Anxiety Disorder and Depression Co-occurrence
Anxiety disorders and depression are both serious forms of mental illness, but they are distinctly different. Anxiety disorders include specific phobias and generalized anxiety disorders and are characterized by intense fear, feelings of apprehension, feelings of being powerless, panic, increased heart rate, sweating, and fatigue1.
Depression has completely different symptoms which include feelings of sadness which don't go away, a lack of interest or pleasure in normal activities, feelings of worthlessness or guilt, reduced sex drive, agitation, indecisiveness, fatigue, trouble thinking, and frequent thoughts about death2. While some of these symptoms overlap, you would probably be surprised to learn how often these disorders overlap.
For those who are diagnosed with anxiety disorders, nearly half will also have depression3. However, people with depression are also likely to have anxiety disorders. One estimate says that two out of three people first diagnosed with depression also have an anxiety disorder. This is a surprise because the disorders are relatively different. Still, there are possible explanations which account for this overlap.
Theories for the Co-occurence of Anxiety Disorder and Depression
There are a few theories which try to explain the relationship between anxiety disorder and depression. However, they are just theories, so none of them have been confirmed as a cause that explains this link. Still, the disorders have a lot in common.
Surprisingly, treatments which work for one of these disorders tend to work for the other disorder as well4. Medications such as SSRIs help to relieve the symptoms of depression and anxiety disorders. Behavioral treatments used for anxiety disorders also tend to help those who suffer from depression. Exercise even helps those who suffer one or both. Even risk factors are similar, and they include a family history of mood disorders, stressful life events, and a decreased amount of available neurotransmitters in the brain. It was these facts which led to a few theories.
The first theory is that the same neurotransmitters are related to both disorders. This theory suggests that the disorders both originate at the same neuron level where neurotransmitters have become scarce leading to decreased neuron activity. Once the neurons fire at a normal rate, both disorders show improvement from mostly the same medications.
The second theory actually incorporates the information from the above theory. It's simple, and it suggests that anxiety disorder and depression are not two distinct disorders which regularly co-occur. This theory says that they are actually one disorder, and we have mistakenly identified them as two. This could be a definite possibly, but it requires further study.
The last idea is that one of these disorders leads to another. As someone becomes generally anxious, they are likely to become sad and hopeless. They would also have trouble enjoying anything. Likewise, someone who is depressed may become uncomfortable in almost any situation and begin to fear it leading to a problematic level of anxiety.
Even though there are theories which try to explain the relationship between anxiety disorder and depression, they may be wrong. It's also possible that no relationship exists between them, but it is likely that there is a relationship.