PEDIATRICS Vol. 104 No. 4 October 1999, pp. 905-910

Relative and Attributable Risks for Moderate to Profound Bilateral Sensorineural Hearing Impairment Associated With Lower Birth Weight in Children 3 to 10 Years Old

Kim Van Naarden, MPH and Pierre Decouflé, ScD

From the Developmental Disabilities Branch, Division of Birth Defects and Developmental Disabilities, National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, Georgia.

	ABSTRACT

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Objective.  To determine the prevalence, relative risks, and attributable fractions for congenital bilateral sensorineural hearing impairment in relation to lower birth weight among children born in the 1980s and living in the metropolitan Atlanta area from 1991 through 1993.

Methods.  We used the population-based, active case ascertainment Metropolitan Atlanta Developmental Disabilities Surveillance Program that conducts surveillance in the five-county metropolitan Atlanta area. Hearing impairment was defined as a bilateral, pure-tone hearing loss at frequencies of 500, 1000, and 2000 Hz averaging 40 dBs or more, unaided, in the better ear. Case children, 3 to 10 years of age, with sensorineural loss of presumed congenital origin were included in these analyses (n = 172). Prevalence rates and relative risks were computed for various birth weight categories by hearing level, sex, race, the presence or absence of coexisting developmental disabilities, and gestational age. Attributable fractions were calculated for low birth weight and very low birth weight children by race.

Results.  The overall prevalence rate of presumed congenital bilateral sensorineural hearing impairment was 5.3 cases per 10 000 3-year survivors. The prevalence was 4.1 per 10 000 among children weighing 4000 g, 3.7 per 10 000 among those weighing 3000 to 3999 g, 6.6 per 10 000 among those 2500 to 2999 g, 12.7 per 10 000 among those 1500 to 2499 g, and 51.0 per 10 000 among those <1500 g. There was virtually no difference in birth weight-specific rates of hearing impairment across three hearing levels. The presence of coexisting developmental disabilities was associated with a much stronger inverse trend with birth weight. Black children weighing <2500 g had much higher rates of hearing impairment than comparable white children. The overall percentage of moderate to profound congenital bilateral sensorineural hearing loss in the entire study population that was attributable to children weighing <2500 g or <1500 g was estimated to be 18.9% and 9.4%, respectively. Prematurity did not alter the magnitude of risk among children weighing <2500 g.

Conclusions.  The results presented here provide recent estimates of the rates, relative risks, and attributable fractions for congenital bilateral sensorineural hearing impairment by birth weight among children in the United States. The elevated relative risks among children weighing 2500 to 2999 g and 1500 to 2499 g may have implications for future newborn hearing screening criteria.  Key words:  birth weight, congenital, sensorineural, hearing impairment, deafness, prevalence, relative risk, attributable risk.

Congenital bilateral sensorineural hearing loss is a serious and common problem affecting 5.0 to 12.0 children per 10 000 live-births annually^1-4 and its prevalence has been found to be unusually high among low birth weight (LBW) (<2500 g) children, particularly very low birth weight (VLBW) (<1500 g) children.^5,6 The high rate of hearing loss among LBW children has been linked to several factors that may result in brain damage and hearing loss: short gestational age, administration of ototoxic drugs, ambient incubator noise, and perinatal complications (eg, hypoxia, acidosis, and so forth). More recent literature focuses primarily on neonatal and postneonatal complications associated with lower birth weight that may put an infant at high risk for hearing loss.^7-9

Recent improvements in obstetrical and neonatal care have led to decreased mortality rates among LBW infants in developed countries.¹⁰ The increased probability of LBW and VLBW infants' survival poses a risk of increased long-term morbidity because of developmental disabilities, including hearing impairment.¹¹

Throughout the past 40 years, there have been relatively few population-based studies examining the relationship between LBW and hearing impairment in the United States. Studies conducted in the 1960s and 1970s found conflicting results on the association between LBW and hearing impairment.¹² These inconsistent findings may have been attributable to variable definitions of birth weight and hearing loss categories, small sample sizes, and synonymous definition of prematurity and LBW. Although results from previous studies have varied, the weight of evidence across the various types, levels, and categories of hearing impairment indicates that LBW infants exhibit a greater risk of hearing loss than normal birth weight (NBW) infants.^6,7,12,13 To our knowledge there are no recent reports which investigate racial comparisons, coexisting developmental disabilities, or a wide spectrum of birth weight in the epidemiology of congenital bilateral sensorineural hearing impairment.

The present analyses used data from the population-based Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP) to: 1) investigate the magnitude of association among unconventional birth weight categories; 2) provide comparisons by race; 3) assess the influence of coexisting developmental disabilities; and 4) estimate the relative and attributable risks for congenital bilateral sensorineural hearing impairment among children 3 to 10 years old in a recent birth cohort.

	METHODS

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Established in 1991, the MADDSP is an ongoing system for monitoring the prevalence of selected developmental disabilities: mental retardation, cerebral palsy, hearing impairment, and vision impairment.¹⁴ The MADDSP seeks to ascertain all affected children aged 3 to 10 years whose parent(s) or legal guardian(s) are residents of one of five Georgia counties (Clayton, Cobb, DeKalb, Fulton, and Gwinnett) during the study year of interest. Existing records from multiple sources are reviewed to identify children who meet the surveillance case definition for one or more of the conditions of interest. The surveillance case definition for hearing impairment is a bilateral, pure-tone hearing loss at frequencies of 500, 1000, and 2000 Hz averaging 40 dBs or more, unaided, in the better ear. We chose 40 dBs as the threshold value for MADDSP because we were reasonably certain that the vast majority of such children would be receiving services in the public school systems and thus be easily identifiable. The most recent audiologic test in the child's record was used to determine case status. Case definitions for the other three conditions have been published elsewhere.¹⁴

Children with hearing impairment were ascertained through programs providing services for children with special needs, including special education programs at the nine public school systems serving the study area and other programs sponsored by the Georgia Department of Education, specifically state schools for children with hearing impairment. Additional sources for identifying eligible children included Georgia Department of Human Resources facilities providing services to children with developmental disabilities, as well as one public and two private pediatric hospitals and their associated clinics in the metropolitan Atlanta area. Most children with hearing impairment (95%) were identified through public and state schools, 3% from public and private hospitals and their associated clinics, and 2% from Georgia Department of Human Resources programs.

Hearing Impairment Data

Data collected on all hearing-impaired children included the type and level of hearing loss, presence of the other three developmental disabilities included in the MADDSP, as well as any mention of other medical conditions (eg, seizure disorder). The type of hearing loss was categorized as sensorineural, conductive, mixed, or unknown. Sensorineural hearing loss was defined as a condition of the inner ear or auditory portion of the central nervous system that may be resolved by medical intervention. Conductive hearing loss was defined as the result of reduced conduction of sound through the outer or middle ear to the inner ear. The majority of conductive hearing losses can be treated by medical or surgical intervention. Mixed hearing loss incorporates both conductive and sensorineural components, which may be treated medically or with amplification and educational intervention. Our three categories of types of hearing loss are mutually exclusive. The levels of hearing impairment defined in the MADDSP are moderate (40-64 dBs), severe (65-84 dBs), and profound (85 dBs) in the better ear. Children with hearing impairment who were not known to have other developmental disabilities or a seizure disorder were categorized as having isolated hearing impairment.

The case population for this report was a subset of all children meeting the MADDSP case definition for hearing impairment in 1991, 1992, or 1993 (ie, those who were born between 1981 and 1990 to a resident within the metropolitan Atlanta ascertainment area [n = 249]). Cases most likely to be of prenatal origin were identified. A panel of three physicians independently reviewed available medical data and by consensus identified a subset of case children (n = 226) whose hearing impairment was not caused by a postneonatal event. The review included data from the Metropolitan Atlanta Congenital Defects Program, an active birth defects surveillance system covering the same metropolitan Atlanta ascertainment area as the MADDSP, and provided etiologic information about children in the MADDSP.¹⁵ Cases of hearing loss of a sensorineural type (n = 172) were considered most likely to be of congenital origin. These cases will be described as children with presumed congenital hearing impairment.

Independent Variables

All the independent variables (birth weight, race, sex, and gestational age) we used in the analyses were taken from Georgia birth certificate data. For initial analysis of birth weight-associated risks, we chose to define the following five birth weight categories: large birth weight (LGBW), 4000 g; NBW, 3000 to 3999 g; borderline normal birth weight (BNBW), 2500 to 2999 g; moderately low birth weight (MLBW), 1500 to 2499 g; and VLBW, <1500 g. For computing relative risks (RRs), we used the 3000 to 3999 g (NBW) category initially and later the 3000 g category (NBW + LGBW). We chose this set of birth weight categories to assess risks more fully along the entire birth weight spectrum, because children in the LGBW and BNBW subgroups may have different risk factors for hearing loss, and, if combined with the usual NBW category, might have been missed. Birth weight data were not available for 3 case children. Of the 169 congenitally hearing-impaired cases with birth weight data, 167 were either black or white and 2 were Asian. All 169 case children with birth weight data were included in all analyses except those dealing with race-specific results, which included only the black and white cases.

To determine the influence of gestational age on the risk of hearing loss, we dichotomized gestational age into two categories (preterm [20-36 weeks] and full term [37-44 weeks]) and analyzed four new subgroups (term NBW, preterm NBW, preterm LBW, and term LBW). Each of the last three subgroups was compared with the term NBW subgroup to assess the independent and combined effects of gestational age and birth weight on hearing loss. Gestational age was estimated by using the date of the mother's last menstruation provided on the birth certificate and the child's date of birth. Because gestational age was not available for 15 of the 169 cases, the remaining 154 cases were used in these analyses.

Analyses

To derive the denominator for computing rates, we applied a two-step procedure. First, we obtained a copy of linked birth-infant death certificates from the Georgia Vital Statistics Office. Second, we matched the certificates to state death files to identify children who died between the ages of 1 and 3 in Georgia. This procedure created the denominator, which represents the estimated number of children born to study area residents between 1981 and 1990 and who survived until 3 years of age (the age to which a child must have survived to be identified as a case). The 3-year survivor denominator data were stratified by maternal race and child's sex and gestational age within each birth weight category, depending on the particular analysis. We calculated 95% confidence intervals (CIs) for the prevalence rates and RRs by using the exact binomial method.¹⁶

We estimated the proportion of children with presumed congenital hearing impairment attributable to LBW or VLBW.^17,18 For LBW, the attributable fraction (AF) was calculated by subtracting the prevalence of hearing impairment among children 2500 g from the overall birth prevalence (ie, in all birth weight categories) and dividing that number by the overall prevalence. For VLBW, the AF was calculated by subtracting the prevalence of hearing impairment among children 1500 g from the overall birth prevalence and dividing that number by the overall prevalence. Ninety-five percent confidence intervals for AFs were computed using the method of Walter.¹⁷

	RESULTS

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The overall prevalence rate of presumed congenital hearing impairment was 5.3 cases per 10 000 3-year survivors. The birth weight range for the 169 case children was 840 to 4625 g. Of these children, 7.7% were LGBW (mean, 4220 g); 45.0% were NBW (mean, 3465 g); 21.9% were BNBW (mean, 2787 g); 15.4% were MLBW (mean, 2069 g); and 10.1% were VLBW (mean, 1079 g). The distribution of birth weight categories did not vary appreciably among children with moderate, severe, or profound hearing impairment. Approximately 70% of case children had isolated hearing impairment and 30% had one or more coexisting developmental disabilities. Mental retardation was present in 86% of the case children with coexisting developmental disabilities. Of the 169 case children, 59% were male and 53% were black. The mean birth weight of 2729 g (range, 840-4167 g) of black case children was significantly lower than the mean birth weight of 3127 g (range, 907-4625 g) for white children (P < .004).

Birth Weight and Risk for Hearing Impairment

The rate of hearing impairment among LGBW case children (4.1 per 10 000) was approximately the same as for those children weighing 3000 to 3999 g (3.7 per 10 000) (Table 1). The prevalence rates and RRs for children with presumed congenital hearing impairment were inversely proportional to the birth weight categories. The highest of these estimates was a prevalence rate of 51.0 per 10 000 and a RR of 13.9 among VLBW infants. Across the three levels of hearing loss there was virtually no difference in risk for hearing impairment within birth weight categories (Table 2). In each birth weight category, children with hearing impairment and coexisting developmental disabilities had a higher RR than children with isolated hearing impairment. This difference was especially marked in the VLBW group (Table 3).

When we subdivided the study group by race and gender, we combined the MLBW and VLBW categories into one LBW category (<2500 g) to avoid small numbers of cases in several strata. The prevalence rates of hearing impairment for NBW and BNBW black male children were consistently higher than those of the other three race-sex subgroups (Table 4). Furthermore, hearing impairment rates among LBW black children were higher than those among LBW white children. Finally, the RR for hearing impairment for LBW black females was greater than that for LBW black males. Although the prevalence rates in the LBW black male and female subgroups were the same, the higher prevalence rate in NBW black males than in NBW black females decreased the RR for black males weighing <2500 g. Etiologic data were limited, however, of the 12 NBW black males with a known etiology, 7 were a result of meningitis.

The overall percentage of moderate to profound bilateral sensorineural congenital hearing loss in the population of children ages 3 to 10 years attributable to LBW or VLBW was estimated to be 18.9% and 9.4%, respectively. LBW and VLBW accounted for a higher proportion of hearing impairment among black children than white children (Table 5).

When gestational age was incorporated into the analyses, two findings of note emerged. First, the high risk of hearing impairment among LBW children was not significantly affected by whether they were preterm or not (preterm: RR = 5.9, 95% CI = 3.8-9.2; term: RR = 4.3, 95% CI = 2.4-7.8). Second, the modest increase seen among 2500 to 2999 g children seems to be confined to term infants (RR, 2.2).

	DISCUSSION

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This investigation provides the first population-based data on the association between presumed congenital hearing impairment and birth weight in the United States. The results support previous studies from other countries showing that risk for sensorineural presumed congenital hearing impairment increases as birth weight decreases. Quantitative estimates of the magnitude of increased risk for presumed congenital hearing loss show three important results beyond the central finding of the dramatic increase in risk among infants born weighing <1500 g. First, risk for hearing loss is elevated among children born weighing 2500 to 2999 g compared with those weighing 3000 g. Second, our data show that the presence or absence of coexisting developmental disabilities greatly affects the nature of the inverse relationship with birth weight. Third, we found an increased prevalence of hearing loss among NBW black males. We estimate that ~19% of all childhood cases of presumed congenital hearing impairment is associated with LBW.

A Swedish study conducted on a cohort of children born in the 1970s applied a methodology similar to that of MADDSP (bilateral sensorineural hearing loss, record review case ascertainment, hearing level, and LBW and VLBW category definitions) and found an inverse association between birth weight and sensorineural hearing impairment (LBW: RR = 2.1, 95% CI = 1.1-3.9; VLBW: RR = 6.4, 95% CI = 0.9-45.0), which was similar to our results in white children.⁵

More recent literature focuses on neonatal complications that identify children who might be potential hearing loss cases.^7-9 These high-risk markers indicate that neonates need to be carefully monitored and tested for hearing loss after birth. The current high-risk criteria in the United States for newborn hearing screening limits the recommendation to VLBW infants.¹⁹ Our data support this birth weight criterion, but also show a fourfold increased risk for MLBW children and a twofold increased risk for BNBW children. According to the Monthly Vital Statistics Report, 1.3% of children in the United States in 1995 were VLBW and 6.0% were MLBW.²⁰ Although, MLBW accounts for about the same burden of hearing impairment as VLBW in our study (ie, difference in the AFs for all LBW and VLBW), there are many more MLBW children than VLBW children in the birth population who would have to be screened for hearing impairment.

Neither the prevalence rates nor the RRs differed across severity levels of hearing loss suggesting that birth weight does not differentially affect the severity of hearing loss. Although BNBW and MLBW children had a significantly higher risk for hearing impairment with coexisting developmental disabilities, VLBW children had exceedingly higher risks for both isolated hearing impairment as well as hearing impairment with coexisting developmental disabilities. These findings suggest that the increased risk for hearing impairment accompanied by other developmental disabilities in MLBW children may be attributable to more widespread neurologic lesions associated with the underlying neuropathology.

Two previous reports of the prevalence of hearing impairment in Atlanta children found that black males had a significantly higher overall rate of moderate to profound hearing impairment than other sex/race subgroups.^14,21 Our results show that among black males the increases in prevalence are found in the NBW and BNBW birth weight categories. These results indicate that research into presence or absence of coexisting developmental disabilities and etiologies of congenital hearing impairment by race-sex subgroups might provide insight into the differences in rates among these subgroups. The possible association between meningitis and hearing impairment among NBW black males that we observed is of particular interest, although we cannot explain this particular finding. These sex/race specific results also suggest that investigation into other risk factors may further elucidate the high prevalence of hearing loss among NBW black males. The only sign of possible effect modification for the association between LBW and hearing impairment is the difference in RR between male and female black children. The primary reason for this finding is the great disparity in prevalence rates in the two reference groups (ie, NBW children).

An unexpected finding was the higher than expected risk of hearing impairment among term BNBW children. There are no previously published studies examining this subgroup. Among LBW children, prematurity did not alter the magnitude of increased risk, although small numbers in the preterm cell (RR, 1.2; n = 4) may have limited this analysis.

As far back as 1965, the federal government has been interested in initiatives related to early identification of congenital hearing loss.²² Since October 1973 in the United States, VLBW has been used as one of the high-risk newborn criteria.²³ In 1982, the Joint Committee on Infant Hearing formally established the High-Risk Registry, which lists seven factors for placing a newborn statistically at risk for hearing loss, including a birth weight of <1500 g.²⁴ In 1994, the Joint Committee on Infant Hearing presented a subsequent position statement endorsing the goal of universal identification of newborns with hearing loss and expanded the High-Risk Registry criteria to 10 indicators.²⁵ Even given a perfectly implemented screening program, these criteria identify only 41% to 50% of children with congenital sensorineural hearing loss.²⁶ The criteria's shortcomings result in significant developmental delay in language, speech, cognition, and psychosocial functioning in undiagnosed children.²⁷ Our results show that the extent of the hearing impairment problem extends beyond the scope of the high-risk criteriachildren weighing 2500 to 2999 g (BLBW), who are considered NBW by current high-risk recommendations for hearing screening, are at increased risk for hearing impairment across all race/sex subgroups. Additionally, children weighing 1500 to 2499 g (MLBW) are also at increased risk for hearing impairment. MLBW cases are currently not included in the screening criteria for hearing impairment. In the absence of other high-risk identifiers, which are not discussed in this article, these BNBW and MLBW newborns are not flagged for identification of developmental disabilities by health care providers.

Although in the future the MADDSP plans to review records for children in speech and language classes, we may have missed some case children in these classes as well as those who were mainstreamed into regular education. However, misclassification bias would only be present if their birth weight distribution was markedly different from that of the cases included here. We may have missed other cases because of out-migration from the ascertainment area, yet detection bias would only prevail if birth weight was a correlate of that movement. Further, our results are reflective of children born in the 1980s; results may be different for infants born later. Advances in neonatal medicine since the 1980s may have resulted in increased survival and decreased morbidity of VLBW children; therefore, the results of this study may be limited in generalizability to the time period under study.

This study is, to our knowledge, the most recent report from a population-based surveillance system for hearing impairment in the United States. It is based on objective test data collected from multiple sources and an active surveillance of cases. Moreover, the demographic composition of the metropolitan Atlanta community allowed us to examine racial differences in prevalence. The AFs and RRs derived from this study's data are valuable because of the inherent strength of results from a population-based series of hearing impairment cases. Furthermore, these epidemiologic results provide new insight and guidance for hearing impairment screening criteria in the future.

	FOOTNOTES

Received for publication Nov 12, 1998; accepted Mar 2, 1999.

Reprint requests to (P.D.) Centers for Disease Control and Prevention (F-15), 4770 Buford Hwy NE, Atlanta, GA 30341. E-mail: pxd1{at}cdc.gov

	ABBREVIATIONS

LBW, low birth weight; VLBW, very low birth weight; MADDSP, Metropolitan Atlanta Developmental Disabilities Surveillance Program; LGBW, large birth weight; NBW, normal birth weight, BNBW, borderline normal birth weight; MLBW, moderately low birth weight; CI, 95% confidence interval; AF, attributable fraction; RR, relative risk.

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