METHODS

Design

The source population for this epidemiologic study is made up of all persons who sustained a whiplash injury in a motor vehicle collision in 1987 in Quebec and submitted a claim for compensation to the SAAQ. The study subjects were identified from the SAAQ's computerized information systems by searching for individuals with an ICD-9 diagnostic code of 847.0 (sprains and strains of the neck, including whiplash injury). The year 1987 was selected as the basis for this study because, in that year, the SAAQ achieved virtually complete coding of claimant-injury data by professional medical archivists.

A historical cohort design was used. The primary source cohort was defined as all subjects for whom some form of compensation was received from the SAAQ (i.e., reimbursement for one or more collision-related expenses and/or compensation to replace regular income), rather than simply as those who submitted a claim to the SAAQ. This source cohort was used in all analyses not involving collision-related data, such as the analyses of costs. A source subcohort was formed of all subjects in the primary cohort for whom collision-related data from a police accident report were available in the SAAQ's computerized databases. This subcohort was used as the source in all analyses involving collision-related data, such as the analyses of duration of absence and of recurrence. The date of entry into the cohort was defined as the date of the collision. The exit date from the cohort was the earliest of either the date on which the whiplash subject's file was closed by the SAAQ, or the date on which the data were extracted from the SAAQ's computerized databases (May 1993) for the purposes of this study. Thus the cohort was followed for approximately six years.

Factors Under Study

The potential prognostic factors that were available in the SAAQ's computerized databases for all members were sociodemographic (gender, age, area of residence, marital status, employment status, net income and number of dependents of the whiplash subject). For the subcohort for whom collision-related data from a police accident report were available, collision-related factors (severity, type of vehicle occupied by the subject, position of subject in the vehicle, type of collision, seat belt use, direction of collision, number of vehicles involved and the authorized speed limit at the collision location) were used. In addition, the presence of multiple injuries was indicated.

Outcome Variables

The first outcome of interest was the duration of absence from usual activities for which some financial compensation was given. This was defined as the length of time, in days, between the date of the collision and the last date for which compensation to replace regular income was made by the SAAQ. In other words, this outcome corresponded roughly to the amount of time taken off work by the whiplash subject (if the subject was employed) or the length of time during which the whiplash subject could not carry out his or her usual activities (if the subject was a student, home maker, retiree, or unemployed). SAAQ policy dictates that motor vehicle collision subjects who can return to work or to their usual activities within seven days of the collision are ineligible to receive compensation to replace regular income, but may receive reimbursements for expenses. Such subjects, whose injuries were presumably minor, were assigned a mean duration of 3.5 days, or half of the seven day waiting period required by the SAAQ. For all others, the absence was taken as the duration of compensation plus seven days. Since the duration of compensation is measured cumulatively by the SAAQ and does not allow successive intervals of compensation to be distinguished in cases of recurrence, the study cohort for this first outcome excluded all subjects who experienced a recurrence.

The second outcome of interest was whether or not the subject experienced a recurrence or relapse of the injury. For the purpose of this study, recurrence was defined as the return of apparently resolved symptoms following a single motor-vehicle collision in which the subject was involved in 1987. It did not include "reoccurrences" of whiplash resulting from subsequent motor vehicle collisions. The study cohort for this second outcome included only those individuals diagnosed with whiplash as their sole injury. This was necessary to ensure that the recurrence was related only to the whiplash injury and not to another injury sustained during the collision.

The third outcome under study was the financial cost of whiplash injury to the SAAQ. For descriptive purposes, the distribution of costs according to the type of reimbursement or compensation was determined for all subjects who submitted a claim to the SAAQ for a whiplash injury sustained in 1987. The medical cost associated with whiplash injury (besides those costs covered by Quebec's universal health insurance plan or other private insurance plans) comprised the fees and expenses that are routinely reimbursed by the SAAQ such as: expenses for medical apparatus necessary for the social and professional integration of whiplash patients undergoing rehabilitation; expenses for professional services used in the development of a rehabilitation plan (e.g., psychologists, psychiatrists); expenses for medical and paramedical care not covered by the provincial or other health insurance plans; hospital expenses not covered by the provincial or other health insurance plans; medical expenses (prescriptions); ambulance fees; expenses for orthoses, prostheses; physiotherapy fees; and fees for medical examinations or expert medical consultations and evaluations not covered by the provincial or other health insurance plans. The study cohort for this cost outcome was the entire cohort of subjects who received some compensation from the SAAQ in 1987 for a whiplash injury.

Analysis

Using all whiplash patients who received some compensation from the SAAQ in 1987, the age- and gender-specific incidence rates for whiplash injury in Quebec were calculated using the 1987 stratum-specific Quebec population figures in the denominator. 99 The resulting rates are thus of the incidence of compensated whiplash injury. Region-specific incidence rates were also calculated for the 16 administrative regions in the province. These rates were based on the region of residence of the whiplash subjects rather than the region in which the collision occurred because the collision location information was frequently unavailable from the computerized databases. Driver-specific incidence rates using the number of licensed drivers in the province as the denominator 99 could only be calculated by extrapolation because of the lack of collision-related data on many of the whiplash cases. It was therefore assumed that the proportion of drivers among whiplash cases for whom collision-related data were available was equal to the proportion of drivers among whiplash cases whose collision data were missing from the databases.

Duration of absence and costs were analyzed using simple methods for continuous data. Comparisons were based on the Wilcoxon two-sample non-parametric test. Distributions of the duration of absence were described using standard techniques for survival data, such as Kaplan-Meier curves. Methods for proportions were used to analyze the rate of relapse or recurrence of whiplash symptoms.