ALLERGIES

Allergic reactions occur along a spectrum. 

Hives are mild. 

Urticaria (hives) refers to the dermal swelling seen with an allergic reaction. Urticaria is the classic “wheal-and-flare” allergic skin rash. It occurs almost immediately after touching an allergen. It’s a Type 1 hypersensitivity - an allergen triggers mast cells through cross-linking of IgE receptors, leading to release of their Histamine, which causes vasodilation. The rash looks like smooth, pink, raised wheals which are very itchy. They respond well to antihistamines and steroids. 

IgE-Independent Mast Cell Degranulation - something other than IgE causes mast cells to degranulate. Usually a drug is responsible (Vancomycin, Opioids and IV radiocontrast)

Anaphylaxis is severe. 

Anaphylaxis is a severe allergic reaction. They have urticaria + at least 1 other symptom (wheezing, angioedema, hypotension). The hypotension can be lethal. Treat it with Epinephrine.

Angioedema is facial swelling, which is a worrisome finding. The swelling can lead to obstruction of their airway. Also seen with ACE-inhibitors.

ANAPHYLAXIS

Bad allergic reaction

Anaphylaxis is a very severe allergic reaction. We’re talking about Type 1 Hypersensitivity folks. The most pro-anaphylactic allergens are peanuts, bee stings, shellfish and penicillin. It’s a MEDICAL EMERGENCY. 

What is the pathophysiology behind Anaphylaxis?

It’s simply a severe allergic reaction, so the mechanisms (IgE cross linking on mast cells) are the same as a normal allergy. Mast cells release histamine and other cytokines that result in vasodilation, which results in localized swelling and redness. With a sufficiently large amount of histamine, you can dilate blood vessels all over the body, so much so that the blood pressure can drop to fatal levels. 

What distinguishes Anaphylaxis from a regular allergic reaction?

The definition of Anaphylaxis is difficult to nail down, since there’s no blood test or imaging test, but I think of it as any allergic reaction that’s potentially deadly. Look for the life-threatening and rapid onset of multiple allergic symptoms. 

What are the allergic symptoms?

Urticaria (hives), wheezing, stridor, GI distress (nausea, vomiting) and hypotension. Urticaria is described as splotchy raised erythematous wheals that are intensely itchy. The respiratory symptoms stem from swelling along the airways. The GI distress comes from vasodilation and edema of the intestines. And the hypotension results from widespread peripheral vasodilation.

How do I tell if someone’s respiratory distress is due to allergies?

When in doubt, ask about itching. Everyone with an allergic reaction will itch somewhere. Angioedema, eyelid swelling, wheezes and hives are also pretty helpful.

What’s the cause of death in untreated Anaphylaxis?

It’s tempting to think of airway inflammation, but the usual cause of death is shock (low blood pressure). Histamine and other vasodilators open up the peripheral vasculature so much that the BP collapses. 

How do you treat it?

Epinephrine is a “parachute drug” for treating Anaphylaxis. Just like you wouldn’t go skydiving without a parachute, you would never treat Anaphylaxis without Epi. Quickly give an IM injection of Epinephrine, aka the EpiPen. Epinephrine is an agonist for alpha and beta adrenergic receptors. 

• Alpha 1 → reverses the diffuse vasodilation, which should improve the blood pressure

• Beta 1 → increase CO to further improve peripheral perfusion

• Beta 2 → bronchodilate / stabilize mast cells

Benadryl (Diphenhydramine) and Steroids are of secondary importance. Diphenhydramine is often monotherapy for mild allergic reactions.