Asthma and Injury Risk: A Large Scale Population-Based Study

doi:10.4236/ijcm.2012.35074

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International Journal of Clinical Medicine, 2012, 3, 394-399

http://dx.doi.org/10.4236/ijcm.2012.35074 Published Online September 2012 (http://www.SciRP.org/journal/ijcm) 1

Asthma and Injury Risk: A Large Scale Population-Based

Study

Wenbin Liang*, Tanya Chikritzhs

National Drug Research Institute, Curtin University, Perth, Australia.

Email: *[email protected]

Received May 11th, 2012; revised June 16th, 2012; accepted July 18th, 2012

ABSTRACT

Purpose: Clinical data suggest that asthma impairs sleep quality and further impairs cognitive performance during the

daytime, while there is a causal relationsh ip between impaired sleep quality and injuries. Therefore asthma p atients may

have increased risk of injury, and this is supported by our recent population-based studies conducted in Australia. This

study is to investigate the effect of asthma on the risk of injury at the population level using data from the US National

Health Interview Survey collected in 2008, 2009, and 2010. Method: Data from the 2008, 2009 and 2010 National

Health Interview Surveys were combined and analyzed together. Results: US adults with current or previous asthma

had significantly greater risk of injury compared to those without asthma. The risk of injury was also significantly

higher among children with current asthma. Conclusion: This population-based study provided further evidence on the

positive association between asthma and risk of injury among both adults and children. The increased risk of injury

among asthma patients is at least partly due to impaired sleep quality and quantity caused by asthma symptoms and

asthma medications.

Keywords: Asthma; Sleep; Injury

1. Introduction

Asthma is one of the major public health problems that

affect both children and adults in many different popula-

tions among both industrialized and developing nations

[1-3]. Parallel with ur banization, the prevalence of as thma

has been increasing in all coun tries over the past 40 years.

In 2004, it was estimated that asthma affected 300 mil-

lion people worldwide, [4] and it caused around 250,000

deaths in 2007 [5]. Clinical data suggest that asthma im-

pairs sleep quality and further impairs cognitive per-

formance during the daytime, especially among those

with serious night-time symptoms [6-10]. There is a

causal relationship between impaired sleep quality and

injuries [11,12]. Moreover, it has also been shown that

asthma patients are at higher risk of fracture due to re-

duced bone mass and bruising, [13,14] a known side ef-

fect of corticosteroid medication. Our recent popula-

tion-based studies suggest that a history of asthma is as-

sociated with an increased risk of injury from various

causes and types among Australian children and adults

[15,16]. Both of our previous studies were performed on

Australian populations, and it is necessary to reassess the

associations in other regions through large scale popula-

tion-based studies.

Asthma has high prevalence in the United States [1].

The National Health Interview Surveys (NHIS) collect

information on both asthma status and injuries, this has

provided an opportunity to investigate the association

between asthma status and injuries with a large sample

representative of the US population. Data were obtained

through Internet release of the NHISs data at

http://www.cdc.gov. The present study aimed to investi-

gate the effect of asthma on the risk of injury at the

population level using data from the National Health

Interview Survey (NHIS) collected in 2008, 2009, and

2010.

2. Method

Data from the 2008, 2009 and 2010 National Health In-

terview Surveys (NHISs) were combined and analyzed

together. Details of the su rvey sampling strategy and data

collection methods have been described elsewhere [17-

19]. Briefly, the NHISs are nationally focused and con-

ducted by the National Center for Health Statistics

(NCHS), Centers for Disease Control and Prevention

(CDC). The 2008, 2009 and 2010 NHISs used the same

sample design as the 2006 NHIS survey. The NHISs

were conducted to provide comprehensive estimations of

*Corresponding a uthor.

Asthma and Injury Risk: A Large Scale Population-Based Study 395

health indictors at a national level, state stratified sam-

ples were draw from all 50 states and the District of Co-

lumbia to ensure the samples are representative at state

level [17-19]. Households were the basic unit of the

NHIS. For each selected household, if there were more

than one family residing in a household, all families in

the household were selected, and assigned a unique fam-

ily number. NHIS collected basic demographics, health

status (including injuries and poisoning incidences over

the last three months), use of health services for each fa-

mily member, and health information (including asthma

history) of one random selected adult and one random

selected child (<18 years) if there were children liv ing in

the family. Information was collected via computer-as-

sisted face-to-face interviews. Since asthma histories

were only collected from one randomly selected adult

and one child within the family (rather than every person

in the family), this study included all selected adults and

children from the three waves of the NHIS [17-19].

Defining asthma status: Both of sample adults and

sample children were asked whether they had ever been

told they had asthma by a doctor or nurse. Participants

with positive responses were further asked whether their

asthma was current. Based on the answers to these two

questions asthma status was classified into one of three

categories (without asthma; previous asthma; current).

Defining injuries and poisoning: The NHISs recorded

every reported injury which occurred in the three-month

period prior to the interview and required a medical con-

sultation (i.e. call to a poison control center; use of an

emergency vehicle or emergency room; visit to a doc-

tor’s office). The current study included all recorded in-

juries and poisoning events which occurred to the sample

adults and sample ch ildren. Demogr aphics of participan ts

extracted from the datasets and controlled for in the

analysis included: age, sex, race, marital status (adults

only), education level of the adult with the highest edu-

cation in family and ratio of family income to the poverty

threshold. Multivariate Poisson regression with robust

estimation of variance was used to investigate the asso-

ciation between asthma status and the risk of injuries,

with analyses performed separately for adults and chil-

dren. The sampling weight was used in all multivariate

analyses. Missing values of some selected family demo-

graphic variables were observed in about 10% of the total

76,669 adult participants an d 8% of th e total 31,248 child

participants. Variables with missing values were: 1)

marital status (adult only); 2) education of adult with

highest education in the family; and 3) ratio of family

income to the poverty threshold. Imputation was used to

produce estimations for the cells with missing values.

Ten imputations were produced. The first set of Poisson

models were performed while excluding observations

with missing values. Then the second set of Poisson

models were refitted with imputed estimations. All

analysis was performed with STATA® 11.

3. Results

The adult sample included 76,669 participants. The ap-

proximate crude accumulative incidence rates of adult

injury for 1) adults with current asthma; 2) adults who

previously had asthma; and 3) adults who never had

asthma, were, 222 per 1000 person-years, 153 per 1000

person-years and 111 per 1000 per son-year s respectively.

The crude relative risks were significantly different with

the risk of injury among participants with current asthma

twice as great as those who never had asthma (Table 1).

In the multivariate analysis, most of the control variables

were significantly associated with the risk of injury,

however, the effects of asthma status on the risk of injury

appeared to be largely independent of the control vari-

ables (Table 2). The multivariate relative risk estimates

for injury across the three categories of asthma status

were similar to those obtained in the univariate analysis.

The child sample included 31,248 participants. The

approximate crude accumulative incidence rates of child

injury for 1) children with current asthma; 2) children

who previously had asthma; and 3) children who never

had asthma, were, 169 per 1000 person-years, 126 per

1000 person-years and 100 per 1000 person-years

Table 1. numbers of injuries, participants and relative risk by asthma status.

Asthma status Injuries (N) Participants (N) Rate Ratio 95% CI

Adult

Never had asthma 1858 67,008 1.00

Ever had asthma 140 3670 1.38 1.15 1.65

Current asthma 333 5991 2.00 1.75 2.29

Children

Never had asthma 669 26,869 1.0

Ever had asthma 44 1402 1.26 0.93 1.71

Current asthma 126 2977 1.70 1.38 2.10

Asthma and Injury Risk: A Large Scale Population-Based Study

396

Table 2. Estimations from multivariate Poisson regression model: The association between asthma status and risk of injury in

adults.

Without imputation With imputati on

Rate Ratio 95% CI Rate Ratio* 95% CI

Asthma status

Never had asthma 1.00 1.00

Ever had asthma 1.26 1.01 1.56 1.31 1.06 1.63

Current asthma 1.81 1.54 2.13 1.86 1.59 2.17

Sex

Male 1.00 1.00

Female 0.95 0.84 1.06 0.93 0.83 1.03

Age group

18 - 24 1.00 1.00

25 - 34 0.97 0.76 1.24 0.96 0.76 1.22

35 - 44 1.05 0.80 1.37 1.06 0.81 1.39

45 - 54 1.14 0.87 1.48 1.10 0.85 1.42

55 - 64 1.18 0.90 1.54 1.19 0.92 1.55

65+ 1.30 0.97 1.72 1.26 0.96 1.66

Race

White 1.00 1.00

Black 0.79 0.66 0.95 0.80 0.67 0.95

Other 0.80 0.62 1.04 0.84 0.66 1.07

Marital status

Married or de facto 1.00 1.00

Widowed/divorced/separated 1.62 1.41 1.86 1.62 1.41 1.85

Never married 1.26 1.05 1.52 1.25 1.04 1.51

Ratio of fami l y income to the poverty t hreshold

Less than 1 1.00 1.00

1 - 1.99 0.85 0.70 1.03 0.86 0.72 1.03

2.0+ 0.88 0.75 1.03 0.90 0.77 1.06

Educational level attained for adult with highest education in the family

12 grade or less 1.00 1.00

Had some college 1.25 1.03 1.53 1.19 0.99 1.43

With a degree or above 1.26 1.02 1.55 1.19 0.98 1.44

Whether any family member had delaye d medical care in the last 12 months

No 1.00 1.00

Yes 1.35 1.19 1.54 1.36 1.20 1.54

Year of survey

2008 1.00 1.00

2009 1.06 0.92 1.22 1.05 0.92 1.20

2010 0.98 0.86 1.13 0.98 0.86 1.12

*IRR: incidence rate ratio.

respectively. Compared to participants without asthma,

the risk of injury was significantly (70%) higher for those

children with current asthma but not for those with pre-

vious asthma (Table 1). As found for the adult sample, in

the multivariate analysis, most of the control variables

were significantly associated with risk of injury, however,

the risk of injury remained significantly higher (50%) for

children with current asthma compared to those without

Asthma and Injury Risk: A Large Scale Population-Based Study 397

(Table 3). Estimations from the analysis of the original

data and estimations from the analysis with multiple im-

putatio ns we re found to be ve ry simil a r .

4. Discussion

In this large scale popu lation -based study, US adu lts with

current or previous asthma had significantly greater risk

of injury compared to those without asthma. The risk of

injury was also significantly higher among children

with current asthma. The association between asthma

status and risk of injury was largely unaffected by the

inclusion of control variables. These observations are

consistent with findings from previous studies [15,16,

20-22]. In our recent birth cohort study conducted in

Western Australia, it was observed that for both children

and adults, males with histories of hospitalizations of

asthma were at increased risk of serious injury (injuries

that required hospitalizations) [16].

Table 3. Estimations from multivariate Poisson regression model: The association between asthma status and risk of injury in

children.

Without imputation With imputation

IRR* 95% CI IRR* 95% CI

Asthma status

Never had asthma 1.00 1.00

Ever had asthma 1.08 0.74 1.58 1.08 0.75 1.57

Current asthma 1.55 1.20 2.00 1.52 1.18 1.96

Sex

Male 1.00 1.00

Female 0.83 0.69 0.99 0.81 0.68 0.97

Age group

<1 0.16 0.06 0.41 0.16 0.06 0.39

1 - 4 1.07 0.80 1.43 1.05 0.79 1.39

5 - 9 1.00 1.00

10 - 14 1.61 1.24 2.08 1.57 1.23 2.02

15 - 17 2.02 1.55 2.62 1.99 1.55 2.57

Race

White 1.00 1.00

Black 0.61 0.46 0.82 0.61 0.46 0.81

Other 0.72 0.52 1.00 0.73 0.53 0.99

Ratio of fami l y income to the poverty t hreshold

Less than 1 1.00 1.00

1 - 1.99 0.88 0.64 1.21 0.90 0.66 1.22

2.0+ 1.11 0.84 1.48 1.14 0.87 1.50

Educational level attained for adult with highest education in the family

12 grade or less 1.00 1.00

Had some college 1.63 1.12 2.38 1.72 1.19 2.49

With a degree or above 1.89 1.28 2.80 1.9 7 1.35 2.89

Whether any family member had delaye d medical care in the last 12 months

No 1.00 1.00

Yes 1.06 0.85 1.34 1.07 0.85 1.33

Year of survey

2008 1.00 1.00

2009 0.91 0.73 1.13 0.94 0.76 1.16

2010 0.88 0.71 1.10 0.92 0.74 1.13

*IRR: incidence rate ratio.

Asthma and Injury Risk: A Large Scale Population-Based Study

398

The mechanism(s) underlying this association is not

entirely clear, however, it is likely to be causal. Asth-

matic conditions and the medications used to treat

asthma (including cortico steroids) may impair th e mental

function of patients and alter their responses to hazard,

and further increase the risk of injury. Among the par-

ticipants of the current study, adults with current asthma

or previous asthma were more likely to sleep less than 7

hours every 24 hours at the time of survey compared to

those without asthma (37%, 32% and 28%, respectively,

p value of Pearson chi-square test < 0.001). Previous

research has suggested that asthma patients are more

likely to experience sleepiness during daytime hours

[7,23,24] and that asthma condition and medications for

asthma may impair sleep quality [8,25]. Moreover, there

is substantial eviden ce sugg estin g th at daytime sleep iness,

inadequate sleep or poor sleep quality is an independent

risk factor for injuries in both adults and children [11,

12,26-29]. Combining the evidence on the three rela-

tionships: 1) asthma status and injury; 2) asthma status

and its impairment on sleep quality and quantity and 3)

inadequacy of sleep and injury, indicates a direct link

between asthma status and injury mediated through im-

paired sleep quality and quantity. Interventions that im-

prove sleep among asthma patients are likely further re-

duce the risk of injury among these patients.

5. Conclusion

This population-based study provided further evidence

on the positive association between asthma and risk of

injury among both adults and children. It is highly likely

that the increased risk of injury among asthma patients is

at least partly due to impaired sleep quality and quantity

caused by asthma symptoms and asthma medications. To

reduce the risk of injury among asthmatics, interventions

are required which improve sleep quality and quantity

among patients.

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