Is Adult-Onset ADHD a Distinct Entity?

In this issue, Moffitt and colleagues (1) pose a fundamental question: Is adult attention deficit hyperactivity disorder (ADHD) a childhood-onset neurodevelopmental disorder? Their provocative answer, based on the first follow-back and follow-forward analysis of a longitudinal community sample is: not necessarily. For decades, ADHDand its nosological predecessorswere “known” to afflict only elementary school-age boys. By the early 1990s, longitudinal follow-up studies documented that children with ADHD did not simply “grow out of it” (2), even whenhyperactivity receded.Revisionof thediagnostic criteria in 1994 to encompass thepredominantly inattentive subtypeof ADHD increased the prevalence ofADHDdiagnoses in adults, adolescents, and females of all ages (3). Since then, ADHD investigators have extended their focus to adults. A paradigmatic, albeit untested, assumptionof this growing literature (Figure 1) has been that ADHD in affected adults represents a continuation of the childhood condition. This motivated the DSM-5 ADHD and Disruptive Behavior DisordersWorkGroup to assert that “ADHDbegins in childhood” and to provide formal criteria for its diagnosis in older adolescents and adults using the same items as are applied in children. In their study in this issue, Moffitt et al. examined the assumed continuity of ADHD in the Dunedin representative birth cohort of 1,037 individuals followed to age 38. Contemporaneous parent and teacher ratings identified 61 participants (6%) as meeting DSM-III criteria for ADHD in childhood. At age 38, 31 participants (3.1%) met DSM-5 criteria for ADHD, based on self-reports and informant reports. These prevalences are as expected. Surprisingly, however, the two sets of affected individuals barely overlapped. A grand total of threeparticipants exhibited the expected continuity from childhood to adulthood. This near-total lack of continuity has two parts. First, only about 5% of children in the Dunedin cohort with childhood ADHD continued to manifest the full DSM-5 syndrome in adulthood, although as a group they still exhibited substantial impairment. Moffitt and colleagues suggest that this rate of “decay” in meeting full diagnostic criteria is consistent with the limited extant literature (4). It is certainly far below the rate of 22%whomet full DSM-IV ADHD criteria in the New York Longitudinal Study at about the same age (5). Inevitable differences between representative community samples and clinically ascertained samples may account for some of this gap. This gap further supports a nosological lacuna, corresponding to residual ADHD in adulthood (4). The second unexpected finding is the emergence of a substantial group of individuals who met all DSM-5 criteria for ADHD except that of onset by age 12. About 90% of these individuals were de novo cases—they had not met childhood criteria, nor come close, and they differed in multiple ways from those who had. Childhood ADHD probands exhibited neurocognitive impairments in childhood that were largely maintained in adulthood. By contrast, in adult ADHD probands, general intellectual ability was comparable tothatofnon-ADHD comparison subjects, and neuropsychological impairments were negligible, both in childhood and in adulthood.Still,bothgroups reported marked subjectivecognitivedifficultiesas adults, and both exhibited objective evidence of psychosocial impairment: lower income, poor credit scores, cash flow problems, poor savings behavior, more government support, and higher numbers of insurance claims. The inescapable conclusion is that a substantial number of individuals in a representative community sample exhibit impairing symptoms that are consistent with ADHD in all aspects except childhood onset. What might explain this? The possibility of malingering was excluded by the lack of potential secondary gain. At age 38, prodromal dementia also seems unlikely. The possibility that the impairments might be secondary to substance use disorders cannot be dismissed, but seems insufficient, as 55% of the adult ADHD participants had no other current diagnoses at age 38 (1). As the authors write, the final “intriguing possibility is that adult ADHD is a bona fide disorder that has unfortunately been mistaken for the neurodevelopmental disorder of ADHD because of surface similarities, and given the wrong name” (1). Acceptanceof suchanovel entitywill require independent replication, but that is unlikely to be rapidly forthcoming, as The inescapable conclusion is that a substantial number of individuals in a representative community sample exhibit impairing symptoms that are consistent with ADHD in all aspects except childhood onset.