Comorbidity has been defined as 'the presence of any distinct clinical entity that has existed or that may occur during the clinical course of a patient who has the index disease under study'/1 The key word here is 'distinct'. True comorbidity implies the presence of two completely separate disorders in the same person which are not causally related to each other in any way. Co-occurrence ranges from true comorbidity to the presence of the same disorder in two or more different forms. (2)

Comorbidity is the norm for most personality disorders both with other personality disorders or with mental state disorders. Borderline personality disorder is a major offender in this regard. Only about one in 20 of such disorders constitutes the pure condition, (3) and multiple comorbidity with four or more disorders is more common.

In deciding on the efficacy of any treatment for personality disorder it is impossible to be certain whether observed improvement is in the personality disorder or in a comorbid condition. As personality disorders are generally more persistent than mental state disorders, it is reasonable to suppose that improvement is more likely to be due to a change in comorbid condition than a change in the personality disorder. This problem is made worse because personality assessment is often confounded, or 'contaminated', by the effect of a concurrent mental state disorder. Thus personality status apparently changes during the presence of a mental state disorder such as depression, only to return to the baseline normal subsequently. (45) Apparent improvement in a personality disorder following a treatment may be entirely due to improvement in a concurrent mental state disorder.

In view of these problems, a treatment for a personality disorder should ideally be tested in patents who have that personality disorder only. As these patients are uncommon and atypical, it is difficult to interpret the data from clinical trials.

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