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Adult attention-deficit/hyperactivity disorder: Associations between subtype and lifetime substance use – a clinical study

[version 2; peer review: 3 approved]
PUBLISHED 19 Oct 2016
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Abstract

ADHD is the one of the most prevalent childhood disorders and has been associated with impairments persisting into adulthood. Specifically, childhood ADHD is an independent clinical risk factor for the development of later substance use disorders (SUD). Moreover, adults who meet diagnostic criteria for ADHD have shown high rates of comorbid SUDs. Few studies, however, have reported on the relationship between ADHD subtypes and SUD in adult samples. The purpose of this study was to characterize a clinical sample of adults with ADHD and to identify possible associations between ADHD subtypes, lifetime substance use, and if ADHD subtypes may be preferentially associated with specific substances of abuse. We recruited 413 adult ADHD patients, performed an evaluation of their ADHD and conducted an interview on their use of psychotropic substances. Complete data was obtained for 349 patients. Lifetime substance abuse or dependence was 26% and occasional use was 57% in this sample. The inattentive subtype was significantly less likely to abuse or be dependent on cocaine than the combined subtype. Our findings underscore the high rate of comorbidity between substance use and ADHD in adults. The more frequent abuse/dependence of cocaine by adult patients with hyperactive-impulsive symptoms should be kept in mind when treating this patient group.

Keywords

Attention deficit hyperactivity disorder (ADHD), subtype, presentation, substance-use disorder, cocaine, dual-diagnosis

Revised Amendments from Version 1

We would like to thank the reviewers for their comments on this manuscript, which has been edited to address their feedback concerning the following points: In the Methods section of the paper the assessment of  ADHD subtype (I) substance use (II) and comorbidities (III) was clarified; the Discussion section has been expanded to describe (IV) neurobiological findings among ADHD patients with a comorbid cocaine dependence and (V) to discuss more thoroughly limitations of this study, especially in regards to co-occurring personality disorders (VI).  The conclusion section now includes a more detailed description of findings among patients with ADHD and a comorbid cocaine dependence receiving methylphenidate, in an effort to comment on the relevance of the presented findings for therapeutic approaches (VII). Additionally the references have been updated (VIII).

See the authors' detailed response to the review by Swantje Matthies and Eliza Hoxhaj
See the authors' detailed response to the review by Marc Auriacombe, Jean-Marc Alexandre and Melina Fatseas

Introduction

Attention-deficit/hyperactivity disorder (ADHD) is a complex neuropsychiatric syndrome that is common not only in childhood and adolescence, but in adulthood14. It is characterized by symptoms of inattention (distractibility), hyperactivity, and impulsivity, which all contribute to significant psychosocial impairment in affected individuals of all age groups57. In order to make a diagnosis of ADHD, the two diagnostic manuals, the American Psychiatric Association’s (APA) Diagnostic and Statistical Manual of Mental Disorders Third Edition Revision (DSM-III-R) and the World Health Organisation’s (WHO) International Statistical Classification of Diseases and Related Health Conditions (ICD-10), require the presence of both inattentive and hyperactive-impulsive symptoms8,9.

In 1994, the introduction of the DSM Fourth Edition (IV)10 marked a diversion from this route by allowing for a diagnosis of ADHD when either hyperactive-impulsive or inattentive behaviors were present, and thereby defined three subtypes of ADHD: a) a predominantly inattentive type, b) a predominantly hyperactive-impulsive type, and c) a combined type11. The following years saw a significant amount of research in which the importance of these subtypes in a clinical and epidemiological context was debated. For example, the clinical response to pharmacologic treatment by subtype or symptom clusters was investigated1214, as were subtype differences in psychosocial functioning15,16, and the rate of comorbidity17 in different age groups.

In both pediatric and adult populations, ADHD is significantly comorbid with a wide range of other DSM-IV disorders, irrespective of subtype. The most prevalent of these are mood, anxiety, impulse control, and substance use disorders (SUD)1820. Data that stem primarily from clinical and population-based studies suggest that up to 89% of all adults with ADHD suffer from a psychiatric comorbidity during their lifetime16, and that the comorbidity of SUD in adolescents and adults with ADHD might range from 16%–79%16,2123. The heterogeneity of these data is also reflected in research focused on the association between specific ADHD subtypes and SUDs. While some authors find no evidence of such an association, others have concluded that the hyperactive-impulsive subgroup is more likely to suffer from a comorbid SUD than is the inattentive subgroup15,22,24.

To our knowledge, few studies have reported on the relationship between ADHD subtypes and SUD in adult samples. Furthermore, the limited data available stems primarily from America, while the few European studies focused on several comorbid factors, not solely on SUD16,25. The purpose of this study was therefore to characterize a clinical sample of adults with ADHD and to identify possible associations between ADHD subtypes, lifetime substance use, and preferences for specific substances.

Methods

Sample

Out of all consecutive referrals to the ADHD consultation service of the Zurich University Psychiatric Hospital26 between 2002 and 2011, we included adults with a confirmed diagnosis of ADHD and with available information on substance use (N=413). There were no other inclusion or exclusion criteria.

Assessment of ADHD symptomatology

The diagnosis of ADHD was based on the Utah criteria for diagnostic assessment with the Wender Reimherr Interview (WRI)27, and translated into German and validated for the German language by Rösler et al. and Retz-Junginger et al.2830. The Wender-Reimherr Interview is the German version of the American Wender-Reimherr Adult Attention Deficits Disorders Scale (WRAADDS) for the assessment of adult ADHD. It allows a diagnosis of adult ADHD to be made. It contains seven scales for: attention difficulties, persistent motor hyperactivity, temper, affective lability, emotional overreactivity, disorganization, and impulsivity. Each scale is represented by 3–5 items. A sum score is formed per scale, and each scale has a diagnostic threshold. A diagnosis requires that sum scores for scales 1–2 must each exceed their threshold, and that for scales 3–7, 2 out of 5 sum scores must exceed their threshold. According to DSM-IV Text Revision (TR)31 specifications, three ADHD subtypes were identified: a predominantly inattentive subtype, a predominantly hyperactive-impulsive subtype, and a combined subtype. Subtypes were derived from the Attention Deficit-/Hyperactivity Self-Report Scale (ADHS-SB) questionnaire (see Supplementary material). The ADHS-SB is a self-rating instrument for the assessment of adult ADHD in German. It consists of 18 symptoms of ADHD derived from the DSM-IV and ICD-10 criteria for ADHD. The degree of endorsement is rated on four levels: 0 = not at all, 1 = slightly, 2 = moderately, and 3 = severely. The total score is obtained by summing up the 18 individual item scores. Subtype scores were obtained by first summing the respective items (items 1–9 for “inattentive”, items 10–18 for “hyperactive-impulsive”). Then, a cut-off value of 6 had to be exceeded in order for the respective subtype to be assigned. Subjects exceeding the threshold for both the inattentive and hyperactive-impulsive type were assigned to the combined subtype. Note that not all subjects fulfilled subtype criteria. The total number of subjects with a subtype assignment was 327. As reported elsewhere32, patients also received a number of questionnaires, including German versions of the Symptom Check List 90-Revised (SCL-90-R)33, the Wender Utah Rating Scale (WURS-k)28, and the ADHS-SB34. If patients did not answer all questions on the questionnaire items, they were approached again and asked to supply the missing information. When patients had difficulty answering a question, their therapist helped to clarify it and enable them to provide an answer. In addition, third-party information was sought from family members, spouses, school reports, and childhood medical reports to support the diagnostic procedure.

Assessment of substance use and comorbidity

Assessment of substance use was based on ICD-10 criteria (F10-F19)9. Subjects reported on the lifetime use of alcohol, opioids, cannabinoids, sedatives, cocaine, (non-cocaine) stimulants, hallucinogens, and tobacco. ICD-10 criteria were applied by a highly experienced clinician (DE) in a semi-structured interview. No official instrument was used. Substance use was differentiated into abuse/dependence and sub-threshold, i.e. non-dependent and non-abusive, but more-than-singular, use. Comorbid disorders were diagnosed according to ICD-10 by DE in a semi-structured interview, again without an official instrument.

Statistical analysis

Fisher’s exact tests were used to compare frequency of substance abuse/dependence and comorbidity rates between ADHD subtypes, since small cell sizes were frequent. Kruskal-Wallis tests were used to compare questionnaire scores. Bonferroni correction was applied to all substance-related significance tests. A total of 26 tests were conducted, resulting in a Bonferroni-corrected significance threshold of p ≤.002. P-values surviving this threshold are printed in boldface in the results section. The study has low power: assuming a power of 80%, the minimal detectable difference in substance use frequency among subtypes is between 25–36%, while the power to detect a difference of 10% ranges from 28–48%. Analyses were carried out in Stata 11.2 and Stata 13.135.

Ethical framework

Authorization by the local ethics committee (Cantonal Ethics Committee Zurich; Kantonale Ethik Kommission Zürich (KEK)) was obtained before the study was conducted (04/2005). All participants received a written description of the study procedure and signed a consent form.

Results

A total of 64 subjects had no questionnaire data whatsoever and were dropped from further analysis. These “drop-outs” were compared with the remaining 349 subjects and found not to differ in age and gender distribution. Drop-outs more often had affective disorders (24.9% vs. 12.7%, p=.05). They tended to have less overall substance abuse or dependence (14.1% vs. 27.8%, p=.02). Total substance use excluding abuse and dependence was clearly lower in drop-outs (23.4% vs. 63.6%, p=.000).

The average age of the included sample was 38.7 years (SD = 11.28), with a gender distribution that was 56% male and 44% female. Other than substance use, the most common comorbidities included affective disorders (25%); neurotic, stress-related and somatoform disorders (15%); and personality disorders (6%).

In the sample with questionnaire data (N=332–345, depending on questionnaire participants reached average test scores of 35.4 (SD=14.51) on WURS-k, 28.5 (SD=9.77) on ADHS-SB and 17.6 (SD=7.87) on the newly developed SCL-ADHD scale18. A total of 233 subjects were identified as belonging to the combined subtype of ADHD (test scores: ADHS-SB 32.9 [SD=7.69], WURS-k 37.5 [SD=13.91], SCL-ADHD 19.4 [SD=7.62]), 70 belonged to the predominantly inattentive type (test scores: ADHS-SB 20.7 [SD=5.57], WURS-k 30.1 [SD=13.59], SCL-ADHD 14.2 [SD=6.52]), and 24 belonged to the predominantly hyperactive-impulsive type (test scores: ADHS-SB 23.9 [SD=6.68], WURS-k 40.8 [SD=16.16], SCL-ADHD 16.4 [SD=7.28]). WURS-k (p<.04) and ADHS-SB (p<.0001) scores were different between inattentive and hyperactive-impulsive subtypes, while all scores were different at p<.004 for the comparison of inattentive vs. combined subtype.

According to ICD-10 F1x, 26% of all participants at the time of the study, regardless of subtype, fulfilled the criteria for abuse of or dependence on psychotropic substances other than nicotine. The most frequently misused substances consisted of alcohol (8.9%), opioids (6.0%), cannabinoids (8.3%), and cocaine (8.0%). Nicotine abuse/dependence was found in 20.3% of participants.

Subtype-specific analyses revealed that 36.9% of the combined subgroup, 44.3% of the predominantly inattentive subgroup, and 41.7% of the hyperactive-impulsive subgroup currently suffered from a comorbid psychiatric disorder. Additionally, 31.3% of the combined-type individuals, 15.7% of the predominantly inattentive subjects and 41.7% of hyperactive-impulsive patients were diagnosed with abuse or dependence on a psychotropic substance other than nicotine. Table 1 summarizes the results.

Table 1. Lifetime substance use by ADHD subtype (p-values surviving Bonferroni threshold [p ≤ .002] in boldface).

Inattentive
type
Hyperactive-
impulsive
typea
Combined
type
pb
inatt-hyp
pb
inatt-
combined
N7024233
%%%
Nicotine abuse/dependence12.912.524.91.0.03
Opiates abuse/dependence1.412.57.3.05.08
Stimulants abuse/dependence7.14.29.41.0.64
Alcohol abuse/dependence1.48.311.2.16.01
Cannabis abuse/dependence5.716.78.6.20.61
Cocaine abuse/dependence012.510.3.02 .002
Substances total (w/o tobacco)
abuse/dependence
15.741.731.3.02.01
Substances total (w/o tobacco) use57.162.566.5.81.16

bFischer's exact test

inatt-hyp = inattentive vs. hyperactive-impulsive subtype, inatt-combined = inattentive vs combined subtype

Discussion

The present study investigated associations between the combined and predominantly inattentive subtypes of adults with ADHD and lifetime substance use, within a clinical sample. The most clinically significant result is the finding that the inattentive subtype showed a statistically significantly smaller rate of cocaine abuse/dependence compared to the combined subtype.

These results are in line with earlier work by Sobanski et al., who had characterized a sample of 118 adults with ADHD and found that the combined type suffered significantly more from lifetime SUDs (48.4%) than did patients with a predominantly inattentive type (23.3%)16. On the other hand, our findings contrast with results published by Clure et al., who reported on 43 patients with adult ADHD but found no differences in ADHD subtypes when divided by substance of choice (cocaine, alcohol, and multiple substances)36.

The most frequently consumed substance among all study participants was nicotine. This finding is in accord with results from prior studies3739. With regard to subtype-specific differences, some authors have reported that, at least in young adolescents, the inattentive subtype of ADHD is more likely to correlate with higher levels of nicotine use than does the combined subtype40. It was suggested that nicotine might primarily improve attention but have less influence on hyperactive-impulsive behavior, which might explain this finding41,42. Other researchers, however, suggest that hyperactive-impulsive symptoms present a greater risk for frequent nicotine use than do inattentive symptoms at a later age, and argue that the relationship between ADHD symptoms and nicotine use might change between adolescence and adulthood43.

Our hypothesis that findings would show continuing preferences for the use of specific substances in adulthood according to subtype (beyond cocaine), remains open due to lack of statistical significance. Like earlier reports of (non-cocaine) stimulants being used as self-medication by patients with ADHD, we had also expected to find a higher rate of non-prescribed lifetime stimulant abuse/dependence in the hyperactive-impulsive type, but not in the inattentive one44,45. In this sample, however, we found no evidence for this assumption, but lack of statistical power precludes interpreting this as evidence of no difference. We suspect that adults with both hyperactive-impulsive and inattentive symptoms might initially prefer cocaine to stimulants for self-medication, but there is no direct evidence for this assessment4648.

The possibility of using cocaine as an attempt to self-medicate for ADHD symptoms was originally proposed in the early ’90s49,50. More recently, Saules et al. compared the symptom profile among adult ADHD smokers with and without cocaine dependence, and found that when they corrected for the use of nicotine, adults who used cocaine exhibited a more severe adult ADHD symptom profile, as accounted for by the presence of elevated hyperactive-impulsive but not inattentive symptoms. He therefore suggested that cocaine use in smokers with ADHD might be driven by excesses in hyperactivity50. Despite differences in sampling, our results are in accord with this finding.

On a different note van Wingen et al investigated structural brain abnormalities in this population and reported of significantly smaller grey matter volumes in the occipital cortex as well as smaller volumes in the putamen in ADHD patients with comorbid cocaine dependence when compared to those without this lifetime diagnosis. The authors of aforementioned study suggested that the differences in putamen volumes may reflect alterations in the availability of striatal dopamine transporters that are available for interaction with methylphenidate, thus giving some explanation for the finding that methylphenidate is less effective in patients with ADHD and a comorbid cocaine dependence51.

The main limitation of this study is low power. This means, in particular, that non significant findings cannot be interpreted as evidence of no difference. A further limitation is that our sample was recruited entirely within a university setting, which might contribute to a selection bias. As a result, this clinical sample might have different characteristics than patients would exhibit who are in treatment with a physician in private practice. Nevertheless, the ADHD consultation service of the Psychiatric University Hospital Zurich is the largest institution of its kind in Switzerland and attracts patients from diverse psychosocial backgrounds. Furthermore comorbidities, particularly personality disorders might have confounded the results. For instance, Borderline personality disorder, which often co-occurs with ADHD and is difficult to differentiate, is also known to be associated with SUD52. However in this sample that relied for diagnosis of comorbidity on a semi-structured clinical interview, but not on additional instruments, we found only 6% of patients suffering from a comorbid personality disorder. This is low in comparison to some studies reporting prevalence rates between 25 – 78%5357.

In conclusion, our findings underscore the high rate of comorbidity between substance use and ADHD in adults. The more frequent abuse/dependence of cocaine by adult patients with hyperactive-impulsive symptoms should be kept in mind when treating this patient group. Although a limited number of evidence-based treatment strategies currently exist for the concurrent treatment of ADHD and SUD, some studies suggest that stimulant medication remains an efficacious pharmacological treatment option that improves symptoms of ADHD without increasing the likelihood of relapse into SUD22,58. Furthermore a study among patients with ADHD and a comorbid cocaine dependence receiving methylphenidate, demonstrated an advantage over placebo with regard to reduction in cocaine use in individuals who responded to ADHD treatment59,60.

Data availability

ZENODO: Dataset 1. Contains all the variables necessary to reproduce the results of Adult attention-deficit/hyperactivity disorder: Associations between subtype and lifetime substance use – a clinical study, Liebrenz et al., doi: 10.5281/zenodo.1962361

ZENODO: Stata source code to reproduce analysis, doi: 10.5281/zenodo.1962262

Consent

Written informed consent was obtained from patients.

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Liebrenz M, Gamma A, Ivanov I et al. Adult attention-deficit/hyperactivity disorder: Associations between subtype and lifetime substance use – a clinical study [version 2; peer review: 3 approved]. F1000Research 2016, 4:407 (https://doi.org/10.12688/f1000research.6780.2)
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Reviewer Report 07 Nov 2016
Swantje Matthies, Department of Psychiatry and Psychotherapy, University Medical Center Freiburg, Freiburg, Germany 
Eliza Hoxhaj, Department of Psychiatry and Psychotherapy, University Medical Center Freiburg, Freiburg, Germany 
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Matthies S and Hoxhaj E. Reviewer Report For: Adult attention-deficit/hyperactivity disorder: Associations between subtype and lifetime substance use – a clinical study [version 2; peer review: 3 approved]. F1000Research 2016, 4:407 (https://doi.org/10.5256/f1000research.10511.r17430)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
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Reviewer Report 17 Oct 2016
Joseph O'Neill, Division of Child and Adolescent Psychiatry, UCLA Semel Institute for Neuroscience and Human Behavior, Los Angeles, CA, USA 
Approved
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The investigators determined the effect of ADHD subtype on the prevalence of substance use disorders (SUDs) in a large (413 patients) sample of (medicated?) adult ADHD. In line with expectations, the overall sample evinced high rates of lifetime substance abuse ... Continue reading
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O'Neill J. Reviewer Report For: Adult attention-deficit/hyperactivity disorder: Associations between subtype and lifetime substance use – a clinical study [version 2; peer review: 3 approved]. F1000Research 2016, 4:407 (https://doi.org/10.5256/f1000research.7284.r17021)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
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Reviewer Report 21 Sep 2016
Swantje Matthies, Department of Psychiatry and Psychotherapy, University Medical Center Freiburg, Freiburg, Germany 
Eliza Hoxhaj, Department of Psychiatry and Psychotherapy, University Medical Center Freiburg, Freiburg, Germany 
Approved with Reservations
VIEWS 35
The paper "Adult attention-deficit/hyperactivity disorders: Associations between subtype and lifetime substance use – a clinical study” covers, besides the well-known comorbidity of ADHD with substance use disorders (SUDs), a very interesting field of research concerning the associations between ADHD subtypes ... Continue reading
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Matthies S and Hoxhaj E. Reviewer Report For: Adult attention-deficit/hyperactivity disorder: Associations between subtype and lifetime substance use – a clinical study [version 2; peer review: 3 approved]. F1000Research 2016, 4:407 (https://doi.org/10.5256/f1000research.7284.r16482)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
  • Author Response 19 Oct 2016
    Michael Liebrenz, Psychiatric University Hospital, Research Group on ADHD, Zurich, 8032, Switzerland
    19 Oct 2016
    Author Response
    We appreciate the dedicated and useful comments of Dr. Swantje and Dr. Hoxhaj and the editor
    and the chance they gave us to improve the manuscript.
     
    1. The
    ... Continue reading
COMMENTS ON THIS REPORT
  • Author Response 19 Oct 2016
    Michael Liebrenz, Psychiatric University Hospital, Research Group on ADHD, Zurich, 8032, Switzerland
    19 Oct 2016
    Author Response
    We appreciate the dedicated and useful comments of Dr. Swantje and Dr. Hoxhaj and the editor
    and the chance they gave us to improve the manuscript.
     
    1. The
    ... Continue reading
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Reviewer Report 17 Feb 2016
Marc Auriacombe, Addiction Psychiatry Team, Sanpsy CNRS USR 3413, University of Bordeaux, Bordeaux, France;  Pôle Addictologie, CH Ch. Perrens / CHU Bordeaux, Bordeaux, France 
Jean-Marc Alexandre, Pôle Addictologie, CH Ch. Perrens / CHU Bordeaux, Bordeaux, France;  Addiction Psychiatry Team, Sanpsy CNRS USR 3413, University of Bordeaux, Bordeaux, France 
Melina Fatseas, Pôle Addictologie, CH Ch. Perrens / CHU Bordeaux, Bordeaux, France;  Addiction Psychiatry Team, Sanpsy CNRS USR 3413, University of Bordeaux, Bordeaux, France 
Approved
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In this clinical study, Liebrenz et al. addressed the association between Adult attention-deficit/hyperactivity disorder (ADHD) subtypes and lifetime substance-use disorders (SUD). This topic is of particular interest (Fatseas et al 2012), as there is a strong literature supporting a relationship ... Continue reading
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Auriacombe M, Alexandre JM and Fatseas M. Reviewer Report For: Adult attention-deficit/hyperactivity disorder: Associations between subtype and lifetime substance use – a clinical study [version 2; peer review: 3 approved]. F1000Research 2016, 4:407 (https://doi.org/10.5256/f1000research.7284.r11890)
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
  • Author Response 19 Oct 2016
    Michael Liebrenz, Psychiatric University Hospital, Research Group on ADHD, Zurich, 8032, Switzerland
    19 Oct 2016
    Author Response
    We appreciate the time and effort of Dr. Auriacombe, Dr. Alexandre and Dr. Fatseas and respond to their concerns below: 

    1.The reviewers ask us to explain the usefulness of ... Continue reading
COMMENTS ON THIS REPORT
  • Author Response 19 Oct 2016
    Michael Liebrenz, Psychiatric University Hospital, Research Group on ADHD, Zurich, 8032, Switzerland
    19 Oct 2016
    Author Response
    We appreciate the time and effort of Dr. Auriacombe, Dr. Alexandre and Dr. Fatseas and respond to their concerns below: 

    1.The reviewers ask us to explain the usefulness of ... Continue reading

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VERSION 2 PUBLISHED 28 Jul 2015
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Alongside their report, reviewers assign a status to the article:
Approved - the paper is scientifically sound in its current form and only minor, if any, improvements are suggested
Approved with reservations - A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit.
Not approved - fundamental flaws in the paper seriously undermine the findings and conclusions
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