“Comorbidity” refers to the occurrence of two conditions in the same individual at a frequency greater than would be expected by chance.  Migraine is comorbid with a number of medical, neurologic, and psychiatric disorders. Examples of medical comorbidities include asthma,  coronary heart disease,  and chronic pain disorders. [4–7] Neurologic comorbidities include stroke and epilepsy,  and psychiatric comorbidities include anxiety, depression, panic disorder, and bipolar disorder. [9,10,] Comorbidities are best studied in representative samples because the prevalence of disease and the association among disorders is sometimes altered in clinic-based samples. This phenomenon, known as Berkson bias, can lead to under-estimates or over-estimates of the rates of co-occurrence for various disorders. Berkson bias arises when patterns of symptoms influence patterns of care seeking for a range of medical disorders. For example, someone with migraine and depression may be more likely to seek medical care with complaints of head pain and sadness than someone who experiences only one of these disorders. Clinic-based studies of comorbidities are useful for generating hypotheses about comorbidities and for characterizing patient groups. They cannot be relied upon to determine if two conditions are actually occurring together with frequency greater than chance.
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