Anxiety and depression, shown through clinical studies and experimentation, have been shown to be two distinctly different mental conditions. However, there has been shown to be a link between the two, as many people with one condition have the other.
While linked, there has yet to have been any concrete link found between anxiety and depression. At the present time, the relationship between both anxiety and depression has been called "comorbidity."
This term means "unrelated, but co-existing." The symptoms often overlap, which makes this harder for people to deal with, and makes it hard for mental health practitioners to initially diagnose. The symptoms, with some people, also act independently of each other.
According to a study committed by the National Comorbidity Survey found that 58% of all patients with major depression also exhibit lifetime anxiety. "Agitated depression" and "akathitic depression" are terms commonly used when referring to these results.
Both of these depression states exhibit similar symptoms. There is heightened anxiety, restlessness, a feeling of dread and suicidal thoughts or tendencies. What separates these two is that agitated depression includes both nonclinical and nonspecific panic, while akathitic does not include these panic symptoms.
Anxiety, whether mild or extreme, completely changes depression symptoms. Even the smallest amount of anxiety has been shown to impact the course of depression.
For example, the University of Pittsburg have completed a study on this topic. They found that patients diagnosed both with depression and anxiety take a completely different path in their treatment. It takes much longer, and there are significant delays, in the effectiveness of any psychological treatments.
Not only that, but there have been shown to be differences after treatment. Patients with both depression and anxiety have a much harder time resuming normal life activities after treatment is finished then those who only suffer from depression.
Theories exist stating the possibility that depression and anxiety act similarly and synergistically to attempt avoidance of any potentially harmful situations for the sufferer. Anxiety avoids and prepares sufferers for physical danger, while depression does the same for emotional or psychological danger.