"They're probably two sides of the same coin," says David Barlow, Ph.D., director of the Center for Anxiety and Related Disorders at Boston University. "The genetics seem to be the same. The neurobiology seems to overlap. The psychological and biological nature of the vulnerability are the same. It just seems that some people with the vulnerability react with anxiety to life stressors. And some people, in addition, go beyond that to become depressed."
They close down. "Depression seems to be a shutdown," explains Barlow. "Anxiety is a kind of looking to the future, seeing dangerous things that might happen in the next hour, day or weeks. Depression is all that with the addition of 'I really don't think I'm going to be able to cope with this, maybe I'll just give up.' It's shutdown marked by mental, cognitive or behavioral slowing."
At the core of the double disorder is some shared mechanism gone awry. Research points to overreactivity of the stress response system, which sends into overdrive emotional centers of the brain, including the "fear center" in the amygdala. Negative stimuli make a disproportionate impact and hijack response systems.
Mental health professionals often have difficulty distinguishing anxiety from depression, and to some degree they're off the hook. The treatments that work best for depression also combat anxiety. Cognitive-behavioral therapy (CBT) gets at response patterns central to both conditions. And the drugs most commonly used against depression, the SSRIs, or selective serotonin reuptake inhibitors, have also been proved effective against an array of anxiety disorders, from social phobia to panic and post-traumatic stress disorder (PTSD). Which drug a patient should get is based more on what he or she can tolerate rather than on symptoms.