According to the DSM-IV classification of mental disorders blood-injection-injury type phobias constitute a subtype of specific phobia

Specific phobia

A specific phobia is a generic term for any kind of anxiety disorder that amounts to an unreasonable or irrational fear related to exposure to specific objects or situations...

s. It includes fear of blood (hemophobia), injury phobia

Injury phobia

According to the DSM-IV classification of mental disorders, the injury phobia is a specific phobia of blood/injection/injury type. It is an abnormal, pathological fear of having an injury....

 and fear of receiving an injection

Injection (medicine)

An injection is an infusion method of putting fluid into the body, usually with a hollow needle and a syringe which is pierced through the skin to a sufficient depth for the material to be forced into the body...

 (trypanophobia

Trypanophobia

Trypanophobia is the extreme fear of medical procedures involving injections or hypodermic needles. It is occasionally referred to as aichmophobia, belonephobia,...

 and some other names) or other invasive medical procedure

Medical procedure

A medical procedure is a course of action intended to achieve a result in the care of persons with health problems.A medical procedure with the intention of determining, measuring or diagnosing a patient condition or parameter is also called a medical test...

s.

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

Stimulus (physiology)

In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

 includes the accelerated heart rate

Heart rate

Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....

. 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

Bradycardia

Bradycardia , in the context of adult medicine, is the resting heart rate of under 60 beats per minute, though it is seldom symptomatic until the rate drops below 50 beat/min. It may cause cardiac arrest in some patients, because those with bradycardia may not be pumping enough oxygen to their heart...

, and dropped blood pressure

Blood pressure

Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

. The above may also lead to vasovagal syncope

Vasovagal syncope

A vasovagal episode or vasovagal response or vasovagal attack is a malaise mediated by the vagus nerve. When it leads to syncope or "fainting", it is called a vasovagal syncope, which is the most common type of fainting.There are a number of different syncope syndromes which all fall under the...

(fainting).

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.