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American Journal of Epidemiology Advance Access originally published online on February 8, 2007
American Journal of Epidemiology 2007 165(9):1063-1069; doi:10.1093/aje/kwk110
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American Journal of Epidemiology Copyright © 2007 by the Johns Hopkins Bloomberg School of Public Health All rights reserved; printed in U.S.A.

ORIGINAL CONTRIBUTIONS

Association between Body Mass Index and Recovery from Whiplash Injuries: A Cohort Study

Xiaoqing Yang1, Pierre Côté1,2,3, J. David Cassidy2,3 and Linda Carroll4

1 Institute for Work and Health, Toronto, Ontario, Canada
2 Department of Public Health Sciences and Faculty of Medicine, University of Toronto, Toronto, Ontario, Canada
3 University Health Network Rehabilitation Solutions, Toronto Western Hospital, and Division of Outcomes and Population Health, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada
4 Department of Public Health Sciences and Alberta Centre for Injury Control and Research, University of Alberta, Edmonton, Alberta, Canada

Correspondence to Dr. Xiaoqing Yang, Institute for Work and Health, 481 University Avenue, Suite 700, Toronto, ON M5G 2E9, Canada (e-mail: xyang{at}iwh.on.ca).

Received for publication July 17, 2006. Accepted for publication October 17, 2006.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
It is hypothesized that excess weight is a risk factor for delayed recovery from neck pain, such as from whiplash injuries. However, the association between obesity and recovery from whiplash injury has not been studied. The authors examined the association between body mass index and time to recovery from whiplash injuries in a population-based cohort study of traffic injuries in Saskatchewan, Canada. The cohort included 4,395 individuals who made an insurance claim to Saskatchewan Government Insurance and were treated for whiplash injury between July 1, 1994, and December 31, 1995. Of those, 87.7% had recovered by November 1, 1997. No association was found between baseline body mass index and time to recovery. Compared with individuals with normal weight, those who were underweight (hazard rate ratio = 0.88, 95% confidence interval: 0.73, 1.06), overweight (hazard rate ratio = 1.01, 95% confidence interval: 0.94, 1.09), and obese (hazard rate ratio = 0.99, 95% confidence interval: 0.90, 1.08) had similar rates of recovery, even after adjustment for other factors. The results do not support the hypothesis that individuals who are overweight or obese have a worse prognosis for whiplash.

body mass index; body weight; neck pain; obesity; recovery of function; sprains and strains; whiplash injuries


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 
Affecting more than 80 percent of individuals injured in traffic collisions (1, 2), whiplash is the most common traffic injury. The trauma leads to soft tissue injuries that result in a variety of symptoms, such as neck pain, back pain, headache, and dizziness (1, 2). Whiplash is a pervasive source of chronic pain and disability and a leading cause of health-care utilization (1–4). However, little is known about the determinants of chronic whiplash. The current literature suggests that its etiology is multifactorial and related to demographics, injury severity, comorbidities, legal and compensation factors, and health-care provision (1, 2, 4–6).

It is hypothesized that excess weight is a risk factor for chronic pain and disability (7–12) and that weight reduction is a valuable treatment option for the rehabilitation of spinal pain (13–15). However, the existing evidence on the association between obesity and spinal pain is conflicting (16). Moreover, the association between body mass index and recovery from whiplash injuries has not been studied. It is postulated that obesity predisposes individuals to develop chronic spinal pain through its association with intervertebral disc disorder (17–19), poor general health status (20, 21), numerous physical and psychological comorbidities (20–25), and a low level of physical activity and function (20, 24, 26).

Understanding the association between body weight and recovery is important, because both whiplash-associated disorders and obesity are highly prevalent in our society (1, 2, 27, 28). The objective of our study is to determine whether body mass index is associated with time to recovery from whiplash injury. We hypothesized that overweight and obese individuals have a slower recovery compared with those with normal weight.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 References
 
Design and target population
We used data from a population-based inception cohort study of traffic injuries in Saskatchewan, Canada. Saskatchewan is a province of approximately 1 million inhabitants with a single, provincially administered, automobile insurer (Saskatchewan Government Insurance) and a universal health-care system. The target population included Saskatchewan residents, aged 18 years or older, who submitted an insurance claim for a traffic injury that occurred between July 1, 1994, and December 31, 1995 (1). Individuals who reported their injury to the Saskatchewan workers' compensation board and those who did not speak English were not eligible. The date of entry into the cohort was the date of injury. Participants were followed until their insurance claim was closed or they died, or until November 1, 1997, when all remaining observations were censored. Therefore, the duration of follow-up varied from 22 months to 39 months. The University of Saskatchewan's Advisory Committee on Ethics in Human Experimentation approved the study.

Baseline data were collected when participants completed the insurance proof-of-claim form (1). These data included sociodemographic information, self-reported measures of collision, injury severity and pain intensity, health history, lawyer involvement, and health-care utilization data.

Inclusion and exclusion criteria
To be included, participants had to answer "yes" to the following two questions: 1) "Did the accident cause neck/shoulder pain?" and 2) "Have you felt neck/shoulder pain or reduced or painful neck movement since the accident?" We restricted the inclusion to claimants who reported their injury within 30 days of the collision to minimize the measurement error associated with injury severity and other confounders. Excluded were individuals who made multiple claims, those who reopened their claim during the study period, and claimants with medical conditions that prevented them from completing the questionnaire. Finally, we excluded individuals who spent more than 2 days in the hospital, because they sustained severe injuries and therefore represent patients with a prognosis that is different from those with soft-tissue injuries.

Body mass index
Body mass index was measured using self-reported height and weight. We used the World Health Organization classification to group the body mass index into four categories (21): underweight (<18.5 kg/m2); normal weight (18.5–24.9 kg/m2); overweight (25.0–29.9 kg/m2); and obese (≥30.0 kg/m2). The sensitivity of this method to measure obesity varies from 50 percent to 72 percent, and its specificity varies from 98 percent to 99 percent (29–32).

Outcome
We measured the time to recovery as the number of days between the date of injury and the date corresponding to the closure of the insurance claim (1, 5). In Saskatchewan, the closure of a claim results from a negotiation among the insurer, health-care providers, claimants, and sometimes a lawyer. It corresponds to the end of treatment, the attainment of maximal medical improvement, or the termination of income replacement benefits.

We validated claim closure as a marker of health recovery in our population by studying its relation to clinically important levels of improvement in neck pain intensity, physical functioning, and depressive symptoms (33). Our analysis demonstrated that claimants who close their claims have significantly lower levels of neck pain and better physical functioning and are less likely to have depression than those who have not closed their claim (5). This finding supports the use of claim duration as a valid marker of health recovery.

Potential confounders
The potential confounders include variables collected from the proof-of-claim questionnaire: sociodemographic variables (age, gender, marital status, number of dependents, educational level, combined family income, employment status, main work activity); postcollision symptoms (headache, reduced jaw movement, dizziness, nausea, vomiting, vision problems, memory problems, concentration problems, ringing in the ears, difficulty swallowing, low back pain, loss of consciousness, numbness, pain in the legs and feet, limitations in the activities of daily living as measured through self-report of work absenteeism); health-care utilization (medical doctors, chiropractors, physical therapists); and precollision general health status (self-rated general health before injury, history of previous injuries in car collisions, precollision symptoms: neck/shoulder pain, headache, jaw pain, low back pain, bodily discomfort, anger, depression, anxiety, fearfulness, tiredness and lack of energy, frustration, concentration problems, memory problems, body discomfort, and/or sleeping problems).

We used self-reported postcollision symptoms to assign a grade of injury severity to each participant. The three grades of injury severity were computed on the basis of the recommendations of the Quebec Task Force on Whiplash-associated Disorders (2); grade I corresponds to neck/shoulder pain only; grade II corresponds to neck pain and reduced/painful neck movement; and grade III includes neck pain and numbness or pain in the arms or hands. We also controlled for average and usual pain intensity in the neck, head, and other locations using 100-mm visual-analogue scales (34, 35). Finally, we controlled for multiple injuries by computing the percentage of the body in pain (36, 37).

Statistical analysis
The crude association between body mass index and time to recovery was assessed with Kaplan-Meier curves and the log-rank test. The median time to recovery and 95 percent confidence intervals were obtained from the Kaplan-Meier estimates. We built Cox regression models to compute the association between body mass index and time to recovery while controlling for confounders. In our final model, confounders were variables that led to a 5 percent change in one of the body mass index's crude hazard rate ratios (38). We plotted the log (-log(survival function)) against time to test the proportional hazard assumption. All analyses were conducted using SAS, version 9.1, statistical software (SAS Institute, Inc., Cary, North Carolina).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 References
 
Between July 1, 1994, and December 31, 1995, a total of 9,006 eligible individuals reported a traffic injury. Of those, 7,462 (82.9 percent) reported whiplash. Among those whiplash claimants, 2,064 (27.7 percent) reopened claims and were excluded. Among the remaining 5,398 participants, 4,395 (81.4 percent) filled out the questionnaires within 30 days of the collision and reported their height and weight. Overall, 3,854 (87.7 percent) participants recovered during the study period.

Sample characteristics
The sample characteristics are presented in table 1. In our sample, 3.1 percent of participants were underweight, 47.8 percent were normal weight, 31.6 percent were overweight, and 17.5 percent were obese. A higher proportion of overweight and obese participants were males. Overweight and obese participants were older than those normal weight and underweight individuals. A higher proportion of obese than normal weight participants reported that they had been injured in traffic collisions in the past. Finally, the proportion of grade 3 injuries was the highest among obese participants and the lowest in underweight individuals.


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  		TABLE 1. Baseline characteristics of 4,395 whiplash participants, Saskatchewan, Canada, 1994–1995*

 
Association between body mass index and time to recovery
The median time to recovery suggests that underweight participants had the slowest recovery and that normal weight, overweight, and obese individuals had similar rates of recovery (figure 1; table 2). However, the adjusted hazard rate ratios suggest that body mass index was not associated with time to recovery (table 3). All hazard rate ratios are close to unity with overlapping 95 percent confidence intervals.


Figure 1
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  		FIGURE 1. Kaplan-Meier curves of time to recovery stratified by body mass index (BMI) categories, Saskatchewan, Canada, 1994–1995.

 

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  		TABLE 2. Median recovery time for each body mass index category, Saskatchewan, Canada, 1994–1995

 

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  		TABLE 3. Association between body mass index categories and time to recovery, Saskatchewan, Canada, 1994–1995

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
References
 
In the current study, body mass index was not associated with delayed recovery from whiplash injury. Compared with individuals who were normal weight, participants who were underweight, overweight, or obese had similar rates of recovery. The results challenge the view that being overweight or obese is a risk factor for chronic pain and disability following whiplash injuries.

Our study has limitations. First, it is possible that a proportion of overweight and obese participants underreported their weight. This would have led to a differential misclassification of these individuals as normal weight and an underestimation of the association between body mass index and time to recovery (29–32, 39–45). We examined the possibility that the exclusion of reopened claims resulted in selection bias. We compared the baseline characteristics of reopened and not reopened claims and found no systematic differences in body mass index and injury severity. Moreover, we previously reported that the median time to claim closure for a reopened claim was 12 days (95 percent confidence interval: 9, 15). Most likely, these claims were reopened for administrative reasons (1). Therefore, we are confident that the exclusion of reopened claims did not significantly bias our results.

Our study also has strengths. First, all eligible participants were included in the study, reducing the potential for selection bias due to nonparticipation. Second, no systematic loss to follow-up occurred, limiting the impact of attrition on our results. Third, we used a validated outcome measure of time to recovery (1, 5). Finally, we collected an exhaustive list of potential confounders that were tested in our analyses.

Future research should test the validity of our results by using different measures of obesity. Other simple and inexpensive measures, such as waist circumference (32), waist/hip ratio, or skinfold testing, may help to reduce the misclassification bias related to self-reported weight. Moreover, the use of different body mass index cutoff points based on age, sex, and ethnicity might be helpful in reducing the possible misclassification caused by universal body mass index cutoff points.


   ACKNOWLEDGMENTS
 
Supported by a grant from Saskatchewan Government Insurance. The participation of Pierre Côté was made possible by a New Investigator Award from the Canadian Institutes of Health Research and by the Institute for Work and Health by the Workplace Safety and Insurance Board of Ontario. Dr. Carroll is supported by a Health Scholar Award from the Alberta Heritage Foundation for Medical Research.

The authors are indebted to Dr. Sheilah Hogg-Johnson for assistance with the statistical analysis.

Conflict of interest: none declared.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 References
 

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