Autonomic Function in Fragile X Syndrome: A Systematic Review.

Results could not be combined into one overall figure, because the studies used different comparison groups, different age ranges and different recording methods. Most included studies were small case-control studies, with sample sizes from 2 to 73 people. The smallest are unlikely to have had enough participants to detect a real difference, and their results should be treated with caution. Only nine of the 25 case-control studies matched groups on age, sex and medication together; most accounted for age and sex but not medication, and many medications used in Fragile X syndrome (stimulants, alpha-2 agonists) and psychotropic drugs generally affect the autonomic measures being recorded. No included study looked only at adults, so this review says nothing specific about autonomic function in adults with Fragile X syndrome; the evidence is mostly from children, or mixed child and adult samples. Comparison groups varied (typically developing, autism, ADHD, intellectual disability, Turner syndrome), so a difference found against one comparison group does not carry the same meaning as a difference against another. Whether the autonomic differences matter for anxiety, autistic traits or behaviour is unclear, because the studies that looked for those links disagreed with each other, and the review's own conclusions about clinical associations rest on that inconsistent set of studies.

The review was funded by the Winefride and Booth Smith Studentship at the University of Edinburgh. The authors declare no conflicts of interest.

Do not read this as evidence that calming a person's arousal will change their behaviour or learning in Fragile X syndrome. The review pooled measurements, not treatments: no study in it tested an arousal-lowering intervention in Fragile X syndrome, and the vagus nerve stimulation success it mentions was in Prader-Willi syndrome, a different condition.