Sentence:  Involutive depression or bipolar <WSD>dysthymias</WSD> must be ruled out....Two characteristics of bipolar <WSD>dysthymias</WSD> are distinguishable: first, the bipolar disorders are cyclical in character and develop over several days to several weeks, which is quite different from the sudden and sometimes multiple changes of chronic depression; second, the productive phases of <WSD>dysthymias</WSD> are accompanied by a considerable decrease in the duration of sleep, to less than 6 hours per day. 
Meaning of the word marked with <WSD> tags:  A form of clinical depression, characterized by low-grade depression which lasts at least 2 years.