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The long-term impact of ADHD has been the subject of considerable deliberation, in particular the manifestation of symptomatology in adulthood. Some 50 – 80% of diagnosed children will continue to experience ADHD symptoms into adolescence (Barkley) and approximately one third of the individuals diagnosed with ADHD in childhood will meet the criteria in adulthood (Swanson et al.). In an attempt to recognize the heterogeneity of the disorder, DSM introduced three subtypes of ADHD, predominantly inattentive type (ADHD-I), predominantly hyperactive – impulsive type (ADHDHI), and combined type (ADHD-C), which distinguished between individuals in terms of symptoms and area of impairment (Lahey et al.; Wolraich, Hannah, Pinnock, Baumgaertel, & Brown). There has been some debate about the appropriateness of the subtypes, for example Barkley has argued that inattention related to ADHD-I is qualitatively different to that found in the other two subtypes and may actually be a separate disorder. Barkley also suggested that ADHD HI and ADHD-C are developmental stages of the same subtype. Others have argued that the high prevalence of comorbid disorders warrants that ADHD be subtyped according to comorbid patterns (e.g., Biederman et al.; Hudziak & Todd; Jensen, Martin, & Cantwell). An example is the proposed ADHD + CD subtype (e.g., Faraone, Biederman, Mennin, Russell, & Tsuang; Kuhne, Schachar, & Tannock) which is distinct in terms of its earlier age of onset, greater female to male ratio, decreased likelihood of remission, and poorer long-term outcome (Jensen et al.). Moreover, the ADHD + CD group is at greater risk for substance abuse and antisocial personality disorder (APD) than either disorder alone (Hechtman; Whitmore et al.).
There does appear to be a relationship between ADHD and drug use problems, but the association is not definitive and is complicated by the high rate of comorbidity between ADHD and CD. Nevertheless, the association between ADHD and drug use is a significant issue and warrants further investigation, particularly given the high estimated rates of SUD (Substance Use Disorder) among adults with ADHD. Barkley approximated that between 10% and 37% of ADHD adults meet the criteria for SUD and Wilens, Biederman, Mick, Faraone, and Spencer and Milin, Loh, Chow, and Wilson reported somewhat higher SUD prevalence rates of approximately 50%. Factors hindering the investigation of this relationship lie in the lack of definitive criteria for diagnosing ADHD in adulthood and the array of drug usage definitions employed, which range from measures of use to abuse to dependence. This study examined the prevalence of ADHD and CD among a sample of drug-using adults and investigated the relationship of adult ADHD with a number of variables including several different measures of drug use. The strength of this study lies in its use of DSM- based ADHD measures and in its acknowledgement of the heterogeneity of drug use, which allowed examination of potential relationships of ADHD to various aspects of drug involvement: associations that might well be overlooked if a single facet of drug use was considered. It was hypothesized that drug users with ADHD would experience more severe symptomatology on measures of childhood ADHD, CD, antisocial behavior, and drug use when compared to non-ADHD drug users.
Participants were men aged 18 years or older currently using alcohol or illicit substances and engaged with drug use agencies. Because ADHD, CD, and antisocial behavior all occur much more frequently in the male population, it would have been difficult to locate a sufficient number of female participants. Data were obtained from 95 participants aged from 18 to 65 years.
This study found high rates of both childhood and adult ADHD symptomatology in a sample of male drug users, approximately 36% and 46% respectively, which is consistent with previous research suggesting up to 50% co-occurrence of adult ADHD and drug use problems (Milin et al.; Wilens, et al.). The general population has significantly lower rates of ADHD, with the ATAP study (Hay et al.) finding a prevalence of approximately 5% in children and the QIMR study (Hay et al.) identifying rates of approximately 7% in adults. The present study found a high prevalence of CD (68%), compared with only 9.2% in the ATAP sample. It is noteworthy that 97% of participants in this study who met the criteria for childhood ADHD also had CD, which unfortunately precluded analyses exploring differences between ADHD + CD, ADHD-only, and CD-only groups. Examination of differences between adult ADHD subtypes revealed that the combined type and hyperactive – impulsive type had higher CD symptomatology than the inattentive type. Other studies have similarly found a lower incidence of CD in children with ADHD-I when compared to ADHD-C (Eiraldi, Power, & Nezu; Lalonde, Turgay, & Hudson) and ADHD-HI children (Lalonde et al.), but more than children in the normal population (Eiraldi et al.; Wolraich et al.). The remaining comparisons between the subtypes were found to be not significant. This finding should be considered preliminary at best given inadequate power.
Participants with adult ADHD had higher childhood symptomatologies in terms of inattention, hyperactivity – impulsivity, and conduct disorder as well as antisocial behavior in adulthood. Moreover, their drug use was more severe in terms of psychological dependence, substance use (excluding alcohol), and quantity and frequency of use. Similarities can be drawn between the findings of this study and others that have identified associations between underlying symptomatology, severity of drug use, and ADHD (e.g., Burke et al.; Fischer & Barkley; Molina & Pelham; Tercyak et al.). Interestingly, there were no differences between drug users with ADHD and those without in terms of alcohol use, highlighting the importance of recognizing the heterogeneity in drug involvement and considering different aspects of drug use when examining the relationship between ADHD and substance use.
The issue of comorbidity is worthy of comment. Comorbidity occurs when two or more disorders cooccur more frequently than the expected rate of occurrence of the individual disorders in the general population (Angold, Costello, & Erkanli). Comorbidity can manifest in several ways. Substance use disorders, for example, may be associated because one disorder is the cause of the other. Or there may be no causal relationship but rather two disorders share common underlying risk factors, such as biological and environmental vulnerabilities (Degenhardt, Hall, & Lynskey). Nosological considerations that might impact on the rate of comorbidity have also been raised, such as overlapping diagnostic criteria (Caron & Rutter) where the presence of one disorder equates to partial fulfillment of the criteria for the second disorder (Angold & Costello). APD, for example, includes criteria that are both directly and indirectly related to substance misuse such as engaging in illegal behavior, being arrested, conning others, and stealing. Such behaviors, however, are significant features of both pathologies and if consideration is given to ‘‘. . .only the criteria not shared between disorders we will end up ignoring many key symptoms’’ (Angold et al.). Thus although this study has demonstrated high rates of ADHD and CD among drug users, the issues underlying the co-occurrence should be borne in mind and further research targeting the source of the comorbidity would enable exploration of the theoretical underpinnings of the demonstrated empirical relationships.
The findings of the present study indicate an overrepresentation of ADHD among
drug users, both in childhood and adulthood, compared to the rates typically
identified in the general population. Similarly, the comorbidity between ADHD
and CD is significantly higher than general population estimates. Neither of
the comparison groups likely provides exact population data but are all that
is available given the dearth of Australian normative ADHD data. The high co-occurrence
of problematic drug use, ADHD, and CD has significant treatment implications.
In light of the finding that there are differences between drug users with
and without ADHD in the extent of their drug usage, clinicians should investigate
the presence of adult ADHD among drug users seeking treatment as part of their
assessment. In doing so, attention should be given to differential diagnoses
and conditions whose symptoms may mimic or overlap with ADHD symptomatology
or be a consequence of substance use (e.g., withdrawal or intoxication). The
difficulties differentiating some psychiatric disorders from substance-related
symptomatology and behaviors related to a broader substance-abusing lifestyle
have been well documented (e.g., Ball; Carey & Correia; Shaner et al.).
Hence although this study has demonstrated that comorbidity between ADHD and
CD among drug users is a valid and important consideration, clinicians should
remain alert for ADHD-like symptoms that may be a manifestation of substance
abuse or other disorder. Where appropriate, intervention should be tailored
to address both drug use issues and ADHD symptomatology with particular attention
to the impact that ADHD symptomatology may have upon drug use behaviors. The
utility of pharmacological treatment of ADHD as a component of intervention
should also be evaluated. In addition, the high occurrence in this sample of
childhood ADHD and CD and their association with adult drug use highlights
the importance of monitoring the extent of drug use behaviors throughout childhood
and adolescence. Further research is required to determine whether ADHD subtypes
can be differentiated in terms of drug use and thus allow intervention to be
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