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Idiopathic anaphylaxis


A fifteen-year-old boy came into the ED, brought in by his parents after having to use his EpiPen at home. He was at school. He was well in the morning. Suddenly, while walking between classes, he started to feel nauseous, sort of breath and tight-chested. Once again, he was having an anaphylactic reaction, but he did not know what he was reacting to. He has idiopathic anaphylaxis.

Key learning points

Anaphylaxis can occasionally be idiopathic, although this is rare.

Idiopathic anaphylaxis has the same pathology and presentation as allergen-mediated anaphylaxis.

Patients with idiopathic anaphylaxis need the same acute treatment.

Long term treatment involves a lot of education about the condition


Anaphylaxis is a common severe allergic reaction to an allergen. IgE binds to the allergen which causes degranulation of mast cells and basophils, leading to histamine release.  This, in turn, causes smooth muscle constriction and bronchoconstriction as well as angioedema. This is a type 1 immunological reaction.

This reaction leads to facial swelling, bronchoconstriction, airway obstruction, rash, vomiting, diarrhoea and fluid redistribution, leading to shock and respiratory failure. Anaphylaxis is a life-threatening medical emergency and needs prompt treatment as well as education to prevent further attacks. So, the prevention of anaphylactic shock is important. Usually, this can be done by tracing common allergens the patient was exposed to before they developed a reaction. However, in some cases, this is more difficult to determine and may not be obvious.

Idiopathic anaphylaxis

Idiopathic anaphylaxis is a condition where patients develop an anaphylactic reaction without a causative agent. The patient develops an anaphylactic reaction without any form of trigger. This can be dangerous, as patients can spontaneously develop anaphylaxis without any warning, putting them in life-threatening situations regularly and without warning. It is unclear what causes this, and although patients are commonly very atopic and will have other atopic conditions and allergies.

How common is idiopathic anaphylaxis?

Idiopathic anaphylaxis is a relatively rare condition. It represents a subset of anaphylactic reactions where, despite thorough investigations, no identifiable allergen or trigger can be found. A significant portion of anaphylaxis cases, ranging from 6% to 59%, have been attributed to idiopathic causes in various studies​​.

The prevalence of idiopathic anaphylaxis among all cases of anaphylaxis is not precisely known, largely due to its nature as a diagnosis of exclusion.

How is idiopathic anaphylaxis classified?

Idiopathic anaphylaxis has been classified into two primary categories: generalized (IA-G) and angioedema (IA-A).

IA-G includes symptoms like urticaria, angioedema, acute bronchoconstriction, voice change, syncope, or hypotension, while IA-A is characterized by significant upper airway obstruction due to tongue or laryngeal swelling.

These episodes may be classified as either frequent or infrequent​​. A significant portion of anaphylaxis cases, ranging from 6% to 59%, have been attributed to idiopathic causes in various studies​​.

How do you treat idiopathic anaphylaxis?

Idiopathic anaphylaxis was first described in 1978, with early observations showing that treatment with prednisone and hydroxyzine could reduce the severity and frequency of episodes. This early evidence supported the understanding that idiopathic anaphylaxis responds to empirical treatment despite the unclear mechanistic cause.

Treatment for idiopathic anaphylaxis is the same as for any other cause of anaphylaxis in the acute setting. Patients need treatment with adrenaline and steroids. However, in the long term, they need to be well educated and prepared, including education on what anaphylaxis is and how important it is, using EpiPens and how important it is to always carry one in case of a reaction developing.

The child in our case underwent multiple allergen and immunological investigations, and although multiple allergens were identified, he continued to have anaphylactic reactions without apparent triggers.


Greenberger, P.A., 2007. Idiopathic anaphylaxis. Immunology and allergy clinics of North America27(2), pp.273-293.

Greenberger, P.A. and Lieberman, P., 2014. Idiopathic anaphylaxis. The Journal of Allergy and Clinical Immunology: In Practice2(3), pp.243-250.

Nwaru, B.I., Dhami, S. and Sheikh, A., 2017. Idiopathic anaphylaxis. Current Treatment Options in Allergy4, pp.312-319.


  • Abbey Ward is a a foundation doctor, interested in teaching, paediatrics, emergency medicine and intensive care, but she has not decided what to do when she grows up! You can find her on twitter as @AbbeyWardDr


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