Obviously the DSM doesn't see it as such, but the idea of Irvin D. Yalom appears to have some traction at least in magazines:
Irving Yalom, in 1981, wrote about near necessity of an existential depression. He basically suggested that a certain level of depression is normal, if not essential. He suggested that this depression is the byproduct of negotiating some key elements of our world and lives. These elements are death, freedom, isolation, and meaninglessness. He argued that the inner conflict that arises in negotiating these forces leaves us depressed and disillusioned.
Are there more rigorous studies of the concept? I'm guessing that a first/necessary step would be to able to reliably distinguish it for other (proposed) subtypes.
Kerr's Encyclopedia of Giftedness, Creativity, and Talent mentions some diagnostic instruments (p. 340):
A number of measures have been developed to assess aspects of existential depression. The Life Purpose Questionnaire and Purpose of Existence Scale specifically address the presence or lack of existential meaning in an individual's life. These measures are positively correlated with several forms of psychopathology, including depression. The Existential Anxiety Questionnaire is based on the work of the philosopher Paul Tillich and asses one's apprehension of meaninglessness and death. This measure was created to assess existential anxiety, but it can also be applied to existential depression. The Quality of Existence Scale has been used as a measure of existential depression, and the widely used Quality of Life Scale has a particular existential subscale. Finally, the Existential Depression Scale is a general measure of potential existential etiological factors in the development and maintenance of depression.
Alas it says nothing of the validity/reliability of these and also lacks inline citations, so it'll be a while before I can figure out what published studies (if any) have been using these.
The end of the "existential depression" entry (itself authored by Thomas C. Motl) in that encyclopedia has this statement regarding treatment
Treatment for existential depression includes traditional "talking" therapies [...] Pharmacological interventions are less effective because the root cause of existential depression is nonbiological. Those treated with drug therapies have higher relapse rated and a greater chance of recurrence.
No indication there of what evidence that is based on though.
While looking into this, I found two studies with noteworthy (IMHO) results, which don't quite answer my question though.
- One Dutch study found a significant score difference on an Existential Concerns Questionnaire between nonclinical and clinical (diagnosed with depression and/or anxiety) subjects.
- A US study using The Scale for Existential Thinking only on non-clinical subjects found that subjects of Indian origin scored higher, suggesting a possible cultural influence. Other variables like age, gender, income, or level of education did not have a significant impact on the score.