Psychobiology of Impulse-Control Disorders
Not Otherwise Specified (NOS)
Stefano Pallanti, Nicoló Baldini Rossi, Jennifer Friedberg and Eric Hollander
Impulsivity can be defined as the failure to resist an impulse, drive or temptation that is harmful to oneself or others (Hollander et al., in press). Impulsivity is a measurable aspect of behaviour, manifesting as impatience (including the inability to delay rewards), carelessness, risk-taking, sensation- and pleasure-seeking, an underestimated sense of harm, and extraversion. The subjective experience of an impulse involves also an increasing sense of arousal or tension before committing/engaging in the act and an experience of pleasure, gratification or release of tension at the time of committing the act. Aggressive behaviour is a conduct that inflicts harm upon oneself or others. Behaviour can be impulsive without being aggressive (for example, a person who engages in pathological gambling is behaving in an impulsive manner, but not in an aggressive manner). Likewise, aggressive behaviour can lack impulsivity (as in a premeditated murder). Impulsive disorders cause large costs to society, and are associated with substantial morbidity, mortality, social/family/job dysfunction, accidents, suicide, violence, aggression, criminality, and excessive utilization of health-care, government and financial resources (Hollander et al., in press).
Largely on the basis of the varying theoretical and clinical approaches of the myriad scientific and professional disciplines studying impulsivity and aggressive behaviours in animal models or in humans, impulsivity and aggressiveness are conceptualized and diagnosed in an unusually broad and disparate fashion. Just as anxiety and depression may be conceptualized either as symptoms or as specific disorders, impulsivity may be distinguished as a symptom or as a distinct disorder.
In psychiatric classification, impulsivity is a core symptom of a broad spectrum of disorders, including the impulse-control disorders (impulse-control disorders not elsewhere classified, comprising pathological gambling [PG], intermittent explosive disorder, pyromania, kleptomania, and trichotillomania, and impulse-control disorder not otherwise specified [NOS]), the impulsive-aggressive personality disorders (borderline, antisocial), the neurological disorders that can be associated with disinhibition of behaviour (such as epilepsy), and substance abuse (Hollander and Rosen, 2000). Of interest, addictive behaviour could also be described as conduct resulting from failure to inhibit impulses that urge and seek tension relief or pleasure. Furthermore, other psychiatric conditions also contribute to the expression of impulsivity, notably attention deficit/hyperactivity disorder (ADHD), mania, and eating disorders (Hollander and Rosen, 2000). The impulse-control disorders may belong to a family of compulsive-impulsive spectrum disorders lying at opposite ends of the dimension of risk avoidance, with impulsive disorders driven by pleasure or arousal, and compulsive disorders driven by reduction of anxiety (Hollander, 1998). Both impulsive and compulsive disorders involve a failure to resist a drive to act in a way that is potentially self-damaging, escalation of anxiety before engaging in the act, and relief of anxiety following the act. In fact, one of the few differences between the two types of disorders is that most compulsive behaviour disorders are perceived by the patient as ego-dystonic, whereas impulsive behaviours are usually viewed as ego-syntonic, at least in the impulsive setting. Rather than being the dimensional opposite of obsessive-compulsive disorders, impulse-control disorders may represent a different phenomenological manifestation of a group of disorders sharing the feature of decreased ability to inhibit motor responses to affective states.
We are at a very early stage in our understanding of the neurobiology of impulsivity and aggression (Kavoussi et al., 1997). Thus, it is noteworthy that meanings and definitions of impulsivity and aggressiveness differ greatly in psychiatry and in neurobiology. Moreover, no simple extrapolation of animal subtypes to humans is possible, mainly because of the influence of complex cultural variables on behaviour. On the whole, research into the subtypes of human impulsivity has been rather limited. Much of this has been conducted in children. Clinical observation, experimental paradigms in the laboratory, and cluster/factor analytical statistics have all been used in an attempt to subdivide impulsivity and aggression. A consistent dichotomy can be identified for aggression between an impulsive-reactive-hostileaffective subtype and a controlled-proactive-instrumental-predatory subtype. Although good internal consistency and partial descriptive validity have been shown, these constructs still need full external validation, especially regarding their predictive power for comorbidity, treatment response, and long-term prognosis (Vitiello and Stoff, 1997).
In attempting to find parallels between aggression in humans and aggression in animals, Gregg and Siegel (2001) observed that whereas both humans and animals exhibit aggressive behaviour, animals do not engage in the premeditated aggressive acts that humans often display. Vitiello et al., (1990), in distinguishing between affective aggression and predatory aggression, emphasize that the first is impulsive, occurring as a result of autonomic arousal, while the second is cold-blooded and premeditated, and is not a result of arousal. Due to the presence of the frontal cortex, humans have the capacity to engage in both affective and predatory aggression, while animals engage only in affective aggression, which is aggression related to survival and increased arousal. Impulsivity has been classified in to three different types: motor impulsivity, impulsivity without programming, and attentive impulsivity; the last may be represented by an exaggerated alert reaction (Barratt and Stanford, 1995).