THE LATENT STRUCTURE AND COMORBIDITY PATTERNS OF GENERALIZED ANXIETY DISORDER AND MAJOR DEPRESSIVE DISORDER: A NATIONAL STUDY

Generalized anxiety disorder (GAD) and major depressive disorder (MDD) are highly prevalent, disabling conditions associated with considerable personal and societal burden.(1) Because MDD and GAD have high symptom overlap(2) and often co-occur,(3) there is considerable controversy regarding their separation as distinct diagnostic entities. Some studies and meta-analyses have suggested that MDD and several anxiety disorders have a modest familial aggregation and moderate levels of cross heritability,(4) but other studies have found mixed evidence for shared familial risk.(5) Other studies have found distinguishable patterns of comorbidity between GAD and MDD,(6) although whether co-occurrence of MDD and GAD has any influence on these differences is unknown. Several twin studies have also examined the relationship between MDD and GAD in large population samples.(7) The Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD) found that the correlation for environmental risk factors between MDD and GAD was 0.51, indicating that environmental risk factors for these disorders are moderately correlated. The VATSPSUD estimated the genetic correlation between MDD and GAD to be 1, indicating a very strong relationship between the genetic risk factors for both disorders.(7) Data from the Australian and Dutch Twins and Siblings Study also suggest considerable overlap in genetic risk factors for GAD and MDD,(8) and data from the Swedish Twin Registry(9) indicate that GAD and MDD share the same genetic factors but their environmental determinants are mostly distinct. Three large prospective epidemiological studies have also indicated that the developmental risk factors for GAD and MDD do not fully overlap,(10-12) suggesting that despite their similarities, the etiological pathways of these two disorders may differ. Application of latent variable techniques to three large community surveys in which psychiatric diagnoses were used as indicators have suggested that MDD and GAD may be part of an underlying factor named “anxious misery”,(13-16) but other studies have found divergent results.(17) In addition, a recent meta-analysis of the relationship between anxiety sensitivity and internalizing disorders found that anxiety sensitivity is more strongly correlated with agoraphobia, GAD and posttraumatic stress than with depression, raising new questions regarding the relationship between GAD and MDD.(18) The present study seeks to build on existing knowledge by examining data from the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC), a large, nationally representative sample of US adults. The specific goals of our study were: 1) to compare the sociodemographic characteristics and comorbidity patterns of individuals with GAD, MDD, and those with both disorders (GAD-MDD); and, 2) to determine the factor structure of the individual DSM-IV criteria for GAD and MDD.