Depression, Pain and Sleep

Some studies have reported associations between depression and a number of long-term medical painful conditions including pain syndrome:

• An epidemiological study performed in the UK reported a prevalence of 16.9% of participants with chronic widespread pain had a psychiatric diagnosis (13).
• A seven-year longitudinal study reported that the presence of multiple physical symptoms is predictive of a new onset of depression (8) and conversely, depressed subjects are three to seven times more likely to develop multiple physical symptoms than are non-depressed subjects.
• More recently, a one year longitudinal study reported that non-depressed subjects with a long-term medical condition (migraine headaches, sinusitis or back problems) are twice as likely to develop a major depression within the next year than are other individuals (11).

Few studies have attempted to determine the role of obesity, a factor found to be associated with increased pain severity in studies on pain (14,15) and with increased likelihood of major depressive disorders in studies on depression (16, 17), in the association between pain and depression.

RESEARCH

In this study, we explored further the relationship of chronic painful physical condition with depression in a large community sample composed of 18,980 Europeans aged 15 years and older. We wanted to find:

• the prevalences of pain, depressive symptoms, and major depression in this representative sample;
• the degree of association between chronic painful physical condition and depressive symptomatology and major depressive disorder diagnosis;
• other factors such as a comorbid non-painful medical condition modifying the association between pain and depression;
• if pain symptoms are more common than classic vegetative symptoms in a community based sample of subjects with major depressive disorder diagnosis.

METHODS

The participants in five European countries were interviewed by telephone between 1994 and 1999 with the broader purpose of investigating sleep habits, sleep-related symptoms and psychiatric and sleep disorder diagnoses according to DSM-IV. The target population was all non-institutionalized residents ages 15 years or over living in the United Kingdom, Germany, Italy, Portugal. These five countries totaled approximately 206 millions inhabitants. A two-stage sampling design was used for all five countries using the geographical distribution in the first stage and the Kish selection method (18) in the second stage. The Kish method is based on eight selection tables. It maintains the representation of the sample in terms of age and gender and avoids bias related to non-coverage error.

The participation rate was 79.6% (4,972 of 6,249 eligible individuals) in the UK; 68.1% (4,115 of 6,047 eligible individuals) in Germany; 89.4% (3,970 of 4,442 eligible individuals) in Italy; 83% (1,858 of 2,234 eligible subjects) in Portugal; and 87.5% (4,065 of 4,648 eligible individuals) in Spain. Altogether, 18,980 subjects participated in the study. The overall participation rate was 80.4%.

RESULTS

The sample included 18,980 subjects between 15 and 100 years old:

• Subjects from the United Kingdom represented 26.2% of the sample;
• German subjects 21.7%;
• Spanish subjects 21.4%;
• Italian subjects 20.9%;
• and Portuguese subjects 9.8%.

Limitations of the study

This study was designed primarily to address sleep and mental disorders in the general population and the questionnaire did not include extensive questions about pain and their sites. Therefore, it was not possible to distinguish between the different clinical forms of pain with great specificity.

Second, the data collected on pain and non-painful medical conditions relied on self-reporting. However, since pain involves mainly a subjective perception and experience, self-reports are commonly considered accurate for the report of pain. Studies that have examined the concordance between subjective reports of pain and external measures of pain have found good agreement between the two (27). Furthermore, a physician or another health specialist had already diagnosed non-painful medical conditions reported in this study.

The pain-depression association

Among the 4% of subjects in the sample with a major depressive disorder diagnosis, over 40% also had a chronic painful physical condition. A third of subjects with a major depressive disorder diagnosis had a non-painful medical condition. Therefore, only 38.4% of subjects with a major depressive disorder had neither a chronic painful physical condition nor a non-painful medical condition; approximately 15% had both. In subjects with a major depression diagnosis, the odds of having a chronic painful physical condition were greater than for depressive symptoms ranging from 2.0 to 5.0 (crude odds ratios).

Conversely, in subjects with a chronic painful condition, the highest odds of having a depressive disorder diagnosis were found among those who reported backache or headaches. These results confirm those in other clinical studies that examined the prevalence of a depressive disorder diagnosis in patients with low back pain or headaches (37-40).

Interestingly, 24-hour presence of pain made an independent contribution to the presence of a major depressive disorder diagnosis, indicating that continuous pain by itself increases the likelihood of having a major depressive disorder diagnosis.

Furthermore, when controlling for the presence of a non-painful medical condition, a chronic painful physical condition remains an important factor related to the presence of a major depressive disorder. Both, the influence of 24-hour pain and comorbid chronic painful physical condition and non-painful medical condition were never investigated in the general population prior this study.

Other results are noteworthy:

• We found that shift working was a predictive factor for major depressive disorders as were unemployment and being homemaker. Previous epidemiological studies on major depressive disorders did not pay attention to separate working individuals according to their work schedule (1-5). However, some studies on shift workers have reported high rates of major depressive disorders in this population (41,42).
• We also investigated the influence of obesity on the association between pain and major depressive disorders because obesity has been demonstrated to be related to the severity of pain on one hand (14,15) and to the presence of major depressive disorder on the other hand (16,17). Interestingly, in bivariate analyses, these two associations were observed; i.e., obese individuals are more likely to report a chronic painful condition and they have a higher prevalence of major depressive disorders than the normal weighted individuals. However, when introduced into a multivariate model, the association obesity and major depressive disorder disappeared. Instead, we found that being over-weighted was a protective factor for major depressive disorder.

Should depression be assessed differently in subjects with chronic pain?

The pain-depression interaction is not yet fully understood. Some argue that chronic pain should be viewed as a potent physical and psychological stressor that may influence mood. Thus, pain can be viewed as a potential trigger of the depressive illness. Some longitudinal studies partly support that hypothesis (8-12).

The relationship, however, is far more complex: if pain precedes the depressive illness in as many as 40% of cases, the reverse is also true: depressive illness often precedes physical pain (7,8,10,12).

Our data are cross-sectional; therefore, they offer limited information about the course of pain and depression. We found, however, that depressed mood is significantly longer - about six months - in subjects who have reported a chronic painful physical condition. Therefore, pain could also contribute to the prolongation of a depressive episode. This result has also been reported in clinical studies (44).

On the other hand, others argue that depression should be assessed differently in subjects with physical pain or, at least, that special attention should be brought to somatic symptoms of depression, that are also often associated with physical pain and can in fact be attributable to the painful physical condition rather than to depression (45-47). Therefore, there is a question as the extent to which painful physical condition can induces a false-positive diagnosis of a major depressive disorder.

In this study, we found that changes in appetite or weight, fatigue, insomnia/hypersomnia and feelings of worthlessness or guilt were frequently associated with pain alone. About 88% of subjects with a depressive disorder diagnosis reported somatic depressive symptoms: fatigue, sleep and appetite disturbances. If we exclude subjects who reported both a chronic painful physical condition and somatic symptoms of depression and who did not have sufficient depressive symptoms to fulfill the criteria of a major depressive episode, the prevalence of a depressive disorder diagnosis drops to 3.4%. Therefore, about 15% of depressive disorder diagnoses would be discarded.

There is no clear consensus on how to manage depressive somatic symptoms in the context of painful physical conditions.

Some argue that the overlap between pain and depressive symptoms may even compromise the validity of diagnostic questionnaires in that specific population (48-50). In this regard, Endicott (51) has recommended that alternative diagnostic criteria be used e.g., using non-somatic substitute symptoms to make the diagnosis of major depression.

In summary, this study clearly shows the importance of chronic painful physical condition in major depressive disorders.

Pain is a common symptom that remains strongly associated with major depressive disorder even when controlling for the presence of non-painful medical conditions. Furthermore, the co-occurrence of chronic painful physical condition and non-painful medical condition increased the likelihood of having a major depressive disorder.

Further studies on the relationship of pain to major depressive disorders appear warranted.