“Comorbidity” (the occurrence of two or more disorders in the same individual) has been a popular topic since it was discovered that many individuals meet DSM criteria for multiple disorders.
Efforts to develop and evaluate treatments focused on particular childhood disorders, such as ADHD, have been greatly complicated by the difficulty of finding enough children who have only the target disorder without symptoms of other disorders. It has been widely hypothesized that comorbidity may undermine the effectiveness of treatments designed for particular disorders.
However, this hypothesis has not been well tested. One obstacle to testing the effects of comorbidity on treatment outcome is the difficulty of recruiting (a)groups of children who meet DSM criteria for a particular target disorder without other disorders and (b) comparison groups of children who meet DSM criteria for the target disorder, plus certain other disorders. To overcome this obstacle, Doss and Weisz (2006) used scores on CBCL syndromes to test the degree to which “co-occurring syndromes” affected the outcomes for 325 7- to 17-year-olds treated in six California mental health clinics.
Pre-treatment and post-treatment CBCL scores were subjected to hierarchical linear modeling (HLM) analyses that took account of differences among therapists, length of treatment, initial CBCL scores, and the co-occurrence of scores among different syndromes. The HLM analyses showed that co-occurring syndrome scores had negligible effects on outcomes. In other words, contrary to the hypothesis that comorbidity would undermine the effectiveness of treatment, co-occurring syndromes did not result in poorer outcomes. Instead, the initial scores on each syndrome were the best predictors of outcomes for that syndrome. Doss and Weisz concluded that “in general, co-occurrence did not seem to impact usual care outcomes over and above the effects of initial severity” (p. 424). They also concluded that the “dimensional approach has several theoretical and statistical advantages over traditional categorical approaches” (p. 424) to assessing psychopathology and comorbidity.
Reference: Doss, A.J., & Weisz, J.R. (2006). Syndrome co-occurrence and treatment outcomes in youth mental health clinics. Journal of Consulting and Clinical Psychology, 74, 416-425.