• First, the age range is broader in this sample (15 to 100 years old) than in the NCS study (15 to 54 years old).

• Second, the time frame is also different (point prevalence in this study, 1-month, 6-month, 1-year or lifetime prevalence in the NCS and ECA studies), and

• Third, in this study, a differential diagnosis procedure was applied to all the subjects with a major depressive episode.

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, among subjects with a chronic painful condition, the highest odds of having a depressive disorder diagnosis were found in subjects 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 in that study 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 pain and major depressive disorders because obesity has been demonstrate to be related to the severity of the 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, it offers 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.