There is a growing interest (and controversy) about ‘adult’ onset ADHD. No current diagnostic system allows for the diagnosis of ADHD in adulthood, yet clinicians sometimes face adults who meet all criteria for ADHD, except for age at onset. Although many of these clinically referred adult onset cases may reflect poor recall, several recent longitudinal population studies have claimed to detect cases of adult onset ADHD that showed no signs of ADHD as youth (Agnew-Blais, Polanczyk et al. 2016, Caye, Rocha et al. 2016). They conclude, not only that ADHD can onset in adulthood, but that childhood onset and adult onset ADHD may be distinct syndromes (Moffitt, Houts et al. 2015).
In each study, the prevalence of adult onset ADHD was much larger than the prevalence of childhood-onset adult ADHD). These estimates should be viewed with caution. The adults in two of the studies were 18-19 years old. That is too small a slice of adulthood to draw firm conclusions. As discussed elsewhere (Faraone and Biederman 2016), the claims for adult onset ADHD are all based on population as opposed to clinical studies. Population studies are plagued b the “false positive paradox”, which states that, even when false positive rates are low, many or even most diagnoses in a population study can be false.
Another problem is that the false positive rate is sensitive to the method of diagnosis. The child diagnoses in the studies claiming the existence of adult onset ADHD used reports from parents and/or teachers but the adult diagnoses were based on self-report. Self-reports of ADHD in adults are less reliable than informant reports, which raises concerns about measurement error. Another longitudinal study found that current symptoms of ADHD were under-reported by adults who had had ADHD in childhood and over-reported by adults who did not have ADHD in childhood (Sibley, Pelham et al. 2012). These issues strongly suggest that the studies claiming the existence of adult onset ADHD underestimated the prevalence of persistent ADHD and overestimated the prevalence of adult onset ADHD. Thus, we cannot yet accept the conclusion that most adults referred to clinicians with ADHD symptoms will not have a history of ADHD in youth.
The new papers conclude that child and adult ADHD are “distinct syndromes”, “that adult ADHD is more complex than a straightforward continuation of the childhood disorder” and that that adult ADHD is “not a neurodevelopmental disorder”. These conclusions are provocative, suggesting a paradigm shift in how we view adulthood and childhood ADHD. Yet they seem premature. In these studies, people were categorized as adult onset ADHD if full-threshold ADHD had not been diagnosed in childhood. Yet, in all of these population studies there was substantial evidence that the adult onset cases were not neurotypical in adulthood (Faraone and Biederman 2016). Notably, in a study of referred cases, one-third of late adolescent and adult onset cases had childhood histories of ODD, CD and school failure (Chandra, Biederman et al. 2016). Thus, many of the “adult onsets” of ADHD appear to have had neurodevelopmental roots.
Looking through a more parsimonious lens, Faraone and Biederman (2016)proposed that the putative cases of adult onset ADHD reflect the existence of subthreshold childhood ADHD that emerges with full threshold diagnostic criteria in adulthood. Other work shows that subthreshold ADHD in childhood predicts onsets of the full-threshold ADHD in adolescence (Lecendreux, Konofal et al. 2015). Why is onset delayed in subthreshold cases? One possibility is that intellectual and social supports help subthreshold ADHD youth compensate in early life, with decompensation occurring when supports are removed in adulthood or the challenges of life increase. A related possibility is that the subthreshold cases are at the lower end of a dimensional liability spectrum that indexes risk for onset of ADHD symptoms and impairments. This is consistent with the idea that ADHD is an extreme form of a dimensional trait, which is supported by twin and molecular genetic studies (Larsson, Anckarsater et al. 2012, Lee, Ripke et al. 2013). These data suggest that disorders emerge when risk factors accumulate over time to exceed a threshold. Those with lower levels of risk at birth will take longer to accumulate sufficient risk factors and longer to onset.
In conclusion, it is premature to accept the idea that there exists an adult onset form of ADHD that does not have its roots in neurodevelopment and is not expressed in childhood. It is, however, the right time to carefully study apparent cases of adult onset ADHD to test the idea that they are late manifestations of a subthreshold childhood condition.
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