Allergic Reactions And Atopy

Introduction

For Canadian medical students and international graduates preparing for the MCCQE1, understanding Allergic Reactions and Atopy is crucial. Allergic diseases affect a significant portion of the Canadian population, with allergic rhinitis alone affecting approximately 20-25% of Canadians.

This guide focuses on the pathophysiology, clinical presentation, diagnosis, and management of atopic disorders, tailored specifically to the Medical Council of Canada (MCC) objectives.

Definitions

• Atopy: The genetic tendency to develop allergic diseases such as allergic rhinitis, asthma, and atopic dermatitis (eczema). It is characterized by the production of specific IgE antibodies to common environmental allergens.
• Allergy: A hypersensitivity reaction initiated by specific immunological mechanisms (usually IgE-mediated).
• Anaphylaxis: A severe, life-threatening, generalized or systemic hypersensitivity reaction.

Pathophysiology: Type I Hypersensitivity

Atopic reactions are primarily Type I (Immediate) Hypersensitivity reactions. Understanding this mechanism is vital for the MCCQE1.

The Mechanism

1. Sensitization: Antigen presentation to Th2 cells $\rightarrow$ IL-4 and IL-13 production $\rightarrow$ B-cells switch to IgE production.
2. Binding: IgE binds to high-affinity receptors (Fc$\epsilon$RI) on mast cells and basophils.
3. Re-exposure: Allergen cross-links IgE on sensitized cells.
4. Degranulation: Release of preformed mediators (Histamine, Tryptase) and synthesis of lipid mediators (Leukotrienes, Prostaglandins).

Clinical Presentations: The Atopic Triad

The “Atopic Triad” consists of Atopic Dermatitis, Allergic Rhinitis, and Asthma. Patients often progress through these conditions in a sequence known as the “Atopic March.”

1. Atopic Dermatitis (Eczema)

A chronic, relapsing inflammatory skin disease.

• Infants: Extensor surfaces, face, scalp.
• Children/Adults: Flexural surfaces (antecubital/popliteal fossae), neck, hands.
• Key Symptom: Pruritus (“The itch that rashes”).

2. Allergic Rhinitis

Inflammation of the nasal membranes.

• Symptoms: Rhinorrhea, nasal congestion, sneezing, ocular pruritus.
• Physical Exam: “Allergic shiners” (dark circles under eyes), “Nasal salute” (transverse nasal crease), pale/boggy turbinates.

3. Food Allergy

IgE-mediated reactions to specific foods.

Priority Allergens in Canada:

• Peanuts
• Tree nuts
• Sesame
• Milk
• Eggs
• Fish
• Crustaceans and molluscs
• Soy
• Wheat
• Mustard (Specific to Canadian labeling regulations)

Anaphylaxis: A Medical Emergency

Anaphylaxis is a high-yield topic for the MCCQE1. You must be able to recognize and manage it immediately.

Diagnostic Criteria

Anaphylaxis is highly likely when any one of the following 3 criteria are fulfilled:

1. Acute onset (minutes to hours) involving skin/mucosa (hives, itch, swollen lips) AND at least one of:
   • Respiratory compromise (dyspnea, wheeze, stridor, hypoxemia).
   • Reduced BP or associated symptoms (hypotonia, syncope).
2. Two or more of the following occurring rapidly after exposure to a likely allergen:
   • Skin/Mucosal involvement.
   • Respiratory compromise.
   • Reduced BP.
   • Persistent GI symptoms (crampy abdominal pain, vomiting).
3. Reduced BP after exposure to a known allergen for that patient.

Management Steps

Diagnosis of Atopic Disorders

While history is the gold standard, confirmatory testing is often required.

Comparison of Diagnostic Modalities

Management Strategies

Effective management involves a multi-pronged approach: Avoidance, Pharmacotherapy, and Immunotherapy.

Canadian Guidelines (CSACI & CPS)

Familiarity with guidelines from the Canadian Society of Allergy and Clinical Immunology (CSACI) and the Canadian Paediatric Society (CPS) is essential for the exam.

1. Early Introduction of Allergenic Foods

• Old Guideline: Delay peanuts/eggs.
• Current Canadian Guideline: Introduce allergenic solids (peanut, egg) early, around 6 months of age (but not before 4 months), to prevent allergy, especially in high-risk infants (those with eczema or egg allergy).

2. Epinephrine Autoinjectors in Schools

• Canadian legislation (e.g., Ryan’s Law in Ontario) mandates that students with anaphylaxis must be allowed to carry their own epinephrine autoinjectors.
• Generic names: Epinephrine autoinjector (EpiPen, Allerject).

3. Biphasic Reactions

• Canadian guidelines suggest observing patients with respiratory or cardiovascular compromise for at least 4-6 hours post-anaphylaxis to monitor for a secondary wave of symptoms (biphasic reaction).

Key Points to Remember for MCCQE1

• Epinephrine is the first-line treatment for anaphylaxis. There is no absolute contraindication to epinephrine in anaphylaxis.
• Second-generation antihistamines are preferred over first-generation for allergic rhinitis and urticaria due to a better safety profile.
• Atopic Dermatitis management relies on hydration (emollients) and topical corticosteroids.
• Oral Allergy Syndrome (Pollen-Food Allergy Syndrome): Itching of mouth/throat upon eating raw fruits/vegetables due to cross-reactivity with pollens (e.g., Birch pollen $\leftrightarrow$ Apple/Carrot). Usually mild; cooked forms are tolerated.
• Serum Sickness (Type III): Fever, rash, arthralgia 1-2 weeks after drug exposure (classic cause: Cefaclor or Penicillins).

Sample Question

Question

A 24-year-old male presents to the Emergency Department brought by friends. He was at a restaurant and started complaining of shortness of breath and dizziness shortly after eating a meal containing shrimp. He has a history of asthma and shellfish allergy. On examination, he is anxious and diaphoretic. Vital signs are: HR 115 bpm, BP 85/50 mmHg, RR 28/min, O2 Sat 92% on room air. There is diffuse wheezing on auscultation and urticaria on his chest and neck.

Which one of the following is the most appropriate immediate management step?

• A. Administer intravenous (IV) diphenhydramine 50 mg
• B. Administer intramuscular (IM) epinephrine 0.5 mg
• C. Administer intravenous (IV) hydrocortisone 200 mg
• D. Administer nebulized salbutamol 5 mg
• E. Administer subcutaneous (SC) epinephrine 0.3 mg

Explanation

The correct answer is:

• B. Administer intramuscular (IM) epinephrine 0.5 mg

Detailed Explanation:

This patient is presenting with anaphylaxis, characterized by acute onset of illness involving skin/mucosa (urticaria) along with respiratory compromise (wheeze, hypoxia) and cardiovascular compromise (hypotension/shock).

• Option B (Correct): Intramuscular (IM) epinephrine is the first-line treatment for anaphylaxis. It acts on alpha-1 receptors (vasoconstriction to improve BP), beta-1 receptors (increased cardiac output), and beta-2 receptors (bronchodilation and inhibition of mast cell mediator release). The correct site is the anterolateral thigh (vastus lateralis). The dose for an adult is 0.3 mg to 0.5 mg.
• Option A (Incorrect): Antihistamines (H1 blockers) like diphenhydramine relieve cutaneous symptoms (itch, hives) but have no effect on the life-threatening respiratory or cardiovascular obstruction. They are adjunctive therapy only.
• Option C (Incorrect): Corticosteroids (like hydrocortisone) have a delayed onset of action (4-6 hours). They may help prevent biphasic reactions (though evidence is weak), but they play no role in the immediate resuscitation of an anaphylactic patient.
• Option D (Incorrect): Nebulized salbutamol is indicated for bronchospasm that is resistant to epinephrine, but it does not address the hypotension or upper airway edema. It is an adjunctive therapy.
• Option E (Incorrect): Subcutaneous (SC) absorption of epinephrine is unreliable and slower compared to the IM route. Current guidelines strongly advocate for the IM route over SC.

References

1. Canadian Society of Allergy and Clinical Immunology (CSACI). Anaphylaxis in Schools & Other Settings. 3rd Edition.
2. Sampson HA, et al. Symposium on the definition and management of anaphylaxis: summary report. J Allergy Clin Immunol. 2005.
3. Canadian Paediatric Society (CPS). Dietary exposures and allergy prevention in high-risk infants. Position Statement. 2019 (Reaffirmed 2021).
4. Medical Council of Canada. Objectives for the Qualifying Examination Part I.
5. Health Canada. The Canadian Food Labeling Guidelines regarding Priority Allergens.