Endogenous opioids and chronic pain intensity: interactions with level of disability.
OBJECTIVE
To test whether endogenous opioid antinociceptive system dysfunction evidenced in response to acute pain stimuli is associated with increased clinical pain intensity in chronic pain sufferers, and to determine whether this association is moderated by disability level.


METHODS
A double-blind, placebo-controlled, randomized crossover design. Subjects underwent laboratory acute finger pressure pain stimulation and ischemic pain stimulation under placebo and under opioid blockade with naloxone. The primary independent measures, reflecting degree of endogenous opioid antinociception, were opioid Blockade Effects derived to reflect the change elicited by naloxone in pain intensity ratings for the acute pain tasks. High and Low Disability groups were derived based on Pain Disability Index scores to allow examination of the influence of disability level on the relationship between Blockade Effects and chronic pain intensity.


METHODS
Twenty-eight chronic low back pain sufferers.


METHODS
Seven-day diary ratings of overall chronic pain intensity based on McGill Pain Questionnaire-Short Form total scores.


RESULTS
Greater daily chronic pain intensity was associated with greater placebo acute pain sensitivity in the laboratory (P < 0.05). Positive Blockade Effects (ie, presence of opioid analgesia) were associated as expected with lower placebo-condition acute pain sensitivity in the laboratory (P < 0.05). In main effects analyses, Blockade Effects were not associated significantly with daily chronic pain intensity. This absence of overall main effects was accounted for by significant opposing interactions between disability level and Blockade Effects (P < 0.05). Negative Blockade Effects (ie, absence of endogenous opioid analgesia to acute pain) in the High Disability group were associated with greater daily chronic pain intensity, consistent with the hypothesized effects of chronic pain-related opioid dysfunction. In contrast, Positive Blockade Effects (ie, effective opioid analgesia to acute pain) were associated with higher daily chronic pain intensity in the Low Disability group.


CONCLUSIONS
These results suggest that endogenous opioid antinociceptive system dysfunction may contribute to elevated acute and chronic pain sensitivity among more disabled chronic pain patients. Among less disabled patients, chronic pain may serve as a primer producing up-regulated opioid antinociceptive responses to acute pain
Department of Anesthesiology, Vanderbilt University School of Medicine, Nashville, TN 37232-1557, USA. Stephen.Bruehl@vanderbilt.edu
Stephen
S
Bruehl
Ok Yung
OY
Chung
Pamela
P
Ward
Benjamin
B
Johnson
D000328:Adult; D001794:Blood Pressure / Q000187:drug effects; D002908:Chronic Disease; D018592:Cross-Over Studies; D004185:Disability Evaluation; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D017116:Low Back Pain / Q000503:physiopathology* / Q000523:psychology; D008297:Male; D008875:Middle Aged; D009270:Naloxone / Q000097:blood / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000097:blood / Q000494:pharmacology; D018847:Opioid Peptides / Q000502:physiology*; D010147:Pain Measurement / Q000187:drug effects; D017288:Pain Threshold / Q000187:drug effects; D010552:Personality Assessment
D009292:Narcotic Antagonists; D018847:Opioid Peptides; D009270:Naloxone