"Psychiatric comorbidities in children with autism spectrum disorder (ASD) are rather a rule than an exception."
That was the opening sentence to the paper by Verheij and colleagues  (open-access available here) who charted the stability of such comorbidity in a "7-year follow-up of 74 6-12 year old children with Pervasive Developmental Disorder-Not Otherwise Specified [PDD-NOS]."
Continuing a theme from this research group  looking longitudinally at what happened to individuals who "were initially referred for diagnostic evaluation to the Department of Child and Adolescent Psychiatry/Psychology of the Erasmus Medical Center—Sophia Children’s Hospital between July 2002 and September 2004", researchers examined various comorbid psychiatric disorders such as anxiety and mood disorder(s) via the use of the Diagnostic Interview Schedule for Children IV Parent Version (DISC-IV-P) over the course of two testing waves (aged 6-12 years and aged 12-20 years).
They found that: "The rate of comorbid psychiatric disorders dropped significantly from childhood (81 %) to adolescence (61 %)." Specifically, the frequency of anxiety disorders dropped from 55% in wave 1 (6-12 years) to 31% in wave 2 (12-20 years) (based on an average follow-up time between waves of nearly 7 years). The frequency of social phobia also seemed to drop between the testing waves (~11% vs. 1%) as did the frequency of specific phobias (40% vs. 25%); these differences were noted to be significant.
But it was not all good news, as we are told that: "The rates for externalizing (i.e. disruptive) disorders did not significantly change from childhood (i.e. 61 %) to adolescence (i.e. 51 %)"; this categorisation including diagnoses such as attention-deficit hyperactivity disorder (ADHD) (various types) and conduct disorder. Indeed also, when it came to issues such as major depressive disorder, the authors noted a non-significant increase in the frequency of this label examined over the course of the testing waves. Further: "Of the individuals who had no comorbid psychiatric disorder in childhood (n = 14), 50 % (n = 7) stayed free of a comorbid psychiatric disorder in adolescence, whereas 50 % (n = 7) of the individuals developed at least one comorbid psychiatric disorder in adolescence."
In terms of predicting stability across the comorbidity and non-comorbidity groups - "the “persistent presence” group (n = 38) versus the “presence to absence” group (n = 22)" - parent-reported stereotyped behaviours and reduced social interest in childhood as measured on the Children’s Social Behavior Questionnaire (CSBQ) seemed to play something of a role, albeit with the requirement for much further study.
Set within the context of PDD-NOS falling into at least some descriptions of the autism spectrum (newer diagnostic criteria don't mention this category) these are interesting results. That certain psychiatric comorbidity wax and wane as a function of issues such as maturation for example, provides something of a ray of hope that these often disabling comorbidity (yes, anxiety can be utterly disabling) may not always be set in stone. At least that is, with regards to their reaching clinical significance and diagnostic thresholds.
Following a trend in autism research suggesting that the core traits of autism are also probably more dynamic than anyone has hitherto appreciated (see here), I might also advance the idea that where issues such as anxiety wane, so to this might have an effect on the core presentation of autism in terms of things like intervention success for example (see here). I might also refer you back to the idea of optimal outcome (OO) in relation to autism and what this might also mean for the presence of psychiatric comorbidity (see here).
The suggestion that certain psychiatric elements however may not be as likely to retreat when it comes to some autism, such as certain types of ADHD and/or major depressive disorder, remains a worrying prospect. As per the Myriam De-la-Iglesia / José-Sixto Olivar discussion piece  (see here for my take) on depression and at least some autism, the often far-reaching effects of depression in conjunction with autism is something that really does need to be tackled, and tackled effectively. Not least because of the "high index of depression in this collective emphasises the need to detect suicidal tendencies." A sad but all too real outcome I'm afraid (see here).
I end with a few caveats to bear in mind about the Verheij data: "Results are based on parental interviews in a relatively small sample of individuals with PDD-NOS with an average to high IQ who were referred to one particular center, thus clinicians should make careful considerations regarding their own specific clients, and further research on samples with more phenotypic variation in ASD severity and cognitive ability using multiple informants is needed." That also there were gaps in important variables such as medication history and other more socially-related variables should also be noted.
Still, this kind of longitudinal research is the kinda thing that autism science really needs to do a lot more of, allied to the idea that psychiatric comorbidity is probably over-represented when it comes to the label of autism .
To close, the Japanese trailer for the next Star Wars instalment has some new scenes to tantalise...
 Verheij C. et al. The Stability of Comorbid Psychiatric Disorders: A 7 Year Follow Up of Children with Pervasive Developmental Disorder-Not Otherwise Specified. J Autism Dev Disord. 2015 Oct 12.
 Louwerse A. et al. ASD Symptom Severity in Adolescence of Individuals Diagnosed with PDD-NOS in Childhood: Stability and the Relation with Psychiatric Comorbidity and Societal Participation. J Autism Dev Disord. 2015 Sep 22.
 De-la-Iglesia M. & Olivar JS. Risk Factors for Depression in Children and Adolescents with High Functioning Autism Spectrum Disorders. ScientificWorldJournal. 2015;2015:127853.
 Russell AJ. et al. The mental health of individuals referred for assessment of autism spectrum disorder in adulthood: A clinic report. Autism. 2015 Oct 15. pii: 1362361315604271.
Verheij C, Louwerse A, van der Ende J, Eussen ML, Van Gool AR, Verheij F, Verhulst FC, & Greaves-Lord K (2015). The Stability of Comorbid Psychiatric Disorders: A 7 Year Follow Up of Children with Pervasive Developmental Disorder-Not Otherwise Specified. Journal of autism and developmental disorders PMID: 26456972