Measuring the shadows: A systematic review of chronic emptiness in borderline personality disorder

Background Chronic feelings of emptiness is an under-researched symptom of borderline personality disorder (BPD), despite indications it may be central to the conceptualisation, course, and outcome of BPD treatment. This systematic review aimed to provide a comprehensive overview of chronic feelings of emptiness in BPD, identify key findings, and clarify differences between chronic feelings of emptiness and related constructs like depression, hopelessness, and loneliness. Method A PRISMA guided systematic search of the literature identified empirical studies with a focus on BPD or BPD symptoms that discussed chronic feelings of emptiness or a related construct. Results Ninety-nine studies met criteria for inclusion in the review. Key findings identified there were significant difficulties in defining and measuring chronic emptiness. However, based on the studies reviewed, chronic emptiness is a sense of disconnection from both self and others. When experienced at frequent and severe levels, it is associated with low remission for people with BPD. Emptiness as a construct can be separated from hopelessness, loneliness and intolerance of aloneness, however more research is needed to explicitly investigate these experiences. Chronic emptiness may be related to depressive experiences unique to people with BPD, and was associated with self-harm, suicidality, and lower social and vocational function. Conclusions and implications We conclude that understanding chronic feelings of emptiness is central to the experience of people with BPD and treatment focusing on connecting with self and others may help alleviate a sense of emptiness. Further research is required to provide a better understanding of the nature of chronic emptiness in BPD in order to develop ways to quantify the experience and target treatment. Systematic review registration number: CRD42018075602.

Introduction 'To define accurately what the word [emptiness] means in any context can feel like trying to find a light switch in a totally dark and unfamiliar room' [1, p. 331].
Borderline personality disorder (BPD) is a complex mental disorder characterised by a pervasive instability of self-concept, emotions, and behaviour [2]. Globally, lifetime prevalence of BPD is estimated at approximately 6% [3], but individuals with BPD can account for up to 20.5% of emergency department presentations and 26.6% of inpatient psychological services [4]. Within personality disorder research, the landscape of formulation and diagnosis is evolving, and there is a need to research features of BPD which are important in both traditional categorical and emerging dimensional approaches [5]. Current diagnosis for BPD involves identifying a minimum five of nine possible criteria in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) [2]. One criterion is labelled chronic feelings of emptiness. This symptom remains in the alternative diagnostic model for BPD in DSM-5, where it is associated with identity disturbance.
Feelings of chronic emptiness have always been included in the conceptualisation and diagnosis of BPD [6]. In an early seminal paper, Deutsch [7] described a group of people who experience inner emptiness in their emotional life, a feeling where 'all inner experience is completely excluded. It is like the performance of an actor who is technically well trained but who lacks the necessary spark to make his impersonations true to life' [7, p. 328]. This experience was described as resulting in a 'chameleonlike quality' in interpersonal relationships, where pretence and adaptability masks the emptiness underneath [8]. Chronic feelings of emptiness has also been described as akin to 'deadness', 'nothingness', a 'void', feeling 'swallowed' [9], a sense of 'vagueness' [10], a feeling of internal absence [11], 'woodenness' [12], a 'hole' or 'vacuum', 'aloneness' [1], 'isolation' [13], 'numbness' and 'alienation' [14].
There are several theoretical views of chronic emptiness in BPD. According to early theoretical literature, people who experience chronic feelings of emptiness lack the capacity to experience themselves, others, or the world fully and there is 'a profound lack of emotional depth or sense of not being in the experience' [9, p. 34, 11]. Kernberg [8,15] suggested that emptiness results from a loss of, or disturbance in, the relationship of self with object relations, with a lack of integrated representations leading to an absence of 'self-feeling' [16,17]. Other analysts similarly proposed that emptiness results from deficits in maintaining stable object relations [18][19][20] and an inability to develop soothing and holding introjects-meaning difficulties with internalising positive and nurturing experiences [21,22], perhaps resulting from the absence of a 'good enough' caregiver [23,24]. Overall, these analysts attributed emptiness to the absence of a good maternal presence, resulting in unstable object-and self-representations and a feeling of inner emptiness. This theory was supported in part by an early study by Grinker and colleagues [25] which found inadequate awareness of self was sufficient for predicting BPD group membership, including a deficiency in recognising internal thoughts and affects as belonging to oneself and an associated feeling of chronic emptiness. Chronic feelings of emptiness were proposed to drive 'the basis of his attempt to appropriate from others, or of his feeling of danger of being engulfed by others. Some try to borrow from others, become satellitic to another, merge with a host or lay skin to skin. Others attempt to fill up with knowledge or experience' [25, p. 16]. These early concepts are still utilised within contemporary psychodynamic approaches to personality assessment, diagnosis and treatment, with a focus on emptiness reflecting disturbances of identity [26,27].
Biosocial models of BPD suggest that chronic feelings of emptiness are reflective of a dysregulation of identity [28]. Emptiness is conceptualised as an attempt (whether conscious or not) to inhibit intense emotional experiences, which leads to a lack of development in personal identity [29]. It is hypothesised chronic emptiness results from insecure attachments with caregivers [30], and transactional models propose emptiness is the experience of an individual not knowing their own personal experience, resulting from inconsistent validation and invalidation responses by caregivers [31]. This understanding is similar to attachment and mentalisation perspectives, where feelings of emptiness reflect a failure in mentalisation. Specifically, emptiness is a consequence of the absence of the psychological self-the secondary representation of self which allows an understanding of one's own internal world, and the world seen through the eyes of others [32].
Despite the numerous theories that mention emptiness, there remains no unifying theory of chronic emptiness in BPD, and it is not typically accounted for in aetiological models of BPD [33]. Further, the symptom has rarely been the focus of empirical research [1]. Substantial empirical literature exists for other symptoms of BPD (e.g. affective instability [34] and impulsivity [35]), but until recently there has been a limited focus on chronic feelings of emptiness. There appears to be confusion within the field regarding what chronic emptiness actually is, with vague boundaries between constructs like hopelessness, loneliness, or boredom [36] and with research often referring to each term interchangeably.
Despite this lack of clarity within the research, recent studies have shown an increased focus on chronic emptiness, suggesting the experience may be associated with vocational and interpersonal dysfunction [37, 38] and self-harm and suicidal behaviours [39]. Research has also linked chronic emptiness to depressive experiences unique to people with BPD-a possible 'borderline depression' [40].
In order to better understand what chronic emptiness is and the importance of chronic feelings of emptiness to the conceptualisation, course, and outcomes of BPD it is important to first analyse the current literature to provide a baseline for future work. In particular, it is important to identify any research that supports theoretical claims that chronic emptiness is a reflection of impaired relationships with the self and others. It is also important to identify research that reports on whether chronic emptiness represents a single construct or if it encompasses other experiences, such hopelessness, loneliness or depression. In order to achieve this, the current study aimed to systematically review empirical research on chronic emptiness and related terms in populations with features or a diagnosis of BPD. Considering there are currently no detailed reviews, a broad focus was employed that is unrestrictive to interventions and outcomes. A cohesive analysis of the empirical literature will enable an understanding of the current state of the field, and provide directions for future research.

Information sources
Electronic databases searched included PsycINFO, PubMed, Scopus, and Web of Science. The last search date was February 2019. Additional records known to authors which were not captured in original database searching were added.

Search
The search strategy for online studies remained the same across databases and included (Empt � or isolat � or vacuum or dead or deadness or nothing � or void or swallowed or bored � or numb � or alien � or wooden � or hole or alone � or vague � or hopeless � or lonel � ) AND (borderline personality disorder or BPD or emotionally unstable personality disorder). Truncation was used in search terms to capture variations in terminology.

Eligibility and inclusion criteria
Studies were eligible for analysis if they met the following criteria: a) Research focusing on individuals with features or diagnosis of BPD and community populations endorsing features of BPD that b) contain novel empirical data (quantitative, qualitative, or mixed methods, excluding systematic reviews and case studies), c) are peer-reviewed, d) discuss findings related to chronic emptiness or a related construct in their results or discussion, and e) meet quality assessment.
Due to the limited nature of the research on chronic feelings of emptiness, eligible studies were not restricted by intervention type, comparison, or outcomes. Further, there was no time limit set on searches in order to capture early data regarding emptiness. Language was not restricted as translating software was used.
Study selection. Articles which did not meet initial screening criteria were excluded. Articles were then screened by title and abstract by two reviewers for inclusion in full-text review. Disagreement on inclusion of articles for screening was resolved by discussion and advice with another reviewer. Following full text screening, articles were further excluded if they a) were unable to be translated and authors could not be contacted, and b) contained keywords which were discussed only in the context of Schema therapy and Schema modes (e.g. lonely child mode).
Risk of bias in individual studies. Following the selection of articles for full-text review, quality was assessed using the Mixed Methods Appraisal Tool (MMAT)-Version 2011. The MMAT has good interrater reliability and content has been validated [42][43][44]. Although the MMAT is yet to be validated in clinical samples, the absence of a gold standard quality assessment for appraisal of observational studies necessitates the use of modified assessments [45]. Two screening questions were asked for all study types prior to further quality assessment; 'are there clear research questions or objectives?' and 'do the collected data address the research questions?' The observational descriptive quantitative component of the MMAT was used to examine quantitative studies. This encompasses several factors including appropriate sampling methods, justification of methods and acceptable response rates [43]. The qualitative component of the MMAT was also used, which similarly included factors of appropriate sampling and justification of methods, in addition to understanding the context of information and influence of researchers' on results.
Studies which satisfied all other eligibility criteria were given an overall rating of quality. Quality scores for quantitative studies ranged from a possible zero to eight, while qualitative study scores ranged from zero to six. Studies with a score of four or higher (quantitative studies) and three or higher (qualitative studies) were deemed appropriate for detailed data extraction and synthesis. Two authors independently assessed study quality, and consensus was reached by discussion. To reduce possible bias towards the previous study published by the authors' which was included in the review, an independent researcher who had not been involved in the previous study assessed all studies for quality.

Summary measures and synthesis
Following the quality assessment one researcher extracted data from included studies which was independently checked by a second researcher. Information extracted from articles included aims of the study, study design, participant details, measures, and key results. Quantitative and qualitative studies were summarised in tabular form. One researcher thematically analysed the data to identify key themes in relation to each key word.

Study selection
A total of 7435 articles were found by electronic database searching (n = 7431) and additional records known to authors (n = 4). Following the removal of duplicates (n = 2786) and exclusion based on article type (n = 404), articles were excluded by title relevance (n = 2597). 1648 article abstracts were screened, and articles were excluded if they had no novel empirical data or were a case study (n = 355), did not have a focus on BPD or Emotionally Unstable Personality Disorder (n = 264), or if there was no mention of emptiness or related keyword in abstract (n = 911). 118 full-text articles were assessed for eligibility. Articles were excluded if they had no novel empirical data (n = 8), no mention of emptiness or related keyword in the results or discussion (n = 3), no focus on BPD (n = 2), if keywords were only used in the context of Schema therapy (n = 3), and if the article was not translatable using software and authors could not be contacted (n = 1). The study selection process is depicted in Fig 1. Following application of MMAT quality assessment, two studies did not meet quality criteria. One study did not meet screening questions and was excluded from further assessment. The remaining studies (n = 100) were evaluated on the additional four dimensions of the MMAT quantitative descriptive or qualitative tool. One study scored a one and was excluded from further analysis due to low quality. There was a 97.98% agreement between raters for quality assessment; two articles were discussed with a third rater to achieve consensus. All remaining studies (n = 99) scored at least half of the quality criteria and are included in the table of study characteristics, but articles with lower scores should be interpreted with caution (S1 Table).

Study characteristics
Ninety-nine studies were included in data extraction representing a total of 98,340 participants, with a range of seven to 36,309 participants across individual studies. Eighty-three studies reported on average age of their sample, and the overall average age across studies was 32.1 (SD = 11.0). Eighty-seven studies reported on gender ratio within their studies. Participants were predominantly female, with a mean of 77.6% (SD = 16.9, range = 36.7-100%). Of the 34 studies reporting participant cultural background, Caucasian participants accounted for an average of 77.2%. Further details of study characteristics are included in Table 1. Studies utilised a wide range of measures to quantify the experience of chronic emptiness and related terms (see Table 2). Table 3 presents a detailed overview of study characteristics.
Study focus. Thirty studies chosen for inclusion focused on chronic feelings of emptiness. A further 14 studies focused on chronic feelings of emptiness in addition to at least one other symptom (i.e. chronic feelings of emptiness and hopelessness, chronic feelings of emptiness and loneliness). Thirty-one studies reported on feelings of hopelessness, eight studies reported

PLOS ONE
Chronic emptiness in borderline personality disorder on loneliness, one study reported on loneliness and aloneness, six studies focused on aloneness, four studies focused on isolation, three studies reported on alienation, and two studies focused on boredom.

Key findings from studies focusing on chronic emptiness
The findings from 99 included studies were categorised according to construct and key findings were extracted. The forty-four studies which focused on chronic feelings of emptiness alone or in conjunction with another key word were analysed, then key findings for similar constructs including hopelessness and loneliness were analysed separately. Difficulties in defining chronic emptiness. A predominant finding of this review was the difficulty in understanding and defining the nature of chronic emptiness, and inconsistent findings regarding its relation to other symptoms of BPD. Studies investigating symptom clusters in BPD were highly variable in their results, theorising chronic emptiness was a component of: psychological process [50], affective instability [52], painful affect and defenses [63], disturbed relatedness [117], internally oriented criteria [124], and self-other instability [128].                       SCID-I, DIB-R, DIPD-R Chronic hopelessness, loneliness, and emptiness all had low rates of remission and high recurrence over follow-up period. These may be a response to impaired function in relationships and work.
DIB-R, DIPD-R (Continued ) DIB AAP-Adult Attachment Projective; AAQ-Acceptance and Action Questionnaire; ACS-Affective Control Scale; ACT-Acceptance and Commitment Therapy; ADP-IV-These disparate results may be indicative of the absence of a working definition of emptiness in the field, and associated difficulties in measurement. Similarly, when investigating discriminative symptoms for a diagnosis of BPD and networks of symptoms, emptiness was often identified as an important symptom for distinguishing people with BPD from other samples [66,103,104,123]. However, one study found that chronic emptiness was the least distinguishing factor of BPD [83]. The authors of this study noted that this result may be more reflective of the lack of definition of emptiness, and the difficulty in rating an internal experience that may have little behavioural manifestations, in comparison to symptoms such as unstable relationships.
Two studies discussed the difficulty of defining chronic emptiness. One study suggested that people with BPD may have difficulty defining and articulating the experiences of emptiness [112], while the other study found low correlations between chronic feelings of emptiness and other BPD symptoms, and postulated this may be due to the absence of a definition of chronic emptiness [83]. Only one recent study investigated what the features of chronic emptiness entails [111]. This study reported on feelings of emptiness transdiagnostically, and determined core features of emptiness include a sense of detachment from self and others, hollowness, aloneness, disconnection, and unfulfillment. Some studies examined the relationship of chronic feelings of emptiness as a construct to similar terms, with mixed findings. One study found no significant association between chronic feelings of emptiness and hopelessness or depression [138]. However, another study found that there were high correlations between feelings of emptiness and feelings of hopelessness, isolation, and loneliness (although these correlations did not meet multicollinearity, suggesting the construct of emptiness was still distinct) [85]. Overall, this points to a lack of cohesion in the field and a sense of confusion regarding not only the experience of emptiness, but its boundaries with related concepts.

Measurement of chronic emptiness within studies.
In several studies, chronic emptiness was quantified by one item from a wider measure of all BPD symptoms, including structured clinical interviews. The UCLA loneliness measure was used to measure both loneliness and emptiness [113], despite some studies differentiating these concepts [85].
One scale providing some measure of emptiness is the Orbach and Mikulincer Mental Pain Scale (OMMP), which aims to measure mental pain [142]. One factor of the OMMP is labelled emptiness-measuring the loss of subjective and personal meaning due to mental pain. The emptiness factor, however, only explains 2.3% of variance in the scale and the items have not been validated as individual measures of emptiness. As such, inferring severity of chronic feelings of emptiness from the OMMP may not accurately capture the experience of chronic emptiness in BPD.
Price and colleagues [111] recently developed a transdiagnostic measure of emptiness. The resultant Subjective Emptiness Scale (SES) is a seven item self-report measure. Internal consistency of items were high across clinical samples of people with psychiatric disorders (.91-.93) and covariance analyses indicated a unidimensional construct which was able to discriminate people who experienced varying severity of emptiness. The scale included central features of emptiness as a 'pervasive and visceral sense of detachment spanning intrapersonal, interpersonal, and existential domains of experience' which results in 'encompassing feelings of hollowness, absence from one's own life, profound aloneness, disconnection from the world, and chronic unfulfillment' [111, p. 18]. The development of this measure represents a significant contribution to the field, but as it is a transdiagnostic measure it requires validation within a BPD sample to test the symptom of chronic feelings of emptiness. Overall, the difficulties with defining and measuring chronic emptiness may partly explain the mixed findings within many reviewed studies, and points to further research aimed at elucidating the nature of chronic emptiness and the use of appropriate measures.
Age and gender. The prevalence of chronic feelings of emptiness was found to be higher in females with BPD compared to males [78], however a study of parent ratings of BPD in their male sons found that emptiness was reported for 97% of the male BPD group compared to 8% of the control group [74]. This study, however, did not compare genders and was reliant on parent report rather than self-report. It is not possible to provide a judgement of the effect of gender on chronic emptiness, and more study is needed in this area. One study found that chronic emptiness was more severe in older adults compared to younger adults with BPD [97]. Several factors may influence this-firstly, more 'acute' symptoms tend to resolve more quickly while emptiness is more chronic [141]. Perhaps once there is an absence of acute symptomology, chronic emptiness is more noticeable or more severe. Secondly, older adults in this study had poorer social function, which possibly results from a sense of disconnection from others and a feeling of emptiness.
Detachment from self and others. The limited number of studies on emptiness as a disconnection or deficiency in relating to self and others was surprising given the theoretical import placed on this relationship. The strongest support for this model was found by Price and colleagues [111] who proposed that transdiagnostic emptiness was a sense of detachment from interpersonal, intrapersonal and existential spheres. In terms of detachment from self, three studies linked chronic emptiness to identity disturbance, reflecting a detachment from sense of self. A qualitative study which asked for the life stories of people with BPD found an emergent theme of chronic feelings of emptiness relating to disturbances in self-identity [101]. This theme included two subthemes-distorted self-image and lack of identity-resulting in chronic emptiness. One study explored identity diffusion within personality disorder presentations, and found it was associated with feelings of chronic emptiness [129], while another study typified a subtype of people with BPD as the 'empty' type characterised by deficits in identity [112]. These studies suggest that chronic feelings of emptiness are the expression of an underlying diffuse identity, reflecting theoretical claims [11]. In relation to detachment from others, one study reported that social dysfunction was associated with feelings of emptiness [37]. Other studies noted that chronic emptiness occurred most often when individuals with BPD were alone [96] or during interactions with others in close social relationships [126].
Course of chronic emptiness. Five studies presented results relating to the course of chronic feelings of emptiness in BPD. Zanarini and colleagues [139] found that feelings of chronic emptiness were experienced frequently and severely for people with BPD. They also found that when investigating symptoms of BPD over ten years follow-up, feelings of chronic emptiness took the longest time to remit at an average of 8-10 years compared to more acute symptoms [141]. In a similar study, authors further found that over 16 years, chronic emptiness had relatively poor remission rates compared to other symptoms, and high recurrence rates [140]. These studies suggest that feelings of emptiness are difficult to alleviate due to being a 'temperamental' symptom enduring over time rather than an acute symptom. Similarly, another longitudinal study aiming to identify the core clinical features of BPD found that after one year of treatment feelings of emptiness were chronic compared to more acute symptoms, suggesting that chronic feelings of emptiness may represent a core underlying factor in BPD or is not targeted by current treatment [95]. A further study found that in a cohort of people with BPD categorised into a younger age group (18)(19)(20)(21)(22)(23)(24)(25) and older age group (45-68), older adults were more likely to report chronic feelings of emptiness [97]. The authors hypothesised that chronic emptiness may be more difficult to change as people age compared to symptoms like mood dysregulation. Overall, the studies focusing on the course of chronic feelings of emptiness reported it as slow to change over time, hypothesising it is a core problem for people with BPD. However, another factor in the chronicity of emptiness could be that it is not targeted in most treatments, and as such it remains untreated for a long period of time. Chronic emptiness, impulsivity, self-harm and suicide. Ten studies investigated behaviours which followed chronic feelings of emptiness. Both qualitative and quantitative studies supported chronic feelings of emptiness preceding impulsive behaviours. A qualitative study reported that women with BPD attempt to fill the 'void' they experience by acting impulsively [101], while a longitudinal study found that impulsivity and self-harm mediated the relationship between chronic emptiness and days out of work over time, suggesting that chronic emptiness may underlie and result in behavioural symptoms of impulsivity including self-harm [38]. A study in a sample of college students found that 67% of participants reported feelings of emptiness prior to engaging in self-harm behaviours [85]. Another study supported these findings with a different college sample, reporting that chronic feelings of emptiness and identity disturbance were associated with a history of self-harm behaviour, and may be the motivation for engaging in these maladaptive behaviours [39]. Overall, these studies may suggest that the void of emptiness is distressing and a common way to tolerate this distress is to engage in self-harm or impulsive behaviour.
Studies also reported a link between chronic feelings of emptiness and suicidal behaviours. One study hypothesised suicidal behaviours and suicide attempts are engaged in by people with BPD to relieve the tension of feeling empty inside [50]. Supporting this hypothesis, studies have found a strong relationship between chronic emptiness and both suicidal ideation and behaviour [85,131]. In one study people with BPD who experience chronic emptiness, mood dysregulation and identity disturbance made up the largest proportion of people who had made more than three suicide attempts [134]. Another study found that the presence of chronic emptiness increased the odds of suicide attempts [75]. It is possible that when selfharm and impulsive behaviours no longer relieve the distress of emptiness, suicidal ideation and behaviours arise.
Chronic emptiness as linked to depressive experiences. Seven studies investigated the relationship between chronic emptiness and depressive experiences. One study reported a moderate correlation between feelings of emptiness and depression [85], while another found that individuals endorsing chronic emptiness had significantly more severe depression than those who did not experience chronic emptiness [83]. Chronic emptiness was experienced frequently as a dysphoric affect for individuals with BPD [139], and was significantly associated with a diagnosis of dysthymia [131]. Individuals with diagnoses of BPD and MDD had higher rates of chronic emptiness and suicide attempts than people who met diagnosis for BPD only [114]. Two studies viewed depression in BPD as qualitatively different to that of MDD [115,137]. Borderline depression was characterised by chronic emptiness and self-condemnation [115]. Emptiness, rejection sensitivity, and dependency were positively associated with more severe depression in BPD which was also related to disturbances of self-concept [137].
Impact of chronic emptiness on social and vocational function. Several studies discussed the impact of chronic feelings of emptiness on vocational and social functioning for people with BPD. One study hypothesised that chronic feelings of emptiness was an understandable response to a life of relational difficulties and impaired work function [140]. A study by Ellison and colleagues [37] found that people presenting for psychiatric treatment who endorsed the single chronic feelings of emptiness symptom had the poorest psychosocial outcomes-the highest number of days out of work and lowest social functioning-compared to groups with any other individual symptom of BPD. Groups with both chronic emptiness and impulsivity had missed more work in the last five years, and groups with chronic emptiness and anger had poorer social functioning compared to people presenting to care with no BPD symptoms. This supported results from a previous study which found that compared to both other personality disorder presentations and people with no personality disorder, people with BPD reported higher levels of chronic emptiness during social interactions with close relationships [126]. A recent study further found that chronic feelings of emptiness predicted days out of work or normal activities over a one year follow-up, suggesting that chronic emptiness may account for psychosocial dysfunction over time [38]. Interestingly, another study found that after investigating BPD symptoms within a community sample over three times points within 18 months, chronic feelings of emptiness were associated with less stressful life events in the preceding six months compared to more acute symptoms [110]. This is perhaps a reflection of impaired social relationships and subsequent social isolation, leading to minimal stressful interpersonal events.
Treatment for chronic emptiness. Three studies discussed psychological treatment of chronic feelings of emptiness. A range of therapeutic modalities were used, including Supervised Team Management plus Sequential Brief Adlerian Psychodynamic Psychotherapy [47], Systems Training for Emotional Predictability and Problem Solving (STEPPS) [56], and Dialectical Behaviour Therapy (DBT) [138]. Each of the studies found that following treatment chronic feelings of emptiness significantly decreased in BPD samples. The follow-up period of these studies ranged from three months to two years. Authors speculated that chronic emptiness may be alleviated due to an increase in mentalisation skills, decrease in idealising and devaluing patterns within relationships, and an increased capacity to tolerate ambiguity and ambivalence [47]. In the STEPPS study, identity disturbance and mood instability also decreased alongside chronic emptiness [56]. Within the DBT treatment, participants experiencing chronic emptiness at baseline (94% of the sample) improved over the three months of treatment, while participants who did not endorse chronic emptiness (6%) demonstrated statistically significant deterioration of depressive symptoms, dissociative symptoms, and general mental health [138]. Authors postulated there may be two factors influencing the change in chronic emptiness. Firstly, they speculated that the core skill of mindfulness in DBT targets feelings of chronic emptiness. Secondly, they noted the model within which DBT was practiced 'offered a validating community to women' [138, p. 9] with high levels of engagement between participants and practitioners, which may have increased feelings of connection with others and self. It is important to note that there is as yet no causal empirical evidence that supports these hypotheses.

Similar constructs
Hopelessness. Thirty-six studies reported on hopelessness or a combination of hopelessness and another keyword. Hopelessness was typically defined as a disconnection from meaning and disconnection from life [90,92]. Eleven studies discussed the role of hopelessness in self-harm and suicidality. Overall, severity of hopelessness was associated with suicidal behaviours [68,107,121,136] and in some studies predicted suicide attempts for people with BPD [122]. Several studies focused on feelings of hopelessness as a disconnection from or lack of meaning in life. Low meaning in life was associated with more suicidal ideation and attempts, and hopelessness was also positively associated with suicidal behaviours [68,107,121,122,136]. Low meaning in life predicted hopelessness [72], and meaning in life also moderated the relationship between previous suicide attempts and hopelessness [92]. One study found that hopelessness mediated the relationship between BPD and suicide attempts [64]. For people who had attempted suicide, severity of hopelessness was higher for those who met diagnosis for BPD [53]. Within a BPD sample, individuals with a history of self-harm expressed higher severity of hopelessness compared to those without self-harm history [125].
Several studies reported on the link between feelings of hopelessness and depressive experiences. Depression was found to predict hopelessness for people with BPD, and was mediated by a sense of meaning in life [90]. Low meaning in life was correlated with feelings of both hopelessness and depression [91] and predicted both depression and hopelessness [72]. One study found people with comorbid BPD and MDD had more severe hopelessness compared to people with MDD only [46], and people with BPD had higher ratings of depression and hopelessness than people with depressive disorders [69]. However, one study found that hopelessness was unable to distinguish adolescents with and without BPD, suggesting it is not a unique experience of BPD [109]. An additional study found there were no differences in hopelessness and depression between people with BPD and people with MDD [79]. Multiple studies reported on the change in hopelessness throughout treatment for BPD. Severity of hopelessness decreased for people with BPD following DBT treatment-including both intense and adapted DBT programs [70,80,86,94,108], although one trial found DBT was not superior to Collaborative Assessment and Management of Suicidality treatment [48]. Severity of hopelessness also decreased following Acceptance and Commitment Therapy group treatment [98] and cognitive therapy [60,67]. One study found that severity of hopelessness did not decrease following treatment with antipsychotic medication for three months [135].
Loneliness. Eighteen studies discussed loneliness or a combination of loneliness and another keyword. Loneliness has been conceptualised as a 'feeling of being alone' [89, p. 1], which is a central feature within the network of BPD symptoms [123]. One study reported that people with BPD perceive loneliness as an inherent trait, not a state, which reflects a feeling of disconnection with the world and can only be temporarily alleviated [116]. Among personality disorders, BPD had the strongest association with loneliness [77], and adolescents who selfharm reported higher rates of loneliness compared to those who did not self-harm [73]. One study reported that loneliness, in addition to chronic emptiness, was a core factor of depression for people with BPD [137], while two other studies clustered chronic emptiness, loneliness and boredom as a discriminating factor of BPD [103,104]. Loneliness was found to have high recurrence and low remission rates over both 10-and 16-years follow-up [140,141]. People with BPD demonstrated higher dysregulation compared to healthy controls following presentation of attachment pictures which may induce loneliness, suggesting an intolerance of loneliness [54]. Similarly, people with BPD demonstrated a higher intolerance to loneliness compared to people with dissociative or conversion disorders [105]. One study reported that loneliness in BPD was related to poor social and relational function, but after controlling for these deficits loneliness was still high for people with BPD, suggesting there are multiple factors which contribute to feeling lonely [89]. Feelings of loneliness may also be associated with deficits in facial emotion recognition and behavioural mimicry. Lower confidence in rating facial emotions has been associated with both higher levels of loneliness and higher levels of rejection sensitivity [130]. For people with BPD with the highest scores of loneliness, behavioural mimicry-an important factor in fostering connection between people-was the lowest, suggesting the capacity or desire to connect with others may be impaired when people with BPD feel lonely [76].
Intolerance of aloneness. Intolerance of aloneness broadly relates to the intolerable distress of being alone with one's own thoughts and feelings and an associated incapacity for solitude [132]. Overall findings indicate that people with BPD experience the feeling of aloneness more frequently and severely compared individuals with neurotic disorders [113] and have an intolerance to being alone [61,105]. A recent study developed a measure for the experience of being alone for individuals with BPD and they report the intolerance of this experience as a salient feature of the disorder [132]. Being alone accounted for 39% of aversive emotions [127] and triggered all BPD symptoms except self-harm [96]. Over ten years intolerance of aloneness was the slowest interpersonal symptom of BPD to remit and still declined less than other features of BPD [65]. Interestingly, an article found that both intolerance of being alone and intolerance of relating to others were salient features of the experience for people with BPD [132].
Alienation and boredom. Three studies reported on feelings of alienation. Alienation was found to be a discriminating feature of BPD [51] and was a risk factor for development of BPD [59]. It was also associated with disturbed identity [87]. Five studies reported on feelings of boredom or boredom in conjunction with chronic emptiness. Most of these studies were published when the symptom of chronic emptiness or boredom remained in the DSM. Boredom was found to be related to core identity diffusion [129], and suicidal behaviour [131]. Boredom was also associated with feelings of depression [81,114], however was not associated with feelings of shame [118].

Discussion
This review sought to examine empirical literature and provide a detailed understanding of the symptom of chronic feelings of emptiness in BPD. It also aimed to identify similar constructs to chronic feelings of emptiness such as hopelessness, and provide clarification around the relationship between these experiences. A broad focus was used in this review-articles needed to be peer-reviewed, contain novel empirical data, and needed to have a focus on BPD or BPD symptoms. However, all articles that mentioned emptiness or a similar construct in their abstract and results or discussion were included, even if the main focus of the study was not on these experiences. This allowed an in-depth analysis within a field where chronic feelings of emptiness is often discussed tangentially and is not a common focus of articles. However, this also resulted in a wide array of study methodology and quality, and findings should be interpreted with caution until further research is conducted.
Overall, 99 articles met the inclusion criteria and quality assessment, and key findings were presented. The review identified a number of gaps within the literature, particularly relating to defining and measuring chronic feelings of emptiness. As such, findings extrapolated from this data should be interpreted with caution, as there are significant limitations with measurement within the field. Nevertheless, the included studies provide a good foundation of knowledge regarding chronic feelings of emptiness.

The difficulty in defining and delineating chronic feelings of emptiness
The available research on chronic feelings of emptiness demonstrated a difficulty in understanding the nature of chronic emptiness, defining the experience, and determining its importance to a BPD conceptualisation or diagnosis. Despite the inclusion of 44 studies discussing chronic feelings of emptiness, only one recent study investigated what chronic emptiness is and how it is experienced, although this was not exclusive to individuals with BPD but included all psychiatric diagnoses [111]. It is clear from included studies that it remains difficult to define and measure an absence of experience, and perhaps this has resulted in the reliance on single-item measures that may not adequately capture the true experience of chronic emptiness. Factor analyses differentially placed chronic emptiness with most other symptoms of BPD, perhaps a further indication of the absence of a definition of chronic emptiness. There were minimal personal accounts of people with BPD across the studies. Only three qualitative studies focused on individual experiences, with most other studies utilising prescribed questions which are often developed by clinicians or researchers and may not accurately reflect the experience of individuals with BPD. The lack of understanding about the nature of chronic emptiness may also contribute to the mixed findings of chronic feelings of emptiness within the broader conceptualisation of BPD.

A conceptualisation of the cause and effect of chronic feelings of emptiness within BPD
Despite difficulties defining and delineating chronic emptiness, this review is able to provide a synthesis on the current understanding of chronic emptiness in the theoretical and empirical literature. Across differing theoretical frameworks, a common theme in the conceptualisation of chronic emptiness is that it results from a disconnection from the self and from other people. This is described differentially in terms of unstable object relations [8,15,16,143,144], an inability to develop soothing and holding introjects [22], a false self [23], a lack of personal identity [26,27,29], insecure attachments [30], invalidation and confusion about internal experiences [31] and deficits in mentalisation [32]. These theories hypothesised the cause of emptiness is inconsistent responses from caregivers resulting in difficulties in knowing oneself and others. Empirical literature that focused on emptiness as a sense of detachment from self and others was not detailed enough to be conclusive, but provided some empirical indications that support these theories. In particular, Price and colleagues [111] found a unidimensional construct of emptiness that was defined as a sense of detachment both from self and others, hollowness, aloneness, disconnection, and unfulfillment [111]. Qualitative narratives have begun to demonstrate in small samples that people with BPD may also associate feelings of chronic emptiness with identity disturbance [101]. Further, treatment that focuses on establishing a more coherent sense of identity and empathic responding to others (e.g. mindfulness [138], mentalisation [47]) also appears to decrease the severity of chronic emptiness, suggesting a possible link between chronic emptiness and disconnection from self and others.
The research was more conclusive on the effects of emptiness for people with BPD. Chronic emptiness was linked to several aversive outcomes including vocational and social function [37, 38], impulsivity, self-harm [85] and suicidal behaviours [75]. A review of the relationship between emptiness and suicidal behaviour found that feelings of emptiness was among the most frequent affect experienced before suicide attempt and after non-fatal suicide attempts [36]. It is possible that deficits in connecting with oneself and others leads to an intolerable sense of emptiness, which is avoided or alleviated by engaging in self-destructive behaviours. Likely, both the feelings of detachment from self and other people and the resultant behaviours impair both social and vocational functioning.
The experience of chronic emptiness has been conceptualised as a component of depression in BPD [40,115,145]. Depression has never been a criterion for meeting a diagnosis of BPD [2,6], but there is high occurrence of both reported depressive experiences and diagnosable depressive disorders including major depressive disorder (MDD) in BPD [146,147]. There are also indications, however, that there exists a 'borderline depression' which is qualitatively different to the experiences of affective disorders [9]. Current theoretical models purport that the experience of depression in BPD is intrinsically linked to an insecure and negative self-identity, which is exacerbated by dysregulation of emotion, anger, anxiety, and importantly-emptiness [9,40]. Borderline depression is centred on these experiences of loneliness, anger, impaired self-concept and relationships rather than the characteristic feeling of guilt in MDD [40,137,146,148]. Specifically, it is suggested a discriminating factor between borderline depression and unipolar depression is the experience of emptiness [145]. Borderline depression is hypothesised as a 'feeling of isolation and angry demandingness rather than true depression' [9, p. 36] and represents a more dependent-anaclitic form of depression [20]. This is considered distinct from other depressive disorders, and reflects an experience where a common characteristic is feelings of chronic emptiness.
The proposition that chronic emptiness is a component of 'borderline depression' still needs to be clarified in future research, but at the very least there is a positive association between chronic feelings of emptiness in BPD and severe depression [115,137]. Two studies which investigated the experience of depression in BPD found that a 'borderline' depression was associated with poor self-concept and a sense of 'void' or 'inner badness' [115,137]. These feelings of chronic emptiness and perhaps the experience of borderline depression may then result in impulsive behaviours including self-harm or suicidal behaviours to reduce the feeling of emptiness or depression [38,39,85,101]. The literature in this area remains inconclusive, with recent research with participants with severe and recurrent depression indicating feelings of chronic emptiness are also an important component of their experiences [149].
Research on the cause and effects of chronic emptiness highlights the importance of increasing knowledge of this symptom. Specifically Brickman and colleagues [39] suggests individuals who experience substantial feelings of emptiness should be identified and targeted for interventions, as they may be more likely to engage in maladaptive behaviours and may have a poorer functional prognosis.

A difference in connection-separating chronic emptiness from related constructs of hopelessness, loneliness and intolerance of aloneness
There have been limited efforts to distinguish chronic feelings of emptiness from similar or related constructs. One study investigated the relationship between feelings of chronic emptiness and hopelessness, isolation, loneliness, uselessness, worthlessness, and grief before and after self-harm incidents with university students [85]. It found high correlations between feelings of chronic emptiness and feelings of hopelessness, loneliness and isolation. The authors proposed that these four states all represent a low positive affect and low rates of arousal. Other studies included chronic emptiness, loneliness and hopelessness together as temperamental affective experiences of BPD [139][140][141], considering them highly related symptoms of BPD.
Based on the reviewed literature, it seems that chronic feelings of emptiness may be distinguishable from similar constructs. We hypothesise that chronic feelings of emptiness is a sense of disconnection from both self and others, hopelessness is a sense of disconnection to meaning or life, loneliness is a sense of disconnection from the world and a feeling of being alone and intolerance of aloneness is the incapacity to be alone. All have a similar basis in a sense of disconnection or detachment but represent different types of disconnect. This hypothesis of emptiness as a sense of detachment and disconnection from self and others echoes that of Price and colleagues [111].
Studies which discussed feelings of hopelessness often viewed it as a disconnection from or lack of meaning in life [72,[90][91][92]. Less meaning in life was associated with more suicidal behaviours. Interestingly, meaning in life-a sense of purpose to life-has been shown as a factor in decreased suicidal ideation [150] and gratefulness towards life has been shown as a buffer between suicidal ideation and hopelessness [151]. Perhaps a sense of hopelessness may reflect low meaning in life and a disconnection from life.
Studies focusing on loneliness in BPD discussed it as a sense of disconnection from others that people with BPD perceive as a sense of disconnection with the world [116]. Feelings of loneliness were associated with deficits in facial emotion recognition [130] and behavioural mimicry [76]-suggesting impairments in fostering connection with other people. Loneliness may both arise from a sense of social disconnection and perpetuate deficits in social interactions. Similarly, people with BPD demonstrated an intolerance to being alone and feelings of aloneness, but also experienced being in the company of other people as dysregulating [132].
While we hypothesise that chronic emptiness, hopelessness and loneliness may be distinguishable from one another, this is based on limited data which has not explicitly investigated these differences. While Klonsky's [85] research began the process of demarcating these experiences, further research is needed to further investigate the differences in constructs and to test the hypothesis.

Treating the chronically empty: Hypothesising a possible treatment focus
Chronic feelings of emptiness seems to be an affective symptom of BPD that is temperamental-meaning it takes significantly longer to remit compared to more acute symptoms [139,141]. This may be due to the nature of chronic emptiness itself, or it may be that most current treatments do not focus specifically on alleviating the symptom.
A limited number of studies discussed treatment for chronic feelings of emptiness. Those that did hypothesised that a reduction in chronic feelings of emptiness was related to an increase in mindfulness skills, mentalisation skills, and a decrease in patterns of idealisation and devaluation [47,138]. Yen and colleagues [138] also considered the impact of validation from clinicians in fostering a sense of community and belonging to self and others. It may be that developing mindfulness skills in DBT within a supportive and safe environment fosters a sense of identity and purpose, and similarly mentalisation-based and transference-focused therapies focus on making sense of the internal world of individuals [152], their self-representations [153], and their connections to others. We hypothesise that work on self-integration including strengthening an understanding of autobiographical history, personal preferences, and sense of self as a unique personality which is allowed to just 'be' may have a flow-on effect and reduce the severity of chronic emptiness. Further, a focus on increasing holding others in mind in addition to basic behavioural strategies may assist in developing social connection. This speculation of the possible treatment for chronic emptiness remains a preliminary hypothesis until research can be conducted testing this specific model.

Study design and methodological limitations
Findings within this review are dependent upon our interpretation of available data. It is important to note that articles included in the review had a wide variance in both scope and quality. In considering the limitations of the field, this systematic review is also limited by the nature of studies reporting of chronic feelings of emptiness; in that findings regarding chronic emptiness were often presented tangentially to other main findings, and as such were often not interpreted at an in-depth level within studies.
Study quality within this area of research is also limited. Few studies stated their sampling procedure or justified their sample size, and reasons why eligible participants chose not to participate were rarely stated. While this is an important area of research, our findings should be interpreted with some caution due to the differences in quality of the included studies. Most studies included in the review presented cross-sectional data (n = 73). Although cross-sectional data is an efficient way to collect data at one time point, it does not allow an analysis of change over time or causal relationships, weakening the conclusions of these articles. However, the findings from longitudinal studies (n = 23) within the included articles were generally consistent with findings from cross-sectional findings.
Despite the importance of being able to identify individuals who experience significant feelings of chronic emptiness, there has historically been a lack of comprehensive methods to measure emptiness. This may reflect the difficulty in defining or measuring an absence of experience which has been described as a sense of 'nothing' [9,112]. Within included studies, there was a higher proportion of studies utilising measures which were specific to a BPD sample (n = 49) or both specific measures and more general measures (n = 10), compared to general measures only (n = 39). This may have allowed for investigation into features and experiences that are specific to BPD, while also allowing an understanding of difficulties with chronic emptiness or a related construct that are not unique to BPD. However, a significant weakness of the included studies is that the majority of articles employed a single-item measure to quantify presence or severity of chronic feelings of emptiness or a related experience. Emptiness has typically been measured using one individual item from semi-structured interviews or diagnostic tools [154][155][156]. This may not adequately capture the nature and severity of chronic emptiness and restricts generalisability of findings. Themes arising from the data in this review should be interpreted cautiously due to the limitation of single-item measurements. The recent development of the transdiagnostic Subjective Experience of Emptiness scale [111] provides a good future direction for further studies which require a more thorough and indepth understanding of feelings of chronic emptiness.

Implications for future research
The findings of this review support several areas of further research. First, there is a need to better understand the nature of chronic emptiness for people with BPD. Qualitative studies are needed to provide an in-depth account of the personal experience of chronic feelings of emptiness to support the development of better ways to measure or quantify chronic emptiness. Second, research in this area could expand on the recent work of Price and colleagues [111] to validate their transdiagnostic measure of emptiness in a BPD sample or add an extension to this measure that is specific to people with BPD. It may be of use to explore transdiagnostic research into emptiness for other presentations, such as chronic depression [149], eating disorders and substance use to further inform our understanding of and interventions for emptiness. Third, once there is a more thorough understanding of chronic feelings of emptiness and a way to quantify its presence and severity, we may be able to test intervention models targeting chronic emptiness.
Despite the inclusion of chronic feelings of emptiness as a diagnostic marker for BPD, it has not been subjected to the same level of interrogation as other symptoms of BPD. This review provided a detailed analysis on literature regarding the construct of chronic feelings of emptiness. Results demonstrated that while there remains many gaps in our knowledge about chronic emptiness, it is clear that as a whole studies point to it as a signal symptom to consider in conceptualisation and treatment of BPD. Further studies are needed to provide a deeper understanding of chronic emptiness and its clinical significance in order to develop effective interventions.
Supporting information S1