or hard of hearing with additional conditions, some of whom were children with a Newborn Intensive Care Unit (NICU) history. About one third of the 38.7% (13.6 %) would be attributable to syndromic causes of hereditary deafness, such as Down syndrome (trisomy 21), followed by Usher, Pierre Robin, Treacher Collins, and C.H.A.R.G.E.3 syndromes, to name only the most frequent conditions. Table III summarizes the prevalence of these syndromes and some of the associated conditions which may affect learning. Interestingly, Usher syndrome is the second most prevailing condition, when permanent hearing loss is part of the syndrome. Therefore, parents and professionals must be alert to any early signs of vision loss (acuity or visual field).
An estimate of 38.7% is also consistent with GRI data. The 1999-2000, 2000-01, 2001-02, and 2002-03 annual surveys indicate that 40% to 42% of the respondents had some associated disability. The more frequent additional conditions were learning disability and mental retardation, each declared by 9% to 10% of respondents. Attention deficit disorders follow next with 6.6% of declarations. Serious visual problems, such as Usher syndrome (the most prevalent cause of deaf-blindness according to Northern & Downs, 2002), were reported less frequently, being acknowledged by 3% to 3.5% of respondents. According to Cone-Wesson et al. (2000) the most prominent causes of visual problems in addition to hearing loss within first years of life are: 1) the triad of very low birth weight, very low gestational age and assisted ventilation for 10 days or more; 2) confirmed sepsis; 3) a history of retinopathy; and, 4) seizures with intra-ventricular haemorrhage.
The extraordinary and successful efforts undertaken to reduce mortality in premature babies (Streppel et al., 1998) is producing more vulnerable children, the number of which is on the rise in developed countries for a variety of reasons (Cano, Fons, & Brines, 2001). Predictably, these children will also be at much greater risk for complex developmental problems or disabilities, due to the variety of neuromotor, sensory, and cognitive sequelae, including hearing loss and learning difficulties. As a result, multiple and more severe functional limitations are prevailing in this group (Saigal, Rosenbaum, Stoskopf, Hoult, Furlong, & Feeny, 1994; Saigal, Feeny, Rosenbaum, Furlong, Burrows, & Stoskopf, 1996), although outcomes on quality-of-life remain unclear at this time (Saigal et al., 1996).
A review of the last four years of recent GRI data does not show any particular trends as to the changing course in prevalence of hearing loss that also have some associated condition. This may have been too short an observation period for changes to be noted. One likely exception in this regard, however, may be syndromic conditions that can be identified early during pregnancy (by amniocentesis, in particular). A typical example in highly industrialized countries that facilitate motherhood at an increasingly advanced age is trisomy 21 (Down syndrome). Adler and Kushnic (1982) examined 15 mothers between the ages of 33 and 44 years carrying trisomy 18 or 21 foetuses. Fourteen of them elected to terminate the pregnancy in a joint decision with fathers. If these findings are representative, one can reasonably assume that interruption of pregnancy in these circumstances will likely act to reduce the number of children with these types of etiologies.
Finally, the particularly high number of children with hearing loss of unknown origin (over 50% since 1992; see Table II) makes identification and diagnosis of any additional disability more challenging.
A New Group of Children with Hearing Loss Functioning as if Multiply Disabled
Migration of large groups of individuals from developing countries, with higher rates of childhood hearing loss, to more developed countries has created new challenges in service delivery – both medical and educational. In many cases, parents may be addressing their children in a language different from the one used for instruction (leading to the particular reality of developing language in a bi-lingual or a multi-lingual environment). Non-native speakers of English currently represent a large and growing proportion of children with hearing loss in more developed countries. These children require special assistance to keep up with instruction and training (Walker-Vann, et al., 1998; Rhoades, Price & Perigoe, 2004).
Immigration to more developed countries is a relatively recent demographic trend linked to the phenomenon of shrinking birth rates. In response to unprecedented low birth rates, more developed countries are increasing immigration as a means to avoid decline of their population. This is coming as a new statistical reality in Canada (Statistics Canada, 2003), and the United States may face a similar situation in the not too distant future with newcomers of Hispanic origin. In March 2002, the Hispanic population in the United States was the largest, fastest growing minority group, representing 13.27% of the total population (Ramirez & de la Cruz, 2003). This is already having an impact on the delivery of services to students who are deaf or hard of hearing as indicated by GRI analyses. Between 1977 and 1997, the proportion of students with hearing loss of Hispanic origin increased from 9% to 18% (Holden-Pitt & Diaz, 1998) and data for years 1999 through 2003 show no reversal in trend. On the contrary, the proportions of Hispanic students who are deaf or hard of hearing has been consistently increasing, rising to 20.8% in 1999-2000, to 21.8% in 2000-01, to 22.8% in 2001-02, and to 24.5% in 2002-03 (GRI, 2001, 2002, 2003a, b).
Within this context, it is reasonable to assume that Spanish is the prevailing language used by these parents at home. In fact, this particular description fits responses of 45%-46% of students of Hispanic origin to the recent GRI Annual Surveys (1999-2000, 2000-01, 2001-02, and 2002-03). Given that the language of instruction is English, a sizable proportion of students with hearing loss living in the United States are actually learning English as a second (or possibly third) language, after Spanish and/or American Sign Language. Considering the complexities of bilingual language acquisition (Gordon, 2000), it is clear that these candidates are at a greater risk of experiencing major listening difficulties understanding speech in noise in the second language (Mayo, Florentine, & Buss, 1997; Van Wijngaarden, Steeneken, & Houtgast, 2002). Inevitably, increased effort will be required to cope with conditions of low speech predictability due to noise interference and to a lack of familiarity with the language of instruction. Thus, greater cognitive effort is needed to achieve the same level of performance in processing the second language as would be expected in the maternal tongue (Hasegawa, Carpenter, & Just, 2002). The problem is all too well documented to be ignored and is putting at risk the course of language acquisition by young children with hearing loss (Picard & Bradley, 2001).
This trend gives cause for concern within current educational systems (Walker-Vann, 1998). Research suggests that students who are deaf or hard of hearing of Hispanic origin demonstrate a lower rate of scholastic success than peers who are either hearing or deaf (Bennett, 1988). Consequently, as a group, they behave as if they are slow learners. Thus, the very nature of their linguistic difference may cause them to function in some respects like students with hearing loss and additional learning challenges (Rhoades, Price, & Perigoe, 2004).
The main conclusions of this review are as follows:
- Sensorineural hearing loss in the newborn population remains high worldwide, requiring improvement in diagnosis and treatment.
- Level of development of a country is related to prevalence of sensorineural hearing loss in the newborn population, with higher rates in less developed countries and in some communities that have migrated to highly industrialized countries.
- In more developed countries, there is a decline in the prevalence of permanent hearing loss in the newborn population. However, this benefit is negated by increases of hearing loss in the 3 to 9 year age group.
- Numbers of children with causes of deafness which may also be associated with additional special needs seem fairly stable for some etiologies (e.g. prematurity, trauma after birth), on the decline for some (e.g. mumps, measles, Rh incompatibility, and possibly meningitis) and on the increase for others (e.g. cytomegalovirus and to a lesser extent, maternal drug use, and complications of pregnancy).
- Very low birth weight as a result of less than 25 weeks gestation may currently rank as the fifth or sixth most common identifiable cause of permanent hearing loss, comparable to cytomegalovirus.
- There remains a high proportion (approximately 40%) of children with permanent hearing loss and multiple disabilities, among which a sizable proportion have NICU history. Within this particular group, those with Down syndrome would outweigh all other origins combined.
- There is currently no evidence substantiating changing prevalence of permanent hearing loss in children with associated disabilities.
- Migration to highly industrialized countries brings a new group of children with hearing loss, functioning as if learning disabled, because of the particular difficulty of processing speech in noise in the non-native language of instruction.
Continued study of hearing loss worldwide is an important aspect of determining trends and migration in this population. Better understanding of these trends can be helpful in developing medical and educational service delivery models that are sustainable and can adapt to the changing demand.
Source: The Volta Review, 2004