The comorbidity of anxiety and depression

INTRODUCTION

The frequency and severity of psychiatric illness, both in the United States (US) and worldwide, were not fully appreciated until the last few decades of the twentieth century. A 1996 study from the World Health Organization reported that mental illness accounted for 15.4 percent of total disability in economically developed countries, exceeded only by cardiovascular disease (18.6 percent) [1]. Disability due to mental illness was greater than that attributed to malignant disease (15.0 percent), respiratory disease (4.8 percent), and infectious disease (2.8 percent). Worldwide, in 1990, 5 of the 10 leading causes of disability were psychiatric. Depression was predicted to become the second leading cause of disability worldwide by the year 2020.

This discussion will focus on diagnosis and treatment of patients who have symptoms suggesting both anxiety and depression. Further discussion of each condition individually is presented elsewhere. (See "Depression: Clinical manifestations and diagnosis" and "Initial treatment of depression in adults" and "Overview of generalized anxiety disorder".)

EPIDEMIOLOGY

Three large epidemiologic studies of psychiatric illness in the United States have been influential in establishing the prevalence of psychiatric illness: the Epidemiological Catchment Area (ECA) study [2], the National Comorbidity Survey (NCS) [3], and the National Comorbidity Survey Replication (NCS-R) [4]. The ECA study found a lifetime prevalence of approximately one in three individuals having some psychiatric disorder. The NCS, completed about 15 years later and with an expanded list of disorders, reported a lifetime prevalence of 50 percent. The prevalence in past year was 30 percent for at least one episode of psychiatric illness in US adults (ages 15 to 54 years) in the NCS. The one-year rate was 26 percent in the NCS-R, with about 60 percent of these considered at least moderate in severity. A report from the US Surgeon General in the late 1990s estimated that, including substance abuse, approximately two of every seven individuals have an active diagnosable psychiatric syndrome [5].

Comorbidity, the presence of two or more disorders, is common in patients with psychiatric illness, and possibly more common than a single diagnosis. The ECA study (lifetime) found that approximately 60 percent of those with at least one disorder qualified for two or more diagnoses, while in the 12-month NCS-R, approximately 45 percent did. In the NCS (lifetime), 14 percent of the sample qualified for three or more diagnoses, and represented 54 percent of the total number of diagnoses in the population, while in the 12-month NCS-R 23 percent of those with any disorder qualified for at least three. One study reported that the presence of any disorder increased the odds of having another psychiatric disorder [4,6]. This observation suggests the possibility of a generalized and non-specific vulnerability towards psychopathology.

Both mood (or affective) disorders and anxiety disorders are prevalent. In the NCS, the lifetime rate of any mood disorder was almost 20 percent, with almost 90 percent of those being unipolar. The 12-month risk for any anxiety disorder in the NCS-R was 18 percent, and the risk for any mood disorder was 9.5 percent. There was a strong association between lifetime risks for panic disorder and major depression (odds ratio 6.8) in the NCS [7]. There was also a positive correlation between any mood disorder and co-occurrent any anxiety disorder in the NCS-R. Generally, it is estimated that if an individual has one of these disorders, he or she has a 25 to 50 percent chance of developing the other. Odds ratios were elevated for other depressive and anxiety disorders as well [8-12].