The criteria for the two disorders follow, directly from the DSM-IV TR. (emphasis added) An explanation follows at the end.
The criteria for Dysthymic Disorder is:
A. Depressed mood for most of the day, for more days than not, as indicated either by subjective account or observation by others, for at least 2 years. Note: In children and adolescents, mood can be irritable and duration must be at least 1 year.
B. Presence, while depressed, of two (or more) of the following:
1. poor appetite or overeating
2. insomnia or hypersomnia
3. low energy or fatigue
4. low self-esteem
5. poor concentration or difficulty making decisions
6. feelings of hopelessness
C. During the 2-year period (1 year for children or adolescents) of the disturbance, the person has never been without the symptoms in Criteria A and B for more than 2 months at a time.
D. No Major Depressive Episode (see p. 356) has been present during the first 2 years of the disturbance (1 year for children and adolescents); i.e., the disturbance is not better accounted for by chronic Major Depressive Disorder, or Major Depressive Disorder, In Partial Remission.
Note: There may have been a previous Major Depressive Episode provided there was a full remission (no significant signs or symptoms for 2 months) before development of the Dysthymic Disorder. In addition, after the initial 2 years (1 year in children or adolescents) of Dysthymic Disorder, there may be superimposed episodes of Major Depressive Disorder, in which case both diagnoses may be given when the criteria are met for a Major Depressive Episode.
E. There has never been a Manic Episode (see p. 362), a Mixed Episode (see p. 365), or a Hypomanic Episode (see p. 368), and criteria have never been met for Cyclothymic Disorder.
F. The disturbance does not occur exclusively during the course of a chronic Psychotic Disorder, such as Schizophrenia or Delusional Disorder.
G. The symptoms are not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication) or a general medical condition (e.g., hypothyroidism).
H. The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
* Early Onset: if onset is before age 21 years
* Late Onset: if onset is age 21 years or older
The criteria for a major depression episode is:
A. Five (or more) of the following symptoms have been present during the same 2-week period and represent a change from previous functioning; at least one of the symptoms is either (1) depressed mood or (2) loss of interest or pleasure.
Note: Do not include symptoms that are clearly due to a general medical condition, or mood-incongruent delusions or hallucinations.
1. depressed mood most of the day, nearly every day, as indicated by either subjective report (e.g., feels sad or empty) or observation made by others (e.g., appears tearful).Note: In children and adolescents, can be irritable mood.
2. markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day (as indicated by either subjective account or observation made by others)
3. significant weight loss when not dieting or weight gain (e.g., a change of more than 5% of body weight in a month), or decrease or increase in appetite nearly every day.Note: In children, consider failure to make expected weight gains.
4. insomnia or hypersomnia nearly every day
5. psychomotor agitation or retardation nearly every day (observable by others, not merely subjective feelings of restlessness or being slowed down)
6. fatigue or loss of energy nearly every day
7. feelings of worthlessness or excessive or inappropriate guilt (which may be delusional) nearly every day (not merely self-reproach or guilt about being sick)
8. diminished ability to think or concentrate, or indecisiveness, nearly every day (either by subjective account or as observed by others)
9. recurrent thoughts of death (not just fear of dying), recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide
B. The symptoms do not meet criteria for a Mixed Episode (see p. 365).
# The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
C. The symptoms are not due to the direct physiological effects of a substance (e.g., a drug of abuse, a medication) or a general medical condition (e.g., hypothyroidism).
D. The symptoms are not better accounted for by Bereavement, i.e., after the loss of a loved one, the symptoms persist for longer than 2 months or are characterized by marked functional impairment, morbid preoccupation with worthlessness, suicidal ideation, psychotic symptoms, or psychomotor retardation.
So there is a very marked difference between Dysthymic Disorder and a Major Depression Episode. A huge difference is that in order to get the dysthymic diagnosis, one has to display the symptoms almost everyday for two years! For a major depression episode, the individual must show five symptoms nearly every day for two weeks. Personally, I think the time frame difference is a bit too much... two weeks for one and two years for another? I suppose if I wrote the book I'd lessen the time period for Dysthymic Disorder to maybe a year. But once again, the writers of the book did not ask me.
There are some other key points to remember here. When any mood change happens, be it depression, agitation, mania, etc., it is important to screen for substance use; both prescription and recreational. I had a therapist (who was a social worker) tell me that chronic marijuana use can mirror Dysthymic Disorder. I haven't seen the research on this, but it sounds plausible. And having experienced taking Topamax, I can say first hand that prescriptions can not only make you feel depressed but actually slow your psychomotor functioning. There are also medical conditions that would cause mood changes, such as hyper or hypothyrodism, brain tumors, even UTIs in the elderly. Lastly, when a patient presents with depressive symptoms it is imperative to screen for possible manic or hypomanic episodes.