Chronic Pain in the General Population of Chile

I. Aims of the study:

1. Estimate the prevalence of chronic pain (CP) and subtypes in the
2. Explore the association between mental health indicators, perceived work stress and the presence of CP in the urban population of Chile.
3. Asses the functional, occupational and social impact of CP in Chile´s urban population reporting CP.
4. Describe the use of management strategies in those who suffer from CP.

II. Design:

The design is longitudinal prospective and involves following up the same group of participants from the MHT study to monitor their mental health and CP parameters.

III. Study variables include:

3.1. Main outcome measures (instrument): Presence of chronic pain in the last 3 months, Severity of chronic pain (Chronic pain grade questionnaire; CPG), chronic pain interference in daily activities, social activities and work (CPG), depression (Patient Health Questionnaire; PHQ-9), anxiety (General Anxiety Disorder (GAD-7) 3.2. Secondary outcome measures: chronic pain management strategies, migraine disability (Migraine Disability Assessment, MIDAS), neuropathic pain (Neuropathic pain DN4 Questionnaire; DN4), fibromyalgia (Fibromyalgia Survey Questionnaire; FSQ), insomnia (Insomnia Severity Index; ISI), loneliness (3-item UCLA Loneliness Scale; R-UCLA) 3.3. Variables of interest: job-relates stress (Copenhagen Burnout Inventory; CBI), childhood adversity (Adverse childhood experiences; ACE), negative life events.

IV. Groups:

For the longitudinal analysis, participants will be classified into four groups:

• Group 1 (no pain): participants who do not declare CP at baseline or follow-up
• Group 2 (incident): participants who do not declare CP at baseline but declare it at follow-up.
• Group 3 (remitted): participants who declare CP at baseline but do not declare it at follow-up.
• Group 4 (chronic pain at follow-up): participants who declare CP at baseline and follow-up

V. Analysis plan:

5.1. Cross-sectional analysis (at follow-up)

• Prevalence of CP will be calculated (with 95% confidence interval (CI)) for the Chilean urban population by sex, age group and employment status (aim 1).
• Prevalences of the most common CP syndromes (i.e. migraine, fibromyalgia, neuropathic pain) with 96% CI will also be calculated.
• In the subset of the population with CP, descriptive analyzes will be carried out (percentages, means and standard deviation) of the interference levels (aim 3) and chronic pain management strategies (aim 4) by sex, age group and employment status.
• Calculation of Odds Ratios (ORs) with 95% CI for the main mental health variables associated with the presence of CI (i.e. symptoms of depression and anxiety) and for work variables (i.e. work-related stress), using logistic regression, adjusting for age, sex, physical health characteristics (aim 2).

5.2. Longitudinal analysis (baseline and follow-up)

• The database will be integrated with the baseline data and based on the distribution of certain sociodemographic and health variables in the two times, an attrition analysis will be carried out to conclude on the selective loss of subjects.
• Four groups will be created according to the pain trajectory between the two measurement points (see group section) and the proportion of new cases in the 12 months will be calculated (i.e. annual incidence).
• The difference in the chronic pain intensity reported at the measurement points will be calculated.
• Several multilevel models will be analyzed to examine the relationship between mental health (PHQ-9 and GAD- 7), sleep difficulties (ISI), perception of loneliness (UCLA-R), health variables (alcohol consumption, tobacco, body mass) and work variables (i.e. job satisfaction, work-related stress) and changes in the magnitude of perceived CP at 12 months, using linear and non-linear mixed effects models (aim 3).