Why is depression comorbid with chronic myofascial face pain?

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Abstract
A number of explanations have been proposed to account for findings that rates of depression are elevated in persons with chronic, non-malignant pain disorders (CNPDs); for example, that CNPDs are variants of depression (e.g. 'masked depression'), that the stress of living with CNPDs contribute to the onset of depression ('diathesis-stress'), or that the correlation of CNPDs and depression is a methodological artifact of studying treatment-seeking samples. These alternative hypotheses are tested for one specific CNPD, chronic myofascial face pain, using a family study methodology. The procedure was to conduct direct psychiatric interviews with 106 patients with a history of carefully diagnosed myofascial face pain, 118 acquaintance controls without personal histories of myofascial face pain, and a random sample of adult first degree relatives of these case and control probands. The probands were further subdivided into four roughly equal samples consisting of cases with and without personal histories of major depressive disorder (MDD), and controls with and without personal histories of MDD. Dates of initial onsets of myofascial face pain and MDD in patient probands were obtained from interviews and records. The main results were that, compared to control probands without personal histories of MDD, MDD and depressive spectrum disorders (DSD) were elevated in the first degree relatives of control probands with personal histories of early onset MDD, but not in the first degree relatives of myofascial face pain probands with or without personal histories of early or late onset MDD. This outcome is consistent with the hypothesis that living with chronic myofascial face pain contributes to elevated rates of depression. It is inconsistent with the alternative hypotheses that this CNPD is a variant of depression or that the elevated MDD rates are simply an artifact of selection into treatment. The implications of these results and additional results consistent with them are discussed.