Medical science has established that depression and anxiety are closely related. Between 35 percent and 85 percent of all patients diagnosed with one condition show symptoms of the other [source: Seltzer]. Conditions that occur in this way are said to be comorbid:They occur simultaneously in the same person, but neither one causes the other.
A comparison sometimes used is the relationship between first cousins, whose parents are siblings. First cousins have several important similarities. They share a certain amount of DNA and they therefore resemble each other to a certain extent. However, having different parents and growing up in different families create differences in their appearances and personalities. In a sense, each cousin is a different expression of the family.
Likewise, depression and anxiety are different expressions of the same family of mental distress. Clinical depression refers to mental disorders characterized by abnormally intense and long-lasting negative thoughts and moods [source: Panzarino]. Examples include dysthymia, which is constant feelings of sadness or dullness,and bipolar disorder, in which bouts of intense energy and recklessness alternate with slumps of listlessness and despair.
Anxiety disorders -- as opposed to normal anxiety -- are characterized by overwhelming, irrational senses of worry, dread and fear [source: Nemours Foundation]. Generalized anxiety disorder (GAD), for instance, is a chronic state of worry. Other disorders, such as panic attacks, phobias and post-traumatic stress disorder (PTSD), are marked by sharp, sudden episodes of terror.
Further, just as cousins are born to closely related parents, clinical depression and anxiety both arise from similar causes. Both may be initially triggered by a traumatic event or situation, especially in cases of PTSD. Both are associated with an imbalance of neurotransmitters, the chemical messengers in the brain. Three neurotransmitters are most commonly involved: dopamine, epinephrine and serotonin.
Both conditions also target certain populations. Up to 50 percent of people with an anxiety disorder have a close relative with the same disorder, for example.[source: University of Maryland Medical Center]. However, it isn't clear whether that's due to genetics, environment or those factors combined. Likewise, people with other vulnerabilities, including mental disabilities and histories of abusive relationships, seem more prone to depression. Again, the depression may result from the person's other issue, or some common cause may underlie both.
A growing number of experts suggest that treating depression and anxiety as one illness would lead to better research and improved patient care. The current standards of treatment are described on the next page.