Consistent with epidemiologic studies which have identified a direct association between chronic pain and depression, experimental studies have demonstrated that patients with depression have altered pain thresholds and tolerances.6 In a recent study, 30 patients diagnosed with major depressive disorder, using DSM-IV criteria, were matched with 30 nondepressed control subjects.7 Pain thresholds and tolerances were assessed bilaterally in response to experimentally induced thermal, electrical, and ischemic pain. Compared to controls, the depressed group was hypoalgesic to heat and electrical pain, but hyperalgesic to ischemic muscle pain. Similar findings have been reported for a group of patients diagnosed with adjustment disorder, depressed subtype.8
These differences in pain thresholds and tolerances suggest patients with depression may experience a differential analgesic response to opioid medications compared to patients without depression. In a randomized, cross-over, double-blind, placebo-controlled study, 60 patients with chronic low back pain were stratified into three groups based on the severity of depressive, anxiety, and neurotic symptoms.9 Subjects in each of the three groups were administered 4-6 mg of morphine intravenously and pain severity was assessed over three hours. The total analgesic response was significantly greater in the low psychopathology group compared to the high psychopathology group. Additionally, the analgesic placebo response was significantly greater in the high psy-chopathology group compared to the low group. While the mechanisms mediating the association between chronic pain, depression, and analgesia remain to be fully elucidated, alterations in emotional processing could be important contributing factors.10,1112
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