Acute bronchiolitis in infancy as risk factor for wheezing and reduced pulmonary function by seven years in Akershus County, Norway
© Fjærli et al; licensee BioMed Central Ltd. 2005
Received: 17 March 2005
Accepted: 18 August 2005
Published: 18 August 2005
Acute viral bronchiolitis is one of the most common causes of hospitalisation during infancy in our region with respiratory syncytial virus (RSV) historically being the major causative agent. Many infants with early-life RSV bronchiolitis have sustained bronchial hyperreactivity for many years after hospitalisation and the reasons for this are probably multifactorial. The principal aim of the present study was to investigate if children hospitalised for any acute viral bronchiolitis during infancy in our region, and not only those due to RSV, had more episodes of subsequent wheezing up to age seven years and reduced lung function at that age compared to children not hospitalised for acute bronchiolitis during infancy. A secondary aim was to compare the hospitalised infants with proven RSV bronchiolitis (RS+) to the hospitalised infants with non-RSV bronchiolitis (RS-) according to the same endpoints.
57 infants hospitalised at least once with acute viral bronchiolitis during two consecutive winter seasons in 1993–1994 were examined at age seven years. An age-matched control group of 64 children, who had not been hospitalised for acute viral bronchiolitis during infancy, were recruited from a local primary school. Epidemiological and clinical data were collected retrospectively from hospital discharge records and through structured clinical interviews and physical examinations at the follow-up visit.
The children hospitalised for bronchiolitis during infancy had decreased lung function, more often wheezing episodes, current medication and follow-up for asthma at age seven years than did the age matched controls. They also had lower average birth weight and more often first order family members with asthma. We did not find significant differences between the RSV+ and RSV- groups.
Children hospitalised for early-life bronchiolitis are susceptible to recurrent wheezing and reduced pulmonary function by seven years compared to age-matched children not hospitalised for early-life bronchiolitis. We propose that prolonged bronchial hyperreactivity could follow early-life RSV negative as well as RSV positive bronchiolitis.
Acute viral bronchiolitis is one of the most common causes of hospitalisation during infancy in Akershus County, Norway, with respiratory syncytial virus (RSV) historically being the major causative agent. RSV causes respiratory disease in young children worldwide and by the age of two years most children have been infected. In temperate climates the infection occurs as yearly winter epidemics and the impact of RSV on human health is demonstrated annually when infants are admitted to hospitals in large numbers. Symptoms vary from a mild upper respiratory tract infection to severe bronchiolitis with hyperinflated lungs and hypoxemia . The first infection is usually the most severe but milder reinfections are common throughout life. The risk of severe illness is highest in infants born prematurely and in those with chronic lung disease, certain congenital heart defects and immunodeficiency disorders [2, 3]. Many infants with severe RSV bronchiolitis experience recurrent wheezing in later childhood and there is growing evidence that early-life RSV bronchiolitis may predispose some infants to the development of childhood asthma [4–6]. The genetic background of the infant, intermittent changes in host cellular immune responses and neural control leading to sustained bronchial hyperreactivity and recurrent wheezing, timing of RSV infection with respect to allergen exposure, environmental conditions and exposure to endotoxin are all factors suggested to contribute to RSV induced asthma [7–9].
Human metapneumovirus (hMPV) is now recognised as one of several other viral pathogens that can cause acute bronchiolitis, the remaining being mainly rhinovirus, parainfluenzavirus, influenzavirus and adenovirus . hMPV have recently been found to be an important cause of acute bronchiolitis in infants and children. However, the long-term effect of this and other viral agents on lung function and symptoms in later childhood is not yet fully investigated [11, 12].
The primary aim of this study was to compare the airway symptoms and lung function of children hospitalised for early-life bronchiolitis with age matched controls without such a history at age seven years. We also aimed to describe possible differences between those with proven RSV bronchiolitis and those with non-RSV bronchiolitis regarding the same endpoints.
During two consecutive years in 1993–1994 a sample of nasopharyngeal aspirate (NPA) was collected from infants hospitalised with acute respiratory disease. The doctors on call conducted, as part of the admission routine, clinical interviews using structured questionnaires and performed physical examination of all infants admitted with suspected acute bronchiolitis.
In the present follow-up study at age seven years hospital records, questionnaires and microbiological results of all children hospitalised during infancy with acute bronchiolitis were analysed in retrospect. 109 infants presenting with respiratory insufficiency such as tachypnoe, intercostal retractions, increased mucus production and soar coughing were eligible for follow-up. However, we excluded 11 infants who were born before completed 37 wGA (weeks of gestational age), 11 infants who had divergent results in two different microbiological tests, 22 infants who had insufficient NPA material for testing, two children who had died from unrelated causes and six children that did not want to participate or had moved far away from our region.
The remaining 57 children were willing and able to take part in the study and came for a follow-up visit at age seven years. Children hospitalised during infancy with verified RSV bronchiolitis (RS+) had to be positive for RSV in two different microbiological tests, and children hospitalised with non-RSV bronchiolitis (RS-) had to be negative for RSV in the same two different microbiological tests.
Two different tests for detection of RSV in NPA were used. These were our own in-house Abbott TestPack RSV enzyme immunoassay (EIA) and immunofluorescens (IF) staining performed in our reference laboratory at the Department of Microbiology, Rikshospitalet, Oslo [13, 14].
Structured clinical interview at the follow-up visit
A clinical interview based on the structured questionnaire used when the children with bronchiolitis were admitted to hospital were performed by three of the authors in the study (H.O.F, T.F., G.R.) to give information on episodes of wheezing (defined by us as episodes of difficult breathing accompanied by a whistling noise in the chest during expiration), number of visits to a family physician, practicing paediatrician and resubmissions to any hospital for wheezing or asthma (as diagnosed by a medical doctor) during the 6–7 years that had passed since the hospitalisation in infancy. Hospital records confirmed number of hospitalisations, whereas the parents gave information regarding visits to a family physician or practicing paediatrician. Information on failure to thrive, regular medications and physical activity was given in the interviews, as was information on parental smoking habits, number of preschool (<6 years old) siblings and prevalence of allergy, eczema and asthma in first order family members (mother, father or siblings) that had been diagnosed by a medical doctor.
Physical examination and pulmonary function tests at the follow-up visit
The medical doctors who conducted the structured clinical interviews performed weight and height measurements and a general clinical examination including inspection and auscultation at the follow-up visit. All children performed a standard pulmonary function test (PFT) under the supervision and guidance of a skilled paediatric nurse (G.K.U.). Forced vital capacity (FVC), forced expiratory volume in one second (FEV1) and forced expiratory flow rate at 50% of FVC (FEF50) were measured with the Vmax 20 equipment (SensorMedics). The equipment was calibrated daily according to the specifications of the producer. Measurements of pulmonary function were performed both before and 10 minutes after inhalation of 0,2 mg salbutamol (Airomir Autohaler) and the one attempt (out of three) with the best technical profile both before and after salbutamol was used for analysis.
The control group
An age-matched control group of seven years old children were recruited from a nearby primary school. A letter was distributed by the school principal to the parents of 92 children (three school classes) providing information about the study and explaining the criteria set for participation. All children had to be healthy term infants born at completed 37 wGA and without history of hospitalisation for respiratory problems during the first year of life. 64 children fulfilled the inclusion criteria, gave informed consents and took part in the study. At the follow-up visit the inclusion criteria were confirmed, an identical structured clinical interview with the parents of the control children were undertaken and the children performed the same PFT as the other children in the study.
For comparison of continuous data between two groups the independent samples T-test was used and for comparison of ordinal and nominal data between two groups we used chi square tests. A p-value <0.05 was considered statistically significant and continuous data were given with 95% confidence intervals (CI). The statistical programme SPSS, version 12.0.1, was used in all calculations.
The regional ethics committee for medical research approved the study and the parents gave oral and written informed consent.
Structured clinical interviews
Among the 57 children hospitalised with acute bronchiolitis in infancy, 35 children (61%) tested double RSV positive while 22 (39%) tested double negative for RSV in NPA. Mean birth weight, mean age at hospitalisation and mean length of hospitalisation including admission day were not significantly different between these groups. According to data collected at admission, seven (32%) in the RS- and 15 (43%) in the RS+ group had experienced one or more episodes of a possible lower respiratory tract infection prior to the first hospitalisation for bronchiolitis.
Follow-up of children hospitalised or not with acute bronchiolitis in infancy
Reported at child age seven years
Not hospitalised group
RS- (n = 22)
RS+ (n = 35)
Both (n = 57)
Controls (n = 64)
Age, mean (range) at hospitalisation, months
Length, mean (range) of hospitalisation, days
Birth weight, mean (range), gramsb
Weight, mean (range) at follow-up visit, kg
Height, mean (range) at follow-up visit, cm
Age, mean (range) at follow-up visit, months
No. boys (%)
Risk factors in family members of children hospitalised or not with acute bronchiolitis in infancy
Reported at child age seven years
Not hospitalised group
No. families with risk factorsa
RS- (n = 22)
RS+ (n = 35)
Both (n = 57)
Controls (n = 64)
Allergy before seven years
Eczema before seven years
Asthma before seven years
Siblings <6 years before seven years
Current siblings <6 years
Parental smoking before seven years
Current parental smoking
Wheezinga ever and current asthma in children hospitalised or not with acute bronchiolitis in infancy
Reported at child age seven years
Not hospitalised group
RS- (n = 22)
RS+ (n = 35)
Both (n = 57)
Controls (n = 64)
≥3 episodes of wheezing ever during follow-upd
Current follow-up for asthma by a medical doctorb,e
Current medication for asthmac,e
At the follow-up visit 31 children in the hospitalised group were currently under supervision for asthma by a practicing paediatrician or a family physician, in contrast to five children in the control group (54% vs 8%, p < 0.001). More children in the hospitalised group than in the control group used asthma medication currently (35% vs 8%, p < 0.01). No statistical differences were found between the RS+ and RS- groups for these parameters (tab 3).
Pulmonary function tests
Spirometry in children hospitalised or not with acute bronchiolitis in infancy
Age seven years
Not hospitalised group
RS- (n = 22)
RS+ (n = 35)
Both (n = 57)
Ctr (n = 64)
FEV 1 (liters)
FEF 50 (liters/min)
We also performed all analyses excluding the children with prior infection, first from the RS- group, then from both groups. There were not any substantial changes in observations or conclusions. These data are not shown.
The results of this follow-up study show that children hospitalised with acute bronchiolitis during infancy in our region are at increased risk for subsequent childhood wheezing and for reduced pulmonary function, at least up to age seven years. The children were all admitted to the same hospital and from two consecutive winter seasons.
Mean length of hospitalisation including admission day is in accordance with other studies . Mean age at first hospitalisation is higher than in other studies, and 39% of the children had experienced episodes of possible RSV infections before hospitalisation [16, 17]. These children were not unevenly distributed on the RS+ and RS- groups. We were not able to observe substantial changes in the results when excluding children with prior infections. This is not surprising. First, since the number of observations is reduced by this procedure, larger differences are necessary to provide significant changes. Secondly, none of these prior infections were severe enough to require admission to hospital. Accordingly, they did probably not meet the criteria of Ruuskanen and Ogra  or Court , and we assume it rather unlikely that they were due to RSV. Nevertheless, there is a small possibility that we have underestimated differences between the RS negative and positive groups.
We were surprised to find that many of the hospitalised children were deemed RSV negative in NPA, since they were examined in the middle of an RSV epidemic. Obviously, the two microbiological techniques used in our study may not have been sensitive and specific enough due to properties of the tests themselves . In 1993–1994 we did not have access to more sensitive microbiological tests, such as detection of nucleic acid by reverse-transcriptase polymerase chain reaction (PCR) . However, using new knowledge about the performance of the tests we used compared to PCR [22, 23], we have calculated that the overall probability of misclassification in our study is 4%, restricted to one or two false negatives. Thus, we find it unlikely that the results in the RSV negative group are contaminated by misclassification of the microbiological agent. For all variables studied, the results in these two groups are so similar, that even a much larger study would not be likely to detect differences.
Several studies have shown that hospitalisation for acute RSV bronchiolitis in infancy is an important risk factor for subsequent wheezing [24, 25]. We show that hospitalisation for RSV negative bronchiolitis is an equal important risk factor for subsequent wheezing as is RSV positive bronchiolitis. The intensity of the host inflammatory response, and not only the virus itself, is probably a major determinant to whether or not bronchiolitis develops at the time of infection. This has been well documented in a recent study comparing infants with bronchiolitis due to RSV and influenzavirus . Also, a high frequency of dual infection by RSV and hMPV has been reported in infants with severe bronchiolitis receiving mechanical ventilation .
The incidence of childhood asthma, when defined as three or more episodes of wheezing ever during follow-up, was in our study 51% in the hospitalised group and 14% in the control group. Correspondingly, as reported by parents, 54% in the hospitalised group and 8% in the control group were at age seven years consulting for asthma with a family physician or a practicing paediatrician. Our findings further support that hospitalisation for early-life bronchiolitis, not only those due to RSV, is associated with increased prevalence of childhood asthma, at least up to age seven years [28, 29].
The percentage of smokers among parents of the hospitalised children in our study was high. However, a high percentage of parents currently smoking was also recorded in the control group. Exposure to tobacco smoke seemed not to be an independent risk factor for childhood wheezing in our study. Several studies have found parental smoking to be an important risk factor for wheezing in early childhood but not in older children. In the Tucson Children's Respiratory Study the greatest impact was found in children under three years of age when mothers were smoking during pregnancy .
No statistically significant differences were found between the hospitalised group and the control group in current prevalence of allergy or eczema in first order family members and number of families with preschool siblings. Heredity for asthma in the hospitalised group seemed to be the only significant risk factor in first order family members for childhood wheezing in our study. Several other studies have shown that the prevalence of asthma among family members of infants with acute viral bronchiolitis is slightly higher than in the general population .
Early-life RSV bronchiolitis has in many studies been associated with sustained impaired lung function. Our study supports this by demonstrating significantly reduced test results for several parameters in the hospitalised group compared to the control group. Our results are in accordance with a study from Noble et al in which the index children had significantly reduced lung function at nine to ten years of age . According to a study by Sigurs et al severe RSV bronchiolitis in infancy was significantly associated with reduced lung function even at 13 years of age .
Studies have shown that pulmonary function at birth of newborns suffering from acute bronchiolitis later on during infancy are reduced compared to pulmonary function at birth of newborns not suffering from acute bronchiolitis later on during infancy [33, 34]. This might indicate a congenital pulmonal predisposition for bronchiolitis susceptibility and later obstructive lung disease. A significant lower birth weight in the hospitalised infants in our study might indicate that they had smaller lungs or delayed pulmonal development compared to the controls.
Acute RSV bronchiolitis is a major health problem in our region with a mean hospitalisation incidence for the period 1993–2000 in infants of 23.6 per 1.000 . Many infants with acute viral bronchiolitis, not only those due to RSV, will have episodes of recurrent wheezing, resubmissions to hospital and reduced lung function, at least up to age seven years. The results of the present study indicate that host factors may be as important as the nature of the infecting agent for the development of bronchiolitis and later childhood wheeze and need further studies.
- Law BJ, Carbonell-Estrany X, Simoes EA: An update on respiratory syncytial virus epidemiology: a developed country perspective. Respir Med. 2002, 96 Suppl B: S1-S7.View ArticlePubMedGoogle Scholar
- Glezen WP, Greenberg SB, Atmar RL, Piedra PA, Couch RB: Impact of respiratory virus infections on persons with chronic underlying conditions. JAMA. 2000, 283: 499-505. 10.1001/jama.283.4.499.View ArticlePubMedGoogle Scholar
- Welliver RC: Review of epidemiology and clinical risk factors for severe respiratory syncytial virus (RSV) infection. J Pediatr. 2003, 143: S112-S117. 10.1067/S0022-3476(03)00508-0.View ArticlePubMedGoogle Scholar
- Carlsen KH, Larsen S, Orstavik I: Acute bronchiolitis in infancy. The relationship to later recurrent obstructive airways disease. Eur J Respir Dis. 1987, 70: 86-92.PubMedGoogle Scholar
- Kotaniemi-Syrjanen A, Reijonen TM, Korhonen K, Korppi M: Wheezing requiring hospitalization in early childhood: predictive factors for asthma in a six-year follow-up. Pediatr Allergy Immunol. 2002, 13: 418-425. 10.1034/j.1399-3038.2002.02091.x.View ArticlePubMedGoogle Scholar
- Sigurs N, Gustafsson PM, Bjarnason R, Lundberg F, Schmidt S, Sigurbergsson F, Kjellman B: Severe respiratory syncytial virus bronchiolitis in infancy and asthma and allergy at age 13. Am J Respir Crit Care Med. 2005, 171: 137-141. 10.1164/rccm.200406-730OC.View ArticlePubMedGoogle Scholar
- Martinez FD: Respiratory syncytial virus bronchiolitis and the pathogenesis of childhood asthma. Pediatr Infect Dis J. 2003, 22: S76-S82. 10.1097/00006454-200302001-00011.PubMedGoogle Scholar
- Openshaw PJ, Dean GS, Culley FJ: Links between respiratory syncytial virus bronchiolitis and childhood asthma: clinical and research approaches. Pediatr Infect Dis J. 2003, 22: S58-S64. 10.1097/00006454-200302001-00009.PubMedGoogle Scholar
- Piedimonte G: Neural mechanisms of respiratory syncytial virus-induced inflammation and prevention of respiratory syncytial virus sequelae. Am J Respir Crit Care Med. 2001, 163: S18-S21.View ArticlePubMedGoogle Scholar
- Freymuth F, Vabret A, Gouarin S, Petitjean J, Campet M: [Epidemiology of respiratory virus infections]. Allerg Immunol (Paris). 2001, 33: 66-69.Google Scholar
- Dollner H, Risnes K, Radtke A, Nordbo SA: Outbreak of human metapneumovirus infection in norwegian children. Pediatr Infect Dis J. 2004, 23: 436-440.View ArticlePubMedGoogle Scholar
- Williams JV, Harris PA, Tollefson SJ, Halburnt-Rush LL, Pingsterhaus JM, Edwards KM, Wright PF, Crowe JEJ: Human metapneumovirus and lower respiratory tract disease in otherwise healthy infants and children. N Engl J Med. 2004, 350: 443-450. 10.1056/NEJMoa025472.View ArticlePubMedPubMed CentralGoogle Scholar
- Holter E, Abrahamsen TG, Rod G, Holten E: Discrepancy between results of a commercial enzyme immunoassay kit and immunofluorescence staining for detection of respiratory syncytial virus antigen. Eur J Clin Microbiol Infect Dis. 1998, 17: 595-596. 10.1007/s100960050140.View ArticlePubMedGoogle Scholar
- Wren CG, Bate BJ, Masters HB, Lauer BA: Detection of respiratory syncytial virus antigen in nasal washings by Abbott TestPack enzyme immunoassay. J Clin Microbiol. 1990, 28: 1395-1397.PubMedPubMed CentralGoogle Scholar
- Behrendt CE, Decker MD, Burch DJ, Watson PH: International variation in the management of infants hospitalized with respiratory syncytial virus. International RSV Study Group. Eur J Pediatr. 1998, 157: 215-220. 10.1007/s004310050798.View ArticlePubMedGoogle Scholar
- Eriksson M, Bennet R, Rotzen-Ostlund M, von SM, Wirgart BZ: Population-based rates of severe respiratory syncytial virus infection in children with and without risk factors, and outcome in a tertiary care setting. Acta Paediatr. 2002, 91: 593-598. 10.1080/080352502753711740.View ArticlePubMedGoogle Scholar
- Sigurs N, Bjarnason R, Sigurbergsson F, Kjellman B, Bjorksten B: Asthma and immunoglobulin E antibodies after respiratory syncytial virus bronchiolitis: a prospective cohort study with matched controls. Pediatrics. 1995, 95: 500-505.PubMedGoogle Scholar
- Ruuskanen O, Ogra PL: Respiratory syncytial virus. Curr Probl Pediatr. 1993, 23: 50-79. 10.1016/0045-9380(93)90003-U.PubMedGoogle Scholar
- SD C: The definition of acute respiratory illnesses in children. Postgrad Med J. 1973, 49: 771-776.View ArticleGoogle Scholar
- Abels S, Nadal D, Stroehle A, Bossart W: Reliable detection of respiratory syncytial virus infection in children for adequate hospital infection control management. J Clin Microbiol. 2001, 39: 3135-3139. 10.1128/JCM.39.9.3135-3139.2001.View ArticlePubMedPubMed CentralGoogle Scholar
- Fan J, Henrickson KJ, Savatski LL: Rapid simultaneous diagnosis of infections with respiratory syncytial viruses A and B, influenza viruses A and B, and human parainfluenza virus types 1, 2, and 3 by multiplex quantitative reverse transcription-polymerase chain reaction-enzyme hybridization assay (Hexaplex). Clin Infect Dis. 1998, 26: 1397-1402.View ArticlePubMedGoogle Scholar
- Hindiyeh M, Hillyard DR, Carroll KC: Evaluation of the Prodesse Hexaplex multiplex PCR assay for direct detection of seven respiratory viruses in clinical specimens. Am J Clin Pathol. 2001, 116: 218-224. 10.1309/F1R7-XD6T-RN09-1U6L.View ArticlePubMedGoogle Scholar
- Kehl SC, Henrickson KJ, Hua W, Fan J: Evaluation of the Hexaplex assay for detection of respiratory viruses in children. J Clin Microbiol. 2001, 39: 1696-1701. 10.1128/JCM.39.5.1696-1701.2001.View ArticlePubMedPubMed CentralGoogle Scholar
- Kneyber MCJ, Steyerberg EW, de GR, Moll HA: Long-term effects of respiratory syncytial virus (RSV) bronchiolitis in infants and young children: a quantitative review. Acta Paediatr. 2000, 89: 654-660. 10.1080/080352500750043945.View ArticlePubMedGoogle Scholar
- Wennergren G, Kristjansson S: Relationship between respiratory syncytial virus bronchiolitis and future obstructive airway diseases. Eur Respir J. 2001, 18: 1044-1058. 10.1183/09031936.01.00254101.View ArticlePubMedGoogle Scholar
- Garofalo RP, Hintz KH, Hill V, Patti J, Ogra PL, Welliver RCS: A comparison of epidemiologic and immunologic features of bronchiolitis caused by influenza virus and respiratory syncytial virus. J Med Virol. 2005, 75: 282-289. 10.1002/jmv.20268.View ArticlePubMedGoogle Scholar
- Semple MG, Cowell A, Dove W, Greensill J, McNamara PS, Halfhide C, Shears P, Smyth RL, Hart CA: Dual infection of infants by human metapneumovirus and human respiratory syncytial virus is strongly associated with severe bronchiolitis. J Infect Dis. 2005, 191: 382-386. 10.1086/426457.View ArticlePubMedGoogle Scholar
- Montalbano MM, Lemanske RFJ: Infections and asthma in children. Curr Opin Pediatr. 2002, 14: 334-337. 10.1097/00008480-200206000-00010.View ArticlePubMedGoogle Scholar
- Welliver RC: Respiratory syncytial virus and other respiratory viruses. Pediatr Infect Dis J. 2003, 22: S6-10. 10.1097/00006454-200302001-00002.PubMedGoogle Scholar
- Stein RT, Holberg CJ, Sherrill D, Wright AL, Morgan WJ, Taussig L, Martinez FD: Influence of parental smoking on respiratory symptoms during the first decade of life: the Tucson Children's Respiratory Study. Am J Epidemiol. 1999, 149: 1030-1037.View ArticlePubMedGoogle Scholar
- Burke W, Fesinmeyer M, Reed K, Hampson L, Carlsten C: Family history as a predictor of asthma risk. Am J Prev Med. 2003, 24: 160-169. 10.1016/S0749-3797(02)00589-5.View ArticlePubMedGoogle Scholar
- Noble V, Murray M, Webb MS, Alexander J, Swarbrick AS, Milner AD: Respiratory status and allergy nine to 10 years after acute bronchiolitis. Arch Dis Child. 1997, 76: 315-319.View ArticlePubMedPubMed CentralGoogle Scholar
- Lodrup Carlsen KC, Carlsen KH, Nafstad P, Bakketeig L: Perinatal risk factors for recurrent wheeze in early life. Pediatr Allergy Immunol. 1999, 10: 89-95. 10.1034/j.1399-3038.1999.00028.x.View ArticlePubMedGoogle Scholar
- Murray CS, Pipis SD, McArdle EC, Lowe LA, Custovic A, Woodcock A: Lung function at one month of age as a risk factor for infant respiratory symptoms in a high risk population. Thorax. 2002, 57: 388-392. 10.1136/thorax.57.5.388.View ArticlePubMedPubMed CentralGoogle Scholar
- Fjaerli HO, Farstad T, Bratlid D: Hospitalisations for respiratory syncytial virus bronchiolitis in Akershus, Norway, 1993-2000: a population-based retrospective study. BMC Pediatr. 2004, 4: 25-10.1186/1471-2431-4-25.View ArticlePubMedPubMed CentralGoogle Scholar
- The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1471-2431/5/31/prepub
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.