The association of allergic symptoms with sensitization to inhalant allergens in childhood

Although it is generally agreed that sensitization is an important risk factor for allergic diseases, the extent to which sensitization accounts for allergic symptoms in children is controversial. As part of the Aalst Allergy Study, this cross‐sectional study investigated the prevalence of allergic symptoms and their association with sensitization in an unselected population of Flemish children aged 3.4–14.8 yr. Skin prick testing with the most common aeroallergens was performed and allergic symptoms were documented by a parental questionnaire. In the children older than 6 yr, a significant association of current wheezing, current dyspnea, airway hyperreactivity, rhinoconjunctivitis, and current eczema with sensitization was found, while in the pre‐school children these associations were less pronounced. The association with sensitization was strongest for rhinoconjunctivitis and current respiratory symptoms – the association was less striking for children with current eczema. The impact of a positive family history of allergy on the association with sensitization was more important for eczema than for the other analyzed allergic symptoms. Persistent and late‐onset wheezers were significantly more likely than non‐wheezers and transient early wheezers to be associated with sensitization and a personal history of rhinoconjunctivitis. Late‐onset wheezing was associated with a positive family history of allergy, while transient early wheezing was associated with day‐care attendance. An association with eczema was found for all three childhood wheezing phenotypes. The association of allergic symptoms with sensitization is significant in the older but less pronounced in pre‐school children and is more pronounced for current allergic symptoms. Diagnosis and disease definition of allergy symptoms remains difficult at pre‐school age. The influence of a positive family history of allergy on the association of the respective allergic symptoms with sensitization was most important for eczema. Our data confirm the atopic characteristics of the different wheeze phenotypes.     

Breastfeeding and the risk of hospitalization for respiratory disease in infancy: a meta-analysis

OBJECTIVE: To examine breastfeeding and the risk of hospitalization for lower respiratory tract disease in healthy full-term infants with access to modern medical care. DATA SOURCES: MEDLINE, personal communication with researchers, the OVID databases, Dissertation Abstracts Online, and BIOSIS. STUDY SELECTION: The titles, abstracts, and text of studies from developed countries were explored for breastfeeding exposure measures and lower respiratory tract disease hospitalization rates. For summary statistics, we required 3 inclusion criteria: (1) a feeding contrast of a minimum of 2 months of exclusive breastfeeding (no formula supplementation) vs no breastfeeding and (2) study populations that excluded sick, low birth weight or premature infants and (3) reflected affluent regions; 27% of studies met these criteria. DATA EXTRACTION: We abstracted data from all relevant reports. DATA SYNTHESIS: Data from all primary material (33 studies) indicated a protective association between breastfeeding and the risk of respiratory disease hospitalization. Nine studies met all inclusion criteria, and 7 cohort studies were pooled. The feeding contrasts in these 7 studies were 4 or more months of exclusive breastfeeding vs no breastfeeding. The summary relative risk (95% confidence interval) was 0.28 (0.14-0.54), using a random-effects model. This effect remained stable and statistically significant after adjusting for the effects of smoking or socioeconomic status. CONCLUSION: Among generally healthy infants in developed nations, more than a tripling in severe respiratory tract illnesses resulting in hospitalizations was noted for infants who were not breastfed compared with those who were exclusively breastfed for 4 months.     

Does breast feeding help protect against atopic disease? Biology, methodology, and a golden jubilee of controversy

To help shed some light on the 50-year-old controversy concerning the possible protective effect of breast feeding on subsequent atopic disease, I developed 12 standards pertaining to both biologic and methodologic aspects of exposure (infant feeding), outcome (atopic conditions), and statistical analysis for studies of atopic eczema, asthma, allergic rhinitis, cow milk allergy, and other food allergy. Among the published studies on atopic eczema, the nine claiming a protective benefit of breast feeding performed less well than the 12 not making such a claim on "methodologic" standards relating to strict diagnostic criteria and blind ascertainment of outcome. The positive studies were somewhat stronger, however, on the "biologic" standards bearing on sufficient duration and exclusivity of breast feeding and on separate analysis of children at high risk. For the other atopic conditions, there were no important differences between positive and negative studies. In few negative or positive studies was there adequate control for confounding variables or examination of potential benefits relating to the severity or age at onset of atopic disease. To avoid another 50 years of unresolved controversy, future studies should improve both the biologic and methodologic aspects of their design and analysis.     

Association of autistic spectrum disorder and the measles,mumps, and rubella vaccine: a systematic review of current epidemiological evidence

OBJECTIVE: To systematically review the evidence for and against the existence of an association between autistic spectrum disorder (ASD) and the measles, mumps, and rubella (MMR) vaccine.Study DESIGN: We conducted a systematic review of the medical literature to identify all controlled epidemiological articles examining for an association between ASD and the MMR vaccine. We extracted data from the articles on the characteristics and objectives of the study as well as evidence of an association. RESULTS: Twelve articles met the inclusion criteria. One study found no difference in the rates of ASD and the MMR vaccine in children who were vaccinated and those who were not. Six studies examined for evidence of an increase in ASD associated with an increase in the MMR vaccine coverage, none of which showed evidence of an association. Four studies examined if a variant form of ASD was associated with the MMR vaccine, none of which showed evidence of an association. Eight studies attempted to determine if there was a temporal association between developing ASD and receiving the MMR vaccine. Of these, 1 study identified an increase in parental concern in the 6-month period following vaccination with MMR in one of its analyses. The results of all other studies showed no association between ASD and the MMR vaccine. CONCLUSIONS: The current literature does not suggest an association between ASD and the MMR vaccine; however, limited epidemiological evidence exists to rule out a link between a rare variant form of ASD and the MMR vaccine. Given the real risks of not vaccinating and that the risks and existence of variant ASD remain theoretical, current policies should continue to advocate the use of the MMR vaccine.     

Tuberculosis, bacillus Calmette-Guérin vaccination, and allergic disease: findings from the International Study of Asthma and Allergies in Childhood Phase Two

Some have suggested a protective effect of tuberculosis (TB) infection on allergic disease risk, but few studies have examined the association between the two. We therefore investigated whether TB disease and bacillus Calmette-Guérin (BCG) vaccination in early life protect against allergic disease. Information on allergic disease symptoms, past TB disease, and BCG vaccination as well as potential confounding factors was gathered by parental questionnaire from a randomly selected subset of 23,901 8- to 12-yr-old schoolchildren in 20 centers in both developed and developing countries. Children were also physically examined for flexural eczema and underwent skin prick testing. Pooled odds ratio (OR) estimates and corresponding 95% confidence intervals (CIs) across study centers were calculated, using random effects meta-analysis models. There were 245 (1.0%) reported cases of TB disease, and 66.3% (15,857) of all children received the BCG vaccine. Asthma, hay fever, and flexural eczema symptoms in the past year as well as flexural eczema on skin examination were all positively linked to a history of TB (adjusted pooled OR 'wheeze in the past year' = 2.27, 95% CI 1.52-3.41; adjusted pooled OR 'hay fever symptoms in the past year' = 2.23, 1.22-4.09; adjusted pooled OR 'flexural eczema symptoms in the past year' = 3.21, 2.01-5.12; adjusted pooled OR 'flexural eczema on skin examination' = 4.04, 1.71-9.56). Even higher risk estimates were seen for severe asthma and eczema symptoms [adjusted OR = 4.02 (2.17-7.47) and adjusted OR = 6.31 (2.19-18.17), respectively]. There was no significant association between past TB and skin prick test positivity (adjusted pooled OR = 1.32, 0.87-2.02). BCG vaccination during the first year of life was also not associated with any of the allergy outcomes. We found a uniform positive association between TB and all allergic disease outcomes, including eczema on skin examination. As this was a cross-sectional study, it is unclear whether this positive association is attributable to a causal relationship, and further longitudinal studies are required.     

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??There is strong evidence implicating the intestinal flora disturbance in developing allergicdisease including allergic rhinoconjunctivitis??asthma??eczema and food allergy. Experimental studies with animal model of allergy have shown that probiotics can improve allergic manifestations by induceing immune regulation Treg cell??inhibiting the secretion of allergen-induced IgE and Th2 cytokines??attenuating eosinophils infiltration and allergic inflammation in target organs. Probiotics have been proved effective in treatment of IgE-mediated eczema;however??because of the conflicting results??probiotics are recommended for prevention of allergy only in populations at high risk of allergy.     

A systematic review of the role of hydrolyzed infant formulas in allergy prevention

OBJECTIVE: To critically examine the published literature to determine whether feeding hydrolyzed infant formulas from birth has a role in allergy prevention. DATA SOURCES: We identified data through a MEDLINE search using allergy prevention and infant formulas as indexing terms. The search was restricted to 1985 through the present, English-language articles, and human subjects. STUDY SELECTION: Criteria for inclusion in the review were prospective controlled trials published in peer-reviewed journals. DATA EXTRACTION: Symptoms of allergy were defined and observed by health care providers (physicians and nurses). DATA SYNTHESIS: Nine published trials evaluated the use of extensively hydrolyzed formulas, 12 evaluated the use of partially hydrolyzed formulas in high-risk infants, and 1 evaluated the use of partially hydrolyzed formulas in an unselected infant population. The reports compared hydrolyzed formulas with breastfeeding, cow's milk formulas, soy formulas, and combinations thereof. The cohort of studies consistently showed reductions in the cumulative incidence of atopic disease from 12 to 60 months of age among high-risk infants fed extensively hydrolyzed casein formulas or partially hydrolyzed whey formulas vs cow's milk formulas. No studies showed an increase in allergy risk with any hydrolyzed formulas. CONCLUSIONS: Extensively hydrolyzed casein formulas and partially hydrolyzed whey formulas are appropriate alternatives to breast milk for allergy prevention in infants at risk. Because atopic disease in children cannot be predicted, the use of these formulas in the general population should be considered, and one must weigh cost, compliance, and long-term benefits.     

Comparison of fecal microflora in children with atopic eczema/dermatitis syndrome according to IgE sensitization to food

Atopic eczema/dermatitis syndrome (AEDS) commonly often arises during early infancy. In several intervention studies a beneficial influence on AEDS course of certain intestinal bacteria, administered as 'probiotics', has been described. To evaluate the possible role of the natural intestinal microflora in children with allergic eczema/dermatitis syndrome regarding immediate type hypersensitivity to food allergens, children with food allergy (AAEDS, n = 68) have been compared with children without detectable food allergy (NAEDS, n = 25). All children (n = 93) in preschool age, mean age of 2.6 (+/-1.8) years, diagnosed with AEDS who were treated as inpatients in 2003 in a dermatological hospital were included. The correlation between fecal microflora, parasites and specific immunoglobulin E (IgE) antibodies against common food allergens was analyzed. A similar composition of intestinal microflora in children with AAEDS and NAAEDS was found. The food allergens that were most frequently detected were egg white, cow milk, casein, peanut and hazelnut. Furthermore, a significant association between IgE sensitization against important food allergens and components of the fecal microflora could not be demonstrated. With aging changes occur in the intestinal microbiota [Proteus/Klebsiella and age (rho = -0.607) and Enterococcus and age (rho = -0.428)]. In two subjects of the AAEDS group Blastocystis hominis was found. The composition of natural intestinal microflora in children with AAEDS and NAAEDS was similar. Hence, there is no evidence of a role of the intestinal microflora with regard to the development of infant (food) allergy in children with AEDS. The possible consequences for allergic diseases later in life require further investigation.     

What if it is the other way around? Early introduction of peanut and fish seems to be better than avoidance

For many years, the advice to prevent food allergy was to postpone the introduction of allergens like egg, fish and peanut. However, elimination of food allergens during pregnancy and infancy failed to prevent food allergy. Instead, several studies indicate that early introduction of food like fish and peanuts may be beneficial. The most compelling illustration of this has been presented for peanuts. The prevalence of peanut allergy is lower in children in Israel than in the UK, despite introduction of peanut during infancy in Israel. Other studies have reported that early introduction of fish reduced the risk of allergic sensitization and allergic diseases like eczema.
Conclusion:  Early introduction rather than avoidance may be a better strategy for the prevention of food allergy. The mechanism may be that early introduction of food allergens during infancy might induce tolerance, thereby preventing the development of allergy.     

Cross-sectional study of allergic disorders associated with breastfeeding in Japan: The Ryukyus Child Health Study

Uncertainties remain as to whether breastfeeding is protective against childhood allergic disorders. Positive relationships of breastfeeding with asthma and atopic eczema were observed in two previous Japanese studies. This cross-sectional study investigated the association between the feeding pattern after birth and the prevalence of allergic disorders during the past 12 months in Japanese schoolchildren. Study subjects were 24,077 children aged 6-15 yr in Okinawa. The outcomes were based on diagnostic criteria from the International Study of Asthma and Allergies in Childhood. Allowance was made for age, sex, number of siblings, smoking in the household, paternal and maternal history of asthma, atopic eczema, and allergic rhinitis, and paternal and maternal educational level. Breastfeeding, regardless of exclusivity, for 13 months or longer and exclusive breastfeeding for 4-11 months were independently associated with a higher prevalence of atopic eczema, particularly among children without a parental allergic history. A clear positive dose-response relationship was observed between prolonged duration of breastfeeding, regardless of exclusivity, but not exclusive breastfeeding, and the prevalence of atopic eczema. We found a significant positive trend for atopic eczema across the three categories (formula milk, partial and exclusive breastfeeding) in the first 4 months of life although the odds ratio for exclusive breastfeeding was not statistically significant. No material association was found between the feeding pattern after birth and the prevalence of wheeze or allergic rhinoconjunctivitis. Prolonged breastfeeding may be associated with a higher prevalence of atopic eczema in Japanese children.     

Food allergy as seen by an allergist

The clinical expression of allergic disease is the consequence of a series of complex gene-environment interactions that occur at the materno-fetal interface and throughout infancy, leading to persistence of the Th2 immune response. It has been proposed that atopic eczema is the cutaneous manifestation of a systemic disorder that also gives rise to asthma, food allergy, and allergic rhinitis. The recent emergence of genes regulating epidermal barrier function has raised the question of whether the skin barrier in atopic eczema is defective from the outset, rendering the epidermis "leaky," thereby increasing the risk of allergen penetration and the succeeding inflammatory reaction that contributes to atopic eczema. Food allergic sensitisation and eczema frequently coexist during the first 2 years of life, and food allergy is more prevalent in infants and children with moderate to severe eczematous inflammation. The majority of food allergic reactions are caused by 8 foods, with milk, egg, and peanut occurring with greatest frequency. The acquisition of food-specific tolerance occurs predominantly with foods in which the epitopes are grouped together in a conformational structure (milk, egg, wheat, soy), whilst it rarely occurs in patients allergic to foods in which the epitopes are arranged in a linear fashion (nuts, seeds, fish). Better tests and novel therapies, such as immunotherapy and oral tolerance induction, are required for the management of food allergy.     

Dietary prevention of allergic diseases in infants and small children.

Because of scientific fraud four trials have been excluded from the original Cochrane meta-analysis on formulas containing hydrolyzed protein for prevention of allergy and food intolerance in infants. Unlike the conclusions of the revised Cochrane review the export group set up by the Section on Paediatrics, European Academy of Allergology and Clinical Immunology (SP-EAACI) do not find that the exclusion of the four trials demands a change of the previous recommendations regarding primary dietary prevention of allergic diseases. Ideally, recommendations on primary dietary prevention should be based only on the results of randomized and quasi-randomized trials (selection criteria in the Cochrane review). However, regarding breastfeeding randomization is unethical, Therefore, in the development of recommendations on dietary primary prevention, high-quality systematic reviews of high-quality cohort studies should be included in the evidence base. The study type combined with assessment of the methodological quality determines the level of evidence. In view of some methodological concerns in the Cochrane meta-analysis, particularly regarding definitions and diagnostic criteria for outcome measures and inclusion of non peer-reviewed studies/reports, a revision of the Cochrane analysis may seem warranted. Based on analysis of published peer-reviewed observational and interventional studies the results still indicate that breastfeeding is highly recommended for all infants irrespective of atopic heredity. A dietary regimen is effective in the prevention of allergic diseases in high-risk infants, particularly in early infancy regarding food allergy and eczema. The most effective dietary regimen is exclusively breastfeeding for at least 4-6 months or, in absence of breast milk, formulas with documented reduced allergenicity for at least the first 4 months, combined with avoidance of solid food and cow's milk for the first 4 months.     

The association between a genetic risk score for allergy and the risk of developing allergies in childhood—Results of the WHISTLER cohort

Background

Several genetic variants have been associated with the susceptibility to allergic disease in adults, but it remains unclear whether these genetic variants are also associated with the onset of allergic disease early in life. The aim of this study was to develop a genetic risk score (GRS) for allergy based on findings in adults and study its predictive capacity for allergy in children.

Methods

A GRS was constructed based on 10 SNPs previously associated with allergies in adults. The GRS was tested in children who participated in a population‐based newborn cohort (WHISTLER) and were followed from birth to school age. Logistic regression analysis was used to study the association between the GRS and the parental‐reported allergies at age 5 (based on a reported allergy to ≥1 of the following allergens: pollen, house dust mites, or pets). A Cox regression model was used to study the association between GRS and a physician‐diagnosed allergy during follow‐up (allergic conjunctivitis, allergic rhinitis, and eczema/dermatitis). Cohen's kappa coefficient was calculated to study the agreement between physician‐diagnosed allergy and parental‐reported allergy at age 5.

Results

The GRS was significantly associated with parental‐reported allergy (odds ratio: 15.9, 95% confidence interval (CI): 1.07‐233.73) at age 5, as well as with a physician‐diagnosed allergy during follow‐up (hazard ratio: 1.89, 95% CI: 1.05‐3.41). The overall agreement between physician‐diagnosed and parental‐reported allergies was 70.5% (kappa: 0.10, 95% CI: 0.03‐0.18).

Conclusions

An adult‐derived GRS for allergy predicts the risk of developing allergies in childhood.     

Allergic disease in the first year of life is associated with differences in subsequent neurodevelopment and behaviour

Background

Recent trials suggest a link between neuropsychological function, atopy and allergic disease particularly in early childhood; however the nature of this association remains unclear.

Aims

To investigate the relationship between early allergic disease and sensitisation at 12 months of age and neurodevelopmental outcomes at 18 months.

Study design

Linear or binary logistic regression analysis was used to determine whether allergic diseases or sensitization at 12 months of age was a significant predictor of neurodevelopmental test scores at the 18 months.

Subjects

Infants with a maternal history of allergic disease (n = 324).

Outcome measures

Allergic outcomes at 12 months of age included allergen sensitisation, eczema, IgE-mediated and food allergy, and neurodevelopmental outcomes at 18 included the Bayley Scales of Infant Toddler Development III Edition, the Achenbach Child Behaviour Checklist and the Macarthur Scales of Infant Toddler Development.

Results

Children with any diagnosed allergic disease at 12 months had evidence of reduced motor scores (p = .016), and this was most apparent for a diagnosis of eczema (p = .007). Non-IgE mediated food allergy was significantly positively associated with problem Internalising Behaviours (p = .010), along with a trend for effects on the Social–Emotional composite score for IgE-Mediated food allergies (p = .052). Allergic sensitisation was not independently associated with any effects on neurodevelopmental outcomes.

Conclusion

This study provides evidence that an allergic phenotype in infancy is associated with effects on neurodevelopment. Further research is required to investigate the nature of this relationship.     

A matched patient-sibling study on the usage of paracetamol and the subsequent development of allergy and asthma

A number of studies have suggested that intake of paracetamol during pregnancy and during the first months of life is associated with an increased risk of childhood asthma. We aimed to determine the association between paracetamol usage during pregnancy and the first 6 months of life, and childhood allergy (i.e. positive skin prick tests), allergic asthma, and asthma, using a matched patient-sibling study comparing patients with allergic asthma with their healthy siblings without any symptoms of allergic diseases. Allergy in patients and their siblings was determined by skin prick tests. Children having at least one positive skin prick test were considered to be allergic. Intake of paracetamol was assessed by standardized, interviewer-administered, questionnaire. Nineteen pairs of allergic asthma patients vs. non-allergic siblings were compared to determine the risk factors for allergic asthma, while 15 pairs of allergic asthma patients vs. allergic siblings were compared to determine the risk factors for asthma. Moreover, 33 pairs of allergic asthma patients vs. non-asthmatic siblings (with and without allergy) were compared to determine the risk factors for asthma. In addition, 17 allergic siblings (without asthma) were compared with 19 non-allergic siblings (without asthma) to determine the risk factors for allergy. Usage of paracetamol during pregnancy was associated with allergic asthma (p = 0.03). Furthermore, usage of paracetamol between birth and 6 months of age, and between 4 and 6 months of age, was also found to be associated with non-allergic asthma (p = 0.008 and p = 0.03 respectively). Usage of paracetamol during pregnancy and during the early months of life may play a role in the development of allergic and non-allergic asthma in children. However, due to obvious ethical reasons, direct evidence for this association (i.e. a double-blind, prospective study) is not available.     

Does breastfeeding protect against allergic rhinitis during childhood? A meta‐analysis of prospective studies

The effect of breastfeeding on the development of allergic rhinitis and other atopic conditions has been assessed in many studies but remains controversial. To elucidate this issue, a systematic review was conducted of prospective studies that evaluated the association between exclusive breastfeeding during the first 3 mo after birth and allergic rhinitis. The 1966-2000 MEDLINE databases were searched and the reference lists of relevant articles were reviewed according to predetermined inclusion criteria. The methodological aspects of each study, duration and exclusivity of breastfeeding, outcome measures, control for potential confounding variables and other factors were assessed, and estimates of the association between breastfeeding and allergic rhinitis were abstracted independently by the investigators using a standardized approach. Six prospective studies met the inclusion criteria. The summary odds ratio for the protective effect of breastfeeding was 0.74 (95% confidence interval 0.54-1.01). The effect estimate in studies of children with a family history of atopy was 0.87 (95% confidence interval 0.48-1.58).

Conclusion : Exclusive breastfeeding during the first 3 mo after birth protects against allergic rhinitis in children, both with and without a family history of atopy. The protective association, although of borderline statistical significance, was substantial. Larger prospective studies with strict methodology and longer periods of follow-up are needed.     

Food allergy in children suffering from atopic eczema

Whilst the association between eczema and food allergy is well established, the role of dietary manipulation in children with eczema remains controversial. These case histories highlight the differing outcomes that dietary manipulation may have in an infant with early onset, severe eczema and an older child with milder eczema. Management strategies and the evidence to support them are presented, followed by a review of clinical recommendations.     

Effect of breast feeding on risk of coeliac disease: a systematic review and meta-analysis of observational studies

Background

Coeliac disease (CD) is a disorder that may depend on genetic, immunological, and environmental factors. Recent observational studies suggest that breast feeding may prevent the development of CD.

Aim

To evaluate articles that compared effects of breast feeding on risk of CD.

Methods

Systematic review and meta‐analysis of observational studies published between 1966 and June 2004 that examined the association between breast feeding and the development of CD.

Results

Six case‐control studies met the inclusion criteria. With the exception of one small study, all the included studies found an association between increasing duration of breast feeding and decreased risk of developing CD. Meta‐analysis showed that the risk of CD was significantly reduced in infants who were breast feeding at the time of gluten introduction (pooled odds ratio 0.48, 95% CI 0.40 to 0.59) compared with infants who were not breast feeding during this period.

Conclusions

Breast feeding may offer protection against the development of CD. Breast feeding during the introduction of dietary gluten, and increasing duration of breast feeding were associated with reduced risk of developing CD. It is, however, not clear from the primary studies whether breast feeding delays the onset of symptoms or provides a permanent protection against the disease. Long term prospective cohort studies are required to investigate further the relation between breast feeding and CD.     

Early introduction of allergenic foods for the prevention of food allergy from an Asian perspective—An Asia Pacific Association of Pediatric Allergy,Respirology & Immunology (APAPARI) consensus statement

Emerging evidence for the early introduction of allergenic foods for the prevention of food allergies, such as peanut allergy in Western populations, has led to the recent publication of guidelines in the USA and Europe recommending early peanut introduction for high‐risk infants with severe eczema or egg allergy. Peanut allergy is, however, much less prevalent in Asia compared to the West. Varying patterns of food allergy are seen even within Asian countries—such as a predominance of wheat allergy in Japan and Thailand and shellfish allergy in Singapore and the Philippines. Customs and traditions, such as diet and infant feeding practices, also differ between Asian populations. Hence, there are unique challenges in adapting guidelines on early allergenic food introduction to the Asian setting. In this paper, we review the evidence and discuss the possible approaches to guide the timely introduction of allergenic food in high‐risk infants in Asia.     

Benefits and harms associated with the practice of bed sharing: a systematic review

OBJECTIVE: To examine evidence of benefits and harms to children associated with bed sharing, factors (eg, smoking) altering bed sharing risk, and effective strategies for reducing harms associated with bed sharing. DATA SOURCES: MEDLINE, CINAHL, Healthstar, PsycINFO, the Cochrane Library, Turning Research Into Practice, and Allied and Alternative Medicine databases between January 1993 and January 2005. STUDY SELECTION: Published, English-language records investigating the practice of bed sharing (defined as a child sharing a sleep surface with another individual) and associated benefits and harms in children 0 to 2 years of age. DATA EXTRACTION: Any reported benefits or harms (risk factors) associated with the practice of bed sharing. DATA SYNTHESIS: Forty observational studies met our inclusion criteria. Evidence consistently suggests that there may be an association between bed sharing and sudden infant death syndrome (SIDS) among smokers (however defined), but the evidence is not as consistent among nonsmokers. This does not mean that no association between bed sharing and SIDS exists among nonsmokers, but that existing data do not convincingly establish such an association. Data also suggest that bed sharing may be more strongly associated with SIDS in younger infants. A positive association between bed sharing and breastfeeding was identified. Current data could not establish causality. It is possible that women who are most likely to practice prolonged breastfeeding also prefer to bed share. CONCLUSION: Well-designed, hypothesis-driven prospective cohort studies are warranted to improve our understanding of the mechanisms underlying the relationship between bed sharing, its benefits, and its harms.