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I had a few thoughts after reading the Siegel paper and their findings based on the use of Clinical Global Impressions - Improvement (CGI-I) ratings that "Forty-three percent of patients who received lithium were rated as "improved"". Mood disorders, or the symptoms of mood disorders, covers quite a bit of diagnostic ground. My recent discussions on bipolar disorder being fairly frequent in cases of Asperger syndrome (see here) coincide with the Siegel findings and particularly the case report by Frazier and colleagues  discussing a treatment regime which mentions the use of lithium. Other reports have similarly described the use of lithium as a possible management option where bipolar disorder and autism are comorbid . What this tells me is that Siegel et al were not the first to look at lithium and autism (with comorbidity).
A quick glance at the other peer-reviewed literature in this area suggests that lithium is also finding some favour where less idiopathic types of autism are present. The paper by Luiz & Smith  talking about lithium as a promising treatment for Fragile X syndrome represents another potentially important area. The precise mode of action is still the subject of some conjecture but the overview provided by Chiu & Chuang  (open-access) gives some indication of what might be going on and could be similarly mapped on to potential biological mechanisms linked to autism and mood disorder if and when comorbid.
Finally, I have to make some mention about the important links being made between the use of lithium and the prevention of suicide in mood disorders . I know it's not exactly a topic which makes great dinner party conversation but the emerging evidence base, alongside other important compounds, could potentially be life-saving for some people. Without trying to brush everyone on the autism spectrum as being at risk from suicide, the growing body of evidence suggesting that suicide ideation (see here) or suicide attempts (see here) might be more frequent for those on the autism spectrum  is something that needs to be taken seriously. This may imply that alongside appropriate societal support being provided, lithium might also have some important role to fulfil for some people...
 Siegel M. et al. Preliminary Investigation of Lithium for Mood Disorder Symptoms in Children and Adolescents with Autism Spectrum Disorder. J Child Adolesc Psychopharmacol. 2014 August 5.
 Frazier JA. et al. Treating a child with Asperger's disorder and comorbid bipolar disorder. Am J Psychiatry. 2002 Jan;159(1):13-21
 Kerbeshian J. et al. Lithium carbonate in the treatment of two patients with infantile autism and atypical bipolar symptomatology. J Clin Psychopharmacol. 1987 Dec;7(6):401-5.
 Liu Z. & Smith CB. Lithium: A Promising Treatment for Fragile X Syndrome. ACS Chem Neurosci. 2014 May 15.
 Chiu CT. & Chuang DM. Molecular actions and therapeutic potential of lithium in preclinical and clinical studies of CNS disorders. Pharmacol Ther. 2010 Nov;128(2):281-304.
 Cipriani A. et al. Lithium in the prevention of suicide in mood disorders: updated systematic review and meta-analysis. BMJ. 2013 Jun 27;346:f3646.
 Paquette-Smith M. et al. History of Suicide Attempts in Adults With Asperger Syndrome. Crisis. 2014; 35: 273-277.
Siegel M, Beresford CA, Bunker M, Verdi M, Vishnevetsky D, Karlsson C, Teer O, Stedman A, & Smith KA (2014). Preliminary Investigation of Lithium for Mood Disorder Symptoms in Children and Adolescents with Autism Spectrum Disorder. Journal of child and adolescent psychopharmacology PMID: 25093602