Priority Topic: Allergy

Key Feature 10: In a patient with unexplained recurrent respiratory symptoms, include allergy (ex: sick building syndrome, seasonal allergy) in the differential diagnosis.
Skill: Clinical Reasoning
Phase: Hypothesis generation

Allergic reactions present in multiple ways. They can cause symptoms in varying combinations that commonly involve the respiratory tract (upper and/or lower), skin and/or mucous membranes, gastrointestinal tract, cardiovascular system, and nervous system. Unless someone is having an episode of anaphylaxis (for which there should be a high index of suspicion), the symptoms and signs that are the manifestations of an allergic reaction have their own differential, and allergy may be only one of the possible etiologies causing the symptoms based on the clinical presentation. Usually these signs and symptoms are not life-threatening, however, so there is time to get to the root cause. 

In general, in order to diagnose an allergy as contributing to a patient's symptoms/signs, it is important to be aware of all of the possible ways in which an allergy may present, and it should be on the differential when a patient presents with respiratory symptoms and skin rashes/pruritus and/or mucous membrane swelling in particular (as these are the more common ways that allergies manifest). As indicated above, other organ systems can be affected, and with recurrent symptoms the possibility of allergy should be heightened (such as repeated paroxysms of gastrointestinal upset, for which one isolated episode would be much less suggestive of allergy). Besides understanding how allergy may present, it is important to gather a comprehensive history to assess for potential exposure to allergens. This should include a review for possible

• Drugs ingested
  
• Food associations
  
  • 8 most common food allergens: Nuts, tree nuts, milk, eggs, wheat, soy, fish, shellfish
• Environmental triggers (related to specific environments or seasons)
  
• Insect bites/stings
• Occupational triggers
• Exercise as a trigger
• Family history (there is a genetic component to one's risk of having allergies)

If the clinician thinks that an allergy may be causing or contributing to the paroxysmal and recurrent symptoms the patient is experiencing, two options are: 1. To trial empiric therapy and see if this improves the patient's symptoms*, and 2. To refer to an allergy and immunology specialist for a more extensive workup (see the American Academy of Allergy, Asthma & Immunology Consultation and Referral Guidelines for when a referral may be warranted).

Empiric therapy (first-line)

• For a skin rash or pruritus: oral antihistamine
• For upper respiratory tract symptoms (rhinorrhea and/or congestion): intranasal antihistamine
• For lower respiratory tract symptoms (dyspnea and/or wheeze): inhaled short-acting beta agonist

*When providing empiric therapy, it is important to consider the natural history of disease processes and the placebo effect, both of which may lead to improvement in symptoms unrelated to the active ingredient in the medication.