Overview: Allergic & Anaphylactic Reactions
An allergy is the immune system overreacting to something foreign. Most reactions are itching or a patch of hives. A few go bad fast. The range runs from a local reaction at the sting site to anaphylactic shock that needs epinephrine right now.
Epinephrine first, not last. Once you recognize anaphylaxis, epinephrine goes in immediately.
Key points
Anaphylaxis is a severe allergic reaction that comes on fast and hits more than one body system.
Epinephrine is the first-line treatment for anaphylaxis. Diphenhydramine (Benadryl) isn't.
On the trail, the usual triggers are bee, wasp, and fire ant stings, food, and medications.
Anaphylaxis can come back after it settles (biphasic anaphylaxis). Most second waves hit within the first hours, but some come up to 72 hours later. All anaphylaxis evacuates.
Symptoms build over minutes, so the sooner the epinephrine goes in, the better your patient's odds.
Allergic Reaction Spectrum
How the reaction works
- IgE-mediated: Immunoglobulin E antibodies sit on mast cells. When the allergen shows up, the mast cells break open (degranulate) and dump histamine and other chemicals into the body.
- What histamine does:
- Makes blood vessels leak, so tissue swells (edema)
- Makes vessel walls more permeable
- Narrows the airways (bronchoconstriction)
- Widens the blood vessels (vasodilation)
- Sets off itching and hives
Mild (localized) allergic reaction
- Signs: Hives (urticaria), itching, or mild redness (erythema), all on the skin at or near the sting or contact site. Nothing anywhere else in the body.
- Example: A mosquito bite that itches and raises a small bump.
- Treatment:
- Diphenhydramine (Benadryl) 25-50 mg by mouth
- Hydrocortisone cream on the skin for itch
- Ice or a cool compress
- Monitoring: Watch closely for at least 30 minutes and keep rechecking. Look for spread beyond the site or new symptoms in another body system. If it progresses, it's anaphylaxis.
More than skin-only = anaphylaxis
- Treat as anaphylaxis if any of these follow a likely allergen exposure:
- Any lower-airway involvement: wheezing, chest or throat tightness, trouble breathing
- Signs in 2 or more body systems (for example, hives plus nausea, or hives plus swelling of the lips, tongue, or face)
- Any cardiovascular sign: dizziness, faintness, a weak pulse, collapse
- Treatment:
- Epinephrine immediately. Don't wait to see if it gets worse.
- Antihistamines (diphenhydramine 25-50 mg) only after epinephrine, as an add-on for skin symptoms. They don't treat anaphylaxis.
Severe (anaphylaxis)
Definition: A severe allergic reaction that comes on fast and involves 2 or more body systems, or a single life-threatening reaction.
- Criteria for anaphylaxis:
- Skin involvement (hives or flushing) plus breathing symptoms (wheezing, stridor, shortness of breath), cardiovascular symptoms (low blood pressure, dizziness, fainting), or GI symptoms (vomiting, cramping)
- Or breathing or cardiovascular symptoms alone, after a likely or known allergen exposure
- Either one means epinephrine and an emergent evacuation.
Anaphylaxis Recognition
Signs and symptoms
- Skin: Hives (urticaria) all over the body, flushing, intense itching (pruritus)
- Respiratory:
- Bronchospasm: wheezing, shortness of breath, chest tightness
- Upper airway swelling (edema): stridor (high-pitched breathing), voice changes, trouble swallowing
- Shortness of breath (dyspnea)
- Cardiovascular:
- Low blood pressure (hypotension): dizziness, fainting (syncope), shock
- Fast heart rate (tachycardia)
- Chest pain or palpitations
- GI: Nausea, vomiting, cramping, diarrhea
- Neurologic: Anxiety, confusion, a sense of impending doom
Time course
- Onset: Usually within minutes of exposure, up to 1 hour
- The faster the onset, the more severe the reaction.
- Symptoms within 5 minutes: severe and life-threatening
- Symptoms at 15-30 minutes: more time to intervene, but once the signs meet the criteria, epinephrine goes in now
- Why speed matters: A swelling airway can close off breathing, and shock builds quickly.
Common triggers in the wilderness
- Insect stings (the most common trigger out here):
- Hymenoptera: bees, wasps, hornets
- Fire ants
- Anaphylaxis risk: about 1% of people stung
- Food: Peanuts, tree nuts, shellfish, eggs, milk
- Medications: Penicillin, NSAIDs, aspirin
- Latex: Gloves (yours included) and other equipment
- Venom: Snake venom (rare)
Biphasic anaphylaxis
- What it is: A second wave of anaphylaxis after the first reaction settles. Most come within the first hours, but some come up to 72 hours later.
- How often: 5-20% of anaphylaxis cases
- Why: More mast cells break open, or the allergen keeps acting.
- What it means for you: Nobody can predict who'll get a second wave, so every anaphylaxis patient gets evacuated and watched in a hospital. That includes the one who feels fine.
EpiPen (Epinephrine Auto-Injector) Administration
At this level, you help the patient use their own prescribed epinephrine auto-injector. It's their device and their prescription, and you're helping with their medication. Many states also protect trained laypeople who give epinephrine in an emergency, so learn the rules where you live and where you travel.
How epinephrine works
- Alpha-1 effects: Vasoconstriction, raising blood pressure and reversing shock
- Beta-1 effects: Faster, stronger heartbeat (rate and contractility), improving cardiac output
- Beta-2 effects: Bronchodilation, opening the airways and reversing bronchospasm
- Net result: It reverses the main signs of anaphylaxis: low blood pressure, bronchospasm, and deep swelling (angioedema).
EpiPen dosing
- Adult auto-injector: 0.3 mg
- EpiPen Jr (pediatric): 0.15 mg, for children about 15-30 kg (33-66 lbs)
- Smaller children, under 15 kg: 0.15 mg or specialty pediatric dosing
Injection technique
- Site: The outer side of the thigh, halfway between hip and knee (lateral mid-thigh)
- Why the thigh: There's plenty of muscle to absorb the drug, you can reach it through pants, and nothing vital sits under it.
- Steps:
- Pull off the blue safety release cap at the top.
- Hold it firmly, orange tip (the needle end) pointing down.
- Swing and push the orange tip firmly against the outer thigh at 90°, straight in.
- Hold in place for 3 seconds (per current auto-injector labeling).
- Remove it. Massaging the site is no longer recommended.
- Other brands work differently: Colors, hold times, and steps vary between devices. Follow the instructions printed on the one in your hand.
- Through clothing is fine: Denim and other fabric won't stop the needle. Leave the pants on.
Learn it before you need it. Figure out how your EpiPen works at home, not on the trail. Carry a trainer (practice auto-injector) on trips and run through the steps with it.
Onset and duration
- Onset: 5-15 minutes
- Duration: 20-30 minutes
- Second dose: At 5-15 minutes if there's no improvement or symptoms return
- Carry more than one: Anyone with known anaphylaxis risk should carry at least 2 EpiPens
Side effects
- Palpitations (feeling the heart pound)
- Tremor and shakiness
- Headache and anxiety
- These are expected, and they pass. None of them is a reason to hold the dose.
Anaphylaxis Field Management
Treatment priority: epinephrine first
Epi before Benadryl. Give epinephrine before diphenhydramine (Benadryl). Don't hold it for any other medication.
- Right away: Epinephrine IM (into the muscle of the thigh)
- Then follow with:
- Oral diphenhydramine 25-50 mg for skin symptoms
- Help the patient use their own inhaler for wheezing that won't quit. After epinephrine, never instead of it.
Positioning
- Breathing trouble: Sitting upright, which makes breathing easier
- Low blood pressure or shock: Flat on their back (supine). Raising the legs may help if breathing isn't impaired. This part is specific to anaphylaxis, because blood pools when the patient stands.
- Both at once: Keep them flat, and let them sit up a little if breathing is hard.
- Nausea or vomiting risk: Recovery position (on their side) if unconscious.
- Never stand or sit the patient up abruptly. In anaphylaxis, a sudden position change can cause fatal collapse.
Field management, start to finish
- Recognize anaphylaxis. Don't wait for "confirmation."
- Give epinephrine IM immediately (0.3 mg adult, 0.15 mg child).
- No better in 5-15 minutes, or getting worse? Give a second dose.
- Position the patient: upright for breathing trouble, flat for shock.
- Give supplemental oxygen if you have it.
- Follow with diphenhydramine for skin symptoms, by mouth only once they're swallowing normally.
- Watch vital signs and the airway the whole time.
- Emergent evacuation, even if epinephrine worked. The reaction can come back, and all anaphylaxis needs a hospital to look at it.
Watching for a second wave
- A patient who bounces back after epinephrine can still relapse, most often within the first hours but possibly up to 72 hours later.
- You can't watch them long enough in the field. They need hospital monitoring.
- Evacuate them even if they feel fine.
What not to do
- Don't give fluids by mouth if the airway is swelling. They can inhale it (aspiration).
- Don't rely on antihistamines alone to stop anaphylaxis. They won't.
- Don't delay the evacuation because the patient "feels better" after epinephrine.
- Don't hold back epinephrine because of its side effects. They're temporary, and anaphylaxis kills.
Case Scenarios
Scenario 1: Bee sting with anaphylaxis on the trail
Setting: A hiker is stung by multiple bees. Within 5 minutes she's wheezing, hives are spreading across her body, and her lips are swelling. She's getting anxious and short of breath. Her EpiPen is in the group first aid kit.
Recognition: This is anaphylaxis. She was stung, and within minutes her breathing (wheezing) and skin (hives) are both involved.
Immediate actions:
- Get her EpiPen out of the first aid kit.
- Pull the blue cap and point the orange tip at her thigh.
- Inject into the outer thigh muscle. Through the pants is fine.
- Hold for 3 seconds, then remove.
- Sit her upright for the respiratory distress.
- No better in 5-15 minutes, or worse? Second dose, if the kit has one.
- Keep watching her airway, and be ready to manage it if the swelling gets worse.
Follow-up:
- Diphenhydramine if you have it, after epinephrine, for skin symptoms. Only by mouth once her lips and throat have settled and she's swallowing normally.
- Help her use her own inhaler if the wheezing continues. After epinephrine, never instead of it.
- Call for an emergent evacuation, even if she improves.
- Watch for a biphasic reaction on the way out.
Scenario 2: Hidden food allergen at camp
Setting: A backpacker with a known peanut allergy eats a groupmate's trail mix, not realizing it has peanuts. Fifteen minutes later her throat feels tight, she's dizzy, and she's breaking out in hives. She has an EpiPen but says, "Maybe I'm overreacting."
Recognition: This is anaphylaxis. She ate a known allergen, and now she has throat tightness (airway), dizziness (cardiovascular), and hives (skin). That meets the criteria.
Don't let her talk you out of it. "Maybe I'm overreacting" isn't a vital sign. It's likely to get worse fast.
Management:
- Get the EpiPen out now. Do not delay.
- Inject into the thigh.
- No better in 5-15 minutes, or worse? Second dose.
- Lay her flat. Legs can go up if her breathing isn't impaired. Never let her stand or sit up abruptly.
- Oral diphenhydramine after epinephrine, for skin symptoms only, and only once her throat has settled and she's swallowing normally.
- Call for an emergent evacuation and plan for a helicopter. Her dizziness means the cardiovascular system is involved.
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Lesson Summary
The short version
Anaphylaxis is a severe allergic reaction that hits more than one body system, and it can kill.
Epinephrine is first-line. Give it immediately, IM into the outer thigh.
EpiPen doses are 0.3 mg for adults and 0.15 mg for kids. No better in 5-15 minutes? Second dose.
Antihistamines are an add-on after epinephrine, never a substitute.
The usual triggers are bee and wasp stings, food allergies, and medications.
A reaction that builds over minutes is an emergency. Get the epinephrine in fast.
Most biphasic reactions come within the first hours, but they can come up to 72 hours later. Always evacuate.
Sit them upright for breathing trouble. Lay them flat for shock.