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Current Research Findings and Theoretical Speculations
The consensus of those writing on the subject is that though anxiety and depression are intimately related, the deepest causal understanding of this linkage is still being investigated. Certainly, statistics calculating the co-occurrence (or "co-morbidity") of these two disorders abound. For instance, it's been estimated that 85% of depressed patients are also afflicted with generalized anxiety disorder (GAD)--and, more generally, that over 60% of depressives experience varying levels of anxious feelings and behavior. Further, no fewer than 53.7% of individuals with anxiety disorders concurrently experience major depression, and 35% of depressives are also estimated to suffer from panic attacks.
In short, odds are that if you're afflicted with serious anxiety, you're apt to be burdened with depression as well--and vice versa. And if you're plagued with both disorders, it's likely that your daily functioning is especially impaired, for the combination of the two almost always indicates a disorder more severe than either one separately. That is, depression can significantly accentuate anxiety, just as anxiety and worry can intensify depression. Moreover, suicide rates are much higher when marked anxiety co-occurs with depression--meaning that the two combined have the potential to be particularly lethal.
Consider the intolerable stress of what I referred to in Part 1 as "agitated depression." Beset with inner turmoil, you can hardly sit still--wringing your hands, biting your lips or finger nails, picking at your clothes or fingers, and exhibiting facial expressions similar to some of those I've employed as illustrations for this extended post. And because your mind is racing, you can't focus or concentrate. Your body and brain may be in frenzied motion, yet you feel almost paralyzed--deeply discouraged and almost totally unmotivated to do anything that might free you from your internal morass.
The fact that the National Institute of Mental Health (NIMH) posits that 19.2 million Americans suffer from anxiety, while 18.8 million are beset with depression, is also telling. Just as these two mental/emotional disorders are entwined symptomatically, they also seem to be interrelated numerically. And if depression is close behind anxiety quantitatively, there's no doubt (as many researchers have observed) that the more extreme the anxiety, the more likely that depression will follow in its wake. And--yet again--vice versa.
At some point it's difficult not to entertain the possibility that in many instances anxiety and depression may be different facets of a more general disorder. At the very least, it's impossible to deny that many substantive similarities and overlaps define their close kinship. And it's worth noting that writing for WebMD, R. Morgan Griffin quotes Ian A. Cook (Director of the Depression Research Program at UCLA) as reflecting, "It's possible that in time, we'll come to see anxiety and depression together as representing a distinct illness, separate from either condition by itself."
So, just how might researchers account for such an "integrated" anxious-depressed illness? Well, in multiple ways. For instance, many researchers have speculated that serotonin imbalance in the brain is the root cause not only of anxiety and panic attacks but of depression as well. In other words, this "chemical imbalance" theory is posited as "key" for both distressed states (see, e.g., Healthy Place.com). So if--as Lisa Ann Schleipfer cites The National Institute of Mental Health as postulating--patients with both anxiety and depression experience a decrease in the number of brain receptors processing serotonin (a neurotransmitter understood to regulate mood, pain perception, memory, and other brain functions), then the co-occurrence of anxiety and depression can be seen as signifying their same biological foundation.
Moreover, Schleipfer speculates (as I suggested earlier in Part 2 of this post) that patients with combined anxiety and depression probably have an over-reactive stress response system. With such neurological abnormalities, they may well react to life stressors with an anxiety that itself can prompt a depressive episode. And finally, since stress is a potent measure of the "wear and tear" on your system, co-occurring anxiety and depression deserve to be appreciated as particularly detrimental to your health.
Canadian researcher Anthony Phillips states that "according to the World Health Organization, depression, anxiety and other related mood disorders now share the dubious distinction of being the most prevalent causes of chronic illness." Phillips also cites the work in molecular biology of Stephen Ferguson and his colleagues, which studied the interaction between corticotrophin releasing factor receptor 1 (CFFR1) and specific types of serotonin receptors (5-HTRs), which can trigger abnormal brain signaling. Inasmuch as "CRFR1 activation leads to anxiety in response to stress, and 5-HTRs lead to depression, the research shows how stress, anxiety, and depression pathways connect through distinct processes in the brain."
Independent of how much your biology, or mix of brain chemicals, may determine your anxiety and/or depression (and we should be ever-mindful of the fact that your every thought influences your chemistry), what should be vividly clear by now is that the way these two disorders have been officially differentiated necessitates major revision. The writer who has most convincingly summed up the reasons that DSM-V (presently scheduled for publication in 2013) needs to correct the misleading distinctions between anxiety and depression made in DSM-IV is David Watson (Professor at the University of Iowa, and also Editor of APA's Journal of Abnormal Psychology).
In an excellent, painstakingly comprehensive article (that includes a bibliography of over 100 articles and books!), Watson discusses all the most important research in this area. His exhaustive review makes a powerful case for collapsing mood and anxiety disorders into "an overarching category of emotional disorders [emphasis added]." Further, Watson recommends that this broadly encompassing heading be subdivided into three subclasses: namely, the bipolar disorders (bipolar I, bipolar II, & cyclothymia); the distress disorders (including major depression, dysthymic disorder, generalized anxiety disorder, & posttraumatic stress disorder); and the fear disorders (including panic disorder, agoraphobia, social phobia, & all the specific, delimited phobias).
Such a conclusion is derived from a penetrating analysis of a large variety of studies attesting to the affinities between the so-called mood disorders and the various anxiety disorders--particularly generalized anxiety disorder. Perhaps the most notable of these examinations are several twin studies that demonstrate such close biological connections between major depression and GAD that S. Mineka and others (1998) concluded they were "genetically indistinguishable."
Note 1: For those readers "tuning in" late for this multi-part post, here are links to and brief descriptions of its earlier parts. Part 1 described the broad distinctions between the syndromes of anxiety and depression, while Part 2 discussed the many crucial similarities between the two syndromes. Part 3 elaborated upon the various negative beliefs about self and one's place in the world that can be seen as giving rise to both these distressful mental/emotional states. And, finally, Part 5 will take up the treatment of combined anxiety and depression, as it involves both medication and talk therapy.
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