Our main finding was that the estimated prevalence of undiagnosed ADHD within substance use disorder clinic populations in South London is around 12%. This is of importance as we also found evidence that those individuals with both SUD and ADHD had significantly higher self-rated impairments across several domains of daily life; and higher rates of substance abuse and alcohol consumption, suicide attempts and depression, recorded in their case notes. Taken together these two findings highlight the negative impact of ADHD on the individual and the increased burden they most likely place on services. When considering the generalisablity of these findings, a limitation of this study is the focus on in-patient detoxification units in London. Further work is therefore required to evaluate rates and impact of ADHD among out-patient SUD populations and other national and international regions.
Impairments associated with comorbid ADHD and SUD
The finding of higher levels of impairment for ADHD cases within the SUD sample appears to be robust since we found indicators of impairment from both self-report and more objective (case note) measures. Appropriate assessment and management of ADHD in SUD patients would therefore seem to be potentially important to improve the general level of functional impairments and, particularly, given our finding that ADHD is associated with increased frequency of suicide attempts and depression. Whether treating ADHD in the context of SUD improves depression has yet to be adequately studied; yet we know that in people with ADHD symptoms such as mood instability and low self-esteem respond well to treatments for ADHD
[25–28]. Another finding was that the ADHD group had significantly higher use of stimulants such as cocaine and amphetamine (although not crack cocaine). One possible (but untested) explanation for this, given that stimulants are routinely used to treat ADHD in the general population, is that people with ADHD are using stimulants as a form of self-treatment. However an additional mechanism could be that people with ADHD have a preference for drugs that are more ‘stimulating’, especially when injected or taken at high dose. Further work is needed to address this issue.
Definition of ADHD and methodological issues
Our work attempted to address, as best we could, the inevitable methodological difficulties that might impact on accurate estimation of prevalence rates for ADHD within people with SUD. These include the potential for unreliable information from self-report questionnaires, difficulties with retrospective recall of childhood symptoms and the direct impact of drug and alcohol intoxication and withdrawal states on ADHD-like symptoms. We therefore attempted to measure ADHD symptoms before and after detoxification, included independent informant ratings whenever possible, compared these to population control data, and completed research diagnostic interviews. One additional confounder might arise if the study participants considered a positive diagnosis of ADHD as a way to obtain stimulant medication. However the study was not linked directly to the treatment of ADHD and expectations for treatment with stimulants are currently low in the UK, because in most cases ADHD in adults goes unrecognised and untreated
. Based on the percentage of patients who screened positive for ADHD in childhood and adulthood and the proportion of those interviewed we estimated an overall rate of ADHD of around 12% in our sample, much higher than the equivalent estimated worldwide prevalence of around 2.5% to 3.4% in non SUD populations
[2, 4, 29]. Using slightly broader criteria for ADHD to reflect different approaches to the diagnosis of adult ADHD being taken by different investigators and clinicians, we found the estimated prevalence in our sample increased to a maximum of 15%. Overall, the estimated prevalence of ADHD in our sample is far higher than population rates, yet lower than those cited in some previous studies of ADHD in SUD populations. There are several potential reasons for these differences in the findings from this and previous studies. One potential reason was our stringent application of the DSM-IV criteria, which might have led to an underestimate of the true rate of ADHD in the SUD population for several reasons. First, the inherent problem in collecting childhood data retrospectively might mean that some participants who met current criteria for ADHD may have been unable to recall sufficient examples of childhood symptoms. We recorded ‘unknown’ ratings in the diagnostic interview assessments if participants could not provide sufficient information to conclude that a symptom was present or absent and found that on average 15% of childhood symptoms could not be scored. Secondly, DSM-IV criteria only require that ‘some’ symptoms and impairments were present during early childhood and do not specifically require ≥6 symptoms from childhood so long at the symptom count criteria are currently met as an adult. Nevertheless, we decided to take the more stringent approach because of the lack of prospective data from childhood and to guard against inclusion of ADHD-like syndromes that might arise as a result of chronic drug abuse in the absence of an underlying ADHD diagnosis. Finally, we know that the current DSM-IV criterion are not adjusted to take into account age-related changes in the development of ADHD and there is evidence that ≥ 4 symptoms in adults, rather than the current ≥6 symptoms, is sufficient; indeed this change is being considered for the fifth revision of the DSM that is currently in preparation
[3, 30, 31]. However, taking all these alternative thresholds into account had only a minor impact on our estimate of the prevalence of ADHD in the SUD population.
The impact of drug detoxification on ADHD symptoms
We investigated the impact of drug intoxication and/or the detoxification process by evaluating self-rated ADHD symptom scores a few days after admission to the detoxification unit and one week later, when they were detoxified or stabilized on long term medication and no longer in a withdrawal state. We found significant decreases in ADHD symptoms of around 8-points (15% of the total score), which is a clinically significant reduction in ADHD symptoms and comparable to a one-level drop of the Clinical Global Impression Scale
. In terms of our screening criteria this led to 40% of patients no longer meeting screening criteria for ADHD at T2 compared to the T1. Hence prior studies may have reported a higher prevalence than we found due to the confounding effect of drug use and/or withdrawal symptoms. Other researchers have noted mood disturbances during alcohol detoxification
 and it is therefore possible that ADHD-like symptoms are also part of the withdrawal syndrome. Despite this, we found that in 60% of cases self-rated ADHD symptoms remained clinically significant following completion of the detoxification process. One implication is that although the withdrawal process may impact on the level of ADHD symptoms (and this should be taken into account when evaluating ADHD in SUD patients) there still remain a significant number of individuals with clinically significant symptoms of ADHD – and these require treatment.
Our findings suggest that clinical evaluations for ADHD are probably best completed once detoxification or stabilization for drug or alcohol dependency has been completed. However this suggestion may be difficult to implement in community patients. Furthermore, once diagnosis of ADHD has been established it will be important to offer treatment. Use of some pharmacological treatments (such as use of stimulants) is complex, however, in those with a current SUD. Nevertheless therapeutic nihilism is not an option as treatment of underlying ADHD may be important to the success of drug treatment programs for some individuals. It might be advisable to use the more stringent screening criteria of 6+ symptoms in either domain to identify those that need full clinical evaluation for ADHD, or to recognise the need for detailed ADHD assessment in a higher proportion of cases. Moreover it may be more appropriate to use non stimulant medications such as atomoxetine, as a first line treatment. Further work is required to address this issue, and the potential for using CBT based therapies for ADHD in this complex population. Obtaining informant data on ADHD symptoms for childhood and for current symptoms proved difficult in this population. The reasons for this were not investigated here, but likely reflect the often poor relationships that many SUD patients have with their family and friends. Therefore in clinical practice it will also often be the case that the diagnostic assessment of ADHD will depend on self-report alone. We were able to investigate the validity of these self-reported data by comparison of self-report with informant reported data in a subset of our sample and a comparison control sample. These showed moderate correlations between raters which were similar to that seen in non-SUD control populations. For the most impaired sub-group however, who were screening positive for ADHD based on their self-report, there was a far higher correlation with informant report for current ADHD ratings of around 0.62–0.65. We therefore suggest that while discrepancies between raters exist for ADHD rating scales, this does not appear to be different for SUD compared to control populations. Furthermore ratings showed moderately high levels of agreement for the group of patients with ADHD. Previous research has shown that in general people with ADHD tend to rate their symptoms lower than informants
 perhaps reflecting difficulties in self-evaluation of ADHD symptoms. Our research does not support this finding within the SUD population investigated here. When we completed diagnostic interviews with 26 people that screened positive for ADHD on the basis of their self-rated symptoms we found that only 61% met full criteria for ADHD. The clinical implication of this finding is that the diagnosis of ADHD in patients with SUD should not depend solely on rating scale data, but rather on the basis of more objective examples of symptoms characteristic of ADHD, as applied here using the DIVA interview. Rating scales are a valuable tool for screening for ADHD but should not be used as a replacement for a full diagnostic assessment by clinical interview. Furthermore, while informant reports are helpful in supporting the diagnosis, the moderately high correlations with self-report suggest that in most cases self-report alone should be sufficient. For in-patient SUD units it should also be feasible to observe patients for level of restlessness, problems with self-organisation, inattentiveness, impulsive responses and poor emotional regulation, that are characteristic of ADHD in adults.