Blood serotonin concentrations and fenfluramine therapy in autistic children.

Whole-blood serotonin concentrations of 31 autistic children, aged 2 1/2 to 16 years, 10 non-autistic retarded children, and 18 children with Down syndrome were measured by a fluorometric method and compared with those of normal children of similar age range. No significant difference in the serotonin concentration per milliliter of whole blood or per 1000 platelets was found between groups for autistic, retarded, or normal children, but the values for those with Down syndrome were significantly lower. A double-blind cross-over study on the effect of fenfluramine versus placebo in seven autistic boys over a period of 8 months demonstrated a significant decrease in blood serotonin levels during the fenfluramine phase in all subjects. Slight improvements were found in short-term auditory memory and some measures of receptive language skills, particularly in children functioning at a high level. There was no significant change in global psychometric measurements of general intelligence during therapy. No adverse clinical effect was observed other than weight loss of 6% in one child. We conclude that fenfluramine may have some selective favorable effects on increasing attention in high-functioning autistic children. Blood serotonin concentration may be followed as an indication of drug compliance during fenfluramine therapy, but does not appear to reflect clinical efficacy.