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Other Biological Factors
In contrast to the situation with other anxiety disorders, little is known about the physiological correlates of specific and social phobias. However, effective drug treatments have been iden-tified for social phobia, leading to an increased interest in the biological factors underlying this disorder. There has been some evidence to suggest a relationship between dopamine and social phobia. Unlike panic disorder, which responds well to a variety of tricyclic antidepressants and monoamine oxidase inhibitors, social phobia tends to have a positive response to MAOIs and shows little response to tricyclic antidepressants. Whereas tri-cyclic antidepressants tend to act on noradrenergic and seroton-ergic systems, MAOIs affect noradrenergic, serotonergic and dopaminergic systems. This finding has led some investigators to suggest that the dopamine system is primarily involved in so-cial phobia, which would explain why biological challenges that appear to affect noradrenergic activity (e.g., sodium lactate infu-sion, carbon dioxide inhalation) have little effect on patients with social phobia, despite having panicogenic effects in patients with panic disorder. The dopamine hypothesis is consistent with find-ings that dopamine metabolite levels correlate with measures of extroversion as well as findings that mice bred to be timid have been shown to be deficient in brain dopamine concentration.
With respect to neuroendocrine correlates in social phobia, studies of the hypothalamic–pituitary–thyroid andhypothalamic–pituitary–adrenal axes in social phobia have found few differences between patients with social phobia and control persons. Some recent studies have found evidence of cortisol dif-ferences associated with social anxiety.
Recent imaging studies have found a number of differ-ences between social phobia patients and controls. One study us-ing functional magnetic resonance imaging (fMRI) found that conditioned aversive stimuli were associated with increased ac-tivation in the amygdala and hippocampus of social phobia pa-tients, whereas decreased activation in these areas was observed in normal controls (Schneider et al., 1999). Another study using single photon emission computer tomography (SPECT) found that after an 8-week trial of citalopram there was significantly decreased activity in the anterior and lateral part of the left tem-poral cortex, the left cingulum and the anterior, lateral and poste-rior part of the left midfrontal cortex in a small (N 5 15) sample of social phobia patients (Van der Linden et al., 2000). Compared with treatment responders, treatment nonresponders had higher activity at baseline in the lateral left temporal cortex and the lateral left midfrontal regions. Further research is necessary to understand the significance of these imaging findings as well as their specificity to social phobia.
A few studies have examined patterns of brain activity as-sociated with shyness. In a study on high and low shyness (anx-ious self-preoccupation and avoidance of social situations) and sociability (preference to be socially active and seek out social situations) in college students, it was found that shyness was as-sociated with greater relative right frontal EEG activity, whereas sociability was associated with greater relative left frontal EEG.
Finally, there may be good reason to consider different underlying mechanisms in patients with performance-related phobias (e.g., public speaking) than in patients with generalized social phobia (i.e., those who fear most social situations). Indi-viduals with performance-related phobias tend to show more autonomic reactivity (e.g., rapid heart beat) in the phobic situa-tion than do patients with generalized social phobia. In addition, beta-blockers such as atenolol may be useful for decreasing per-formance anxiety in normal individuals, although they have little effect on patients with generalized social phobia. These facts have led some investigators to suggest that adrenergic hyperactivity may be involved in performance anxiety but not in generalized social phobia. However, it should be noted that despite limited evidence for the use of beta-blockers in normal groups (e.g., musi-cians with performance anxiety), their utility for treating patients with a diagnosis of social phobia (e.g., performance fears that lead to significant distress or impairment) has not been established.
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