[6] In an individual with BII phobia, expression of these or similar phobic symptoms in response to blood, injection, or injury typically begins before the age of ten.
[3] This can lead to health issues in phobic individuals as a result of avoidance of hospitals, doctors' appointments, blood tests, and vaccinations, or of necessary self-injections in those with diabetes[6][3] and multiple sclerosis (MS).
[4] The phobia is also able to affect the health of those who don't have it; a BII-phobic, for instance, may have difficulty providing aid to someone else in an emergency situation in which blood is present.
There is a body of evidence which suggests the phobia has genetic underpinnings, though many phobics also cite a traumatic life event as a cause of their fear.
[8][9] Applied tension (AT), a method in which individuals alternately tense and relax their muscles while being exposed to a phobic trigger, is widely recognized as an effective form of treatment for BII phobia.
While AT is generally the default treatment suggestion, methods of applied relaxation (AR) and exposure-only cognitive-behavioral therapy (CBT) have been found to be effective in diminishing phobic response in some instances.
[2] While BII-phobics experience a similar reaction initially upon exposure, most ultimately respond to a trigger with a biphasic, or two-phase, fainting response.
[3][6] In the first phase, phobics often experience an anxiety reaction characterized by elevated heart rate and heightened blood pressure, as occurs in most other phobias.
[2] However, with BII phobia, a second phase usually follows closely, in which the phobic individual experiences a massive dip in heart rate and blood pressure[4][10] known as vasovagal response.
[8] Stimulation of the vagus nerve, a part of the parasympathetic nervous system, is responsible for promoting the lowered heart rate and decreased blood pressure.
[3] Other symptoms that may evolve when exposed to phobic triggers include extreme chest discomfort, tunnel vision, becoming pale,[4] shock, vertigo, diaphoresis (profuse sweating), nausea, and in very rare cases asystole (cardiac arrest) and death.
[1] It has been theorized that exhibiting vasovagal response when exposed to blood was evolutionarily advantageous, and that this phobia is a vestige of an ancestral evolutionary mechanism.
[8][9] Fainting may have acted as a form of tonic immobility,[8] allowing primitive humans to play dead in a situation where blood was being spilled, perhaps helping them to avoid the attention of enemies.
[8][9] Individuals typically seek therapeutic treatment for BII phobia in a bid to alleviate symptoms that arise when exposed to a phobic trigger.
Therapists may use a combination of physical and psychological measures, such as cognitive-behavioral-therapy and applied tension (AT), in order to aid in extinguishing the individual's fear response.
A later study tested applied muscle tension as a way of preventing fainting when an individual with a fear of injuries was exposed to triggering visual stimuli.
Lars-Göran Öst expanded upon this research, having BII-phobic individuals engage in applied muscle tension while shown blood stimuli.
[8] An AT treatment program most often involves an individual being instructed to clench their arm, leg, and chest muscles in 10 to 15 second intervals[6] as they are systematically exposed to triggers of increasing likeness to real blood or needles.
[8][6] The method of applied tension remains popular — it is the most common BII phobia treatment suggestion, and has been found to be highly effective in a majority of BII-phobics.
These include the phobic individual crossing their legs, making tight fists with both hands, or engaging muscles of the trunk or arms.