Our results suggest that, for persons with acute SCI, pain intensity alone is not sufficient for understanding the relationship of pain and depression. In each analysis, the effect of pain interference completely displaced the effect of pain intensity on depression, highlighting its importance in the pain experience in acute SCI. The association of pain intensity and depression, before accounting for pain interference, in this study was consistent with the SCI literature16,24,27
as was the relationship of pain interference and depression.1,7,29,30
When taken together, the relationship of pain intensity and interference and depression in the acute setting provides an additional perspective that can provide insight into treatment approaches.
In this study, the presence of depression may amplify the impact of pain on life activities, thereby driving the strong relationship of pain interference and depression. For example, there is considerable evidence that there is an amplification of symptoms in persons with anxiety and depression who also have chronic medical conditions.39
Our results suggest that for individuals in this sample, how pain interferes with life activities has considerably more influence on depression than simply the degree to which pain is present. To further highlight this, Stroud et al40
found that a partner’s negative responses to pain behaviors in the partner with SCI increased the link between pain interference and depression.
The few longitudinal studies of pain and depression in SCI make it difficult to establish a causal link between pain and depression, although there is some evidence to suggest that pain is a likely risk factor for the development of depression in SCI.16,28
This is supported by broader literature across populations indicating that pain likely precedes depression.41
Although we were unable to test causality in this study, our results suggest that pain interference and not just pain intensity should be accounted for in longitudinal studies of pain and depression.
Pain is now considered the “5th vital sign”; numeric pain intensity rating scales are used widely when assessing pain intensity and are also recommended for use in patients with SCI.36
However, others have argued that relying solely on pain intensity rating change (ie, 50% change) is insufficient for evaluating the effectiveness of pain management strategies because pain is a multidimensional experience.42,43
Our results support this argument. Despite the growing recognition of the multidimensional experience of pain, a 2008 consensus meeting on interpreting the clinical importance of treatment outcomes in clinical trials of chronic pain treatments included pain intensity and mood but not pain interference as important outcomes.44
As the understanding of the pain–depression relationship has grown in recent decades, there is greater appreciation for the need to treat pain and depression simultaneously.19
For example, Cardenas et al45
recently reported on the efficacy of pregabalin to significantly reduce neuropathic pain in chronic SCI as well as depression symptoms; pregabalin did not appear to have an effect on anxiety. The acute phase of SCI is also an important period in which pain management is crucial. Acute pain, if poorly controlled, has the potential to develop into chronic pain.46
Kennedy et al47
found that pain at 6 weeks post traumatic SCI was a strong predictor of pain 1 year post injury. High pain levels at the start of depression treatment also can result in poorer response to treatment19
and lower rates of remission.48
As such, effective pain management in acute SCI has implications for the development of chronic pain and depression. Our results also emphasize the importance of addressing pain and depression in the acute setting not as separate entities, but as linked by the impact of pain on important life domains. These results suggest that treating pain intensity alone, typically the primary focus of medical intervention, may not be sufficient to reduce depression and/or reduce future risk. Instead, comprehensive treatment approaches that target pain intensity, pain interference, and depression, in combination and with multidisciplinary collaboration, may be the most effective in the short and long term. This is supported by recent findings from clinical trials that collaborative approaches to treat depression and pain are superior to usual care.21,49,50
Although this study fills some gaps in the understanding of pain and depression in SCI, results should be considered in light of several limitations. This was a cross-sectional study, which limits our ability to make causal inferences. We did not differentiate between those who did and did not agree to be interviewed, so there may be systematic differences between the 2 groups. The measurement of pain interference in the confines of acute rehabilitation limits the variability of experience of the ways in which pain interferes in major life domains. The impact of pain interference, when also accounting for pain intensity, may vary in important ways when the assessment occurs in the chronic phase of injury. The average pain intensity in this sample was relatively low; a sample of persons with high pain levels may produce different findings. Finally, our sample size precluded the examination of whether there is an indirect effect of pain intensity through pain interference; future studies with larger samples should use techniques such as path analysis to test the mediating effects of pain intensity on the relationship of pain interference and depression.