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Cyclothymia and dysthymia matter because they describe chronic, sub-threshold mood patterns lasting years, not the full-blown episodes tested elsewhere. Cyclothymia is a persistent instability of mood with numerous periods of mild depression and mild elation, never severe enough to meet criteria for a full depressive or manic episode. Dysthymia is a chronic, persistently low mood that similarly never reaches the severity of a full depressive episode. For example, imagine a man who has felt subtly low most days for the past four years, functioning throughout but never quite well: that chronic pattern over years is dysthymia. In practice, both diagnoses require the pattern to run for at least 2 years before they can be confirmed.

Both require a minimum duration of about 2 years, one year in children and adolescents, with any symptom-free gaps limited to no more than a couple of months at a time. Cyclothymia shows both the mild highs and mild lows, whereas dysthymia shows only the persistent low mood, without elated phases. When a full depressive episode develops on top of pre-existing dysthymia, the combination is nicknamed double depression. Take a patient with years of low-grade dysthymia who then develops a florid depressive episode, with poor sleep, poor appetite and suicidal thoughts layered on top. In practice, recognising double depression matters because the depressive episode is treated urgently, on top of managing the underlying dysthymia.

• How long must symptoms be present for dysthymia to be diagnosed in an adult, and in a child?
• A man with two years of persistent low-grade low mood now develops a full depressive episode on top. What is this pattern called?
• What is the evidence for antidepressant treatment in dysthymia?

• Cyclothymia, mild mood swings never reaching full maniaA period of abnormally elevated mood, energy and activity, seen in bipolar disorder.Read more → or depression, unlike bipolar disorder's full manic or depressive episodes.
• Dysthymia, chronic mild low mood for at least 2 years, rather than the 2-week threshold required for a depressive episode.
• Double depression, a full depressive episode superimposed on pre-existing dysthymia, unlike dysthymia alone.
• Bipolar disorder, requires at least one full manic or hypomanic episode, whereas cyclothymia's highs never reach that severity.

• Two weeks is the threshold for a depressive episode rather than a persistent disorder.
• Chronicity does not make the disorder unresponsive to medication.
• Six months falls short of the required chronicity.
• Five years exceeds the requirement and would delay recognition.
• One month is far too brief for a persistent depressive disorder.

• Extract the defining phenomenology, duration, severity and functional-impact clues.
• Match the pattern to the most specific diagnostic or assessment construct.
• Actively test the closest look-alike against the missing or contradictory clue.
• Choose the precise term rather than a broader category that is merely compatible.