Serotonin is a chemical that signals information between cells, primarily those cells that are found in the central nervous system and digestive track. In addition, within the blood, serotonin is found in platelets where secretion induces both platelet aggregation and contraction of blood vessel walls. When found in low concentrations serotonin mediates the dilation of blood vessels that helps trigger a migraine attack in susceptible patients. Serotonin is also considered a natural stabilizer for mood (including depression, anxiety, panic and obsessive compulsive behaviors), digestion (helps regulate gastric acid as well as mucus secretion), and socialization (mediates responses to social norms). It is the chemical precursor of melatonin, a hormone that helps regulate the entire sleep/wake cycle. Unsurprisingly, researchers have used whole blood serotonin and plasma melatonin as potential biomarkers for autism and target for treatment.
Many decades of research have supported a link between the serotonergic system and autism. Elevated levels of serotonin have consistently been found in blood/platelets of individuals with autism as well as their relatives. Dietary manipulation meant to deplete serotonin exacerbate repetitive behaviors and elevate feelings of anxiety and unhappiness in autistic individuals. Indeed, too much serotonin causes mild to severe symptoms in any individual, but in autism they seem to be exacerbated. The presence of muscle rigidity, confusion, agitation, tremors, diarrhea, shivering and sweating may attest to the presence of a serotonergic abnormality. These are all the more important symptoms to keep in mind as difficulties in communication may prevent an autistic individual from voicing proper concerns.
In the majority of occasions, the serotonergic syndrome is induced by prescribed medications. This seems to be a common occurrence when combining drugs such as selective serotonin reuptake inhibitors (SSRIs), monoamine oxidase inhibitors, and opioid medications. Autistic individuals seem more susceptible to the side effects of these medications and are the most common cause for withdrawing from therapy. Parents and caretakers should be well aware of the black box warning instituted by the FDA for serotonin reuptake inhibitors (SSRIs). These drugs may lead to excessive emotional arousal and behavioral activation in children. Indeed, in young individuals their usage may lead to increased risk of suicidal thinking, feeling and behavior. The manifestation of any of these symptoms by an autistic individual should lead to urgent consultation and intervention by a physician. Besides withdrawal of any offending medication, treatment may include benzodiazepines, cyproheptadine (a drug that inhibits serotonin production), and intravenous fluids. With proper treatment, symptoms may disappear in less than 24 hours.
Casanova MF. Autism Updated: Symptoms, Treatments, and Controversies. Amazon Publishing, 2019.