Chronic pain and mental illness are common and individually are leading causes of years lived with disability and impact an individual’s quality of life. Despite increasing recognition that chronic pain and mental illness commonly co-occur, the directionality between these, impact on physical health and function outcomes in the NHS are unclear. Initial research has suggested that health and functional outcomes for people with different mental illness and chronic pain may be different according to the type of mental illness and chronic pain diagnose. Specifically, little information is available on the nature of chronic pain and mental illness relationship in the context of primary care in the NHS.
The aim of this study is to better understand the relationship between mental illness and chronic pain and outcomes experienced (e.g. physical health, or social outcomes) across the CPRD database. The CPRD database is an anonymous GP database that represents a broad section of the United Kingdom. We will use the anonymous GP records in the CPRD database by analysing the data over time to understand: 1) How common is mental illnesses in different chronic pain diagnoses?; 2) How does having chronic pain and mental illness together influence physical health and functional outcomes; and 3) what factors may predict the development of mental illness in people with chronic pain. We will also investigate if this differs by different chronic pain diagnoses across different sites (e.g., neck pain, generalised chronic multisite pain) and types (e.g., musculoskeletal pain and non-cancer pain) (data permitting).
Primary objective is to estimate the incidence of mental illness among people with chronic pain. Primary exposure is chronic pain across different sites (e.g., neck pain), including type (e.g., musculoskeletal pain) and severity (e.g., moderate). Primary outcome includes the incidence of different mental illness (e.g., depression, schizophrenia) in people with chronic pain across different sites, including type and severity.
Secondary objectives include 1:2 matching patients with chronic pain who later developed mental illness (age, general practice and index date for mental illness) with controls with chronic pain only and compare their outcomes. Secondary exposure is chronic pain across different sites, including type and severity. Secondary outcomes include the incidence of subsequent physical comorbidities (e.g., diabetes) and functional outcomes (e.g., mobility, frailty, fit to work) in those with chronic pain and mental illness versus those with chronic pain only.
Study design: Retrospective, longitudinal cohort study
We will define a cohort of patients with new diagnoses of chronic pain between 01st of March, 2011 and 28th of February, 2020 and follow them up to estimate the time to study outcomes. For secondary objective, patients with chronic pain and who later developed mental illness will be matched with controls with chronic pain only (defined above). Chronic pain events include pain-specific clinical diagnoses (e.g., neck pain), referrals to secondary care/specialist (e.g., physiotherapy), or medications (e.g., opioids). Pain severity will be assessed using medical codes or prescriptions as a proxy (e.g., yes/no). Predictors of mental health illness include clinical (e.g., age at diagnosis) and socio-demographic (e.g., ethnicity) variables. Data will be analysed in a time to event framework. The start date is the latest of the registration/index date for chronic pain. The end date is the earliest of the study outcomes/death date/transfer out date/study end date. The analyses will adjust for relevant confounders (e.g., comorbidities).
The study primary outcome will be the incidence of specific mental health diagnoses (depression, anxiety, bipolar disorder, schizophrenia) in people with different chronic pain diagnoses across different sites (e.g., lower back pain, neck pain), including type (e.g., musculoskeletal pain and non-cancer pain) and severity (e.g., mild, moderate, severe).
Mental health diagnoses (e.g., ICD-10 diagnosis F20*-25* for severe mental illness, F30*-F39* for affective disorders) will be identified via SNOMED-CT and/or Read medical codes using our team codes published in previous research (Dregan et al, 2015, 2019; Stubbs et al, 2015, 2017) and other codes publicly available (e.g., Cambridge University) following validation by clinical experts within the team.
Specific secondary outcomes will be investigated include:
Long term physical health conditions (e.g., diabetes, heart failure): Physical condition diagnoses will be identified from the Read medical codes used by GPs
Function status: Frailty will be extracted from routinely used scales such as electronic fragility index (eFI). Data permitting, we will also consider extracted clinical events related to mobility (e.g., physically impaired) and/or social functioning (e.g., fit to work).
Brendon Stubbs - Chief Investigator - King's College London (KCL)
Ruimin Ma - Corresponding Applicant - King's College London (KCL)
Alex Dregan - Collaborator - King's College London (KCL)
Eugenia Romano - Collaborator - King's College London (KCL)
Fiona GAughran - Collaborator - King's College London (KCL)
Mark Ashworth - Collaborator - King's College London (KCL)
Robert Stewart - Collaborator - King's College London (KCL)