Recent research has revealed that agoraphobia is often
secondary to
panic disorder (the state of having frequent
panic attacks), not vice versa as these otherwise accurate writeups state (citations for this and other assertions follow). Although they have a variety of causes and predisposing factors, panic attacks are generally caused by
catastrophic misinterpretation of bodily sensations that are inherently
benign. Much research has demonstrated that
subjects with panic disorder are more likely to interpret ordinary,
transient feelings of dizziness or faintness as signs of a heart attack, stroke, or nervous breakdown (among other things). This leads to heightened
arousal, which in turn intensifies their sensations, and the spiral continues upwards until they have a panic attack.
The effect of this is that these people become extremely fearful of future attacks, and worry that if they are away from home and safety they will suffer an attack and something horrible will happen. This
hypervigilance, of course, just increases their chances of having one. In this manner, panic disorder actually
leads to agoraphobia. Similarly, treatment for panic disorder (involving education about the mechanisms above and gradual exposure to sensations like racing heart and hyperventilation, along with relaxation training) can lead to substantial improvement in agoraphobia as the subject's confidence in their basic safety and coping skills increases.
For basic information on the PD-agoraphobia link, see
http://text.nlm.nih.gov/nih/cdc/www/85txt.html.
For information on panic disorder and catastrophic predictions, see Salkovskis, P., Clark, D., & Gelder, M. (1996). Cognitive-behaviour links in the persistence of panic. Behaviour Research and Therapy, 34, 453-458.
For an interpretation of agoraphobia dealing with distance from safety cues, see Rachman, S. (1984). Agoraphobia -- A safety-signal perspective. Behavioural Research and Therapy, 22, 59-70.