Phobias are the most common anxiety disorder. Specific phobia is characterized by extreme and persistent fear of specific objects or situations that present little or no real threat. The condition has behavioral, cognitive, and physiologic manifestations. (See Etiology.)
Functional magnetic resonance imaging (fMRI) findings have suggested a neural network for the processing of threatening stimuli, with increased activation in the prefrontal cortex, insula, and posterior cingulated cortex, when subjects were exposed to phobia-sensitive words (eg, spider phobia), compared with subjects without phobias. (See Etiology, Pathophysiology.)
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The specific nature of anxiety associated with specific phobia is felt to be associated with an increase in tonic cardiac vagal tone, manifested by an increase in heart rate, which is considered to be greater than that in other anxiety disorders.
These responses are mediated by the autonomic nerve system, or more specifically, by its parasympathetic branch. The vagus nerve attenuates the sympathetic nervous system output and is key to responsiveness to environmental cues.
Children with specific phobias are felt to display a greater degree of response to perceived threats, although they do not demonstrate an increase in anticipatory responsiveness. (See History.) However, this physiologic change is not the same with all specific phobias, because different specific phobias have been associated with differences in cardiac vagal tone.
Assessment of specific phobia generally consists of structured or semistructured interviews by the practitioner with the child and his or her parents. Various rating scales are also available to assess anxiety disorders. (See Workup.)
Numerous theories about the etiology of specific phobias have been offered. Psychoanalytic theory offered an early explanation, although it is no longer supported.
Many other theories have been proposed, including learning theories, explanations encompassing environmental influences, and theories regarding genetic factors. (Genetic and environmental factors are generally acknowledged to influence behavior, including anxiety disorders in general and specific phobias in particular.) It has also been proposed that a combination of these factors gives rise to specific phobias.
In the classic conditioning theory, it is believed that a previous neutral stimulus is paired with an aversive stimulus that elicits a strong fear or emotional response. Often, adults in the child's life may be unaware of the aversive episode. Some believe this learning may be direct or vicarious, which entails witnessing an event involving another person that elicits fear.
The operant conditioning theory holds that parents may inadvertently reinforce a child’s phobic behavior by providing the child with increased amounts of social attention surrounding the avoidant behavior. In young children, familial influences have been reported to affect comorbidity between specific phobia and separation anxiety and between specific phobia and social phobia. These influences are felt to reflect shared environmental influences more than genetic factors. Learning and modeling, as well as parent-child interactions, have been proposed as possible explanations.
Because learning theories are not felt to adequately explain the development and persistence of phobias, attention has been focused on the role of cognition. Children with anxiety disorders are more likely to display distorted and maladaptive thoughts, although the extent to which these negative thoughts are causes or consequences of their fears is unclear.
Genetic, familial, and constitutional theories
A familial component of specific phobia is often observed, although the type of specific phobia is usually different.
Genetic factors are felt to contribute to specific phobia. Genetic effects on early onset disorders have been reported to be more substantial than environmental factors. A recent twin study of adolescents found that genetic factors appear to nonspecifically increase risk for all the pediatric anxiety disorders including pediatric specific phobia. 
Increased fears and phobias have been identified in children of parents with major depressive disorder and anxiety, although they have not been found to be associated with parents with major depressive disorder without anxiety. 
Avoidance of new foods has been found to be highly heritable in children aged 8-11 years, although less than one fourth of children with this presentation are also believed to be influenced by nonshared environmental factors. 
Constitutional factors and individual experiences increase the risk for developing anxiety disorders.
Manifestations of constitutional factors may be apparent in individual differences in responsiveness to environmental events. A sudden, loud sound elicits a range of emotional responses in different children, some of which are the result of biological differences.
Children who demonstrate a stable behavioral inhibition (becoming excessively distressed and withdrawn in novel situations) from infancy through childhood have higher rates of anxiety disorders than do children who are not consistently inhibited. These findings suggest that childhood behavioral inhibition may be a risk factor for developing anxiety disorders.
Personality is felt to be heritable.
Incidence and race preference of specific phobia in the United States
The National Institute of Mental Health (NIMH) estimates that 5-12% of Americans have phobias; specific phobias affect approximately 6 million Americans.  Approximately 7-9% of children have been estimated to have specific phobia.
The National Comorbidity Survey-Adolescent Supplement estimated the lifetime prevalence of specific phobia to be 22.1% for adolescent girls and 16.7% in boys. However, only 0.6% of adolescents were rated as severely impaired by a specific phobia. 
No significant differences in the incidence of specific phobias have been noted between whites, blacks, or Latinos.
No conclusive evidence links socioeconomic status with specific phobia.
The prevalence rates and types of phobias vary among various cultural and ethnic groups. The overall reported prevalence rates in children in New Zealand, Puerto Rico, Switzerland, and Germany are low.
Sex preference in specific phobia
Females may be at higher risk for developing specific phobia than males. The sex difference is less notable for certain specific phobias, such as a fear of heights.
Age of onset
The mean age of onset of specific phobia depends on the type of phobia that develops. Animal, blood, and storms and water-specific phobias typically develop in early childhood. Height specific phobia develops in teenagers. Situational specific phobias (eg, claustrophobia) typically develop during the late teenage years and early third decade of life.
Fears and phobias are common in young children. Referral rates tend to increase in mid-to-late childhood and early adolescence. The peak age for referral of children diagnosed with specific phobia is 10-13 years, with the average age of symptom onset at approximately 8 years.
The prognosis for specific phobia is good with appropriate treatment. Poor response to therapy may be secondary to poor compliance, motivation, or understanding of treatment procedures. Interpersonal factors may also interfere with treatment results.
The presence of more than one specific phobia (eg, spider and needle) is associated with early age of onset, increased anxiety severity, and impairment. Additionally, children and adolescents with more than one type of phobic stimulus have higher rates of psychiatric comorbidity. 
In some instances, natural environmental contingencies may extinguish a fear. Specific phobias in children generally attenuate over time, although they may persist into adulthood. In contrast, specific phobias that appear in adolescents and adults tend to persist, with only approximately 20% of these cases resolving without intervention.
Patients with specific phobia may be at increased risk for future anxiety disorders.
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