According to the DSM-IV classification of mental disorders blood-injection-injury type phobias constitute a subtype of specific phobias. It includes fear of blood (hemophobia), injury phobia and fear of receiving an injection (trypanophobia and some other names) or other invasive medical procedures.[1]
A distinctive feature of phobias of this type is their vasovagal manifestation. For most fears (both normal and abnormal) the response to the feared stimulus includes the accelerated heart rate. [1] In the cases of blood-injection-injury phobias a two-phase vasovagal response is observed: first a brief acceleration of heart rate, then its deceleration, bradycardia, and dropped blood pressure. [1] The above may also lead to vasovagal syncope (fainting).[1][2]
These characteristic vasovagal reactions may contribute to the development of a phobia.
The other factors contributing to the development of the blood-injection-injury phobias are the same as for other specific phobias.[1]