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Introduction
Anxiety disorders are disorders that have common features of excessive fear and anxiety with related behavioral disturbances. They commonly coexist with depressive symptoms or major depressive disorder especially in patients with severe anxiety.
Epidemiology
Generalized Anxiety Disorder (GAD)
The prevalence of generalized anxiety disorder is 0.9% among
adolescents and 2.9% in adults in the general population of the United States
(US). While the prevalence in other countries ranges from 0.4-3.6%. Notably,
there is lower incidence of generalized anxiety disorder among Asians,
Africans, native Americans, and Pacific Islanders compared to Europeans. The
morbidity risk is noted to be 9% and usually occurs in middle-aged individuals.
The ratio of occurrence in females to males is 2:1.
Panic Disorder (PD)
The prevalence of panic disorder is estimated to be at 2-3% in adults
and adolescents in the general population across the US and several European
countries. There are lower rates of panic disorder among Latinos, African
Americans, Caribbean blacks, and Asian Americans. There are higher rates among
American Indians and non-Latino whites. The prevalence of panic disorder is
estimated to be 0.1-0.8% in Asia, Africa, and Latin American countries. The ratio of occurrence in females to males is
2:1 with lower prevalence rates in children <14 years of age. Peak
prevalence of panic disorder is during adulthood and then declines in
individuals >64 years of age.
Social Anxiety Disorder
The prevalence rate of social anxiety disorder in the US is
approximately 7% while in Europe it is 2.3%. There are comparable prevalence
rates in children and adolescents with rates decreasing with age. The
prevalence rates among adults range from 2-5%. The odds ratio of females to
males ranges from 1.5-2.2.

Pathophysiology
The pathogenesis of anxiety disorders is not well understood. However, they may stem from a complex interplay of biological, psychological, and social or environmental factors. Firstly, genetic factors have been noted to predispose individuals to the development of GAD. Notably, twin studies have shown significant familial aggregation among GAD, panic disorder, phobias, and obsessive-compulsive disorder. The heritability of anxiety disorders varies from around 30-35% to as high as 50-60%. Secondly, patients with anxiety disorders show hyperactivation of the amygdala in response to anxiety cues. Studies have also shown perturbed coordination of brain circuitry between the amygdala and prefrontal cortex in patients suffering from GAD. Additionally, PET scans show significant increase in glucose metabolism in the amygdala, thalamus, medulla, right posterior temporal love, inferior gyrus, cerebellum, and right frontal gyrus in patients with GAD and PD. Lastly, it has been shown that individuals with anxiety disorders show bias toward threat-related stimuli, with distortions consistent with their anxiety. These biases include hypervigilance for possibly threatening stimuli, overinterpretation of neutral stimuli as threatening, and increased attention for potentially threatening stimuli. This threat bias in cognitive processing of information is consistently seen in anxiety disorders.