Olfaction in complex regional pain syndrome.

OBJECTIVE: Complex regional pain syndrome (CRPS) is associated with a range of sensory disturbances on the symptomatic side of the body but whether this includes olfaction is uncertain. To clarify this, the aims of this study were to compare ratings of intensity and hedonic appeal of household odorants in CRPS patients and controls, and to determine whether ratings differed between the symptomatic and contralateral sides within the sample of patients.

METHODS: Six odorants (vanilla, fish sauce, vinegar, eucalyptus, almond essence and acetone) were presented sequentially in random order on cottonwool buds held in the midline approximately 1 cm from both nostrils in 37 CRPS patients and 21 pain-free controls. Each odour was rated for intensity and hedonic appeal, and participants reported whether the odour was stronger and/or smelt different on one side than the other.

RESULTS: The odorants smelt worse for patients than controls (p < 0.05 for the symptomatic and contralateral sides) but neither the intensity nor the unpleasantness of the odorants was greater on the symptomatic than contralateral side in the group as-a-whole.

CONCLUSION: These findings suggest that the trigeminal component of olfaction interacts bilaterally with pain-sensitized circuits in the thalamus or higher cortical centres to distort odour perception in patients with CRPS. This aberrant process appears to differ from the mechanism that underlies hemilateral hyperalgesia in other sensory modalities.