Fibromyalgia, fatigue, and headache disorders.

Fibromyalgia, chronic fatigue, and primary headaches are common and debilitating disorders, and their related symptoms of widespread pain, fatigue, and headache have complex interactions and different implications for classification, diagnosis, mechanisms, and treatment. The "continuum" or "spectrum" idea and the modular headache theory are fundamental concepts in understanding these interactions. The overlap between symptom-based conditions leads the reasons to consider them as "functional somatic syndromes." Management of these patients includes a correct diagnosis, appropriate investigation for associated conditions, adequate treatment, and considering the therapeutic opportunities and limitations the comorbid disorders may impose.