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Epidemiology of Anxiety Disorders

Abstract and Keywords

This chapter reviews the results of community epidemiological surveys concerning the descriptive epidemiology of anxiety disorders, with a focus on lifetime prevalence, age-of-onset, persistence, and comorbidity. Anxiety disorders are found to be very common despite current DSM and ICD criteria underestimating many clinically significant cases. Anxiety disorders often have early age-of-onset and high comorbidity. They typically are temporally primary to the disorders with which they are comorbid. Young people with anxiety disorders seldom receive treatment prior to the onset of secondary conditions. The chapter closes with a discussion of the importance of long-term studies to determine whether early treatment of primary anxiety disorders would influence the subsequent onset and course of secondary disorders.

Keywords: agoraphobia, anxiety disorder, epidemiology, generalized anxiety disorder, obsessive-compulsive disorder, panic disorder, phobia, posttraumatic stress disorder, separation anxiety disorder, World Mental Health (WMH) surveys

This chapter presents an overview of the descriptive epidemiology of anxiety disorders, with an emphasis on lifetime prevalence, age-of-onset distributions, persistence, subtypes, and comorbidity. The disorders considered include panic disorder, agoraphobia, specific phobia, social phobia, generalized anxiety disorder (GAD), posttraumatic stress disorder (PTSD), obsessive-compulsive disorder (OCD), and separation anxiety disorder (SEPAD). Mixed disorders (e.g., mixed anxiety-depression), acute stress disorder, and adjustment disorder with anxious mood are not considered based on the scant epidemiological data available on them. We focus largely on evidence obtained in general population surveys, although some findings are also reported from clinical epidemiological studies with regard to subtyping. In light of the fact that Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria have been used more widely than International Classification of Diseases (ICD) criteria in epidemiological studies, we focus largely on disorders defined by the DSM system, although a few important differences with ICD disorders are highlighted.

Assessment of Mental Disorders in Epidemiological Surveys

Data on the epidemiology of anxiety disorders have proliferated over the past two decades due to the development of diagnostic criteria in the DSM and ICD systems that are amenable to operationalization and the subsequent development of fully structured research diagnostic interviews based on these criteria. The first such interview was the Diagnostic Interview Schedule (DIS) (Robins, Helzer, Croughan, & Ratcliff, 1981), which was developed for use in a large community epidemiological survey in the United States (Robins & Regier, 1991) and (p. 20) subsequently used in similar surveys in other parts of the world. The DIS was based on DSM-III criteria (Horwath & Weissman, 2000).

The World Health Organization (WHO) subsequently developed the Composite International Diagnostic Interview (CIDI) (Robins et al., 1988), which was based on the DIS, in order to have an instrument that operationalized ICD-10 as well as DSM criteria and that could be used reliably in many different cultures (Wittchen, 1994). As general population surveys were carried out in a number of countries with the first version of CIDI, WHO developed a cross-national research consortium to carry out systematic comparisons of CIDI survey results (Kessler, 1999).

Results based on these comparisons led to the expansion and refinement of the CIDI and to a new generation of cross-national CIDI surveys in the WHO World Mental Health (WMH) Survey Initiative. The latter is an initiative to carry out parallel CIDI surveys in 28 countries throughout the world (www.hcp.med.harvard.edu/wmh). Some preliminary cross-national comparative results have been reported for the first half of participating WMH countries (The WHO World Mental Health Survey Consortium, 2004). However, the full set of baseline WMH surveys has not yet been completed, which means that WMH results are currently available largely in country-specific reports (e.g., Bromet et al., 2005; Karam et al., 2006; Shen et al., 2006).

Other instruments besides the CIDI have been used to carry out recent community epidemiological surveys in various parts of the world (Grant et al., 2004; Jenkins et al., 2003). However, the CIDI is so predominant that a few words need to be said about CIDI validity. Clinical reappraisal studies of the original version of CIDI were quite mixed, some showing concordance of anxiety disorder diagnoses with clinical diagnoses to be low (Brugha, Jenkins, Taub, Meltzer, & Bebbington, 2001) and others moderate to good (Wittchen, Kessler, Zhao, & Abelson, 1995). Concordance has been considerably better for more recent versions of CIDI in clinical reappraisal studies carried out in Western countries (Haro et al., 2006; Kessler et al., 1998; Wittchen et al., 1995; Wittchen, Zhao, Abelson, Abelson, & Kessler, 1996).

This evidence for clinical relevance of CIDI diagnoses applies much more to Western than non-Western countries, as prevalence estimates in CIDI surveys in some non-Western countries are implausibly low (Gureje, Lasebikan, Kola, & Makanjuola, 2006; Shen et al., 2006). Methodological studies that debriefed respondents in these surveys discovered that some respondents are uneasy admitting emotional problems to strangers, with underestimation of disorder prevalence varying geographically and sociodemographically as a function of variation in this reluctance. There also appear to be instances in which some CIDI questions are confusing to respondents in certain countries. Innovative methods have been developed to address these problems and to make post hoc corrections based on recalibration of CIDI diagnoses with diagnoses based on independent clinical reappraisal interviews (Kessler, Abelson et al., 2004). However, no results based on these methods have yet been published for anxiety disorders. As a result, data on the epidemiology of anxiety disorders in less developed countries need to be interpreted with caution.

Three cautions must to be kept in mind when considering estimates of the lifetime prevalence, age-of-onset (AOO), and course of anxiety disorders. The first two apply largely to estimates of lifetime prevalence. First, differences in diagnostic criteria across DSM-III-R, DSM-IV, and ICD-10, even if they seem small, can influence prevalence estimates substantially. Second, differences in survey characteristics and populations can introduce variation in prevalence estimates even when the same interview schedule and diagnostic system are used. Based solely on these two factors, we should expect variation in lifetime prevalence estimates across studies.

The third caution is that estimates of lifetime prevalence, AOO, and course in epidemiological studies are generally based on retrospective reports made by survey respondents who are asked to review their entire life and report if they ever experienced particular syndromes, when they started, and how long they persisted. Lifetime review questions are extremely difficult to answer and are prone to a number of problems, such as underreporting bias with regard to occurrence and “telescoping” bias (i.e., bias in the direction of thinking things happened more recently than they really did) in AOO reports. These biases are especially likely for older people, who have more years of life to review than younger people and might have reduced cognitive capacity to carry out such a review with accuracy (Kessler, Wittchen, Abelson, & Zhao, 2000).

It is noteworthy in light of the considerations in the last paragraph that most community epidemiological surveys find that lifetime anxiety disorder prevalence estimates are lower for older than younger people, although AOO distributions are (p. 21) generally the same among older as younger respondents (International Consortium in Psychiatric Epidemiology, 2000). A number of methodological factors could account for this finding, such as that people living in institutions (including nursing homes and other assisted living facilities) are often excluded from general population surveys and that people who die early are excluded, resulting in the elderly survey participants being especially healthy. It is noteworthy in this regard that anxiety disorders have been shown in prospective research to be significant risk factors for early mortality (M. L. Bruce, Leaf, Rozal, Florio, & Ho, 1994). It is also important to consider the possibility that the lower reported lifetime prevalence of anxiety disorders among older respondents might be due to a genuine cohort effect. Based on the plausibility of both methodological and substantive interpretations, it is impossible to draw a firm conclusion about the relative importance of methodological and substantive factors in the lower lifetime prevalence estimates of anxiety disorders in community surveys. Whichever is more important, though, both sets of explanations agree that the lifetime prevalence estimates found in community surveys should be considered lower bound on the true prevalence of recent cohorts.

Lifetime Prevalence, Age-of-Onset, and Persistence

With these cautions as a backdrop, we now consider estimates of lifetime prevalence, AOO, and course of anxiety disorders reported in recent community epidemiological surveys. Several literature reviews have recently been published that present detailed summary tables of prevalence estimates for individual anxiety disorders across many epidemiological surveys (Fehm, Pelissolo, Furmark, & Wittchen, 2005; Goodwin et al., 2005; Lepine, 2002; Lieb, 2005; Lieb, Becker, & Altamura, 2005; Wittchen & Jacobi, 2005). Rather than reproduce these tables, we refer the reader to those reviews for detailed estimates and present only summary results here.

A number of patterns in the lifetime prevalence data are quite consistent. The three most striking are that anxiety disorders overall are consistently more prevalent than any other class of mental disorders; that specific phobia is consistently the most prevalent lifetime anxiety disorder, with estimates usually in the 6%–12% range; and that OCD is consistently the least prevalent lifetime anxiety disorder, with estimates always less than 3%. Prevalence estimates of other anxiety disorders fall within these extremes. However, as detailed in the remainder of this section, uncertainties exist about diagnostic boundaries for the anxiety disorders, and prevalence estimates could become considerably higher based on future revisions of diagnostic criteria.

Although fewer published data exist on retrospectively reported AOO distributions of anxiety disorders (Burke, Burke, Regier, & Rae, 1990; Christie et al., 1988; Jacobi et al., 2004; Kessler, Berglund, Demler, Jin, & Walters, 2005), a number of noteworthy patterns can be seen in these data. Specific phobia is always found to have a modal AOO in childhood, with the vast majority of lifetime cases having onsets by early adulthood. Social phobia and OCD are always found to have a modal AOO in adolescence or early adulthood, with the vast majority of cases beginning by the start of mid-life. Panic disorder, agoraphobia, and GAD are always found to have later and more widely dispersed AOO distributions, with median AOO in the early to mid-20s and an interquartile range of up to two decades. Finally, PTSD is generally found to have the latest and most variable AOO distribution, presumably reflecting the fact that trauma exposure can occur at any time in the life course. Somewhat earlier AOO estimates are generally found in prospective-longitudinal studies than in retrospective AOO reports (Wittchen, Lieb, Schuster, & Oldehinkel, 1999).

This early onset, coupled with the fact that significant associations exist between early-onset anxiety disorders and the subsequent first onset of other mental and substance use disorders (Zimmermann et al., 2003), has led some commentators to suggest that aggressive treatment of child-adolescent anxiety disorders might be effective in preventing the onset of the secondary mental and substance disorders that are associated with the vast majority of people with serious mental illness (Kendall & Kessler, 2002). It is noteworthy in this regard that despite their generally early ages of onset, first treatment of anxiety disorders usually does not occur until adulthood, often more than a decade after onset of the disorder (Christiana et al., 2000).

Course of illness has been less well studied in epidemiological studies of anxiety disorders than either prevalence or AOO. However, estimates of recent prevalence (variously reported for the year, 6 months, or 1 month before interview) are often reported in these surveys in parallel with estimates of lifetime prevalence. Indirect information about chronicity can be obtained by computing recent-to-lifetime prevalence ratios. The 12-month to lifetime prevalence ratios for anxiety disorders are typically (p. 22) in the range 0.4–0.6, with the highest ratios usually found for specific phobia and the lowest for generalized anxiety disorder (Bijl, Ravelli, & van Zessen, 1998; Kringlen, Torgersen, & Cramer, 2001). Ratios as high as these strongly imply that anxiety disorders are quite persistent throughout the life course. More detailed analyses of these ratios could be carried out by breaking them down separately for subsamples defined by age at interview or by time since first onset, but we are unaware of any published research that has reported such analyses. Our own preliminary analyses of this sort in the WMH data suggest, as one might expect, that the 12-month to lifetime prevalence ratios decline with increasing age. The more striking result, though, is that this decline is fairly modest, suggesting that anxiety disorders are often equally persistent over the life course. The few long-term longitudinal studies that exist in representative samples of people with anxiety disorders show that this persistence is usually due to a recurrent-intermittent course that often features waxing and waning of episodes of different comorbid anxiety disorders (Angst & Vollrath, 1991; Bruce et al., 2005; Hasler et al., 2005; Perkonigg et al., 2005; Yonkers, Bruce, Dyck, & Keller, 2003).

Panic Disorder

The lifetime prevalence of panic disorder is consistently found to be in the range 2%–5% in general population epidemiological surveys (Hoyer, Beesdo, Bittner, & Wittchen, 2003). A much higher proportion of respondents in most of these surveys report having one or more panic attacks in their life (Wittchen, Reed, & Kessler, 1998). In the U.S. National Comorbidity Survey Replication (NCS-R), respondents with a history of panic attacks were divided into four subgroups: history of one or more attacks in the absence of both panic and agoraphobia, attacks in the absence of panic disorder but with agoraphobia, panic disorder without agoraphobia, and panic disorder with agoraphobia. Persistence, number of lifetime attacks, number of years with attacks, clinical severity, and comorbidity were all found to increase monotonically across these four subgroups. These results could be interpreted as suggesting that panic exists along a continuum in which panic attacks and panic disorder differ in degree rather than in kind. It would be premature to conclude from these results that the boundary between panic attacks and panic disorder is arbitrary, as a generally similar sort of pattern can be found for subthreshold symptoms of virtually any disorder. However, given the high prevalence and negative outcomes associated with panic attacks in the absence of panic disorder, there is a need for further research to improve differentiation of pathological and normal panic experiences.

A complicating factor in this line of thinking is that panic attacks are so common and so often found to be precursors of a wide range of other mental disorders that it might make more sense to think of isolated panic attacks as a core psychopathological marker for psychopathology than as a subthreshold manifestation of panic disorder (Goodwin et al., 2005). This possibility broadens the continuum concept to include the consideration of comorbidity among syndromes as well as levels of symptom severity and duration within a single syndrome. We postpone the discussion of this broader consideration of comorbidity among the anxiety disorders and of anxiety disorders with other mental disorders to a later section in the chapter.

Specific Phobia

The lifetime prevalence of specific phobia is as high as 11% in some community epidemiological surveys (Magee, Eaton, Wittchen, McGonagle, & Kessler, 1996). Systematic variation exists in AOO distributions and symptom profiles of specific phobia subtypes (Antony, Brown, & Barlow, 1997; Lipsitz, Barlow, Mannuzza, Hofmann, & Fyer, 2002). Blood-injury phobia is particularly distinct in this regard (Bienvenu & Eaton, 1998). However, other research suggests that the number of specific fears is more important for course than type of fears. In particular, data from a large epidemiological survey showed that the number of fears with avoidance reported by people with specific phobia is strongly related to persistence and severity of overall phobia (Curtis, Magee, Eaton, Wittchen, & Kessler, 1998). We are unaware of any attempt to replicate this finding in other epidemiological surveys. This is not to say that some people who report only one specific fear never have persistent or severe specific phobia, as such cases do exist even though they are considerably less common than persistent-severe specific phobia with a number of specific fears. However, it could be that the former are distinct from the latter in some way that has implications for diagnosis. One possibility is that persistent-severe specific phobia with only one specific fear is more likely to be caused by a trauma, such as being attacked by a dog as a child leading to a dog phobia. If so, then further research to elucidate this distinction might lead to downward revision of current lifetime prevalence estimates for specific phobia by differentiating (p. 23) a generalized subtype that does not require trauma exposure and a pure type that does require trauma exposure.

Agoraphobia Without Panic Disorder

Agoraphobia without a history of panic disorder is another type of phobia that has been the source of controversy regarding prevalence. Agoraphobia is considered by many experts (especially in the United States) to be a response to panic (Klein & Gorman, 1987), which means that agoraphobia without panic disorder would only occur when the agoraphobia was caused by a fear of recurrence of paniclike symptoms rather than by a fear of recurrence of panic attacks. DSM-III-R was revised to embody this perspective, requiring fear of either panic attacks or paniclike symptoms as the precipitating factor for a diagnosis of agoraphobia. The ICD system, in comparison, allows for the possibility that agoraphobia is caused by broader fears about being trapped or about safety when outside the home, such as fear of assault. Many experts outside the United States hold to this broader view of agoraphobia. Consistent with the ICD perspective, community epidemiological surveys consistently find that agoraphobia without a history of prior panic attacks or paniclike symptoms is as common as, if not more common than, agoraphobia with a history of prior panic (Andrews & Slade, 2002; Wittchen, Reed et al., 1998).

This inconsistency was first addressed empirically in a clinical reappraisal study carried out in a sub-sample of 22 respondents from a large U.S. epidemiological survey who were diagnosed by the Diagnostic Interview Schedule (Robins et al., 1981), the fully structured diagnostic interview that was the basis for the later development of the CIDI, as having agoraphobia without panic disorder (Horwath, Lish, Johnson, Hornig, & Weissman, 1993). Diagnoses were based on DSM-III criteria, which have a broader definition of agoraphobia without panic disorder than either later DSM editions or the ICD-10. The researchers concluded that 19 of the 22 respondents actually had a specific phobia rather than agoraphobia, one had panic disorder with agoraphobia, and that the remaining two had agoraphobia with paniclike symptoms. In other words, the high estimated prevalence of agoraphobia without panic disorder in the original version of the CIDI seemed to be due to measurement error—a confusion of specific phobia with agoraphobia. The story does not end here. A subsequent version of CIDI was created that expanded the assessment of specific phobia in an effort to address the problem of confusing specific phobia for agoraphobia. A survey carried out using that version of the instrument found that a substantial proportion of the people originally classified as having agoraphobia without a history of panic actually had specific phobia rather than agoraphobia (Wittchen, Reed et al., 1998). However, the study also found that the number of respondents who genuinely had agoraphobia without a history of panic was quite large even after clinical review of cases—3.5% of the sample. This estimate stands in sharp contrast to the 0.2% lifetime prevalence of DSM-IV agoraphobia without panic disorder reported in a very large recent national survey of the United States (Grant et al., 2006).

Whether the high prevalence of agoraphobia in the absence of any prior paniclike symptoms holds up in future epidemiological investigations is an issue of considerable importance in light of the apparent severity of the syndrome in the recent surveys where it has been rigorously evaluated (Andrews & Slade, 2002; Wittchen, Reed et al., 1998). The WMH surveys have taken this as a topic of central importance and to this end have expanded the number of questions asked about agoraphobic fears and expanded the probes about the focus of fear in the assessment of agoraphobia. For example, adult separation anxiety disorder (SEPAD) is one possible basis for the fear of being out alone and separated from loved ones. This disorder was not assessed in any of the previous CIDI surveys that documented a high prevalence of agoraphobia without any prior paniclike symptoms, but it is assessed in the WMH surveys. Adult separation anxiety disorder as a focus of agoraphobic fears is included in the WMH assessment of agoraphobia along with parallel assessments of such things as fear of crime and fear of an attack of an episodic illness such as asthma or epilepsy. In consequence, although our current understanding is so incomplete that we cannot say whether the lifetime prevalence of independent agoraphobia is vanishingly small or as high as 3%–5%, we can anticipate a reduction in this uncertainty based on analysis of the WMH survey data that should be completed in the near future.

A related issue concerns the temporal priority between panic and agoraphobia. As noted above, the thinking on which the DSM-IV definition is based assumes that agoraphobia occurs as a result of panic rather than the reverse. However, a recent follow-up of the original ECA sample in Baltimore that was re-interviewed 13 years after the baseline survey found that this was not the case (Bienvenu et al., 2006). (p. 24) Psychiatrist follow-up evaluations in this sample documented not only the existence of agoraphobia in the absence of a history of spontaneous panic attacks, but also showed that this type of independent agoraphobia at baseline predicted the subsequent first onset of panic disorder with about the same strength that baseline panic disorder predicted the subsequent first onset of agoraphobia. Such a pattern of equal-sized cross-lagged associations is more consistent with a common causes model (i.e., that panic and agoraphobia are both indicators of a common diathesis) than with a one-way causal model in which panic leads to agoraphobia but agoraphobia does not lead to panic.

Social Phobia

Prevalence estimates of social phobia in recent epidemiological surveys have varied more than those of specific phobia or agoraphobia. Two recent nationally representative surveys of the United States, for example, reported widely differing lifetime prevalence estimates of DSM-IV social phobia: 12.8% in a survey based on the CIDI (Kessler, Berglund et al., 2005) and 5.0% in a survey based on a less widely used fully structured diagnostic interview (Grant et al., 2005). It is noteworthy that a clinical reappraisal study in the CIDI sample, where a probability sample of respondents was blindly administered a follow-up semistructured diagnostic interview by a clinician, confirmed the high prevalence of social phobia suggested by the CIDI. No clinical reappraisal study was carried out in conjunction with the survey that found the lower prevalence estimate. Nonetheless, even in recent CIDI surveys, the estimated prevalence of social phobia has varied between less than 3% (ESEMeD/MHEDEA 2000 Investigators, 2004) and nearly 14% (DeWit, Ogborne, Offord, & MacDonald, 1999).

It is not clear why such large differences in prevalence estimates should exist. Most methodological explanations are excluded by the fact that the same instrument was used in studies that found both high prevalence and those that found low prevalence. It is noteworthy that the highest prevalence estimates were obtained independently in CIDI surveys carried out in the United States and Canada (DeWit et al., 1999; Kessler, Berglund et al., 2005) and that consistently lower prevalence estimates were found in a coordinated series of surveys using an identical version of the CIDI in six Western European countries (ESEMeD/MHEDEA 2000 Investigators, 2004; DeWit et al., 1999). Is it possible that the true prevalence of social phobia is markedly higher in North America than in Europe? As noted above, a clinical reappraisal study carried out in conjunction with the U.S. CIDI survey confirmed the estimated prevalence of DSM-IV social phobia as being greater than 12%. At least a dozen independent epidemiological surveys in Europe, in comparison, have estimated a much lower prevalence of DSM-IV social phobia, with a median lifetime prevalence (6.6%) only about half as high as in the North American studies (Fehm et al., 2005). Based on these results, the most plausible conclusion is that the prevalence of social phobia is genuinely higher in North America than Europe, although direct comparisons with parallel clinical reappraisal studies would be needed to resolve this question definitively.

Separation Anxiety Disorder

We noted above in the discussion of agoraphobia without panic that one hypothesis regarding overdiagnosis is that people with separation anxiety disorder (SEPAD) might be misdiagnosed as having agoraphobia without panic. In DSM–IV–TR, SEPAD is described as a childhood disorder that seldom persists into adulthood. Epidemiological studies have found that 2%–6% of children and adolescents have a lifetime history of SEPAD (Jurbergs & Ledley, 2005). However, empirical studies in clinical samples argue that adult SEPAD is more common than suggested by DSM–IV–TR (Diener & Kim, 2004). This could be due to either of two possibilities. First, a higher proportion of childhood-onset cases might persist into adulthood than assumed in DSM–IV–TR. Second, at least some first onsets might occur in adulthood.

Only a few short-term follow-up studies of clinical samples have evaluated the first possibility (Foley, Pickles, Maes, Silberg, & Eaves, 2004). While these studies showed that the vast majority of childhood cases remit before adulthood, they did not follow cases long enough to determine whether an adult form of the disorder subsequently reemerged in conjunction with the development of adult attachment relationships. A few studies of adult SEPAD have evaluated the second possibility (Cyranowski et al., 2002). These studies consistently found a number of adults with SEPAD who reported never having childhood SEPAD. However, as these studies were all based on small and unrepresentative samples, no generalizations can be made from them about prevalence or correlates of adult-onset SEPAD.

The WMH surveys are the only community epidemiological surveys of which we are aware that assessed adult SEPAD. Only one WMH report (p. 25) has appeared so far on SEPAD (Shear, Jin, Ruscio, Walters, & Kessler, 2006). This report is based on the nationally representative CIDI survey carried out in the United States. Lifetime prevalence estimates of childhood and adult SEPAD in this survey were 4.1% and 6.6%, respectively. Approximately one-third (36.1%) of respondents classified as childhood cases were found to persist into adulthood, while the majority (77.5%) of respondents classified as adult cases were found to have first onsets in adulthood.

In interpreting these results, it needs to be noted that adult SEPAD was found to be highly comorbid with other mental disorders. This is especially important in light of the fact that the DSM gives little guidance on diagnosis of SEPAD among adults. This underscores the need for further exploration of the boundaries between normal response to loss of an attachment figure, separation anxiety as an adjustment reaction, and syndromal separation anxiety disorder in order to arrive at a clear and principled set of criteria for adult SEPAD in future editions of DSM and ICD.

Generalized Anxiety Disorder

Prevalence estimates of GAD have varied widely in community epidemiological surveys over the years due to the fact that the criteria for a diagnosis of GAD have changed substantially in the various editions of the DSM as well as the fact that the DSM system requires worry to be “excessive” but the ICD system does not and that the DSM includes a hypervigilance syndrome that does not exist in the ICD definition (Kessler, Keller, & Wittchen, 2001). Lifetime prevalence estimates of DSM-IV GAD in recent epidemiological surveys have been in the range 1%–6% (Kessler, Brandenburg et al., 2005). However, recent research has shown that variation in the key assumptions of duration and excessiveness has a marked effect on these estimates (Ruscio et al., 2005).

The minimum duration requirement for GAD was 1 month when GAD was introduced in DSM-III (American Psychiatric Association [APA], 1980). However, clinical studies found that DSM-III GAD seldom occurred in the absence of other comorbid anxiety or mood disorders in clinical samples (Breslau & Davis, 1985b), suggesting that GAD might better be conceptualized as a prodrome, residual, or severity marker than an independent disorder.

Based on clinical evidence that comorbidity substantially decreased with episode duration (Breslau & Davis, 1985a), GAD was retained in DSM-III-R (APA, 1987) with an increased duration requirement of 6 months. This change also addressed the problem of distinguishing short episodes of GAD from situational stress reactions (Barlow & Wincze, 1998). A requirement that “unrealistic, hard to control worry” be present was also included in DSM-III-R to sharpen the distinction between GAD and nonspecific distress associated with other anxiety and mood disorders (Barlow, Blanchard, Vermilyea, Vermilyea, & DiNardo, 1986). The ICD-10 criteria for research also require a 6-month duration (World Health Organization, 1993), but the ICD-10 criteria for clinical practice take a middle position on duration by requiring GAD to last “several months” (World Health Organization, 1992).

This variation in required duration could dramatically influence the number of people classified with GAD. Contrary to the motivation for increasing the DSM-III-R duration requirement, epidemiological data have subsequently shown that GAD is not more comorbid than most other anxiety or mood disorders (Kessler et al., 2001; Kessler, Walters, & Wittchen, 2004) and that the extremely high comorbidity of GAD in early clinical studies was due to a help-seeking bias (Wittchen, Zhao, Kessler, & Eaton, 1994). The 6-month duration requirement was nonetheless retained in DSM-IV (APA, 1994), which means that episodes of shorter duration receive no diagnosis even if they recur over many years. It has been suggested that such “orphaned” cases of GAD are substantial in number (Rickels & Rynn, 2001).

Several large community epidemiological studies have examined whether episodes of GAD with durations less than 6 months might be either less impairing or less comorbid with other DSM disorders than episodes lasting 6 months or longer. No significant differences were found in any of these studies. In the largest and most comprehensive of these studies (Kessler, Brandenburg et al., 2005), the lifetime prevalence estimate of DSM-IV GAD more than doubled (from 6.1% to 12.7%) when the duration requirement was changed from 6 months to 1 month. Cases with episodes of 1–5 months were found not to differ greatly from those with episodes of 6 or more months in onset, persistence, impairment, comorbidity, parental GAD, or sociodemographic correlates. These results support the view that a large number of people exist with a clinically significant GAD-like syndrome that is characterized by episodes of less than 6 months duration (Rickels & Rynn, 2001). In saying this, the issue of recurrence is one of obvious importance. Cases with exclusively short episodes typically were (p. 26) found to recur over a number of years, with the average number of years with an episode equal to that of cases with episodes that last 6–11 months, raising the possibility that it might be useful for future studies to consider whether episode recurrence should play a more important part than it currently does as a defining feature of GAD among cases with exclusively short episodes.

A question can be raised whether reverting to a 1-month duration requirement would make sense clinically in that treatment might not be necessary for cases with such short durations. However, as noted in the last paragraph, these cases typically have high recurrence that might be prevented through effective treatment. It is not clear from existing trials whether currently available treatments would be effective in preventing recurrence of cases with short episode durations, as psychotherapy outcome studies and maintenance pharmacotherapy trials have been carried out almost exclusively with cases of longer duration (Pollack, Meoni, Otto, Simon, & Hackett, 2003). However, even if currently available therapies are not effective in this way, it could be argued that this failure should not be reified by defining the problem out of existence by excluding cases with short-recurrent episodes from a diagnosis.

The excessiveness criterion poses many of the same assessment challenges that led to the removal in DSM-IV of the requirement that worry must be “unrealistic” (Abel & Borkovec, 1995; Barlow & Wincze, 1998). For example, there is considerable confusion over what makes worry “excessive” as well as uncertainty over who should determine whether worry is excessive. There is also the question of what leads individuals to appraise their worry as excessive and whether this appraisal corresponds to objective characteristics of the worry experience. It is perhaps not surprising in light of these considerations that one study of DSM-III-R GAD found excessiveness to be the criterion on which assessors most often disagreed, with its elimination leading to a dramatic rise in interrater reliability of diagnosis (Wittchen et al., 1995).

In addition to these problems of conceptual confusion, concerns can be raised about the implications of the excessiveness criterion for diagnostic validity. In particular, critics have noted that the excessiveness requirement excludes from the GAD diagnosis individuals who develop clinically significant generalized anxiety in the context of chronic, objectively stressful situations (Kessler & Wittchen, 2002). Importantly, community epidemiological research has shown that a sizable number of individuals exist who are diagnosed with GAD by ICD-10, but not by DSM-IV, solely because they fail to meet the excessiveness criterion (Slade & Andrews, 2001). Little is known about these individuals or their likely impact on estimates of the prevalence, severity, or correlates of GAD.

An epidemiological study based on a nationally representative U.S. sample found that the estimated lifetime prevalence of DSM-IV GAD increased by approximately 40% when the excessiveness requirement was removed (Ruscio et al., 2005). It also found that GAD with excessive worry begins earlier in life than GAD without excessive worry, that the former has a more chronic course than the latter and is associated with greater symptom severity and psychiatric comorbidity than the latter. However, GAD without excessive worry was nonetheless found to have substantial persistence and impairment and significantly elevated comorbidity compared to respondents without GAD. Nonexcessive cases also were found to have sociodemographic characteristics comparable to excessive cases and, importantly, were found to have the same familial aggregation of GAD as excessive cases assessed with modified family history RDC interviews of respondents.

Posttraumatic Stress Disorder

Prevalence estimates of PTSD are more difficult to estimate than those of other anxiety disorders because the prevalence of PTSD is a joint function of the prevalence of particular types of trauma exposure and the conditional prevalence of PTSD among people exposed to these traumas. An additional complication is that conditional risk of PTSD among people exposed to trauma varies greatly by type of trauma (Kessler, 2000). Most research on the epidemiology of PTSD has focused on the second of these two prevalence estimates by studying victims of specific traumas such as physical assault (Kilpatrick & Resnick, 1992), sexual assault (Pynoos & Nader, 1988), natural disaster (Koopman, Classen, & Spiegel, 1994), and military combat (Solomon, Neria, Ohry, Waysman, & Ginzburg, 1994). Less is known about the total population prevalence of trauma exposure or PTSD. However, it is possible to piece together such a portrait by combining the results of general population surveys on the prevalence of trauma exposure with the results of more in-depth studies carried out in trauma samples on the conditional prevalence of PTSD among people exposed to trauma.

(p. 27) The largest body of general population data on the prevalence of trauma exposure comes from the United States, where recent surveys have shown that trauma exposure is highly prevalent (Breslau, Davis, Andreski, & Peterson, 1991). A national survey of the U.S. household population found that 60.7% of men and 51.2% of women reported exposure to at least one lifetime traumatic event, with the majority of respondents who reported trauma saying that they experienced more than one type of trauma (Kessler, Sonnega, Bromet, Hughes, & Nelson, 1995).

It is unclear whether these U.S. results generalize to other developed countries. The fact that crime statistics for extreme forms of assaultive violence such as murder and rape are considerably higher in the United States than in other developed countries (Langan & Farrington, 1998) means that exposure to traumatic interpersonal violence is likely to be lower in other developed countries than in the United States. However, rates of exposure to natural disasters and life-threatening accidents, two of the most commonly reported traumas in the U.S. surveys, presumably are comparable in other developed countries. The situation is almost certainly quite different in less developed countries, where we know that exposure to traumatic events involving interpersonal violence is much more common. To take but one of many examples in the literature, Husain et al. 1998) evaluated a sample of 791 Bosnian school children aged 7–15 in Sarajevo at the end of the city's siege in 1994. These children reported that during the previous year, 85% had been shot at by snipers, 66% had lost a family member, and between 10% and 48% had experienced various types of physical deprivation, such as water shortage and lack of shelter.

Turning to the conditional risk of PTSD given trauma exposure, comparison of results across epidemiological studies of particular traumatic stressors shows that conditional risk of PTSD among trauma victims varies enormously depending on the type of trauma under investigation. The general pattern is that PTSD risk is much greater after exposure to trauma involving assaultive violence than after other forms of traumas (Breslau et al., 1998) and that the probability of PTSD is associated with the number of preexisting traumas, exposure severity, and exposure duration as well as with the occurrence of secondary stressors (Stein et al., 2002). A meta-analysis of epidemiological surveys in populations directly exposed to terrorism found the prevalence of PTSD to be in the range 12%–16% (DiMaggio & Galea, 2006). Prevalence has usually been considerably lower in surveys of populations exposed to natural disasters (Galea, Nandi, & Vlahov, 2005). The only report from an epidemiological survey to generate a comprehensive list of traumas and to evaluate risk of PTSD for one randomly selected trauma from each respondent was based on a representative sample of young adults (ages 18–44) in a metropolitan area of the United States (Breslau et al., 1998). Conditional risk of PTSD after a representative trauma was 9.2%, while the highest conditional risk (20.9%) was associated with traumas characterized by assaultive violence. In particularly traumatic situations, though, conditional risk can be higher. In the study of Bosnian school children mentioned in the last paragraph, for example, the prevalence of PTSD was greater than 40%.

It is difficult to combine the estimates of the unconditional prevalence of trauma exposure with the estimates of conditional risk of PTSD after trauma exposure into an overall estimate of population-wide PTSD prevalence due to the complexities associated with many people being exposed to multiple lifetime traumas that vary in prevalence as well as in conditional risk of PTSD. An added complication is that some evidence suggests that conditional risk of PTSD associated with a particular trauma varies as a function of prior exposure to other traumas (Breslau, Chilcoat, Kessler, Peterson, & Lucia, 1999). The only nationally representative estimate of lifetime PTSD prevalence in the United States, which is based on DSM-III-R criteria, is 7.8% (Kessler, Berglund et al., 2005).

Lifetime prevalence estimates of PTSD in the general populations of Western European countries are substantially lower than in the United States. The six Western European countries in the WMH survey series estimated that 1.9% of their populations had PTSD at some time in their lives (ESEMeD/MHEDEA 2000 Investigators, 2004). Other recent Western European surveys had estimates between less than 1% and 2% (Hoyer et al., 2003). As with social phobia, where a similar difference exists between the results of surveys carried out in North America and Western Europe, the most plausible conclusion is that the prevalence of PTSD is genuinely higher in the United States, although direct comparisons involving identical lists with all kind of traumatic events in line with DSM-IV and studies with parallel clinical reappraisal would be needed to resolve this question definitively. While comparable surveys have not been carried out in less developed countries, it is likely that PTSD is (p. 28) considerably more prevalent in the subset of those countries that have experienced prolonged sectarian violence.

Obsessive-Compulsive Disorder

As noted in the introduction, epidemiological surveys find that OCD is one of the least prevalent anxiety disorders, with lifetime prevalence estimates consistently less than 3% (Horwath & Weissman, 2000). There is considerable interest, though, in the possibility that a number of other disorders are part of an OCD spectrum that might be far more prevalent than OCD itself (Goldsmith, Shapiro, Phillips, & McElroy, 1998). Conditions thought to be part of the OCD spectrum include tic disorders, body dysmorphic disorder, eating disorders, trichotillomania and related self-harm disorders, and possibly even hypochondriasis. The argument for the existence of this hypothesized spectrum is based on similarities across the different disorders in a subjective sense of compulsion, in difficulty inhibiting repetitive behaviors, in age of onset, in course of illness, in patterns of comorbidity, in family history, and in specificity of treatment response (Neziroglu, Henricksen, & Yaryura-Tobias, 2006).

Although some controversy exists about the range of conditions that fall within the OCD spectrum (Richter, Summerfeldt, Antony, & Swinson, 2003), the notion that there exists such a spectrum is now widely enough accepted that it has been proposed that spectrum disorders should be reclassified in the ICD and DSM systems as subtypes of OCD (Yaryura-Tobias et al., 2000). Needless to say, if this happens, the estimated prevalence of OCD could increase substantially. As far as we know, no large-scale community epidemiological research exists on the prevalence of OCD spectrum disorders, so it is difficult to know how high the prevalence estimate might become if OCD spectrum disorders are eventually redefined as subtypes of OCD. A complication is that other research has documented heterogeneity within OCD (Watson, Wu, & Cutshall, 2004), such as a clear distinction between OCD with and without hoarding (Grisham, Brown, Liverant, & Campbell-Sills, 2005). It is currently unclear how this evidence of within-disorder heterogeneity is related to evidence of an OCD spectrum.

Comorbidity Among the Anxiety Disorders

Comorbidity among anxiety disorders is quite common, with up to one-third of people with a lifetime anxiety disorder in some surveys meeting criteria for two or more such disorders (Kessler, 1995). Furthermore, there is some evidence that anxiety disorders are more highly comorbid than other mental disorders both with each other and with other mental and physical disorders (Toft et al., 2005). Factor analytic studies of diagnostic comorbidity consistently document separate internalizing and externalizing factors in which anxiety and mood disorders have high factor loadings on the internalizing dimension, while most impulse-control disorders and substance use disorders have high factor loadings on the externalizing dimension (Kendler, Prescott, Myers, & Neale, 2003). The internalizing disorders, furthermore, have secondary dimensions of fear disorders (panic, phobia) and distress disorders (depression, dysthymia, GAD) (Watson, 2005). The locations of OCD and PTSD in this two-dimensional space are less distinct; the former appears to be more related to the fear dimension (Watson, 2005) and the latter more related to the distress dimension (Cox, Clara, & Enns, 2002), although neither is strongly indicated by either of these dimensions. Social phobia additionally appears to be somewhat more strongly related to the distress dimension than are the other phobias. Separation anxiety disorder has not been included in factor analytic studies to date.

These results have recently been used by Watson 2005) to call into question the codification of anxiety disorders as a distinct class of disorders in the DSM and ICD systems and to suggest that a more useful organizing scheme in the upcoming DSM-V would be one that distinguished between fear disorders and distress disorders, with the latter including not only GAD and possibly PTSD but also unipolar depression and dysthymia. The argument for a class of fear disorders has the stronger support of the two in neurobiological research based on extensive investigation of fear brain circuitry (Knight, Nguyen, & Bandettini, 2005). The possibility also exists that future research might lead to OCD being distinguished from either fear disorders or distress disorders as part of a spectrum of impulse-control disorders based both on evidence of differential comorbidity and differences in brain circuitry (Whiteside, Port, & Abramowitz, 2004).

Studies of multivariate disorder profiles confirm the complexity of the comorbidity that exists among anxiety disorders. The most comprehensive of these analyses was carried out in the U.S. National Comorbidity Survey Replication (Kessler & Merikangas, 2004) by examining the multivariate profiles among 19 separate DSM-IV disorders (Kessler, Chiu, Demler, Merikangas, & Walters, 2005). (p. 29) Of the 524,288 (219) logically possible multivariate disorder profiles among these disorders, 433 were observed. Nearly 80% of them involved highly comorbid cases (three or more disorders), accounting for 27.0% of all respondents with a disorder and 55.9% of all instances of these disorders.

Importantly, the distribution of comorbidity was found to be significantly different from the distribution one would expect to find if the multivariate structure among the disorders was due entirely to the two-way associations that are the focus of factor analysis, suggesting that the more typical factor analytic studies of comorbidity fail to detect important structure. Based on this result, latent class analysis (LCA) was used to study nonadditive comorbid profiles. A seven-class LCA model provided the best fit to the data, with four classes featuring anxiety disorders prominently.

Results as complex as these clearly need to be replicated before they are taken seriously, but even if accepted as no more than preliminary, they suggest that understanding comorbidity in the anxiety disorders will require considerably more in-depth analysis than has previously been used to study these associations. A dynamic perspective might be useful here in building in information about AOO distributions that allows an investigation of the temporal unfolding of comorbidity. This type of dynamic analysis is currently being used to investigate lifetime comorbidity among anxiety, mood, and impulse-control disorders in the cross-national WMH surveys as well as in a multiwave 10-year prospective analysis of a large cohort of adolescents and young adults (Wittchen, Perkonigg, Lachner, & Nelson, 1998).

The Societal Costs of Anxiety Disorders

We noted earlier in the chapter, but did not emphasize, that early-onset anxiety disorders are powerful predictors of the subsequent onset and persistence of other mental and substance use disorders. It is important to note that these predictive associations are part of a larger pattern of associations that have been documented between anxiety disorders and a much wider array of adverse life course outcomes that might be conceptualized as societal costs of these disorders, including reduced educational attainment, early marriage, marital instability, and low occupational and financial status (Lepine, 2002). A considerable amount of research has been carried out to quantify the magnitude of the short-term societal costs of anxiety disorders in terms of health care expenditures, impaired functioning, and reduced longevity (Marciniak, Lage, Landbloom, Dunayevich, & Bowman, 2004). The magnitude of the cost estimates in these studies is staggering. For example, Greenberg et al. 1999) estimated that the annual total societal costs of active anxiety disorders in the United States over the decade of the 1990s exceeded $42 billion. This estimate excludes the indirect costs of early-onset anxiety disorders through adverse life course outcomes (e.g., the documented effects of child-adolescent anxiety disorders in predicting low educational attainment and consequent long-term effects on lower income) and through increased risk of other disorders (e.g., anxiety disorders predicting the subsequent onset of cardiovascular disorder).

Summary and Conclusions

The results summarized here document that anxiety disorders are commonly occurring in the general population, often have an early age of onset, and are characterized by frequent comorbidity with each other as well as with other mental disorders. We reviewed evidence to suggest that the current DSM and ICD definitions of anxiety disorders might substantially underestimate the proportion of the population with a clinically significant anxiety condition. It is noteworthy that research on comorbidity among anxiety disorders generally ignores the existence of anxiety spectrum conditions, a failing that should be rectified in future research.

Based on these results, along with results regarding the societal costs of anxiety disorders, we can safely conclude that anxiety disorders are common and consequential problems that are deeply interwoven with a wide range of other physical, mental, and broader personal difficulties in the general population. As early-onset conditions, anxiety disorders typically begin prior to the vast majority of the other problems with which they are subsequently associated. Yet, as noted earlier in the chapter, young people with early-onset anxiety disorders seldom receive treatment. This is a situation that has to change if we are to be effective in addressing the enormous public health burden created by anxiety disorders throughout the world. To do this will require a level of political will that has heretofore been lacking in even the most progressive countries in the world. One can but hope that future research focused on the long-term costs of illness and the cost-effectiveness of early effective treatment will correct this situation by demonstrating the wisdom of overcoming the current neglect of this extremely prevalent and important class of disorders.

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