168 Chapter 8 Accordingly, the described etiopathogenesis of NSSI is diverse. Several studies found environmental or social risk factors for NSSI, like childhood trauma, attachment problems and dysfunctional interpersonal relationships, as well as individual risk factors like emotion regulation problems (32, 34). Paivio and McCulloch (2004) (35) showed that difficulties to identify and express emotional experience appropriately (i.e. alexithymia) mediated the relation between childhood trauma (except sexual abuse) and NSSI . Most studies on NSSI describe emotion regulation problems as the primary source. Typically, an increase in negative emotionality is observed before engaging in NSSI, which is reduced after NSSI, resulting in a positive rewarding experience (32). Ammerman e.a. (2018) (36) reviewed experimental studies using physical aversive (painful) stimuli like heat, electric shocks or cutting to induce NSSI related responses and found that when NSSI functioned as self-punishment, NSSI individuals had a less intense pain response to painful stimuli than controls (36). There are several models of NSSI. Favaro & Santonastaso (2000) (37) distinguish two forms of NSSI: 1) impulsive NSSI, described as an impulsive act functioning as an episodic relief after increasingly build up tension (e.g. skin cutting, burning), and 2) compulsive NSSI, described as a compulsive act expressed as a habitual, repetitive, non-functional motor behavior (e.g. hair pulling, skin picking) . Others (38) have proposed a four-factor model, which states that NSSI can be reinforced by either intrapersonal of interpersonal motives, and can be either positively reinforcing (by generating a positive feeling) or negatively reinforcing (by reducing a negative feeling). This model is supported by empirical evidence and is widely used for categorizing functionality of NSSI (29, 33). Klonsky & Muehlenkamp (2007) (29) have extended this model to seven groups of functions or motivations of NSSI: 1) affect regulation, 2) self-punishment, 3) interpersonal influence, 4) anti-dissociation, 5) anti-suicide, 6) sensation seeking, 7) interpersonal boundaries and self-control. In summary, NSSI seems to be a maladaptive emotion regulation strategy with (internal and external) reinforcing and rewarding properties (32). Neurobiology of NSSI There are several neurotransmitter systems involved in NSSI. First, the endogenous opioid system, because it is involved in pain perception, pain relief and reduction of negative affect, reward and motivational processes (39, 40). Evidence for the relation between NSSI and the endogenous opioid system is based on altered endogenous opioid levels, reduced pain sensitivity and successful opioid antagonist treatment in NSSI (36, 40, 41). Risk factors for NSSI such as childhood trauma and disrupted attachment are also related to changes in opioid levels (39, 40, 42).
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