Persistent depressive disorder, known historically as dysthymia, is depression's chronic form. The diagnostic criteria require depressed mood most of the day, more days than not, for at least 2 years (1 year in children and adolescents), along with at least two additional symptoms: poor appetite or overeating, insomnia or hypersomnia, low energy, low self-esteem, poor concentration, feelings of hopelessness.
What makes PDD distinct from MDD is the chronicity rather than severity. Many patients have never had a clearly demarcated major depressive episode — their baseline mood is low but does not meet the more intense MDD criteria most of the time. Some patients have both: chronic PDD baseline punctuated by superimposed major depressive episodes, sometimes called "double depression."
The neurobiology overlaps substantially with MDD — the same DMN, HPA, and hippocampal circuits are involved — but the pattern is sustained rather than episodic. Many patients present with what looks like treatment-resistant symptoms because their chronic depressed baseline has been mistaken for personality or temperament. Some describe never having experienced sustained euthymia in adult life.
First-line treatments mirror MDD with longer trial expectations: SSRIs and SNRIs (often 8-12 weeks before declaring inadequate response rather than the standard 4-6 weeks for episodic MDD), CBT, behavioral activation. Combined treatment is particularly important given the chronicity. Cognitive-behavioral analysis system of psychotherapy (CBASP) and interpersonal therapy have specific evidence for PDD.
The diagnostic challenge: PDD is under-recognized. Patients often don't seek help because they consider their chronic low mood part of who they are, not an illness. Clinicians may miss it because the symptoms are less acute than MDD. Family members may have adapted to the patient's baseline. Recognition often comes when an acute MDD episode brings the patient to clinic and longitudinal history reveals the chronic underpinning.
When you encounter a patient describing a lifetime of "low mood" or "always feeling like this," screen carefully. The diagnostic criteria are specific, the treatment is effective, and the patient may never have experienced what wellness feels like. Treatment can be transformative — sometimes patients describe finding out, in their thirties or forties, what their emotional baseline could actually be.