Anaphylactoid reaction
Synonyms and keywords: Pseudoallergic reaction

# Overview
Anaphylactoid reaction or pseudoallergic reaction is defined as a condition presenting with symptoms similar to an allergic reaction but without detectable immunological sensitization, as found in classical allergic reactions. It is characterized by elevated IgE levels in the blood.

# Pathophysiology
The pathophysiological mechanism behind anaphylactoid reactions is not clear. One of them is direct release of histamine and other mediators caused by some drugs such as opioids, intravenous narcotics, colloid volume substitutes on gelatin basis, radiographic contrast media, and others. Direct activation of complement system and kinin-kallikrein system may also play a role in the development of these reactions. Local anesthetics or stress can induce neuropsychogenic reflexes in the body leading to release of mediators. Involvement of mast cells and basophils has also been found to be involved in the pathogenesis of anaphylactoid reaction.

# Causes
Several factors/substances have been shown to be associated with anaphylactoid reactions. Listed below is a list of common precipitators of anaphylactoid reactions:
- Drugs (almost any drug can cause anaphylactoid reaction)
- Dextran
- Didanosine
- Felbamate
- Imiglucerase
- Lidocaine
- Prednisolone
- Hydrochlorothiazide
- Valdecoxib,
- Foods (foodstuffs such as peanuts, fish, gelatin, etc.)
- Additives in drugs and foods
- Occupational substances (e.g., latex)
- Animal venoms (scorpion, snake, etc.)
- Aeroallergens
- Hemodialysis
- Contrast-induced
- Seminal fluid
- Contact urticariogens
- Physical agents (cold, heat, UV irradiation)
- Hereditary angioedema
- Systemic mastocytosis
- Exercise
- Echinococcal cyst
- Idiopathic

# Differentiating Anaphylactoid Reaction From Other Diseases
The presentation may mimic a number of conditions, although in most cases the signs and symptoms are sufficient to make the diagnosis of anaphylactoid reaction.
These mimickers include:
- Vasovagal syncope
- Syncope
- Drug reactions
- Hypoglycemia
- Seizure
- Foreign body/aspiration
- Psychogenic
- Pulmonary embolism
- Hyperventilation

# Diagnosis

## Symptoms
The clinical features are quite similar to anaphylaxis, however the presentation is almost always milder.  Most commonly the syndrome begins with involvement of the skin, usually as pruritus, flush, urticaria, or angioedema. Paresthesia, itching of pharynx and genital area, and feeling of anxiety are common symptoms. Almost all organ systems may be involved as explained below:
Respiratory
- Sneezing or rhinorrhea is the earliest manifestation.
- Hoarseness or throat tightness
- Cough and wheezing
- Change of voice (dysphonia)
- Dyspnea due to laryngeal obstruction
- Cyanosis or even respiratory arrest may occur
Gastrointestinal
- Nausea and cramping
- vomiting and diarrhea
- Micturition and defecation
Cardiovascular
- Tachycardia
- Hypotension
- Arrythmia
- Shock
- Cardiac arrest
The following system of grading is sometimes used to grade the severity of reactions:
Adapted from Ring  and Mesmer

## Physical Examination

# Medical Therapy

# Prophylaxis
The best way of preventing a reaction is by avoiding the allergen, but this is possible only when allergy diagnosis has been done and patient is informed about the condition and also specific agents involved. It has been found by repeated experiences that drugs are more anaphylactoid when injected as compared to oral administration, so when administering a new drug it's better to do prophetic testing  (testing for allergy in the absence of a prior history). Trying first with oral administration or small injectable test doses are other strategies. In some cases, desensitizing a patient to a particular drug is also helpful (Case of Penicillin use for treating syphilis). The induction of immuno-logical tolerance against the xenogeneic protein has been shown to reduce the frequency of side reactions of antilymphocyte globulin therapy.
Pseudo allergic reactions can be prevented by H1 and H2 anti histaminics, and includes but is not limited to pseudo allergic reactions caused by IV contrast media, analgesics etc.