Transdiagnostic factors of mental disorders.

Abstract

Official nosological systems, such as the DSM-5 and ICD-10, define psychopathology and substance use disorders as distinct, independent, and categorical constructs. In other words, the classification systems imply that a patient either meets the diagnostic threshold for a particular mental disorder or does not (categorical), the disorder does not overlap with other disorders (distinct), and therefore presence of the disorder should not necessarily be associated with a higher probability of having another disorder (independent). Both clinical experience and empirical research indicate that these assumptions are not justified, however. First, sub-threshold disorder manifestations can be associated with significant distress and dysfunction; moreover, there are important severity differences among individuals receiving the same diagnosis. This suggests an underlying dimensionality to mental disorders not captured by categorical diagnoses and highlights the information lost when reducing a complex constellation of signs and symptoms to a present-absent dichotomy. Second, comorbidity is the rule, rather than the exception. Individuals who have one disorder are likely to meet criteria for additional disorders at rates far exceeding what would be predicted from disorder prevalence rates. Research on disorder dimensionality and comorbidity suggests that many mental disorders are manifestations of relatively few core underlying dimensions. Beginning several decades ago, investigations of common symptoms and behaviors in children, and diagnoses in adults, have repeatedly replicated such an underlying cross-cutting transdiagnostic structure: the internalizing-externalizing model. Internalizing accounts for comorbidity among major depression, generalized anxiety disorder, dysthymia, panic disorder, social and specific phobias, post-traumatic stress disorder, and so on, while externalizing accounts for comorbidity among substance use disorders and antisociality-, behavioral-, and impulsivity-related disorders. Unlike the organizations of many official nosologies (e.g., "mood disorders" as separate from "anxiety disorders"), this internalizing-externalizing model provides excellent fit to the data and has been replicated in various populations, from around the world 1, 2. This report highlights recent advances and contemporary directions in transdiagnostic comorbidity research. Studies have shown that internalizing and externalizing are quite stable over time, which has marked implications for understanding successful psychological aging as well as disorder persistence. Indeed, research on this question illustrates that these two transdiagnostic factors are key to understanding disorder continuity. The transdiagnostic variance that related disorders share (captured by the factors) appears to drive disorder persistence. On the other hand, the unique variance of disorders – disorder-specific variance that makes each disorder different from its related disorders – tends to show comparatively low, often negligible, stability. In other words, internalizing and externalizing appear to serve as the primary pathway for homotypic disorder persistence over time. Generalized anxiety disorder, for instance, appears to persist because the internalizing factor variance saturates the diagnosis, and it is this transdiagnostic variance that is stable, not the disorder-specific variance 3, 4. Since the factors account for the majority of homotypic continuity over time, investigations of their role in heterotypic continuity and disorder onset are crucial. For instance, one can conceptualize lifetime transdiagnostic factor levels as a liability for subsequent disorder onset and thus as the key drivers of the development of sequential comorbidity. In longitudinal onset data on eighteen disorders, Kessler et al 4 applied a novel time-lagged latent comorbidity survival model, and found that internalizing and externalizing at time 1 accounted well for subsequent onset of new disorders. This highlights the need for latent structure modeling to move beyond cross-sectional data into well-characterized longitudinal datasets. Researchers have recently addressed three important questions about transdiagnostic factors' characteristics. First, how is the distribution of these factors best conceptualized? This distributional question is important in that it allows for a better understanding of latent internalizing and externalizing generally, and it thus helps us understand the dispersion of these factors in the population. Multiple studies now indicate that these factors are continuously distributed dimensions (vs. liability classes, or dimension-class hybrids) 3. Second, how similar (invariant) are these factors across different groups? Studies of internalizing-externalizing across several populations – comparing individuals by gender, race/ethnicity, age, and sexual orientation – have repeatedly replicated the finding that internalizing and externaliz