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 , 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 
IA is estimated to affect 30,000 patients in the USA alone.
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  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)
Variations of IA Diagnoses
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 
Differential Diagnosis of Idiopathic Anaphylaxis
I. Known causes of immediate generalized reactions
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 anaphylaxis. It 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
- Aspirin & NSAIDs
- Angiotensin-converting enzyme inhibitors (ACE-inhibitors)
- 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