Researchers have proposed that affective instability is comprised of multiple dimensions. These generally include: (1) frequency of affective changes; (2) amplitude of affective changes, particularly shifts from positive into negative affective states; (3) quality of affective changes; (4) temporal dependence, i.e., trends in affective instability related to factors such as time and overall mood intensity; (5) overall negative mood intensity; (6) mood reactivity in response to environmental triggers; and (7) difficulties with affective regulation [7–10]. This chapter will deal primarily with components 1–3 and 6–7.

Renaud has proposed that affective instability be distinguished from mood lability and that affective lability be considered a subconstruct of affective instability [7]. She notes that affects are generally short-lived reactions to stimuli that last only minutes. Moods, in contrast, refer to more sustained affective states. She proposes that mood lability applies to the cycling of affective states that occurs in bipolar disorder. She suggests that affective lability is a temperamental vulnerability toward strong and rapid affective shifts in response to environmental stimuli that would produce less extreme affective changes in normal individuals. She proposes that mood instability, while not defined in the literature, generally refers to fluctuations within an abnormal mood state. She concludes that affective lability is a more accurate term than mood lability because there is no reported association between number of major mood episodes and self-reported mood lability.

The DSM-V discusses affective or mood instability in relation to both BPD and bipolar disorder. It defines affective instability in BPD as follows:

Individuals with Borderline Personality Disorder may display affective instability that is due to a marked reactivity of mood (e.g., intense episodic dysphoria, irritability, or anxiety usually lasting a few hours and only rarely last more than a few days). The basic dysphoric mood of those with Borderline Personality Disorder is often disrupted by periods of anger, panic, or despair and is rarely relieved by periods of well-being or satisfaction. These episodes may reflect the individual’s extreme reactivity to interpersonal stresses [1].

In its discussion of manic states in bipolar I disorder, the DSM-V states: “Rapid shifts in mood may occur over brief periods of time and are referred to as lability (i.e., the alternation between euphoria, dysphoria, and irritability)” [1]. In its discussion of mixed manic states, it states:

Mood may shift very rapidly to anger or depression. Depressive symptoms may occur during a manic episode and, if present, may last moments, hours, or, more rarely, days [1].

These descriptions of affective or mood shifts involve emotions that can be interpreted as overlapping. While the descriptions of a manic episode, a depressive episode, and BPD all include irritability, the description of BPD affective instability also refers to the related emotion anger. The description of affective instability in BPD mentions despair in comparison with the description of a mixed state in bipolar, which includes depression. The description of mood instability in bipolar disorder includes anxiety and panic. The DSM-V describes one type of mania that includes anxious distress [1]. The two descriptions of affective or mood instability differ most clearly in one respect. Whereas the description of borderline affective instability stresses affective shifts that may be reactive, the description of bipolar affective instability makes no reference to such reactivity.

Delineating the interface between BPD and bipolar disorder has proven complex and challenging. One group of investigators has advocated the position that BPD is a form of affective disorder [6]. This group has asserted that the emotional reactivity in BPD arises from a cyclothymic temperament, which they believe to be a variant of bipolar II disorder. This group has proposed the following definition of a cyclothymic temperament based on two criteria sets. The first includes: (1) hypersomnia vs. decreased need for sleep, (2) introverted self-absorption vs. uninhibited people seeking, (3) taciturn vs. talkative, (4) unexplained tearfulness vs. buoyant jocularity, and (5) psychomotor inertia vs. restless pursuit of activities. The second set includes: (1) lethargy and somatic discomfort vs. eutonia, (2) dulling of senses vs. keen perceptions, (3) slow-witted vs. sharpened thinking, (4) shaky self-esteem alternating between low self-confidence and overconfidence, and (5) pessimistic brooding vs. optimism and carefree attitudes [11]. Based on research with subjects suffering from atypical depression, this group has found that patients with cyclothymic temperaments—35 % of whom were diagnosed with some form of bipolar disorder—were substantially more likely to meet criteria for BPD than subjects who did not have cyclothymic temperaments. Although this study employed standardized clinician administered assessments, its validity is limited by the fact that interviewers administering Axis II assessments were not blind to Axis I diagnosis. It should be noted, moreover, that most of the elements of a cyclothymic temperament do not appear to involve the type of affective shifts described above as typical of BPD.

Additional data suggesting an overlap between BPD and bipolar disorder have come from studies of the prevalence of bipolar disorder in populations with BPD and from studies of mood stabilizing medications. Studies have generally shown higher rates of bipolar disorder in subjects with BPD than in the general population. Studies examining rates of bipolar I disorder in BPD have found rates ranging from 5.6 to 16.1 %; studies examining rates of bipolar II disorder in BPD have found rates ranging from 8 to 19 % [5]. Studies looking at the question of whether BPD precedes the onset of bipolar disorder have been mixed. Two studies have found no significant differences between BPD and comparison subjects in rates of new onset bipolar disorder [12, 13]. However, a large longitudinal study of personality disorders found BPD subjects had a significantly higher rate of new onsets of bipolar subjects in comparison to subjects with other personality disorders (8.2 % vs. 3.1 %) [14].

Studies of mood-stabilizing agents previously found to be effective in treating bipolar disorder have shown that these agents reduce affective symptoms in BPD. Studies have shown that valproic acid is effective in reducing anger and depression in BPD [15] and manic symptoms in bipolar disorder [16]. Research has shown that lamotrigine is effective in reducing anger and overall affective instability in BPD [17], as well as reducing relapses of bipolar depression in bipolar disorder [18]. Research has suggested that oxcarbazepine is effective in reducing affective instability in BPD [19] and manic symptoms in bipolar disorder [20]. Finally, studies have shown that aripiprazole may reduce anxiety, depression, and anger in BPD [21] while reducing manic and depressive symptoms in bipolar disorder [22].

Despite evidence that bipolar disorder and BPD may share clinical features and neurobiologies, some researchers assert that evidence does not support an overlap between the two entities [5]. To begin with, they note that, although there is evidence of increased bidirectional comorbidity between the two disorders, this increase is not any greater for BPD as opposed to other personality disorders. Second, they note that each disorder has a distinct phenomenology. Changes from euthymia to elation or depression to elation are characteristic of bipolar disorder, but not BPD. Additionally, affective changes in BPD are considered reactive to environmental events, but the mood shifts in bipolar disorder are less clearly associated with stressors. Third, these researchers note that the frequency of bipolar disorder in first-degree relatives of BPD probands is not necessarily higher than the frequency of bipolar disorder in the general population. Fourth, they note that the courses of BPD and bipolar disorders differ: unlike BPD, bipolar disorder rarely remits within 2 years. Lastly, they write that there is currently no evidence of overlapping biological factors for the two disorders.

One area where there may be particular overlap between BPD and BD occurs in the rapid cycling experienced by some bipolar patients. The DSM-V defines rapid cycling bipolar disorder as having four mood episodes within a 12-month period [1]. Clinical experience suggests that some bipolar patients will have many more than four episodes during that time. There is evidence that, compared to non-rapid cycling bipolar patients, patients with rapid cycling may share some features of BPD. Rapid cycling bipolar patients may make more suicide attempts and have more comorbidity including substance abuse or dependence, eating disorders, and anxiety disorders [23]. Some authors have gone so far as to propose that the same mechanism may cause both rapid cycling and the affective instability in BPD [24]. Nevertheless, the exact relationship between rapid cycling and the affective instability found in BPD remains unclear.

Most studies of affective instability in BPD have compared borderline subjects to healthy controls (HCs) or subjects with psychiatric diagnoses other than bipolar disorder. Most studies of affective instability in bipolar disorder have compared bipolar subjects to healthy controls. Current studies of affective instability in BPD and bipolar disorder have employed self-report measures, prospective assessment, or laboratory measures.

Retrospective studies have assessed frequency, intensity, quality of affective changes, and emotion regulation in BPD. One study found that, compared to subjects with other personality disorders, subjects with BPD report more frequent shifts between: euthymia and anxiety, euthymia and anger, and depression and anxiety [25]. This study did not find any difference in the intensity of affective shifts between the two groups. A second study, assessing emotion dysregulation, found that, even when controlling for overall negative emotions such as depression and anxiety, borderline subjects had more difficulty with emotion regulation, particularly with respect to awareness of emotions and impulsivity [26].

The most recent studies using prospective assessment of affective instability in BPD have used Ecological Momentary Assessment (EMA), involving pencil and pen measures or electronic diaries. EMA has several advantages over retrospective assessment. First, it records information while subjects are in their natural environment. Second, it captures information about immediate or near immediate experiences and thereby minimizes inaccuracies that may occur during retrospective reporting. One study using EMA to compare affective instability in BPD and normal controls found that BPD subjects had significantly more changes in affect from a positive to a negative state [27]. This study was limited in that it covered only a 24-h period. A second study, also covering a 24-h period, used EMA to assess patterns of affective shifts in subjects with BPD and HCs. This study found that BPD patients reported more shifts: (1) between anxiety and sadness and (2) from anxiety to anger [28]. A third study used EMA to assess affective instability in borderline and depressed patients over a 28-day period. This study found that borderline patients reported more significant variability in both positive and negative emotions. Furthermore, borderline patients reported larger increases in sadness, hostility, and fear from one assessment to the next [10].

Laboratory studies examining emotional reactivity in BPD have produced mixed results. Two studies have found that, in response to visual cues, subjects with BPD were no more reactive than either subjects with major depression or healthy controls [29, 30]. But several other studies evaluating affective instability, using both visual and auditory cues, have found BPD subjects to be more emotionally hyperreactive. These studies have measured responsiveness to color slides with different affective valences; videos containing neutral, violent, or abandonment themes; and short stories [31–33]. In general, these studies have found hyperresponsiveness to positive, negative, and neutral stimuli. Of note, the two studies that did not find BPD to be associated with emotional hyperreactivity used inpatient BPD subjects, who may have been affectively blunted by sedating medication [34].

Two laboratory studies have supported the theory that BPD patients have increased difficulty with emotion regulation. One found that, after being instructed to forget a series of words, BPD subjects were more likely than HCs to remember words associated with negative emotional valence [35]. Similarly, a second study found that subjects with BPD were more likely to remember negatively valenced words they had been instructed to forget. In addition, this study found that BPD subjects had more difficulty suppressing attention to aversive irrelevant stimuli [36].

In contrast to BPD, there has been little research on affective instability in bipolar disorder, particularly in euthymic bipolar patients. One study compared affective instability in euthymic bipolar patients and HCs [3]. To measure frequency of affective instability, this study used the Affective Lability Scale (ALS), a 54-item self-report questionnaire measuring affective instability in six dimensions [37]. These dimensions include shifts between euthymia and elation, euthymia and depression, depression and elation, euthymia and anger, euthymia and anxiety, and depression and anxiety. To measure intensity of affective instability, this study used the Affect Intensity Measure (AIM), a 40-item self-report instrument that measures the intensity of both positive and negative affects [38]. Patients in the bipolar group were predominantly bipolar I (78.8 %). Over half the bipolar subjects (58.7 %) had had psychotic episodes. This study found that, compared to HCs, bipolar patients had higher overall ALS scores and higher scores on all dimensional subscales of the ALS. In addition, bipolar patients had higher overall scores on the AIM.

Laboratory studies have found that compared to healthy controls, normothymic bipolar subjects respond to photos and film clips with higher levels of positive emotion [4, 39]. But there is no evidence that bipolar patients differ from healthy controls in terms of negative emotional reactivity. Specifically, studies have shown no differences in emotional responses of bipolar subjects to negative feedback [40], interpersonal criticism [41], or negative photos [42].

Three studies have directly compared affective instability in BPD and bipolar disorder. The first of these studies compared affective instability in type II bipolar (BPII) disorder, BPD, and other personality disorders (OPD) [43]. This study had four subject groups: subjects with BPD alone (N = 29); subjects with BPII and another personality disorder, but not BPD (N = 14); subjects with both BPD and BPII disorder (N = 12); and subjects with other personality disorders but without BPD or BPII disorder (N = 93). The study found that patients with BPD had higher overall scores on the ALS than subjects with OPD and that there was a trend toward subjects with BPII having higher overall ALS scores than subjects with OPD. Furthermore, it found that subjects with BPD endorsed more frequent lability on the ALS euthymia-anger subscale. BPII subjects, on the other hand, endorsed more frequent shifts on three ALS subscales: euthymia-elation, euthymia-depression, and elation-depression. This study found an interaction between BPD and BPII for higher scores on the depression-anger subscale. Finally, the study found a trend for patients with BPD to have higher scores on the AIM. Results of this study are limited by several factors. First, all the subjects in the bipolar group had comorbid personality disorders. Second, the study did not directly compare subjects with BPD without BPII disorder and subjects with BPII disorder without BPD.

A second study of affective instability in BPD and BD used a self-report measure, the Affective Lability Questionnaire for Borderline Personality Disorder (ALQ-BPD), to compare affective instability in college students with elevated bipolar and borderline features. Subjects in the study consisted of 818 undergraduates at a state university [44]. The study classified subjects as having elevated BPD features if they had scores on the Personality Assessment Inventory Borderline Scale (PAI-BOR) [45] that were two standard deviations or higher above the PAI-BOR mean score for the study sample. The study classified the subjects as having elevated features of BD if they had scores on the Personality Assessment Inventory Mania scale (PAI-MAN) [45] that were two standard deviations or higher above the sample mean for the PAI-MAN. Subjects could not have elevated features of both BPD and BD. Twenty-three subjects met study criteria for elevated BPD features; 21 subjects met study criteria for elevated BD features. Two subjects were excluded because they had elevated features of both disorders. Subjects with elevated BPD features endorsed more frequent affective changes in 7 of 9 affective dimensions of the ALQ-BPD. These included shifts between euthymia-depression, euthymia-anger, euthymia-anxiety, depression-anxiety, anxiety-depression, anger-depression, and depression-anger. Subjects with elevated borderline features endorsed more intense affective shifts in 2 of 9 dimensions: euthymia-depression and anxiety-depression. Composite subscale scores for both frequency and intensity were higher for subjects with elevated borderline features.

Results of this study suggest clear differences in affective instability between BPD and bipolar disorder, but the study had several important limitations. Although subjects in the study appeared to have clear elevations in BPD or bipolar features, they were not actually diagnosed with any psychiatric disorder. Consequently, the study did not distinguish between different types of bipolar disorder. Second, the study did not control for severity of symptoms. Third, the study did not assess such factors as medication status, severity of illness, and comorbidity aside from BPD and BD. Finally, because the ALQ-BPD measures affective instability only over the last week, it may not be an accurate indication of affective instability as a trait for these disorders.

One study has used a clinician administered instrument, the Affective Lability Interview for Borderline Personality Disorder (ALI-BPD), to compare affective instability in BPD and BD [46]. Additionally, this study assessed affective instability using the ALS and the AIM. Subjects in the study were 29 subjects with BPD and 25 subjects with BPII disorder. The study assessed subjects diagnostically using the Structured Clinical Interview for DSM-IV Axis I disorders (SCID-I), the Diagnostic Interview for DSM-IV Personality Disorders (DIPD-IV), and the Revised Diagnostic Interview for Borderlines (DIB-R). Subjects diagnosed as borderline met DSM-IV criteria for BPD and had DIB-R scores of 8 or higher. Subjects diagnosed as type II bipolar met DSM-IV criteria for this disorder. The study excluded potential subjects with comorbid BPD and BPII.

As with results using the ALQ-BPD, results of this study suggested clear differences in affective instability between bipolar disorder and BPD. On the ALS, BPII subjects reported significantly more frequent shifts between euthymia and elation. BPD subjects reported more frequent shifts between depression and anxiety. BPII subjects had higher scores on the positive emotion subscale of the AIM. On the ALI-BPD, bipolar II subjects reported more frequent shifts in dimensions traditionally considered more bipolar: shifts between euthymia and elation and shifts between depression and elation. Borderline subjects reported more frequent shifts between euthymia and anger, between depression and anxiety, and between anxiety and depression. BPII subjects reported more intense shifts between euthymia and elation and between depression and elation. Borderline subjects reported more intense shifts between euthymia and anxiety, euthymia and anger, depression and anxiety, and anxiety and depression. Composite subscale scores for intensity and frequency for those affective dimensions considered more typically borderline—euthymia-anxiety, euthymia-anger, depression-anxiety, and anxiety-depression—were higher for subjects in the borderline group.

This study, too, had several limitations. First, the size of the study sample was relatively small. Second, subjects in the BPD group had Global Assessment of Functioning (GAF) scores that were 9 points lower than subjects in the bipolar group. Third, most of the subjects in the study were taking psychotropic medications, which may influence affective instability. Fourth, like the ALQ-BPD, the ALI-BPD is largely a state measure and therefore may not provide valid information on affective instability as a trait.

Overall, results of current studies suggest that the affective instability in BPD and bipolar disorder has different profiles. Not surprisingly the affective instability in bipolar disorder involves more elation. The affective instability in BPD, in contrast, appears to consist more of affective shifts involving anxiety, depression, and anger. The differences between the two groups appear to include differences in both frequency and intensity.

If the affective instability in BPD and BD is different, then presumably the affective instability in each has a different neurobiology. As noted above, there are no studies comparing the neurobiology of affective instability in BPD and BD. There are, however, multiple studies that provide clues about the neurobiology of affective instability in BPD and BD by examining the emotion processing and regulation in each disorder. Studies of affective processing in both BPD and BD have shown that this processing differs from affective processing in healthy controls. Interpretation of these studies for BD is complicated by two factors. First, these studies are diagnostically heterogeneous, including bipolar I and bipolar II patients. Second, the studies include subjects in states of euthymia, mania, and depression. Overall, these studies have found abnormalities of amygdala activation that vary by mood state. In addition, studies have suggested that there is mood-independent hypoactivation of the ventrolateral prefrontal cortex (VLPFC)—a structure responsible for regulation of the intensity of emotional responses, cognitive responses to negative emotions, and overall emotional integration—and that this may be a trait marker of BD [47].

Studies of BPII subjects may be particularly relevant for comparison with BPD because symptoms and associated features of bipolar II disorder appear to more closely resemble BPD. Two studies have used fMRI to examine emotion processing in unmedicated depressed BPII subjects. Both studies have employed emotional face activation paradigms. One study found reduced activation in the bilateral VLPFC and the right amygdala [48]. A second study found decreased activation of posterior cortical midline structures (precuneus, cingulated cortex, and medial parietal cortex) [49]. Notably, no studies have examined emotion processing in euthymic BPII subjects.

Studies of neural correlates of emotional processing in BPD have used presentation of emotional faces, aversive scenes, or memories of negative life events as activation paradigms [50–56]. Most of these studies showed enhanced amygdala activity in BPD, suggesting this may be a trait marker for the disorder [50–53, 55, 56]. Collectively, these studies have shown enhanced activation of multiple prefrontal structures: the middle and inferior temporal cortical areas, anterior and posterior cingulate cortices (ACC and PCC), insula, and medial and inferolateral prefrontal cortical areas. Overall, current neuroimaging research suggests that affective processing in BPD differs from that in BD in that it involves more widespread abnormalities in cortical structures involved in affect regulation, as well as abnormalities in the amygdala, a structure involved in affect generation.

Several case examples may illustrate the differences between affective instability in BPD and bipolar disorder, as well as the complexity of affective instability in each.