Non-suicidal self-injury (NSSI), the self-infliction of bodily harm without apparent suicidal intent, is a potent risk factor for suicidal ideation and suicidal behaviour.^{Reference Chesin, Galfavy, Sonmez, Wong, Oquendo and Mann1} Although NSSI and suicidal behaviour are distinct constructs, both forms of self-harm frequently co-occur. NSSI and suicidal behaviour share key instrumental functions, such as escape from aversive internal states, reducing dysphoria or communicating distress.^{Reference Klonsky2} Some individuals also report engaging in NSSI to ameliorate suicidal thoughts or urges,^{Reference Klonsky2} a possible by-product of NSSI's regulating effect on mood. In an adolescent sample, nearly half of those who self-harmed reported using NSSI to diminish the urge to attempt suicide.^{Reference Nixon, Cloutier and Aggarwal3} However, evidence of NSSI's mitigating impact on suicidal ideation is limited to subjective reports of its perceived effectiveness,^{Reference Brausch and Muehlenkamp4} and it remains unclear whether in fact NSSI is associated with subsequent reductions in suicidal ideation. This important to establish, since the success of NSSI's anti-suicidal function may encourage recurrent self-injury.

Despite NSSI's perceived effectiveness in mitigating suicidal states, self-harming suicidal individuals have demonstrably higher long-term risk of suicidal behaviour relative to non-self-harming suicidal individuals.^{Reference Chesin, Galfavy, Sonmez, Wong, Oquendo and Mann1} Elevated suicide risk among those who self-harm is consistent with the interpersonal theory of suicide,^{Reference Joiner, Ribeiro and Silva5} which suggests that repeated self-harm may promote a capability for suicide over time by habituating individuals to the fear of physical pain or injury that might otherwise discourage suicidal behaviour. In the short term, however, NSSI may serve to lower acute suicide risk, consistent with findings that individuals with suicidal ideation who self-harm take longer to make the transition to attempted suicide compared with non-self-harming suicidal individuals.^{Reference Bryan, Bryan, May and Klonsky6} As yet, the functional relationship between NSSI and suicidal states – particularly its short-term effect on suicidal ideation – is not well characterised, partly because of methodological challenges inherent in observing these experiences in research settings. However, recent advances in smartphone-based technology have stimulated new research using ecological momentary assessment (EMA) to study NSSI in real-world contexts.^{Reference Rodriguez-Blanco, Carballo and Baca-Garcia7} EMA involves repeated, real-time sampling of participants’ thoughts and behaviours over a discrete period, overcoming issues of recall bias, forgetting or post hoc reasoning that influence retrospective reporting.

Prior EMA studies have assessed triggers and consequences of NSSI, largely concentrating on NSSI's affective dynamics,^{Reference Rodriguez-Blanco, Carballo and Baca-Garcia7} but none to date have quantitatively evaluated NSSI's real-world effect on suicidal ideation. We sought to address this gap by using real-time monitoring of NSSI and suicidal ideation over 7 days in adults with borderline personality disorder (BPD), a clinical population with exceptionally high rates of NSSI, suicidal ideation and suicidal behaviour.^{Reference Black, Blum, Pfohl and Hale8} We hypothesised that episodes of NSSI would lead to a reduction in suicidal ideation in the subsequent hours.

Results

Sample description

Participants (n = 82) were predominantly female (91%) and White (57%), with an average age of 29.4 years (s.d. = 9.5 years). All participants had a history of either NSSI (n = 76, median 38 episodes) and/or suicide attempt (n = 59, median 1). A complete sample characterisation is published elsewhere.^{Reference Stanley, Martínez-Alés, Gratch, Rizk, Galfalvy and Choo9}

EMA responses

We obtained a mean of 27 responses per participant (64% completion rate). Ninety-one per cent of epochs had a non-zero level of suicidal ideation (suicidal ideation: median 7, IQR = 4–11); 37 participants (45%) reported one or more NSSI episodes (median 0, IQR = 0–2).

Suicidal ideation before and concurrent with NSSI

We examined change in suicidal ideation prior to NSSI (in addition to following NSSI) to provide a broader view of NSSI's temporal relationship with suicidal ideation. Mixed-effects models indicated that a 1 point increase in suicidal ideation change from the epoch prior to the epoch concurrent with NSSI (t − 1 to t) predicted 15.2% greater odds of NSSI (OR = 1.15, 95% CI 1.11–1.19, P < 0.001). Total suicidal ideation concurrent with NSSI (t) predicted greater likelihood of NSSI in that epoch (OR = 1.15, 95% CI 1.12–1.17, P < 0.001).

Suicidal ideation following NSSI

Participants reported a mean 1.77 unit reduction (i.e. change) in suicidal ideation following epochs with NSSI, which significantly differed from the mean 0.05 unit increase in suicidal ideation reported following epochs without NSSI (Table 1). To assess whether suicidal ideation change following NSSI may be related to suicidal ideation severity, we compared mean suicidal ideation following epochs with or without NSSI; there were no significant differences.

Table 1 Separate single-predictor linear models of (a) the effect of non-suicidal self-injury (NSSI) on mean suicidal ideation in the following epoch and (b) the effect of NSSI on change in suicidal ideation in the following epoch