Irritable bowel syndrome
Early studies addressing antidepressant efficacy in IBS reveal inconsistencies. For example, whereas some suggest that TCAs are effective in mitigating chronic, severe abdominal pain,39,40 others concluded that TCAs failed to demonstrate a significant analgesic benefit.69 A recent meta-analysis that restricted analysis of efficacy to randomized controlled trials (RCTs) with more rigorous methodological adherence found that pain relief in IBS is possible with both TCAs as well as SSRIs. However, adverse effects were more commonly encountered with TCAs than with SSRIs. Some of the inconsistencies in treatment efficacy reported in early studies may be due to variations in responsiveness of subsets of IBS patients. Specifically, the utility of TCAs appears to be best among patients with diarrheal-type (as opposed to constipation-type) IBS, presumably due to TCAs’ anticholinergic effects, whereas SSRIs may provide more of a benefit for patients with predominantly constipation-type IBS.40,70
Other chronic pain conditions
Antidepressants have been used to assist in the management of several other pain conditions, including oral-facial pain, interstitial cystitis, non-cardiac chest pain, and others. The role of antidepressants for such conditions remains unclear due to limitations in the prevailing empirical work, such as few trials, small sample sizes, variations in outcome measures, and insufficient randomization and blinding.71-76 The interpretation of results from systematic reviews and meta-analyses is limited because of these shortcomings.77 Hence, it has not always been possible to determine whether, and to what extent, patients with such conditions may benefit from antidepressants.
Neuromodulatory effects and efficacy for pain
The interplay of norepinephrine (NE) and serotonin (5-HT) neurotransmitter systems and cellular mechanisms involved in the descending modulation of pain pathways is complex. Experimental animal models of pain modulation suggest that 5-HT can both inhibit as well as promote pain perception by different physiological mechanisms, in contrast to NE, which is predominately inhibitory. While 5-HT in the descending modulating system can inhibit pain transmission ascending to the brain from the periphery, it appears that an intact noradrenergic system is necessary for the inhibitory influences of the serotonergic system to be appreciated.16,78,79 Deficiencies in one or both of these neurotransmitter systems may contribute to hyperactive pain processing, and thereby precipitate or maintain chronic pain.
Pain mitigation may be achieved best by enhancing both neurotransmitters simultaneously, less so by enhancing NE alone, and least by enhancing 5-HT alone.6 The ability to impact pain modulation would, therefore, depend on the degree to which an antidepressant capitalizes on both noradrenergic and serotonergic neurotransmission. Antidepressants commonly employed to manage pain are presented in Table 147,60,68,80-88 according to their primary neurotransmitter effects. Thus, the literature summarized above suggests that antidepressants that influence both NE and 5-HT transmission have greater analgesic effects than antidepressants with more specific effects, such as influencing 5-HT reuptake alone.80-85 It is unsurprising, therefore, that the SSRIs have not been demonstrated to be as consistently analgesic.47,60,68,80,86-88
Similarly, pharmacodynamic and pharmacokinetic differences within antidepressant classes may influence analgesic effectiveness. Simultaneous effects on NE and 5-HT are achieved at low doses with duloxetine and milnacipran. By contrast, 5-HT effects predominate at low doses for venlafaxine. To achieve pain-mitigating effects, higher doses of venlafaxine generally are required.89 Therefore, inconsistencies across studies regarding the analgesic benefits of venlafaxine may be attributable to variability in dosing; patients treated with lower doses may not have experienced sufficient NE effects to garner positive results.
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