Parent-reported mild head injury history and behavioural performance in children at 6 years

Abstract

Objective

Mild head and brain injuries have gained increasing attention from health professionals and researchers. Little is known about mild injuries, which may not always be diagnosed or brought to medical attention. This study examines the associations between parent-reported history of mild head injury and behavioural problems in a large community-based sample of Chinese children.

Method

Subjects included 725 children from China. Parents reported on children’s head injury history and behaviour at age 6 years using the Chinese version of the Child Behavior Checklist. Mild head injury was defined as injury without loss of consciousness or hospitalization. Standardized T-scores were compared for each of the seven clinical sub-scales and three summary behavioural measures, adjusting for confounders. Logistic models were used to calculate odds ratios (ORs) between head injury and behavioural problems.

Results

Parents reported that 97 children (14%) had a single injury and 70 (10%) had multiple injuries. Compared to the controls, head-injured children had worse behavioural outcomes and a higher prevalence of behavioural problems. Multiple injuries were associated with higher risk of certain internalizing and externalizing problems.

Conclusion

Mild head injuries, especially incurred repeatedly, may still be a significant risk for adverse behaviours in children.

Introduction

Traumatic brain injury (TBI) is a leading cause of death and disability among children and adolescences worldwide [1]. According to the World Health Organization, 70–90% treated brain injuries are mild, with the estimated incidence of mild traumatic brain injury (MTBI), commonly known as concussion, being greater than 600/100 000 [1]. MTBI is reported to account for ~80% of all children treated in Eastern China hospitals for TBI [2]. However, adverse health outcomes such as social interaction deficits [3], fatigue [4] and anxiety disorders [5] are associated even with mild head injuries. Therefore, further understanding the consequences of MTBI may be an important step for future prevention.

Paediatric MTBI is an increasing concern due to its potential short- and long-term impacts to health [6-8]. Outcomes associated with MTBI in childhood and adolescences may include cognitive and behavioural deficits. Recently, McKinlay [8] demonstrated that, compared to uninjured peers, children sustaining pre-school MTBI exhibited significantly more behavioural problems (e.g. attention, aggression) later in childhood. Notably, even within the study’s MTBI population, a significant difference between behavioural outcomes existed between hospitalized and outpatient cases, thus highlighting an important gradient even within this generally-accepted category of severity. Indeed, mild head injuries cover a large clinical spectrum and concussion may occur without loss of consciousness or external signs of head trauma. According to the Centers for Disease Control and Prevention, a case of MTBI is defined as resulting from blunt trauma or acceleration or deceleration forces, with at least one of the following results from head injury: transient confusion, disorientation, memory dysfunction around the injury event, impaired or loss of consciousness (<30 minutes) or observed signs of neurological or neuropsychological dysfunction, including seizures, irritability, omitting and dizziness [9].

The evidence regarding behavioural and psychiatric outcomes among children experiencing MTBI is mixed [7]. One study, for example found increased incidence of behavioural problems, which extended into adolescence, in paediatric inpatient MTBI subjects, when compared to their outpatient MTBI counterparts and controls [10], while other studies have shown little to no difference between MTBI and healthy controls [11]. Given these mixed results, more research is warranted. In particular, most studies examining MTBI outcomes are, due to recruitment procedures, limited to hospital admissions or emergency department (ED) visits [12].

An unknown number of cases may not be diagnosed or brought to medical attention, especially those on the lower end of the severity gradient, and a recent study, in which the researchers conducted prospective interviews prior to looking at the medical charts of subjects admitted to the ED for possible head injuries, found that over half of MTBI cases identified by researchers were absent from ED medical records [13]. It is, thus, of interest to examine the outcomes of mild injuries which may be overlooked in the ED. Furthermore, the existing literature on behavioural outcomes of childhood TBI is based on evidence from only a handful of cohorts, none of which are from non-Western countries [7]. In the present study, the authors aim to contribute to the literature by examining the associations between parent-reported early childhood mild head injuries (characterized by no loss of consciousness or hospitalization) and behavioural problems in a community-based cohort of Chinese children.

Methods

Participants

The current study was part of a larger population-based community cohort study of 1656 Chinese children (55.5% boys, 44.5% girls) initially recruited between Fall 2004 and Spring o 2005 from Jintan, located in the southeastern coastal region of Mainland China. Participants were drawn from four pre-schools chosen to represent the entire city’s geographical, social and economic profiles. Detailed sampling and research procedures of this cohort study have been described elsewhere [14]. Briefly, the China Jintan Child Cohort Study is an on-going prospective longitudinal study with the main aim of assessing the early health risk factors for the development of child neurobehavioural outcomes. From the original cohort, data for this current study was available for 777 children. The remaining data was not available for various reasons (e.g. children moved to other schools or did not respond or refused to participate). There were no significant differences between children for whom data was available and for whom data was not available.

In children’s last year of kindergarten, parents were asked to report on the history of head injury in their children. Parents also rated child behavioural outcomes with the Chinese version of the Child Behavior Checklist [15, 16] and answered a series of questions regarding child nutrition, behaviour and parental and family characteristics. Parents were not asked to report on their income since people in China still considered this to be private information [17]. Written informed consent was obtained from parents. Institutional Review Board approval was obtained from both the University of Pennsylvania and the ethical committee for research at Jintan Hospital in China.

Measures

History and operational definition of mild head injury

In the last year of kindergarten, when children were 6 years old, parents reported whether their child had a history of head injury (‘has your child had head injury before?’, ‘how many injuries did he/she have?’), mechanism of injury (slips and falls, traffic, exercise and fight), age at injury, hospitalization history and severity of injury (loss of consciousness). Questions were asked in Chinese. Children’s head injury histories were categorized as no head injury, single head injury and multiple head injuries. Injury mechanism was not included in analysis due to low frequencies.

Due to the unavailability of valid medical data, this study could not define head injury using medical diagnoses. Thus, for the purposes of this study, mild head injury was defined as no loss of consciousness and/or no hospitalization for treatment due to injury.

Child behaviour problems

Parents were asked to rate children’s behaviour with the Chinese version of the Child Behavior Checklist (CBCL) [15, 16, 18] at the same time head injury history was collected when children were 6 years old. The CBCL is one of the most widely used rating scales for assessing behavioural and emotional problems in children and consists of 99 items dealing with a child’s behaviour within the past 12 months. Items are rated on a 3-point scale (0 = Not True, 1 = Sometimes True or 2 = Often True) [15]. Factor analysis performed on the CBCL has separately produced two broad bands of factors: internalizing behaviour and externalizing behaviours, which contains seven clinical sub-scale syndromes anxiety/depression, emotional reaction, withdrawn, somatic complaints, sleep problems, attention problems and aggression and two summary syndromes internalizing behaviour problems (the sum of emotional reaction, anxiety/depression, withdrawn and somatic complaints) and externalizing behaviour problems (the sum of attention problems and aggression). The total problems score is the sum score of the all problem items. Standardized T-scores were calculated from raw scores. A child was diagnosed as having a clinical sub-scale syndrome when T-score equals or is greater than 65 (93rd percentile of Chinese norm group) or having a summary syndrome when T-score equals or is greater than 60 (84rd percentile of Chinese norm group) [15, 16].

Statistical analysis

Because this study was focused only on mild head injuries, after excluding 52 children who were reported to have loss of consciousness or hospitalization, a total of 725 children were included in this analysis. Baseline characteristics of parents and children were compared between children with and without head injury histories, using chi-square test for categorical variables and Wilcoxon rank sum test for blood lead levels. Standardized T-scores for each syndrome were compared among three groups (no head injury, single injury and multiple injuries) using one-way variance analysis. The associations between head injury history and behaviour problems were examined using Logistic regression models. Odds ratios (ORs) and 95% confidence intervals (95% CI) were calculated for children with one or multiple head injuries (no head injury as reference). All analyses were conducted in SAS 9.2 (SAS Institute, Cary, NC). Potential confounders were controlled in mean comparisons and Logistic regression models. Potential confounders were selected, a priori, and include characteristics of children (sex, age at time of interview, pre-school area) and parents (father’s education, father’s occupation, whether father smoked after the child was born, mother’s age when the child was born, mother’s education, mother’s occupation, parental psychopathologic problems and marital status).

Results

Sample characteristics

Of 725 children, 97 (14%) had a single injury and 70 (10%) had multiple injuries. Table I compares parental and children’s characteristics of children with and without head injury histories. While there were no differences in children’s characteristics, parental characteristics are correlated with the history of children’s injury. Father’s education was correlated with the probability of head injury, with children of least educated fathers having the highest total rates of head injury (single and multiple injuries). Mother’s characteristics were also likely to relate to child’s head injury history. Mothers who were older, more educated or unemployed (or less skill required jobs) were more likely to have children with a history of head injury.

Association of mild head injury and behaviour outcomes

Standardized T-scores consistently increased with the frequency of head injury across all clinical sub-scales and summary scales, as shown in Table II. More specifically, compared to children without any injury, those with a single injury had statistically significant higher T-scores (worse outcome) for withdrawn behaviour, but not other behaviours. After adjusting for covariates, those with multiple injuries had statistically significant higher T-scores for several scales (emotionally reactive, withdrawn and aggressive problems) and all summary scales (internalizing, externalizing, total problems).

When T-scores were transformed into binary outcomes for behavioural problems (yes vs no) as described above, children with head injury histories tended to have higher prevalence rates of all clinical sub-scales and summary scales (Table III); these rates were even higher among children with multiple head injuries than those with single injury (except attention problems). In unadjusted Logistic models, ORs for children with single injury were not statistically significant for any behaviours except withdrawn, but multiple injuries history was associated with all clinical sub-scales and summary scales except sleep and attention problems (Table IV). The association between single injury and withdrawn behaviour disappeared after taking confounders into account. The associations between multiple injuries and behavioural problems, however, remained, with the exception of somatic complaints. Adjusted ORs (95% CI) were 8.04 (3.18–20.33) for emotionally reactive, 2.24 (1.13–4.46) for anxious/depressed, 2.76 (1.20–6.34) for withdrawn, 6.03 (2.36–15.37) for aggressive behaviours, 2.76 (1.35–5.68) for internalizing problems, 4.03 (1.97–8.25) for externalizing problems and 3.42 (1.66–7.03) for total problems.

Discussion

There has been increased interest in studying the short- and long-term outcomes of MTBI such as neurobehavioural problems among children and adolescences [7, 8]. This study focused on behavioural problems associated with mild head injuries, which were defined as parent-reported head trauma without loss of consciousness or hospitalization of treatment. The findings demonstrate that multiple, but not single, head injuries are associated with higher risks of adverse behaviours in children, including both internalizing (emotional reaction, anxious/depressed and withdrawn) and externalizing (aggressive behaviour) problems.

Internalizing behavioural problems are commonly reported after TBI [7]. In the present study, three (emotionally reactive, anxious/depressed and withdrawn) of the four internalizing behavioural problems measured by CBCL were associated with a history of multiple, mild head injuries. Novel depressive disorders emerging after TBI appear to occur in 10–25% of children up to 2 years following TBI [19-21]. This problem is more apparent in socioeconomically disadvantaged children [22]. Due to some similar pathophysiology, the presentation of novel depression and anxiety disorders often co-exist [20]. Withdrawal, as measured by the CBCL, has also been reported to occur in 12–13% of children with TBI [23]. Furthermore, compared to uninjured peers, 6–11 year old children with a history of moderate–severe TBI have been reported to show greater lability/negativity and poorer emotional self-regulation [24]. These children have less emotional awareness, empathy and situationally appropriate affect and greater poorly-regulated negative affect (e.g. mood swings, socially inappropriate emotional expression).

Several sub-types of externalizing behavioural problems are also reported to be associated with a history of TBI [7]. Notably, new-onset attention problems are common after TBI [19, 25-27] and they have also been reported in children with MTBI [8]. However, no association between a history of head injury and attention problems was found in the present study. While these findings may reflect an actual lack of association due to the mildness of injury, they may also be related to attention deficit assessment and age of assessment [7]. Observational measures may not readily identify a lack of age-appropriate behavioural functioning in younger children [28]. Indeed, in children sustaining MTBI in pre-school, attention problems more noticeably emerged in middle-to-late childhood, with parent-rated impairments peaking ~11–12 years of age [8]. Geraldina et al. [29] further noted that types of post-TBI behavioural outcomes are age-dependent, with the onset of externalizing problems and social relations disorders occurring in school-age children. Thus, longer follow-up may be necessary to detect significant attention problems in the study sample.

In contrast to the findings on attention problems, this study did find the second domain of externalizing behaviours measured by CBCL, aggressive behaviour, to be associated with multiple injuries in this paediatric population. Associations between TBI and aggressive and anti-social behaviours have been observed in several previous studies [10, 24, 30-32]. Children with a history of TBI may engage in more aggressive behaviours such as temper tantrums and destructiveness [24]. Even MTBI in pre-school is reported to put children at significant risk for oppositional defiant and conduct disorders (ODD/CD) at ages 7–13 years [8]. These aggressive behaviours may be associated with neurocognitive damage following TBI, such as impaired self-regulation resulting from abnormalities in the pre-frontal cortex [24] and impaired social problem-solving skills (e.g. tendency to use aggressive and avoidant solutions and to provide impulsive and self-centred solutions) [33, 34].

Indeed, studies have suggested biological mechanisms of MTBI to cause neurobehavioural problems [35]. Cerebrovascular pathophysiological changes following MTBI include decreased cerebral blood flow, impaired cerebral autoregulation, loss or impaired cerebral reactivity, reduced cerebral oxygenation and impairment in neuroautonomic cardiovascular regulation [35]. Inertial forces from TBI may also deform white matter, mechanically disrupt nerve fibres and lead to diffuse axonal injury, regardless of actual impact [36]. Diffuse damage, especially in frontal, temporal and parietal structures, may adversely affect neurocognitive skills, such as executive functioning and social problemsolving skills, which mediate the persistence and even secondary-onset of behavioural problems [7].

Multiple MTBIs are reported to have a cumulative impact in adult [37] and adolescent populations [38]. Studies in animal models have shown that repeated head injury can cause cumulative damage to hippocampal cells [39]. Prins et al. [40], using diffuse tensor imaging (DTI) in a sample of juvenile rats, found that white matter damage and increased reactive astrocytosis were observable after a single, mild impact and that this damage increased noticeably with a second insult endured shortly after. Previous DTI studies in children and adolescents have shown that decreased white matter is linked to severity of post-concussive symptoms and executive function deficits [41, 42], which may significantly mediate behavioural outcomes [43]. Given that the head injuries included in this sample were lower-grade concussions (e.g. characterized by no loss of consciousness, it is possible that only the exaggerated damage caused by repeated injuries manifested as behavioural problems detectable by the CBCL at the time of assessment. Thus, history of single injury was not found to be associated with significant behaviour problems in this study.

The present study makes important contributions to the understanding of mild head injuries and behavioural problems in children. However, several limitations should be noted. A main limitation of this study is that no pre-injury data was collected and that, due to the cross-sectional design, observed associations may not be causal relationships. It is, thus, possible that these results reflect the fact that children with behavioural problems are more likely to incur head injuries, especially repeated injuries. However, much of the belief that behavioural problems pre-dispose children to experiencing a TBI event relies on research using post-injury recall of preinjury behaviour [44], which may be subject to potential recall biases. In fact, prospective studies collecting pre-injury behavioural data before the TBI event have found that preinjury behaviour is not actually a significant predictor for a TBI event [45-47]. Another limitation is that head injury could not be defined by a clinical diagnosis of MTBI (e.g. GCS scores) and was instead defined as a parent-reported history of head trauma without loss of consciousness or hospitalization for treatment. Specific and objective data on the timing, severity and mechanism of reported injuries are unknown and parent-reports of injury may be subject to recall biases, especially if the parents lack medical knowledge regarding TBI. Although a reliance on subjective reports of head injury is not ideal, it is sometimes the only way to study head and brain injuries, such as within military [46-47] or prison populations [48-49]. This is also the case in China, where a lack of a coherent system for TBI documentation and registration in China make it difficult to obtain objective, accurate medical data on head injury [2]. Another limitation was that factors such as injury mechanism, although evaluated, were not included in analyses due to low frequency. In addition, this study did not record time since injury, which could be a potential confounding factor. Finally, both head trauma history and behavioural assessment relied on parent-report, potentially allowing for expectation and recall bias. Moreover, the use of the CBCL as the only outcome measure in this study is another limitation and future studies may include more broad-based measures of behaviour.

To the authors’ best knowledge, this is the first study evaluating the associations between mild head injuries and Chinese children’s behaviours. Although it is not possible to determine the direction of causality from the results of this study, these findings still highlight the need for increasing attention towards the possible effects of even very mild head injuries on children’s development. One of the implications could be educating parents/caretakers on the risks of head injury during participation in recreational sports because this study showed that children of mothers who were more educated were more likely to have a history of head injury, possibly due to increased participation in recreational sports. A previous study conducted in China found an inverse relationship between paternal education and physical inactivity, suggesting that the children of parents with higher education are more active, which may suggest they engage in more recreational sports, which increases the risk for head injuries [50].

Although patients, families and healthcare professionals may be tempted to dismiss incidents of head injury unaccompanied by loss of consciousness or hospitalization, the present study demonstrates that such injuries, especially when repeated, may pose significant risks for adverse behaviours in children. Healthcare professionals should ask parents about their children’s head injury histories and adequate follow-up should be provided. Patients and caregivers should be informed about the potential behavioural consequences of even mild head injury. Prevention of multiple injuries should also be stressed. Of the children in this study with a history of head injury, roughly 40% incurred multiple injuries. Prospective, longitudinal follow-up studies armed with appropriate references, detailed records on pre-injury factors and behaviours and accurate and reliable post-injury clinical behavioural assessment will also be critical to advancing the understandings on the adverse outcomes of mild, paediatric head injuries.

Table I.

Characteristic of children with and without head injury history.

	Head injury
Characteristics	No injury (n = 558)	Single injury (n = 97)	Multiple injuries (n = 70)	p-value
Sex
Boy	285 (74.9)	57 (14.9)	39 (10.2)	0.32
Girl	273 (79.4)	40 (11.6)	31 (9.0)
Pre-school
City	227 (76.2)	33 (11.1)	38 (12.7)	0.13
Suburban	230 (77.4)	45 (15.2)	22 (7.4)
Rural	101 (77.7)	19 (14.6)	10 (7.7)
Father’s education
Less than high school	177 (73.7)	45 (18.8)	18 (7.5)	0.01
High school	199 (81.9)	23 (9.5)	21 (8.6)
College & University	182 (75.2)	29 (11.9)	31 (12.8)
Father’s occupation
Unemployed	14 (63.6)	7 (31.8)	1 (4.6)	0.12
General labour	280 (79.1)	43 (12.2)	31 (8.8)
Technician/Professional	229 (74.8)	44 (14.4)	33 (10.8)
Other	35 (81.4)	3 (6.9)	5 (11.6)
Father’s smoking frequency
Never	126 (80.2)	16 (10.2)	15 (9.6)	0.67
<10/day	293 (75.7)	59 (15.3)	35 (9.0)
10-20/day	94 (75.8)	15 (12.1)	15 (12.1)
20+/day	26 (81.2)	4 (12.5)	2 (6.3)
Mother’s age at the child’s birth
<25	171 (81.8)	24 (11.5)	14 (6.7)	0.09
25-29	356 (75.4)	63 (13.4)	53 (11.2)
≥30	31 (70.5)	10 (22.7)	3 (6.8)
Mother’s education
Less than high school	256 (77.8)	51 (15.5)	22 (6.7)	0.07
High school	172 (77.8)	22 (9.9)	27 (12.2)
College & University	130 (74.3)	24 (13.7)	21 (12.0)
Mother’s occupation
Unemployed	31 (73.8)	9 (21.4)	2 (4.8)	0.04
General labour	224 (76.7)	45 (15.4)	23 (7.9)
Technician/Professional	173 (73.6)	30 (12.8)	32 (13.6)
Other	130 (83.3)	13 (8.3)	13 (8.3)
Parental psychological problems
No	534 (77.3)	93 (13.5)	64 (9.3)	0.27
Yes	24 (70.6)	4 (11.8)	6 (17.7)
Parental divorced/separated
No	496 (76.1)	89 (13.7)	67 (10.3)	0.45
Yes	22 (78.6)	5 (17.9)	1 (3.6)
Blood lead level
<10.0 μg dL−1	517 (76.9)	90 (13.4)	65 (9.7)	0.97
≥10.0 μg dL−1	41 (77.4)	7 (13.2)	5 (9.4)

Table II.

Standardized T-scores among children with and without head injury history.

	Head injury history
CBCL behavioural problems	No injury (n = 558)	Single injury (n = 97)	Multiple injuries (n = 70)	p-valuea
Clinical sub-scales
Emotionally reactive	51.3 ± 1.8	52.2 ± 2.1	56.4 ± 2.2b	<0.0001
Anxious/depressed	56.6 ± 2.2	57.9 ± 2.5	60.5 ± 2.6	0.327
Somatic complaints	52.7 ± 1.8	54.0 ± 2.1	54.5 ± 2.1	0.110
Withdrawn	53.2 ± 1.9	56.2 ± 2.2b	56.6 ± 2.2b	<0.0001
Sleep problems	51.4 ± 1.9	51.5 ± 2.1	53.0 ± 2.2	0.904
Attention problems	57.3 ± 2.6	59.0 ± 3.0	61.8 ± 3.1b	0.055
Aggressive behaviours	52.4 ± 1.9	54.7 ± 2.1	57.8 ± 2.2b	0.003
Summary scales
Internalizing	54.2 ± 1.8	56.0 ± 2.1	58.4 ± 2.3b	<0.0001
Externalizing	53.9 ± 1.9	56.3 ± 2.2	59.5 ± 2.3b	0.004
Total problems	53.5 ± 1.9	55.8 ± 2.1	58.2 ± 2.2b	0.001

^ap-value for GLM model;

^bp<0.05, compared to children without head injury history.

Table III.

Prevalence rates of behavioural problems measured by CBCL among children with and without head injury history.

	Head injury
CBCL behavioural problems	No injury (n = 558)	Single injury (n = 97)	Multiple injuries (n = 70)	p-value
Clinical sub-scales
Emotionally reactive
No	524 (93.9)	86 (88.7)	54 (77.1)	<0.0001
Yes	34 (6.1)	11 (11.3)	16 (22.9)
Anxious/depressed
No	450 (80.7)	76 (78.4)	47 (67.1)	0.032
Yes	108 (19.4)	21 (21.6)	23 (32.9)
Somatic complaints
No	522 (93.6)	90 (92.8)	61 (87.1)	0.147
Yes	36 (6.5)	7 (7.2)	9 (12.9)
Withdrawn
No	504 (90.3)	81 (83.5)	58 (82.9)	0.039
Yes	54 (9.7)	16 (16.5)	12 (17.1)
Sleep problems
No	491 (87.9)	87 (89.7)	59 (84.3)	0.562
Yes	67 (12.0)	10 (10.3)	11 (15.7)
Attention problems
No	516 (92.5)	84 (86.6)	64 (91.4)	0.157
Yes	42 (7.5)	13 (13.4)	6 (8.6)
Aggressive behaviours
No	521 (93.4)	88 (90.7)	55 (78.6)	0.0001
Yes	37 (6.6)	9 (9.3)	15 (21.4)
Summary scales
Internalizing problems
No	476 (85.3)	80 (82.5)	50 (71.4)	0.012
Yes	82 (14.7)	17 (17.5)	20 (28.6)
Externalizing problems
No	471 (84.4)	79 (81.4)	45 (64.3)	0.0002
Yes	87 (15.6)	18 (18.6)	25 (35.7)
Total problems
No	480 (86.0)	79 (81.4)	49 (70.0)	0.002
Yes	78 (14.0)	18 (18.6)	21 (30.0)

Table IV.

Unadjusted and adjusted odds ratios (ORs) for head injury history.

		Unadjusted models				Adjusted models*
CBCL behavioural problems	Head injury history (vs NO)	ORs	95% CIs		p-value	ORs	95% CIs		p-value
Clinical sub-scales
Emotionally reactive	Single injury	1.97	0.96	4.04	0.064	2.62	0.91	7.54	0.075
	Multiple injuries	4.57	2.37	8.81	<0.0001	8.04	3.18	20.33	<0.0001
Anxious/depressed	Single injury	1.15	0.68	1.95	0.600	0.90	0.45	1.78	0.758
	Multiple injuries	2.04	1.19	3.50	0.010	2.24	1.13	4.46	0.021
Somatic complaints	Single injury	1.13	0.49	2.61	0.779	1.91	0.69	5.31	0.215
	Multiple injuries	2.14	0.98	4.65	0.055	2.27	0.77	6.64	0.136
Withdrawn	Single injury	1.84	1.01	3.38	0.048	1.43	0.64	3.17	0.380
	Multiple injuries	1.93	0.98	3.82	0.059	2.76	1.20	6.34	0.017
Sleep problems	Single injury	0.84	0.42	1.70	0.632	0.69	0.29	1.68	0.419
	Multiple injuries	1.37	0.68	2.73	0.376	1.18	0.49	2.85	0.705
Attention problem	Single injury	1.90	0.98	3.69	0.058	1.64	0.67	4.01	0.279
	Multiple injuries	1.15	0.47	2.82	0.757	0.80	0.24	2.61	0.707
Aggressive behaviours	Single injury	1.44	0.67	3.09	0.349	1.13	0.35	3.70	0.839
	Multiple injuries	3.84	1.98	7.44	<0.0001	6.03	2.36	15.37	0.000
Summary scales
Internalizing	Single injury	1.23	0.70	2.19	0.473	0.82	0.38	1.77	0.619
	Multiple injuries	2.32	1.31	4.10	0.004	2.76	1.35	5.68	0.006
Externalizing	Single injury	1.23	0.70	2.16	0.463	0.82	0.38	1.75	0.604
	Multiple injuries	3.01	1.75	5.16	<0.0001	4.03	1.97	8.25	0.000
Total	Single injury	1.40	0.80	2.47	0.241	1.00	0.47	2.11	0.992
	Multiple injuries	2.64	1.50	4.64	0.001	3.42	1.66	7.03	0.001

^*Adjusted for children’s sex, pre-school area, blood lead levels, father’s smoking frequency, mother’s age at child’s birth and parental education, occupation, psychopathologic problems and marriage status.