Bipolar Disorder

understanding them will aid in differential diagnosis and increase the likelihood that a patient will receive appropriate pharmacological and psychosocial treatments. Moreover, such differences, or lack thereof, may also be informative when adapting interventions for bipolar disorder from existing treatments for MDD (e.g. cognitive behavioral therapy).18

Is Bipolar Depression a ‘Different’ Depression?

Despite the growing literature focused on bipolar depression, there has been no clear consensus as to whether it can be differentiated from unipolar depression on the basis of symptoms alone. Some have reported a greater prevalence of atypical features in bipolar versus unipolar depression,19–22 bipolar II disorder.23,24 findings,25

with the highest levels among those with However, others have failed to replicate these

and there is some evidence that bipolar depression might actually be marked by a greater prevalence of melancholic symptoms.26 Additional research suggests that individuals with unipolar depression may tend to endorse higher rates of anxiety and somatization in comparison with those with bipolar depression.27

However, compared

with bipolar II depression alone, others have reported lower rates of anxiety and agitation in unipolar samples.20,28,29

Finally, there is some

evidence that bipolar depression may be characterized by greater rates of psychosis than unipolar depression,21,26 limited to bipolar I depression only.30

although this finding may be

Although suicidal ideation and behaviors may best be conceptualized as transdiagnostic,31

limitations of the extant literature in order to further clarify any differential phenomenology between the two conditions.

Limitations of the Extant Literature

Indeed, the mixed findings reviewed above may be attributed to methodological limitations of prior research. First, the large majority of studies have relied on clinical versus community samples of individuals, some of whom were recruited from inpatient settings21,28,33,46 enrolled in clinical efficacy trials that use narrow inclusion criteria.22

addition, with few exceptions, sample sizes have generally been small, thus rendering statistical parameters unstable.27

or had been In

Another source of

inconsistency across studies may be attributed to type I error, as most studies have performed a large number of comparisons without corresponding alpha-level corrections,27

thereby increasing the risk of

falsely interpreting chance observations as indicative of reliable cross- diagnostic differences.

It should also be noted that several studies have combined individuals with bipolar I and bipolar II depression for purposes of comparison against unipolar depression,22,25,26,33,34

which may account for

they are also Diagnostic and Statistical Manual of

Mental Disorders, 4th Edition (DSM-IV) symptoms of a major depressive episode. Within this context, there is disagreement in the literature as to whether there are differences between bipolar and unipolar depression in rates of endorsement of suicidal ideation and behaviors. For example, in their review of the literature, Rihmer and Kiss32

concluded that

“bipolar patients in general, and bipolar II subjects in particular, carry the highest risk of suicide.” However, published data do not fully support this assertion. Indeed, some33–35

have reported greater suicide

risk in bipolar (I or II) disorder versus MDD, whereas others have reported the opposite effect, with greater suicide risk in MDD.36,37

Still

others have failed to identify any diagnostic difference in levels of suicidal ideation, number of suicide attempts, or intent to die.33,38

With

respect to bipolar II disorder specifically, some investigators have reported higher lifetime history of suicide attempts in bipolar II versus bipolar I disorder,39,40

demonstrate greater suicide risk in bipolar II relative to bipolar I disorder, with no differences reported between subtypes on measures of family history of suicide, suicidal ideation, or suicide attempts.28,41–43

Finally, in contrast to the literature that has emphasized potential diagnostic differences in depression symptom expression, some recent comprehensive reviews have concluded that bipolar and unipolar depression are more similar than different,27,44 differentiable.45 al.45

and may not even be In their two-illness model of bipolar disorder, Joffe et

have suggested that bipolar illness may consist of two separate but inter-related disorders, mania and depression, the latter of which may be “no different from the broad range of depressive disorders that constitute unipolar depression.” In sum, data supporting a unique presentation of depression in bipolar disorder versus MDD remain equivocal, and there is a need for future research to address potential

16

whereas several studies have failed to

inconsistencies when bipolar subtypes have been evaluated independently of one another. In addition, the majority of this research has focused narrowly on atypical or other feature specifiers, and has not comprehensively addressed potential differences across all DSM-IV depressive symptoms. Although such clinical feature specifiers may, indeed, be useful in differentiating between bipolar and unipolar depression, such a focus limits an understanding of how the core symptoms of depression operate in these two disorders.

Perhaps most importantly, several studies have not controlled for overall symptom severity in their comparisons across groups.27

It is

therefore unclear whether any differential symptom expression reported in the literature is due to true phenomenological differences between bipolar and unipolar depression, or whether such differences are instead reflective of greater overall depression severity in one group versus another. This latter point is especially critical in the context of heterogeneity of sample selection in the published literature (i.e. inpatient versus outpatient, clinical versus community), and in light of findings that both atypical features47

and bipolar II disorder48 may be

associated with greater depression severity. For example, the higher risk of suicide in bipolar II versus bipolar I disorder that has been reported in some research39,40

may be better accounted for by greater

depression severity in the bipolar II samples that were evaluated. Consistent with this argument, Cooke et al.49

reported no differences

between MDD, bipolar I, and bipolar II disorders on a number of depression course characteristics when comorbidity (i.e. a proxy for illness severity) was accounted for in the analysis.

Given the limitations reviewed above, we have argued that methods based on item response theory (IRT) might be particularly useful in overcoming some of the challenges that have plagued existing research comparing bipolar and unipolar depression. Although a comprehensive overview of IRT is beyond the scope of the current review, we briefly describe this methodology in the paragraphs that follow and review its potential strengths and limitations as applied to the study of diagnostic group differences in the expression of depressive symptoms. We also

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