Psychiatric Comorbidity in Back Pain Disorders - Risk, Treatment and Pharmaco-epidemiological Analysis
Conditions: Back Pain, Psychiatric Disorder
Sponsor: Spine Centre of Southern Denmark

Psychiatric comorbidity in Back Pain Disorders 
Risk, treatment and pharmaco-epidemiological analysis  
 
1 
 
 
 
Project proposal for the PhD project: 
 
 
Psychiatric comorbidity in Back Pain Disorders  
– Risk, treatment and pharmaco-epidemiological analysis 
 
 
 
 
 
 
 
 
 
 
 
Applicant 
 
Pernille Sanberg Ljungdalh, MscPH 
Medical Research Department, Spine Centre of Southern Denmark 
Last update: 29 of January 2019 
 
 
 

Psychiatric comorbidity in Back Pain Disorders 
Risk, treatment and pharmaco-epidemiological analysis  
 
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Introduction 
Chronic pain is a common health problem negatively affecting sleep, work ability and socialization1-4. Chronic 
pain also increases morbidity, mortality and health care costs and has a direct and severe impact on the quality of 
life3 5-8. The prevalence of chronic pain is up to 46,5% in the general population5. Musculoskeletal back and neck 
pain are one of the most common pain conditions2 9 with a lifetime prevalence of low back pain in the range of 
51%-84%6. Variation in prevalence estimates can be explained by a difference in the definition of pain, research 
methodologies and differences in study samples10.  
 
Several different diagnostic classifications systems for back pain disorders (BPD) exist, but most frequently 
these are divided into specific and non-specific BPD11. This study chooses to establish two subgroups for 
specific BPD based on the presumed cause of illness and available treatment options and one group for 
unspecified BPD (UBD). The specific subgroups consists of inflammatory back diseases (IBD) and degenerative 
back diseases (DBD)12.  
 
Clinical guidelines recommend that treatment consists of a combination of medical, surgical and non-
pharmacological interventions such as patient education, physical- and occupational therapy, cognitive-
behavioral therapy, nutritional counseling and self-management strategies13-16. The most frequent treatment of 
BPD is pharmacological17. Despite not being generally recommended for treatment of back pain disorders18, 
opioids are prescribed in up to 66% of patients19. The prevalence of concomitant substance abuse disorder in 
BPD patients receiving opioid treatment is up to 43%19. Previous studies fail to establish, which type of back 
pain patients are prescribed opioids and which specific other psychiatric comorbidities than substance abuse are 
linked to higher opioid consumption. It does seem, that patients with a high burden of psychiatric comorbidity, 
have a higher rate of opioid misuse, although studies are old, small and heterogenic20-23 Several studies on the 
association between back pain and psychiatric disorders, focusing on depression and anxiety disorders, have 
been conducted. However, these studies have several limitations: they do not include other types of psychiatric 
comorbidity24-29 and they are often based on questionnaires. Including nationwide data sources might minimize 
potential selection bias. Furthermore, many studies have focused on large heterogeneous groups of patients with 
nonspecific back pain symptoms instead of being based on BPD diagnosis as defined by the ICD-1030. The 
majority of studies do not compare different BPD-diagnosis when assessing psychiatric comorbidity. 24 27 31-33 
 
To more accurately assess the risk of psychiatric comorbidity in patients with back pain disorders, population-
based registry data could be utilized. The Danish health registries are generally considered to have a high 
validity, however there are limitations to consider, as described in previous studies on each registry34 35 36 37, 
which has to be taken into consideration when interpreting results based on data from these registries.  
When investigating psychiatric comorbidity in BPD patients, focusing on anxiety and depression, has been the 
norm. However, important information on the relationship between other types of mental illness and back pain 
disorders might be identified, if a dataset of sufficient size and quality, is used for analysis. A population-based 
set of registry data as the ones available in Denmark provides this. By grouping psychiatry comorbidity 
according to ICD-10, mental illnesses with common pathways are analyzed together. In the ICD-10 system 
psychiatric diagnosis are grouped according to symptomology and common etiology30. All psychiatric diagnoses 
are considered relevant in the chosen population, as any psychiatric disorders represents a vulnerably patient 
with possible complicating factors when treating back pain. The limitation of Danish registry-based data is, that 
it does not provide complete data from the primary care sector selecting only severe psychiatric diagnosis. 
In conclusion, back pain disorders and the association with psychiatric comorbidity has primarily been 
investigated using symptomology, not ICD-10 diagnoses and with a focus on anxiety and depression. The 
studies are small, heterogeneous and of varying quality. The use of pharmacological treatment, including 
opioids, in back pain disorders complicated by psychiatric comorbidity is unknown.    

Psychiatric comorbidity in Back Pain Disorders 
Risk, treatment and pharmaco-epidemiological analysis  
 
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Aim of the thesis 
The aim of this PhD thesis is to use population-based registry data to: 
1) to estimate the prevalence of psychiatric comorbidity in patients with back pain disorders (BPD) compared to 
patients with no back pain in the time period 2010-2014; 2) Investigate whether presence of psychiatric 
comorbidity affects utilization of pharmacological, surgical and other types of treatment for patients with BPD 3) 
Investigate whether presence of psychiatric comorbidity affects utilization and dosages of pharmacological 
treatment for patients with BPD. 
 
Clinical contributions of this PhD 
This PhD study will contribute with new knowledge on the association between different types of BPD and 
psychiatric comorbidity. As BPD affects a large part of the population, this is a very important area of research 
from a public health and clinical perspective. Access to nationwide registries facilitates a comprehensive and 
population-based dataset, ensuring the ideal circumstances for investigating the association between BPD and 
potential psychiatric comorbidity.  Furthermore, the results can be used to increase clinical awareness of 
vulnerable patients in the intersection between rheumatology and psychiatry and subsequently improve 
prevention and clinical management for a complex and resource-demanding patient population.  
 
Materials and methods 
 
Setting and study population 
This PhD project includes a population-based nationwide cohort based on data from Danish health registries. 
Danish health registries, including The Danish National Patient Registry (DNPR), a subdivision of the DNPR, 
the National Patient Registry – Psychiatry (NPD-Psych), The Danish National Prescription Database and The 
Danish National Health Service Register, contains complete data on hospital contacts, prescription drug use and 
services provided by health contractors. By using the Danish Civil Registry and the unique personal 
identification number assigned to all Danish citizens at birth, data across registries can be linked on an individual 
level34 35 36 37. 
 
The study population will be defined as all adult patients (18+) identified in the DNPR34, covering all inpatient 
and outpatient services in Denmark, in the time period 1th of January 2010 to 31th of December 2014 with a 
diagnosis of BPD according to the ICD-10 Classification of Disease (DM*). 30  
 
Psychiatric comorbidity will be defined as the presence of any of the following diagnosis, according to ICD-10 
classification30, obtained from the National Patient Registry – Psychiatry (NPD-Psych), covering all inpatient 
and outpatient psychiatric services in Denmark in the time period 1th January 2007 to 31th December 2017: 
Organic, including symptomatic, mental disorders (F00-DF09), Mental and behavioral disorders due to 
psychoactive substance use (F10-F19), Schizophrenia, schizotypal and delusional disorders (F20-F29), Mood 
[affective] disorders (F30-F39), Neurotic, stress-related and somatoform disorders (F40-F48), Behavioral 
syndromes associated with physiological disturbances and physical factors (F50-F59), Disorders of adult 
personality and behavior (F60-F69), Mental retardation (F70-F79), Disorders of psychological development 
(F80-F89), Behavioral and emotional disorders with onset usually occurring in childhood and adolescence (F90-
F98) and, Unspecified mental disorder (F99). To qualify as a psychiatric comorbidity to back pain disorder, we 
define that the psychiatric diagnosis must be given no earlier than three years prior to back pain disorder 
diagnosis and no later than three years after.  
 
 
Study 1) Prevalence of psychiatric comorbidity in back pain disorders  
 

Psychiatric comorbidity in Back Pain Disorders 
Risk, treatment and pharmaco-epidemiological analysis  
 
4 
 
Aim: 1) To estimate prevalence of psychiatric comorbidity in patients with BPD compared to patients without 
BPD and 2) To estimate prevalence of psychiatric comorbidity in patients with  unspecific back pain (UBP) 
compared to the background population and to  patients with a specific back pain  (SBP) in a Danish nationwide 
cohort from 2010-2014.  
 
Hypothesis: 1) There is a higher risk of psychiatric comorbidity in BPD patients compared to the background 
population and 2) UBP patients have a higher risk of psychiatric comorbidity compared to patients with specific 
BPD. 
 
Design: The study will be a nationwide register-based matched cohort study. 
 
Procedure: Selected BPD patients with the following diagnosis will be identified via the DNPR: 
Spondylopathies DM45-49, Other dorsopathies DM 50-54 and Segmental and somatic dysfunction DM99. Each 
patient will be matched 1:5 on age and sex with patients without back disorders by random at Statistics 
Denmark38. The selection of diagnosis is to be able to match the population with non-BPD patients in the Danish 
registries. 
Information on somatic comorbidity for calculation of Charlson Comorbidity index39 (CCI) at time of BPD 
diagnosis, will be retrieved from the DNPR. Data on psychiatric comorbidity will be identified using NPD-
Psych. Information on marital status, vital status, and immigration status will be retrieved from the Civil 
Registration System (CPR)35. Patients that are not alive or have emigrated within three years after BPD diagnosis 
are considered lost to follow-up and excluded from the cohort. Descriptive data on the socioeconomic status, 
education level and equivalent available income will be retrieved from Statistics Denmark38. All data comprising 
each variable will be retrieved from 1th of January 2007 to 31th of December 2017 (three years before and after 
study period).  
 
Measures of outcome: Primary outcome is to estimate prevalence of psychiatric comorbidity in patients with 
BPD compared to the Danish background population. Secondary outcome is to estimate prevalence of 
psychiatric comorbidity stratified by type of BPD (SBP UBP) compared to the Danish background population. 
We will use a presence of psychiatric comorbidity as outcome.  
 
Statistics: We will use Fisher’s exact test and t-test to compare baseline characteristics with 95% confidence 
intervals (CI) and p-values. Crude risks of psychiatric comorbidity will be calculated and logistic regression 
model will be used, to investigate differences between the distribution of risk in BPD patients and the 
background population. A logistic regression model will also be used to investigate differences between the 
distribution of psychiatric comorbidity in the IBD, DBD and UBP group compared to the background 
population. The logistic regressions will be reported using 95% CI and adjusted for the following confounders: 
age, sex, equivalent available income, educational level, CCI, marital status. 
 

Psychiatric comorbidity in Back Pain Disorders 
Risk, treatment and pharmaco-epidemiological analysis  
 
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Study 2) The effect of psychiatric comorbidity on type of treatment for BPD 
Aim: 1) To investigate whether treatment of BPD in patients with psychiatric comorbidity differ from treatment 
of patients with BPD without psychiatric comorbidity; 2)To investigate whether treatment of BPD differs in the 
following subgroups: a) specific BPD and no psychiatric comorbidity; b) specific BPD with psychiatric 
comorbidity; c) unspecific BPD with no psychiatric comorbidity; d) unspecific BPD with psychiatric 
comorbidity.  Investigation will be performed in a nationwide Danish cohort in 2010-2014. 
 
Hypothesis: 1) Psychiatric comorbidity affects type of treatment in BPD patients and 2) psychiatric comorbidity 
affects type of treatment in subgroups of BPD.   
 
Design and study population: The study will be a nationwide register-based cohort study. The study population 
consists of patients with BPD and in the sub-analysis the study population consists of patients with BPD and 
psychiatric comorbidity.   
 
Procedure: BPD patients will be identified via the DNPR. Surgical treatment will be defined as the presence of 
any procedure code for back surgery retrieved from DNPR (NOMESCO: KNA-W)34 40. Pharmacological 
treatment of BPD will be defined as any treatment with the following drugs: NSAID   (ATC: N01A*), 
acitomenophen (ATC: N02BE01), opiods (N02A*), antiepileptica (ATC: N03A) and tricyclic antidepressants 
(ATC: N06AA)13 identified using the Register of Medicinal Products Statistics7. Data on other types of treatment 
(defined as any treatment not surgical or pharmacological) such as physiotherapist, psychologist, chiropractor or 
occupational therapist (covered via tax-funded health insurance) will be obtained via The Danish National 
Health Insurance Service Register (SSR)37. All BPD treatment-related information will be retrieved up to three 
years after the date of BPD diagnosis. 
Information on somatic comorbidity for calculation of Charlson Comorbidity index39 (CCI) at time of BPD 
diagnosis, will be retrieved from the DNPR. Data on psychiatric comorbidity will be identified using NPD-
Psych. Information on psychiatric comorbidity will be used to define whether each patient had a psychiatric 
diagnosis prior to BPD diagnosis, after BPD diagnosis or no psychiatric diagnosis. If a psychiatric diagnosis 
occurs both prior to and after BPD diagnosis, the patient will be defines as having a psychiatric comorbidity 
prior to BPD diagnosis. 
Information on marital status, vital status, and immigration status will be retrieved from the Civil Registration 
System (CPR)35. Patients that are not alive or have emigrated within three years after BPD diagnosis are 
considered lost to follow-up and excluded from the cohort. Descriptive data on the socioeconomic status, 
education level and equivalent available income will be retrieved from Statistics Denmark38. All data comprising 
each variable will be retrieved from 1th of January 2007 to 31th of December 2017 (three years before and after 
study period).  
 
Measures of outcome: The outcome is type of treatment for BPD (surgical, pharmacological and other types of 
treatment )  
 
Statistics: We will use Fisher’s exact test and t-test to compare patients’ baseline characteristics with 95% 
confidence intervals (CI) and p-values. For each treatment type we will use logistic regression model to 
investigate differences in outcome distribution between patients with and without psychiatric comorbidity.The 
logistic regressions will be performed with 95% CI adjusted for the following confounders: age, sex, CCI, level 
of education, equivalent available income, type of BPD (IBD, DBD or UBP) and marital status. 
 
 

Psychiatric comorbidity in Back Pain Disorders 
Risk, treatment and pharmaco-epidemiological analysis  
 
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Study 3) Effect of psychiatric comorbidity in pharmacological treatment of BPD. 
 
Aim: 1) To compare dosages of pharmacological treatment in BPD patients with and without the presence of 
psychiatric comorbidity in a nationwide Danish cohort study in 2010-2014. 2) To compare high vs. low dosages 
of pharmacological treatment for BPD in patients with BPD and psychiatric comorbidity.  
 
Hypothesis: 1) Patients with BPD and psychiatric comorbidity are prescribed a higher dosage of pain 
medication for BPD compared with BPD patients without psychiatric comorbidity. 2) BPD patients with a 
psychiatric comorbidity with a high (defined as above the 50th percentile) vs. low (defined as below the 50th 
percentile) dosage of pain medication are also prescribed higher dosages of psychiatric medication.  
 
Design and study population: The study will be a nationwide register-based cohort study. The study population 
consists of patients with BPD that received pharmacological treatment for BPD. In the secondary analysis the 
study population consists of patients with BPD and a psychiatric comorbidity, who received pharmacological 
treatment for BPD. 
 
Procedure: BPD patients will be identified via the DNPR.  Pharmacological treatment of BPD will be defined 
as any treatment with the following drugs: NSAID (ATC: N01A*), Acitomenophen (ATC: N02BE01), Opiods 
(N02A*)13. Pharmacological treatment for psychiatric comorbidity will be defined as any treatment with the 
following drugs: Antipsychotics (ATC:N05A), Antidepressants (ATC: N06A), Selective Serotonin Reuptake 
Inhibitors (SSRI) (ATC: N06AB), and anxiolytica (ATC: N5B*)41 Pharmacological treatment that can be used in 
the treatment of both BPD and psychiatric disorders will be defined as any treatment with the following drugs: 
Antiepileptica (ATC: N03A) and Tricyclic antidepressants (ATC: N06AA). All data on pharmacological 
treatment will be identified using the Register of Medicinal Products Statistics7. From time of BPD diagnosis and 
the following three years, pharmacological treatment for both somatic illness and psychiatric comorbidity in 
Daily Defined Dosage (DDD) per year will be calculated42. 
Information on somatic comorbidity for calculation of Charlson Comorbidity index39 (CCI) at time of BPD 
diagnosis, will be retrieved from the DNPR. Data on psychiatric comorbidity will be identified using NPD-
Psych. Information on marital status, vital status, and immigration status will be retrieved from the Civil 
Registration System (CPR)35. Patients that are not alive or have emigrated within three years after BPD diagnosis 
are considered lost to follow-up and excluded from the cohort. Descriptive data on the socioeconomic status, 
education level and equivalent available income will be retrieved from Statistics Denmark38. All data comprising 
each variable will be retrieved from 1th of January 2007 to 31th of December 2017 (three years before and after 
study period).  
 
Measures of outcome: Primary outcome is pharmacological treatment in DDDs defined as high (above the 50th 
percentile and low (below the 50th percentile) of pain medication. Secondary outcome is psychiatric medication 
in BPD patients with psychiatric comorbidity in DDD’s defined according to high (above the 50th percentile) and 
low (below the 50th percentile) dosages of pain medication. 
 
Statistics:  We will use Fisher’s exact test and t-test to compare patients’ baseline characteristics with 95% 
confidence intervals (CI) and p-values. Time dependent Cox regression will be performed separately for patients 
with specific and unspecific BPD to examine dosage of pharmacological treatment over time comparing patients 
with and without psychiatric comorbidity. The analysis will be performed with 95% CI and adjusted for the 
following confounders: age, sex, CCI, level of education, equivalent available income, marital status and the 
presence of psychopharmacological treatment.  
To examine dosage of psychopharmacological treatment over time comparing patients with psychiatric 
comorbidity and low vs high dose of pharmacological treatment of BPD, time dependent Cox regression will be 
performed reporting 95% CI and adjusting for the following confounders: age, sex, CCI, level of education, 
equivalent available income, marital status. 

Psychiatric comorbidity in Back Pain Disorders 
Risk, treatment and pharmaco-epidemiological analysis  
 
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Ethics 
All studies will be based on national clinical registries and approval is obtained by The Danish Health Data 
Board and the Danish Data Protection Agency (18/3337). According to Danish law, further ethical approval is 
not required for purely registry-based studies. All data will be stored on Statistics Denmark’s servers and will be 
anonymized ensuring no patients can be identified.  
 
PhD Outline 
The PhD will be conducted within a timeframe of three years (six semesters). 
 
1st semester 
2nd semester 
3rd semester 
4th 
semester 
5th 
semester 
6th 
semester 
Data 
collection 
X 
 
 
 
 
 
PhD 
Courses 
X 
X 
X 
 
 
 
Data 
analysis 
 
X 
X 
X 
X 
 
Writing 
articles 
 
 
X 
X 
X 
X 
 
 
 

Psychiatric comorbidity in Back Pain Disorders 
Risk, treatment and pharmaco-epidemiological analysis  
 
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