PATHOGENETIC ASPECTS OF RESPONSIVENESS TO ONDANSETRON (5-HYDROXYTRYPTAMINE TYPE-3 RECEPTOR ANTAGONIST) IN PATIENTS WITH PRIMARY FIBROMYALGIA SYNDROME - A PRELIMINARY-STUDY

Objective. To study the efficacy of 5-hydroxytryptamine type 3 recepto r (5-HT-3R) antagonist (ondansetron) vs paracetamol in primary fibromy algia (FM) syndrome. Methods, A double blind, crossover, latin square study of 31 patients with FM. Visual analog scale (VAS) and body drawi ngs were used to record pain intensity. Functional symptoms were deter mined for using a Likert-type self-developed protocol. Quantitative do lorimetry was applied to assess the number of painful tender points an d the average pain threshold. Serum serotonin levels were measured by a commercial ELISA. Results. A marked improvement in pain intensity me asured by VAS (p < 0.005), pain score (p < 0.05), tender points (p < 0 .05), and average pain threshold (p < 0.01) was obtained with ondanset ron, whereas no improvement was seen with paracetamol. After ondansetr on treatment, there was also significant reduction in both functional symptoms (p < 0.01) and headache intensity (p < 0.05). In patients who did not respond to ondansetron there was higher baseline pain intensi ty measured by VAS (p < 0.05) and pain score (p < 0.01), and a lower p ain threshold (p < 0.05) compared to those who did respond well. In th e responsive group, no significant differences were seen in the seroto nin level before and after therapy, whereas a significant increase in serum serotonin concentration (p < 0.01) was observed in nonresponders after ondansetron treatment. Conclusion. Ondansetron appears to be an effective drug in about 50% of patients with FM. Then may be 2 subset s of patients with FM that differ clinically and pathogenetically with regard to the disturbance in the 5-HT-3R system.