This study found that comorbidity is common in BDD patients, both in terms of the percentage of patients with at least one comorbid disorder and the mean number of comorbid disorders. Indeed, comorbidity is the rule rather than the exception, as is the case for many psychiatric disorders.25
While the differing comorbidity rates in our phenomenology and treatment studies were expected, these differences underscore that rates may vary in different samples. It is possible, for example, that comorbidity rates in BDD patients in a dermatology or primary care setting might differ from those in the current study. Also, studies from clinical settings tend to find higher comorbidity rates than studies from nonclinical settings, because having more than one disorder may increase the probability of seeking treatment.26
It is therefore important that comorbidity in BDD be studied in a variety of samples, including the general population. Comorbidity rates may also vary depending on the cohort’s age, as older cohorts will have had more time to pass through the period of risk for developing a mental disorder. BDD patients older than those in this study, for example, might evidence even higher rates of lifetime major depression or alcohol dependence.
The significance and meaning of the high comorbidity rates found in this study are unclear. It is unknown, for example, whether certain disorders have high base rates in the study setting and co-occur with BDD randomly, or whether they co-occur with BDD at greater than chance levels, reflecting a meaningful association.27
Various models have been proposed to explain comorbidity among disorders, such as chance association (random co-occurrence), symptom nonspecificity, and shared etiology/pathophysiology.28
In part based on its high comorbidity with OCD, the concept of shared etiology/pathophysiology with OCD is implied in proposals that BDD be considered an “OCD spectrum disorder”—i.e., related to OCD.29
Indeed, studies indicate that BDD and OCD have many similarities30
and that BDD occurs more frequently in first-degree relatives of OCD probands than control probands.1
However, BDD and OCD also have a number of differences30
; in this regard, it is interesting that no BDD subjects in the current study had a history of Tourette’s syndrome, in contrast to the 5% to 10% rate reported for OCD.31
Because of its frequent comorbidity with depression, the question has also been raised of whether BDD is an affective spectrum disorder.32
From a somewhat different perspective, might the high co-occurrence of BDD with depression, social phobia, and OCD reflect a shared broad underlying dimension such as neuroticism, a trait highly characteristic of BDD?11
The very high comorbidity rates of these disorders with BDD in this study and others suggests that their co-occurrence with BDD is not random. However, to establish this, and to clarify their relationship to one another, further prevalence data as well as data from domains other than comorbidity are needed (e.g., family history, course of illness, and pathogenesis).33,34
Regarding age of onset, our finding that major depression usually began after BDD may simply reflect the fact that major depressive disorder typically begins in the mid-20s, whereas BDD usually begins in the early to middle teenage years. An alternative explanation is that major depression may reflect depressive symptoms and demoralization as a secondary complication of BDD. Although it cannot be determined whether one disorder causes another, our clinical impression is that the major depression that so frequently co-occurs with BDD is often due to BDD; however, this does not always appear to be the case. Indeed, in this study major depression preceded onset of BDD in 22% of cases. It is also worth noting that onset of social phobia usually preceded onset of BDD, as would be expected based on social phobia’s usual age of onset. This suggests that in this study, “primary” social phobia was differentiated from the marked social anxiety typically caused by BDD, which would be expected to begin at the time of, or after, onset of BDD. It is sometimes difficult clinically to differentiate primary social phobia from social anxiety secondary to BDD.
Greater comorbidity was associated with greater functional impairment, consistent with findings for other psychiatric disorders.17–19
This association was found primarily for the phenomenology study, which may in part reflect the treatment studies’ exclusion of inpatients, suicidal patients, and patients with a current substance use disorder; substance use disorders are associated with unemployment35
and suicide attempts.36
This study has several limitations. It did not include a control group, making it unclear how specific the findings are to BDD. All participants were assessed by the second author, without use of a consensus process to determine diagnoses. The inclusion of pharmacotherapy study participants may have introduced unknown biases that may have affected comorbidity; indeed, our finding that rates of bipolar disorder and substance related disorders were lower in the treatment sample, while expected, illustrates such a bias. Furthermore, recent findings indicate that depressed patients eligible for treatment trials represent only a subset of individuals with that disorder.37
Such findings encourage caution when generalizing results from treatment studies to clinical settings. In addition, subjects in the present study were obtained from a setting that specializes in BDD, and it is unclear how generalizable the results are to nonspecialty settings. Additional research is needed to identify possible differences between individuals in different settings.
Despite these limitations, our results, in the largest series of BDD patients to date, converge with previous findings in suggesting that axis I comorbidity is common in patients with BDD. Clinicians need to be mindful of the possible clinical impact of comorbid conditions—for example, their association with greater morbidity and the need to consider comorbidity in treatment planning.38,39
Because little is known about comorbidity in BDD, including implications for course of illness, prognosis, and treatment response, additional research is needed in a variety of settings to increase understanding of this important aspect of illness.