The aim of this study was to explore the prevalence of a comorbid ADHD-BPD diagnosis and its impact on the clinical presentation of borderline personality disorder adolescents, and to explore which type of impulsivity is specifically associated with BPD-ADHD adolescents. To our knowledge, this is the first study investigating ADHD in BPD in this specific age group.
Concerning the prevalence of ADHD diagnosis in our sample, we found a current rate of 11%. This result is close to the 16% rate found by Philipsen and colleagues in a sample of adult BPD female patients [19
], notwithstanding some methodological differences between the studies. In the Philipsen's study, current ADHD was diagnosed by self-assessment using the short version of the WURS (for childhood ADHD symptoms) and the adult ADHD-Checklist, whereas in our study diagnosis was ascertained by experienced clinicians using a valid and reliable diagnostic interview integrating all relevant data from the clinical records of the patients, including parental reports. Although the current prevalence observed here may appear not very high, up to 46% of the subjects presented at least one symptom with a clinical or subclinical significance and some impact on functioning in the ADHD screening, eventually qualifying for a diagnosis of ADHD-NOS. It is interesting to note that symptoms of inattention, hyperactivity and impulsivity were evenly distributed across the sample. This points to the fact that all types of ADHD symptoms, not solely impulsivity, are frequently found in BPD adolescents. Moreover, comorbidity rates did not change when diagnosis was made without including impulsivity, thus reducing the criticism of an overestimation of ADHD diagnosis in BPD due to symptom overlap.
The results of this study also show that the presence of a comorbid ADHD diagnosis influences the clinical presentation of BPD in adolescents. ADHD in BPD was significantly associated with a greater likelihood of disruptive disorders (particularly ODD) and with a trend for a greater likelihood of other cluster B personality disorders (histrionic, narcissistic and antisocial personality disorders). This result is not surprising since in longitudinal studies, ODD in childhood as well as antisocial behaviours in adolescence and adulthood have been frequently observed as main outcomes for ADHD children [41
]. Impulsivity has been suggested as an important mediator of this negative outcome among ADHD children [43
]. The role played by impulsivity in the relationship between ADHD and outcome was indirectly suggested in our study by the observation of higher levels of impulsivity on all Barratt subscales (although significant only for the Attentional/Cognitive subscale) and in the specific domain of impulsivity on the DIB-R in the BPD-ADHD group. The impulsivity dimension of the DIB-R includes several externalizing behaviours, driven by impulsivity, such as substance abuse, promiscuous sex, reckless driving or self-harming/suicidal behaviours. A reverse tendency on the DIB-R was observed in the domain of cognition, with borderline adolescents without ADHD showing a clinical profile characterized by more internalising symptoms such as odd thinking, unusual perceptual experiences or paranoid/quasi-psychotic experiences. This dual dissociation on the DIB-R indices between BPD and BPD-ADHD adolescents moderates the conclusions reached by Philipsen and colleagues [19
], suggesting that this association might not be equivalent to a more severe form of the borderline disorder, but could correspond to a specific subtype of BPD with high impulsivity associated with an ADHD profile. This hypothesis is in line with recent conclusions drawn by Ferrer and colleagues [21
] who have suggested that BPD patients should be distinguished in two subgroups according to the presence or absence of ADHD, with the former subgroup showing a specific profile of impulsive comorbidity. Moreover, these results recall the ICD-10 conceptualization of the emotionally unstable personality disorders, which specifically includes an impulsive sub-type alongside the typical borderline profile [45
]. Our study suggests that the ICD-10 impulsive sub-type could be more developmentally driven, with ADHD symptoms persisting since childhood. This proposal could be of interest for the possible inclusion of a developmental perspective in the DSM classification of personality disorders. A similar proposal for differentiating borderline patients according to specific developmental features has already been suggested by Andrulonis [46
] who, in a sample of DSM-III BPD adults, identified a separate group of patients showing severe hyperactivity, distractibility and/or learning disabilities and episodes of behavioral dyscontrol. This group reported hyperactive and aggressive behaviours during childhood and antisocial acting-out with drug/alcohol abuse during adolescence but, like our sample, did not show any micro-psychotic episodes. This association also supports one of the developmental routes to BPD suggested by Nigg [12
], which he has termed as the primary impulsivity route, as opposed to the traumatogenic route more related to severe disruptions in early caregiving experiences and mainly affecting the development of affect regulation. For this author, this impulsive BPD subgroup could arise from weak executive response inhibition mechanisms, leading to extremes of impulsivity, behavioural disturbances during childhood, inappropriate interpersonal relations, and a cascade of negative socialization experiences leading to personality disturbances. From a temperamental perspective, specific features related to impulsivity in ADHD children, such as Novelty Seeking, have also been found to increase the risk of development of BPD in adulthood [47
]. Data supporting this theoretical perspective have been reported by Lampe and colleagues [48
] who assessed various motor and cognitive inhibitory functions in adult ADHD patients, with and without BPD, compared to subjects with BPD alone and controls. In this study, ADHD subjects (whether or not comorbid with BPD) had higher scores than BPD subjects on all behavioural subscales of the BIS and showed impaired inhibition on the Attentional Network Task (Stop and Interference). Conversely, BPD subjects (without comorbid ADHD) did not differ from their matched controls, a result which led the authors to conclude that an impairment of inhibitory control could be a core deficit of BPD only when associated with ADHD. This result suggests that the cognitive component of inhibitory control may play a specific role in the phenomenology of the impulsive/developmental sub-type of BPD. Results from the regression analysis of our study showed a specific association between Barratt's Attentional/Cognitive Impulsiveness and ADHD diagnosis in borderline adolescents. The Attentional/Cognitive impulsivity of the BIS-11 involves several clinical features in the domain of attention and of cognitive stability: the inability to inhibit irrelevant information held in working memory and to focus on the task at hand leading to distractibility [49
]; and an excessive cognitive speed in decision-making [50
] with an aversion to externally imposed delays [51
] leading to cognitive and behavioural mistakes or acting-out behaviours, especially under emotional conditions [52
]. Attentional impulsivity has been linked to the the dorsolateral prefrontal cortex [53
] whereas cognitive impulsivity has been correlated to the orbitofrontal/ventromedial areas of the prefrontal cortex, especially the more anterior sector of this region, the frontal pole [49
]. Some preliminary results support the hypothesis that orbitofrontal/ventromedial prefrontal dysfunction may underlie some of the behavioural manifestations of BPD-ADHD patients [54
], but more data are needed, especially in adolescent samples.
Some limitations of the current study must be taken into consideration when interpreting the findings.
First, the main limit of the study is its cross-sectional design with data on childhood ADHD diagnosis collected retrospectively. Only longitudinal studies can directly support the identification of the developmental pathways leading from childhood to adult psychopathology. This is even more important if we consider that these diagnostic constructs tend to overlap, particularly in the realm of impulsivity. However cross-sectional studies on comorbid disorders in specific populations, such as adolescents, can shed light on their clinical presentation and help identifying their specific therapeutic needs. Moreover, although indirectly, the high diagnostic stability between past and current ADHD diagnosis found in our study supports the hypothesis of a subtype of BPD with a childhood history of ADHD, hypothesis that has been recently confirmed by Stepp and colleagues in their longitudinal study on adolescent girls [25
The second limit concerns the small sample size of the study and the potential sample selection bias of the screening phase conducted by the consulting clinicians without performing a systematic between-center inter-rater reliability. This may have reduced the statistical power of the analyses and the generalizability of the results.
For instance, our sample included a majority of female patients. It is commonly agreed that ADHD is less frequent in females, with a predominance of purely inattentional forms. It is possible that the high levels of impulsive features associated with ADHD could be due to a referral bias of our specific clinical sample composed of severe forms of BPD female adolescents. Although the size of the sample of BPD participants was reasonable compared to other studies, particularly since it was limited to adolescents with a well-characterized BPD diagnosis, results should be interpreted with caution as to know what the likelihood might be that the sample is actually representative of BPD adolescents.
Finally, to assess impulsivity, we used the validated adult version of the Barratt Impulsiveness Scale. Although the use of the adult version of the BIS-11 in adolescents can be found in the literature on impulsivity [56
], it could have been interesting to use the adolescent version of the scale which has been shown to present a different structure from the adult one [58