Heritability of Symptom Domains in Otitis Media: A Longitudinal Study of 1,373 Twin Pairs

Maroeska Rovers1, Mark Haggard1, Mary Gannon1, Gesina Koeppen-Schomerus2 and Robert Plomin2

1 MRC Institute of Hearing Research, Nottingham, England.
2 Institute of Psychiatry, Social, Genetic, and Developmental Research Center, London, England.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Research on risk factors and pathogenesis of otitis media has emphasized the environment, but previous twin studies suggest a strong genetic component. In those studies, no attempt was made to differentiate the role of initial acute infection from the chronic airway blockage that frequently accompanies persistent effusion. The authors estimated genetic and environmental determination of both of these aspects of otitis media histories at three time points. A large and representative subset of a total population cohort of all twins born in England and Wales in 1994, was studied prospectively at ages 2, 3, and 4 years (monozygotic, n = 715; dizygotic, n = 658). For the total symptom score, the estimated heritabilities at ages 2, 3, and 4 years were 0.49, 0.66, and 0.71, respectively. All correlations for the total score found in monozygotic twins were about 0.9, but correlations were lower for dizygotic twins (p < 0.65). Shared environmental influences declined in importance over time (0.41, 0.22, and 0.16 at ages 2, 3, and 4 years, respectively). No gender differences were found in parameter estimates. Symptoms of acute infections showed lower heritability (0.57 on average) and higher shared environment (0.18) than did chronic airway blockage (0.72 heritability and 0.10 shared environment). The strong genetic component, which this study confirms, is not unitary but distributes differently across the phenotypic aspects of otitis media and probably across pathogenetic stage.

otitis media; twin studies; twins, dizygotic; twins, monozygotic

Abbreviations: MEDS, middle-ear disease scale; OME, otitis media with effusion; SD, standard deviation; TEDS, Twin Early Developmental Study


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Middle-ear disease (otitis media) forms a large part of the most common group of diseases in childhood, respiratory infectious diseases. It refers to an inflammation in the middle-ear cavity. The disease can be either symptomatic, with a rapid onset of signs and symptoms, such as otalgia and fever (acute otitis media), or can be asymptomatic, delayed and of long duration (otitis media with effusion (OME)). By age 3 years, most children have experienced at least one episode of otitis media (1Go). Persistent otitis media is associated with a mild-to-moderate hearing loss, which can have an adverse effect on early language development, plus longer-term effects on behavior and quality of life (2Go).

Often replicated environmental determinants of otitis media include number of siblings (3GoGoGo–6Go), upper respiratory tract infections (6GoGoGo–9Go), and day care attendance (10GoGoGo–13Go). These are examples of "shared environment" factors because they are largely shared by children growing up in the same family (14Go). In principle, prevention might reduce exposure to such environmental risk factors, although successful prevention seems doubtful when parents would have to forego lifestyle options or work time (15Go). Less attention has been paid to the possible genetic component, probably because in a largely benign and eventually resolving condition, genetically based interventions would not be considered. However, in infectious disease more generally, heredity gives people differing susceptibilities to infection. Knowledge of the link between this heritability and particular risk factors could lead in future to the rational targeting of environmental interventions or treatment, for example, by using a biomarker to predict likely persistence as justification for surgical intervention. Such progress could help reduce current international differences in thinking on management. In the United States, surgical intervention is often early and is preceded by antibiotics, but there are issues of possible overintervention and bacterial resistance. In much of Europe, surgical intervention is typically later and is more usually preceded by watchful waiting. In the conservative policy more characteristic of Europe, surgical intervention is largely restricted to those cases with persistent otitis media, which time must elapse to document. However, interventions could come too late to be maximally effective in assisting the child's development. For effective case finding, especially in OME, there is a need for affordable biomarkers, for example, DNA tests of polymorphisms that predict the persistence or recurrence of the condition in advance. For a multifactorial condition, high heritability and understanding the timing of expression in the phenotype helps to prioritize further genetic research from which predictive markers may emerge.

Epidemiologic studies of otitis media have long suggested a familial component in the disease (16Go, 17Go). Two recent twin studies have suggested that otitis media runs in families for genetic reasons rather than for reasons of shared family environment (18Go, 19Go). In a retrospective self-report study of 2,750 Norwegian adult twin pairs, Kvaerner et al. (18Go), using a continuous model-fitting approach, estimated the heritability of ear infections at 0.74 in females but only 0.45 in males. The remainder of the variability in females was explained by nonshared environmental factors. In males, the remaining higher variance was explained by both shared environment (29 percent) and nonshared environment (26 percent) effects. Casselbrant et al. (19Go) used a clinical sample of 140 twin pairs, measuring time with middle-ear effusion at ages 1 and 2 years. With the statistical model proposed by DeFries and Fulker (20Go), the estimated heritability of time with effusion was 0.73 (p < 0.001). A slight and nonsignificant trend to lower heritability in boys (0.64 vs. 0.79 for girls) was again found.

For more insight into pathogenesis, the heritabilities for the disease at different stages or in its different manifestations need to be studied. We had the opportunity to do this in a large, longitudinal United Kingdom twin study, the Twin Early Development Study (TEDS), coordinated from the Institute of Psychiatry, King's College, London, England. In TEDS, a large community sample of twins has been studied prospectively at ages 2, 3, and 4 years. Including a set of ear infection and airway questions in TEDS enabled us to study trends in heritability and shared environment over time for total symptoms as well as for separate symptoms of acute infection from chronic airway symptoms. The airway questions were included because they are readily answerable and are strongly predictive of persistent OME (21Go).


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
The source population was all twins born in England and Wales in 1994 (n = 7,756 pairs). The parents were invited to participate in TEDS, and 5,443 families opted for further information. For the following analyses, only same-sex twins were included. Booklets were sent shortly before the children's second birthday, and families of 3,121 same-sex twin pairs subsequently returned them. For age 3 years, 2,048 families responded, and for age 4 years, 1,854 did so. Of the 1,689 families who provided data at all three times, 1,373 completed all items on the middle-ear disease scale (MEDS) at all ages. The TEDS sample is reasonably representative of the United Kingdom population of parents with young children in terms of parental education, occupational status, and ethnicity (22Go). The number of same-sex twins within TEDS is about two thirds of the total sample, which is similar to the distribution for all twins born in England and Wales. Zygosity was determined by parent questionnaire ratings of twins' physical similarity. A previous report using a zygosity questionnaire found accuracy of between 93 and 98 percent (23Go). In a separate analysis on the instrument used in TEDS, zygosity was correctly assigned by parent ratings in 94.7 percent of cases as validated against zygosity assigned by identity of polymorphic DNA markers (24Go).

Measures
At age 2 years, seven otitis media questions were available, and at age 3 and 4 years, six items were available. Principal component analysis on all available data at each age yielded a first principal component accounting for 22, 22, and 23 percent of the total variance at years 2, 3, and 4, respectively. All items loaded above 0.45 on this principal component, reflecting high interrelation and the appropriateness of using a total score.

Statistical analyses are reported on the longitudinal twin sample at each age. All twin pairs with unknown zygosity (0.12 percent) as well as twins with missing data on the item scores (18.6 percent) were excluded. We made cross-sectional comparisons to study whether the sample of twins whose parents completed all three questionnaires is representative of the sample of twins available at each age.

Analyses
The twin method is a quasi-experimental design that compares phenotype concordance between monozygotic twins, who are genetically identical, and dizygotic twins, who share, on average, 50 percent of their genes. The heritability coefficient quantifies the genetic effect by expressing the proportion of phenotypic (observed) variance attributable to genetic variation; it approximates a doubling of the difference between the correlations for monozygotic and dizygotic twins. The remaining phenotypic variance can be attributed to two types of environmental influence: shared environment, which makes family members similar, and nonshared environment, which makes them different. Twin within-pair similarity for the phenotype is assumed to be due to genetic factors plus shared environment factors. Nonshared or unique environment is a residual term that includes those environmental factors that make members of a family different from one another, plus error of measurement. Comparison of the within-pair correlations for monozygotic twins with those of dizygotic twins gives estimates for the contribution to phenotype variance from genes additive in their effect, from shared environment, and from unique environment.

In practice, structural equation models are fitted to the variance/covariance matrices rather than to simple comparisons of twin correlations. This gives estimates of genetic and environmental parameters plus confidence intervals (25Go). The ACE model estimates parameters for additive genetic variance (A), common or shared environment (C), and environmental influences that are not shared (E); it assumes that genetic effects are additive and that monozygotic and dizygotic twins experience equally similar environments (14Go).

Two major variants of this model were considered. It is possible that disease transmission plays a role in otitis media, that is, affected status in one child can cause affected status in the other, but conceivably more so for monozygotic than for dizygotic twins. We therefore compared the ACE model with a PACE model, which quantifies the portion of the variance-covariance structure (P) accountable by adding a path from the phenotype of a twin to that of the cotwin. As a second variant, we examined concordance with respect to the most extreme symptoms rather than for the entire distribution; here, people with high total scores (above the 95 percent percentile of the distribution) were selected and compared with their cotwin (20Go). In addition, comparisons were made between boys and girls, and heritabilities on individual items were analyzed.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Table 1 shows that the sample of twins whose parents completed all three questionnaires (longitudinal data) is representative of the twins available at each age. For control in the age-trend of heritability, we report the longitudinally complete data of 1,373 same-sex twin pairs at ages 2, 3, and 4 years, of whom 52 percent were monozygotic and 48 percent were dizygotic. The mean scores of girls and boys for ear disease did not differ much, at 23.7 (standard deviation (SD) 2.8) versus 23.4 (SD 2.9) at age 2 years, 18.8 (SD 3.0) versus 18.4 (SD 3.0) at age 3 years, and 18.7 (SD 3.1) versus 18.4 (SD 32) at age 4 years.


View this table:
[in this window]
[in a new window]
 
TABLE 1. Cross-sectional comparisons of twins whose parents completed all three questionnaires and the larger sample of twins born in England and Wales in 1994 and available at ages 1 or 2 years

 
The frequencies of the individual items were slightly skewed; "occasionally" and "never" were selected more often that "sometimes" and "often." The distribution was, however, highly similar over time. The monozygotic and dizygotic twin correlations for each item and the estimates (26Go) of additive genetic factors (A), shared environment (C), and environmental influences that are not shared (E) are shown in tables 2 and 3. Estimates on individual items are shown for the best-fitting models. The model for the total MEDS score fitted the data well at age 2 years ({chi}2(3) 5 0.75, p = 0.86), at age 3 years ({chi}2(3) 5 1.28, p = 0.73), and at 4 years ({chi}2(3) 5 1.35, p = 0.72). At all ages, the PACE models provided a fit to the data similar to that of the ACE models; the interaction term was nonsignificant at all ages, indicating that phenotypic interaction (disease transmission) need not to be taken into account.


View this table:
[in this window]
[in a new window]
 
TABLE 2. Longitudinal sample: correlations within monozygotic and dizygotic twins born in England and Wales in 1994 and at ages 2, 3, and 4 years (data present across all ages)*

 

View this table:
[in this window]
[in a new window]
 
TABLE 3. Longitudinal sample: estimates of the genetic influence (a2), shared environment (c2), and nonshared environment (e2) at ages 2, 3, and 4 years for monozygotic and dizygotic twins born in England and Wales in 1994

 
In the children at the upper extreme on the MEDS scale (>95 percentile), the percentage of concordant pairs was significantly higher in the monozygotic twins. At age 2 years, the percentage probandwise concordance in monozygotic twins was 95 percent compared with 63 percent in dizygotic twins. At ages 3 and 4 years, these concordances were 91 and 85 percent for monozygotic twins and 67 and 68 percent for dizygotic twins, respectively. The estimates of the additive genetic factors, shared environment, and nonshared environment in the extremes analysis of DeFries and Fulker (20Go) were 0.44, 0.51, and 0.04, respectively, at age 2 years; 0.49, 0.45, and 0.06 at age 3 years; and 0.59, 0.31, and 0.10 at age 4 years. These estimates display a pattern similar to the estimates in table 3, which use the entire range of the scores. The comparison does not support particular heritability of extreme forms—if anything, it does the opposite.

The results for the individual items in tables 2 and 3 indicate that, as hypothesized a priori, the four acute infection items (earache, ears leaking pus or mucus, pulling or scratching ears, and red or sore ears) show lower heritability on average (0.57) and greater shared environment (0.18) than do the two chronic airway items (breathing through the mouth and snoring or snorting in sleep). The latter have an estimated heritability of 0.72 and a shared environment estimate of 0.10. Hearing/ignoring lie in a third domain not further discussed here.


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Heritability interaction and sex
The correlations on the MEDS total score for monozygotic twins were much higher than those for dizygotic twins for the total score and for each individual item. For the total score, between ages 2 and 3 years, the additive genetic influence (A) increases substantially (from 0.49 to 0.66), while shared environment (C) decreases (from 0.41 to 0.22). Between ages 3 and 4 years, heritability further increases slightly to 0.71, and environmental influences decrease to 0.16. In this prospective twin study, the concordances for several aspects of middle-ear disease histories found in monozygotic twins were all at scores of about 0.9, while those in dizygotic twins were lower (0.65 at age 2 years, 0.54 at age 3 years, and 0.51 at age 4 years). Finding substantial genetic influence for an infectious disease such as otitis media can best be interpreted as genetic influence on susceptibility to the viruses and bacteria that cause otitis media. This is a concrete example of the general point that the mechanisms of genetic influence usually involve how people interact with their environment. The extremes analysis suggested that heritability is relatively stable throughout the range of disease scores and, hence, that further studies using continuous scales and report-based measures will remain relevant to genetic aspects of the clinical condition.

The overall results are consistent with those in a retrospective report study (16Go) and a prospective, instrument-based study (17Go) in finding a large genetic effect, suggesting that differences in method have limited influence on heritability estimates. However, in contrast to the paper by Kvaerner et al. (18Go), we did not find higher heritability in females in our prospective study, and the difference in heritability was not significant in the instrument-based study (17Go). The discrepancy between the two large, report-based studies could possibly be explained by differences between populations in the distribution of risk factor genes (with different sex-linkage). However, there are few genetic precedents for such a difference in heritability between the sexes being found in some populations but not in others, so there also may be some memory bias in the data on adults in the paper by Kvaerner et al. on retrospective report related to the closer bonding in girls coupled with a higher awareness of health issues.

Effects of age and symptom domain on heritability
For the total symptom score, genetic influence increased from 2 to 4 years, while the estimates for the shared environment declined. Once maternally conferred immunity wanes by 6 months, the environmental influence of exposure to pathogens becomes strong, and acute otitis media (infection) becomes most prevalent from this age to age 21/2 years. It is of interest that the heritabilities of the items in both domains (acute infections and airway/anatomic problems) increase with time. This may be due to the necessity of an infection to reveal the airway problem, and it remains consistent with the difference in heritability between the symptom areas.

Symptoms indicative of airway blockage showed higher heritability than did those associated with an acute infection. A possible explanation is that the occurrence of airway blockage reflects a second pathogenetic stage with an additional intrinsic component involving genes expressed in the anatomy of the head or the immunologic response of the mucosa. For this blockage to occur, it still needs to be driven by events in the first stage of the common, environmentally determined, respiratory infections. Acute infection must be directly related to an environmental exposure, although the initial immunoglobulin A and immunoglobulin G response (27Go, 28Go) can display genetic susceptibility. The initial ability of the immune system to counter pathogens could be expected to have low heritability and moderate, shared environmental influence. This is because development of the immune system is partly determined by experiences shared by two children growing up in the same family, including similarity of exposure to viruses and bacteria. However, the genes seem to assert their influence on the response to the environmental exposure increasingly as the immune system matures. The environment, in the original sense of what is outside the organism, is closer to the first pathogenetic stage, so the first stage should display lower heritability and greater shared environment than the second stage, as we indeed find.

The two-domain account of symptoms advanced here (acute ear infection and chronic airway blockage) justifies more detailed genetic analyses in which specific risk factors are also documented. Our results, replicated at ages 2, 3, and 4 years, show greater heritability for chronic airway blockage than for acute ear infection. This suggests that the candidate gene approach to susceptibility in otitis media would do well to consider both of these domains rather than undifferentiated otitis media. Meanwhile, the findings can be used clinically by including questions about family history and chronic airway blockage as well as about acute ear symptoms, especially in the older (>=3 years) child suspect for acute otitis media or OME. In children aged 2 years and less, it is more worthwhile to establish—and subsequently minimize—a history of exposure.

With richer questionnaire data (five or more items per domain), it would be possible to extract several domain factors and rotate them for maximum independence. Rotated factors usually permit easier interpretation than second and third principal components do because principal components often approximate rather abstract differences or ratios between the factors when the latter are considered as absolute construct scores. Here, however, we simply required a rational basis for valid and reliable reduction of six to seven items to a weighted total score representing all aspects of otitis media. For this, the first principal component is an accepted approach making few assumptions and applicable with relatively few items. The two or three per domain are too few for such reduction. The higher heritability for airway problems is consistent across items within domain; at this point, such consistency is more impressive than greater precision from a total domain score.

Use of report data
The strength of otitis media history measured in this study is based on parental report, which might introduce various types of bias. From the literature, we know that, because of low parental alertness to the clues and fluctuations in hearing, reports of recent hearing difficulties by unselected parents predict current actual hearing in audiometry and other disability-related performance measures with only rather low sensitivity (29Go). However, parental reports of physical symptoms, consultation, and treatment are sufficiently valid to put together a good cumulative picture of an otitis media history (30Go, 31Go). Alternatively, symptoms reported by parents can be seen as a style of data complementary to objective tests of hearing loss. The major advantages of parental reports are convenience and cost: It becomes possible to assess symptoms in a large community sample and feasible and affordable to follow the children longitudinally at reasonable expense by comparison with frequent testing or physical examinations. In particular, middle-ear status and hearing loss can fluctuate over a period of days, thus limiting the reliability of single, objective measures. The ability of parental responses to integrate information over time offsets their lower precision and reliability and confers enhanced relevance to the clinical condition of concern, that is, longer-term persistent disease. Previous risk factor studies of otitis media confirm the broader validity of parental symptom history reports in large samples (32Go). The similarity of the present overall heritabilities to those of Casselbrant et al. similarly calibrate the relevance of the report data to be a more detailed demonstration of the age and domain effects. These would have been much less practical to establish in an instrument-based study.

Do concordances refer to facts about the children or ideas of the parents? For example, it could be argued that parents of monozygotic twins are more likely to report concordant results than are parents of dizygotic twins. However, in another TEDS study on body weight (33Go), parental reports of children's weight had an 0.77 correlation with actual measures of the twins' weight taken in their homes by testers (216 families in total), and no difference was found between monozygotic and dizygotic twins.

Interpretation and implications
This study was not designed to identify specific environmental influences, such as the influence of caretaking (day care vs. care at home) and having older siblings. If we had been able to control for these factors explicitly, the residual variation might have been reduced. On the other hand, these factors are mostly shared by twins in a family and therefore contribute to the shared environmental influence. In our results, only 20 percent of the variance was explained by such shared environmental factors.

This study has four main strengths: the prospective data acquisition, the large number of subjects, the a priori distinction between two symptom domains in otitis media that turn out to have different heritabilities, and the longitudinal description of the trajectory of heritability and shared environment during the first years of life. A possible limitation of the study is that we do not have medical data on all twins, so we do not know at precisely what incidence and with what accuracy the children with reported symptoms that yielded a particular score would have resulted in clinical cases with recurrent or persistent otitis media, much less a precise diagnosis. However, given the geographic and social biases in health care uptake, omission of this further source of variability could be seen as a virtue.

In conclusion, the results support a strong genetic influence on the chronic airway blockage that is manifest with respiratory infection and is a risk factor for persistent otitis media with effusion. There is less, but still substantial, genetic influence on acute ear infection. Genetic studies distinguishing these related domains are more likely to be informative than univariate studies of otitis media. Heritability increases over time in a way consistent with the known immunology and epidemiology.


    ACKNOWLEDGMENTS
 
The Twins Early Development Study (TEDS) is supported by a program grant from the United Kingdom Medical Research Council to Robert Plomin. The epidemiologic research on otitis media at the MRC Institute of Hearing Research has also been supported in part by Glaxo-Wellcome United Kingdom and by Defeating Deafness (The Hearing Research Trust).

The authors thank the parents of the twins in TEDS for making the study possible.


    NOTES
 
Reprint requests to Prof. Mark Haggard, MRC Institute of Hearing Research, University Park, Nottingham NG7 2RD, England (e-mail: Mark{at}ihr.mrc.ac.uk).


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Casselbrant ML, Mandel EM. Epidemiology. In: Rosenfeld RM, Bluestone CD, eds. Evidence-based otitis media. Hamilton, British Columbia, Canada: Decker, Inc, 1999:117–37.
  2. Vernon-Feagans L. Impact of otitis media on speech, language, cognition, and behavior. In: Rosenfeld RM, Bluestone CD, eds. Evidence-based otitis media. Hamilton: British Columbia, Canada: Decker, Inc, 1999:353–73.
  3. Haggard M, Hughes E. Objectives, values and methods of screening children's hearing: a review of the literature. London, England: Her Majesty's Stationery Office, 1991.
  4. Zielhuis GA, Heuvelmans-Heinen EW, Rach GH, et al. Environmental risk factors for otitis media with effusion in preschool children. Scand J Prim Health Care 1989;7:33–8.[Medline]
  5. Sassen ML, Brand H, Grote JJ. Risk factors for otitis media with effusion in children 0 to 2 years of age. Am J Otolaryngol 1997;18:324–30.[ISI][Medline]
  6. Rovers MM, Zielhuis GA, Straatman H, et al. Prognostic factors for persistent otitis media with effusion in infants. Arch Otolaryngol Head Neck Surg 1999;125:1203–17.[ISI][Medline]
  7. Birch L, Elbrond O. Prospective epidemiological study of common colds and secretory otitis media. Clin Otolaryngol 1987;12:45–8.[ISI][Medline]
  8. Otten FWA, Grote JJ. Otitis media with effusion and chronic upper respiratory tract infection in children: a randomized, placebo-controlled clinical study. Laryngoscope 1990;100:627–33.[ISI][Medline]
  9. Bylander A. Upper respiratory tract infection and eustachian tube function in children. Acta Otolaryngol Suppl 1984;97:343–9.
  10. Rovers MM, Zielhuis GA, Ingels K, et al. Day-care and otitis media in young children: a critical overview. Eur J Pediatr 1999;158:1–6.[ISI][Medline]
  11. Alho O, Oja H, Koivu M, et al. Risk factors for chronic otitis media with effusion in infancy. Arch Otolaryngol Head Neck Surg 1995;121:839–43.[Medline]
  12. Paradise JL, Rockette HE, Colborn DK, et al. Otitis media in 2253 Pittsburgh-area infants: prevalence and risk factors during the first two years of life. Pediatrics 1997;99:318–33.[Abstract/Free Full Text]
  13. Marx J, Ogstuhorpe JD, Parsons G. Day care and the incidence of otitis media in young children. Otolaryngol Head Neck Surg 1995;112:695–9.[ISI][Medline]
  14. Plomin R, DeFries JC, McClearn GE, et al. Behavioral genetics. 3rd ed. New York: NY: Worth Publishers, 2001.
  15. Bexell A, Rastam L, Isacsson S, et al. Parents' response to recurrent middle ear infection in their children. Scand J Soc Med 1990;18:25–30.[ISI][Medline]
  16. Daly KA, Rich SS, Levine S, et al. The family study of otitis media: design and disease and risk factor profiles. Genet Epidemiol 1996;13:451–68.[ISI][Medline]
  17. Stenstrom C, Ingvarsson L. Otitis-prone children and controls: a study of possible predisposing factors. 1. Heredity, family background and perinatal period. Acta Otolaryngol 1997;117:87–93.[ISI][Medline]
  18. Kvaerner KJ, Harris JR, Tambs K, et al. Distribution and heritability of recurrent ear infections. Ann Otol Rhinol Laryngol 1997;106:624–32.[ISI][Medline]
  19. Casselbrant ML, Mandel EM, Fall PA, et al. The heritability of otitis media: a twin study. JAMA 1999;282:2125–30.[Abstract/Free Full Text]
  20. DeFries JC, Fulker DW. Multiple regression analysis of twin data. Behav Genet 1985;15:467–73.[ISI][Medline]
  21. MRC Multi-Centre Otitis Media Study Group. Sensitivity, specificity and predictive value of tympanometry in predicting a hearing impairment in otitis media with effusion. Clin Otolaryngol 1999;24:294–300.[ISI][Medline]
  22. Dale PS, Simonoff E, Bishop DVM, et al. Genetic influence on language delay in 2-year-olds. Nat Neurosci 1998;1:324–8.[ISI][Medline]
  23. Goldsmith HH. A zygosity questionnaire for young twins: a research note. Behav Genet 1991;21:257–69.[ISI][Medline]
  24. Price TS, Freeman B, Craig I, et al. Infant zygosity can be assigned by parental report questionnaire data. Twin Res 2000;3:129–33.[Medline]
  25. Neale MC, Cardon LR. Methodology for genetic studies of twins and families. Dordrecht, the Netherlands: Kluwer Academic Publications, 1992.
  26. Neale MC. Mx: statistical modeling. Richmond, VA: Virginia Commonwealth University, Department of Psychiatry, 1997.
  27. Faden H. The microbiology and immunologic basis for recurrent otitis media in children. Eur J Pediatr 2001;160:407–13.[ISI][Medline]
  28. Stenfors LE. Non-specific and specific immunity to bacterial invasion of the middle ear cavity. Int J Pediatr Otorhinolaryngol 1999;49 (suppl 1):S223–6.[ISI][Medline]
  29. Anteunis LJC, Engel JAM, Hendriks JJT, et al. A longitudinal study of the validity of parental reporting in the detection of otitis media and related hearing impairment in infancy. Audiology 1999;38:75–82.[ISI][Medline]
  30. Alho OP. The validity of questionnaire reports of a history of acute otitis media. Am J Epidemiol 1990;132:1164–70.[Abstract]
  31. Daly KA, Lindgren B, Giebink S. Validity of parental report of a child's medical history in otitis media research. Am J Epidemiol 1994;139:1116–21.[Abstract]
  32. Bennett KE, Haggard MP. Accumulation of factors influencing children's middle ear disease: risk factor modelling on a large population cohort. J Epidemiol Community Health 1998;52:786–93.[Abstract]
  33. Koeppen-Schomerus G, Wardle J, Plomin R. A genetic analysis of weight and overweight in 4-year old twin pairs. Int J Obes Relat Metab Disord 2001;25:838–44.[Medline]
Received for publication January 2, 2001. Accepted for publication January 11, 2002.