What is double depression?

Double depression is the concurrent presence of dysthymia and major depression, in which acute major depressive episodes appear to be ‘superimposed’ upon the underlying chronic depression.

Double depression is distinct from chronic major depression, in which an individual experiences the symptoms of a major depressive episode continuously for at least 2 years.

This type of depression has a poorer prognosis than either condition alone, and recurrences of major depressive episodes are common.

Patients with double depression seem to recover more rapidly from episodes of major depression than do patients with major depression alone.
What is double depression?