Why Does EpiPen Have to be Injected into the Thigh?

As any severely food allergic patients know, when suffering from a potentially life-threatening anaphylactic reaction, they need to jab an epinephrine needle into the side of their thigh...

But... why the thigh? Why not the arm like most other shots?

Per AAAAI, there have been 3 anatomic sites investigated to determine which location offers the fastest systemic absorption of epinephrine when injected intramuscularly and/or subcutaneously.

• Intramuscular injection in the upper arm (deltoid muscle) just like a flu shot.
• Subcutaneous injection in the upper arm just like an allergy shot.
• Intramuscular injection in the lateral thigh (vastus lateralis muscle).

Of these sites, the intramuscular thigh injection demonstrated the fastest rise in blood epinephrine.

When dealing with a life-threatening anaphylaxis, you need the speed because if the drug is too slow, the person may die before adequate epinephrine levels are achieved... and the thigh muscle provides the speed whereas the other sites do not.

Why might that be?

It's probably because the thigh muscle is large with lots of blood vessels allowing for multiple points of access for the injected epinephrine to enter into the blood circulation.


References:
Epinephrine absorption in children with a history of anaphylaxis. J Allergy Clin Immunology 1998; 101(1):33-37.

Epinephrine absorption in adults: intramuscular versus subcutaneous injection. J Allergy Clin Immunol 2001; 108(5):871-873.

Strong Sneeze Broke Woman's Neck

Fox News reported yesterday about an Australian woman, Monique Jeffrey, who sneezed SO hard that she broke her neck... literally.

According to the neurosurgeon the patient eventually saw, her C1 and C2 vertebrae had dislocated and were displaced which resulted in her symptoms of excruciating pain with her head locked to her right shoulder and in too much pain to move anything but her right arm. Her left arm was also numb with tingling.

She was placed in a halo traction for three months with weights to gradually move the vertebrae back in alignment.

Source:
Super-strong sneeze puts mom in traction for three months. Fox News 3/27/12

Melbourne mum dislocates neck by sneezing. 9 News Melbourne. 3/27/12

Allergy Shots in the Leg? Abdomen?

It is not uncommon that we have patients that ask if allergy shots can be administered in the leg (or other part of the body) rather than the usual behind the upper arm.

Before we answer that question, let's ask why we given it in the upper arm in the first place!

1) Easy to access
2) Easy to monitor for any adverse reaction
3) All the good studies done are based on injections administered to the upper arm
4) No good studies performed determining whether injections done at other body sites has equal efficacy to the arm.
5) If anaphylaxis occurs, a tourniquet can be easily applied to the arm proximal to the injection site which theoretically would decrease allergen traveling systemically

Given these answers, is it possible to give allergy shots in other body locations other than the arm?

Yes... BUT...

1) No good studies performed determining whether injections done at other body sites has equal efficacy to the arm.

2) If anaphylaxis occurs, may not be possible to apply a tourniquet easily (if given in the abdomen for example)

3) May not be as easy to monitor for a reaction (if given in the buttocks for example)

SO...

It is recommended that allergy shots be given in the upper arm and not other locations.

Reference:

Ask the expert. AAAAI 2/6/12