Auckland Allergy & Eczema Clinic

Idiopathic Anaphylaxis: An Update

Idiopathic Anaphylaxis:
An Update

Vincent St Aubyn Crump

Idiopathic Anaphylaxis (IA) is a well-described syndrome of anaphylaxis without any recognised external trigger. These patients present with the same symptoms as patients with other types of anaphylactic reaction. The attacks occur with variable frequency. Fatalities have been reported in patients who have been diagnosed with idiopathic anaphylaxis.

Bacal et al first reported IA in 1978 [1], when they published data on 11 patients whose episode of anaphylaxis could not be explained. The series has been expanded to include over 335 patients. Atopy was noted in 48% of the patients (the same percentage implicated in the Spanish series of 81 patients).

The disease exists in all age groups and proves expensive if not diagnosed and treated early. It is reported to be more common in women, with 65% of 335 idiopathic anaphylaxis patients being women in the largest published series in the literature.

Prevalence of IA

Idiopathic anaphylaxis is one of the most common causes of anaphylaxis, accounting for approximately one third of cases in one retrospective study [2]

IA is estimated to affect 30,000 patients in the USA alone.

Clinical presentation

Symptoms & signs do not differ from other forms of anaphylaxis and include:

  • Hypotension & increased pulse rate
  • Wheezing & stridor
  • Hives, angioedema, flushing, & itching
  • Nausea, vomiting, diarrhea, difficulty swallowing
  • Light-headedness and loss of consciousness

In the series of Ditto et al [3] all of them experienced hives and angioedema, whereas 60% experienced symptoms of upper airway obstruction. Individual patients usually tend to have the same manifestations on repeated episodes. Progression from hives and itching to life-threatening symptoms of wheeze, loss of consciousness, and laryngeal edema may occur in 10 min to hours after onset.

Classification of Idiopathic Anaphylaxis (IA)

(Adopted from Roy Patterson, M.D. textbook on Idiopathic Anaphylaxis)

Classification of Idiopathic Anaphylaxis.png

Variations of IA Diagnoses

Variations of IA Diagnoses.png


Idiopathic anaphylaxis is a diagnosis of exclusion. Only after a thorough history and physical examination should the diagnosis be entertained. Skin prick testing, RAST and other lab tests are usually required to rule out culprit allergens and the numerous disease mentioned below that may masquerade as IA. A proper history (like in all diagnosis in Allergies) is probably the most important tool in the work-up, because it will guide subsequent testing.

Serum (mast cell) Tryptase can be very useful in differentiating anaaphylaxis from the many conditions that can masquerade as anaphylaxis [4]

Differential Diagnosis of Idiopathic Anaphylaxis

I. Known causes of immediate generalized reactions

A. IgE-mediated


Food is the most common cause of anaphylaxis and occurs in 1-2% of the population. Symptoms usually start 5 — 30 min after ingestion, occasionally after 1-2h, but rarely any longer. Consider an alternative diagnosis if symptoms began many hours after ingestion or if the patient has since eaten the suspected food without any reaction. The most common food allergens are milk, eggs, peanuts, tree nuts, fish, shellfish, soy & wheat, which account for over 90% of all food allergies. Foods commonly mistaken for IA include mustard and other spices.


Drugs are another common cause of anaphylaxisIt is important to focus on drugs / supplements / herbal preparation (bee pollen, Echinacea) and other over-the counter formulations (especially aspirin & NSAIDs) in the history taking.

B. Exercise Induced Anaphylaxis

C. Food-dependent, Exercise-induced Anaphylaxis

D. Non-IgE Medication-induced reactions

  1. Aspirin & NSAIDs
  2. Opiates
  3. Angiotensin-converting enzyme inhibitors (ACE-inhibitors)
  4. Radiographic contrast media

II. Asthma masquerading as asthma

III. Systemic Mastocytosis

IV. Hereditary Angioedema

V. Munchausen's stridor / Anaphylaxis

VI. Undifferentiated Somatoform idiopathic anaphylaxis

VII. Miscellaneous Diagnosis

  • Panic Attacks
  • Globus Hystericus
  • Histamine-rich food flushing