We all experience stress, right? It's a fact of nature, and it offers pretty important evolutionary advantages. Without it, we wouldn't be able to sound the proverbial alarm if danger were imminent. Stress is one of the greatest tools an animal has to beat the odds and stay alive.
Human beings are animals, yes, but we contend with totally different environmental pressures than wild animals do. Many of us engage in monogamous relationships, spend long hours in the office, and struggle to keep up with deadlines, bills, and other commitments. At a certain point in human evolution, the adaptive advantage of acute stress was overshadowed by the downsides of that headline-dominating bogeyman, chronic stress.
Robert Sapolsky's book Why Zebras Don't Get Ulcers is the most enthralling treatise on chronic stress I've ever read. It does an excellent job explaining why the daily grind of traffic jams, post office visits, and sitting on hold with the electric company is simply not the "nature of the beast." We didn't evolve with the physiological equipment necessary to withstand these artificial stressors in a healthy way, which is why we (and not zebras) suffer from stress-related chronic illnesses.
Acute stress is adaptive. It helps us make beneficial decisions. The biochemical hallmark of acute stress is the release of epinephrine (adrenaline) from the adrenal glands, which sit atop the kidneys. But if everyday experiences of stress start to aggregate and snowball, they can lead to chronic stress. High levels of circulating epinephrine over the long term, coupled with the release of the stress hormone cortisol, can cause or exacerbate severe health problems, like heart disease, obesity, and suppression of the immune system. Chronic stress can also contribute to the risk of developing depression.
But this month, we're talking about mental illnesses. Acute and chronic stress aren't diagnosable mental illnesses, but anxiety disorders are. So what's the difference? To me, this line is a bit fuzzy. It all comes down to what's happening in the brain, and how that affects an individual's ability to cope.
The brain experiences stress and anxiety in slightly different ways, although they do share some of the same real estate. Anxiety is more akin to fear. An anxiety disorder is diagnosed when that fear is significant enough to interfere with daily functioning, or if it seems to develop without cause. The DSM-IV-TR recognizes the following diagnosable anxiety disorders: generalized anxiety disorder, panic disorder, panic disorder with agoraphobia, phobias, obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), separation anxiety, and childhood anxiety disorders. You can read specific descriptions of each anxiety disorder in a publication by the National Institutes of Mental Health here and in a statement released by the surgeon general here.
The neurocircuitry involved in anxiety disorders is largely indistinguishable from that involved in fear reactions. Limbic structures (deep brain structures involved in emotional processing) such as the amygdala and insular cortex show heightened activity in individuals struggling with anxiety. Specific disorders, such as OCD and PTSD, involve specific pathways and behaviors not seen in other states. For example, individuals with OCD have heightened activity in the caudate nucleus of the basal ganglia, a region involved in learning and memory, as well as emotional processing. It appears to be a gatekeeper of signals to the orbitofronal cortex and thalamus, two regions that are overactive in patients with OCD. The orbitofrontal cortex is a critical region for moral judgment, planning, and general executive functioning. The thalamus integrates and relays sensory information to brain regions necessary for its processing.
With recent pushes toward the medical model in modern psychiatry, I wonder if the lines between the terms "stress," "fear," and "anxiety" will continue to blur. The conventional wisdom is that unchecked acute stress may aggregate to become chronic stress, and that chronic stress underpins diagnosable anxiety in a large percentage of the population. Anxiety disorders are the most common mental illnesses in America, affecting around 18 percent of the U.S. population in any given year, and almost 30 percent of American adults across the lifespan. But in a world where everyday stress is a fact of life, in a body that is not equipped to deal with its cumulative effects, this doesn't really surprise me. What do you think?