Monday, April 14, 2014

Hypersensitivity to Hymenoptera Sting... Not a Little Problem !

Spring is here and with that, the return of the hymenoptera stings coming to the ER. So we are going to review this week's NEJM article on the topic using the question and answer format.

What is the antigenic cross-reactivity between Hymenoptera species?
The Hymenoptera insects whose stings cause allergy are generally from three families: Apidae (honeybees and bumblebees), Vespidae (hornets, wasps, and yellow jackets), and Formicidae (fire ants). The molecular characteristics of the venoms from the three families of Hymenoptera are sufficiently different that there is very little antigenic cross-reactivity. Within families (e.g., vespids), there can be substantial cross-reactivity among the allergens present in the venoms; however, honeybee and bumblebee allergies are distinct.

What is the pathophysiology of an allergic reaction to a hymenoptera sting, and how common is an anaphylactic reaction?
In sensitized persons, a sting can cause the injected venom to bind to venom-specific IgE on mast cells, cross-linking high-affinity IgE receptors and subsequently leading to the rapid release of mast-cell mediators, including histamine, leukotrienes, prostaglandins, and platelet-activating factor. The released mast-cell mediators can cause a spectrum of allergic reactions, from local reactions (affecting small or large [≥10 cm] areas) or urticaria to anaphylaxis and even death. Patients with large local reactions usually do not have a systemic reaction to subsequent stings (with systemic reactions occurring in <10% of these patients), nor do children with isolated urticaria. However, a previous systemic reaction in a patient with venom-specific IgE is associated with a high risk of subsequent systemic reaction, which may occur in 30 to 60% of these patients. Anaphylaxis due to a hymenoptera sting causes at least 40 deaths per year in the United States, although this number is probably an underestimate. Severe systemic allergic reactions occur in approximately 0.4 to 0.8% of children and 3.0% of adults.

What are the risk factors for a severe reaction to a hymenoptera sting and how should it be treated?
Acute systemic reactions typically occur very rapidly after a hymenoptera sting but may be delayed for several hours or be biphasic. The factors associated with an increased risk of severe reaction include being stung by a honeybee (greater risk than with other hymenoptera), underlying mast-cell disorders with elevated serum-tryptase levels at baseline, a previous severe reaction, preexisting cardiovascular disease, and concomitant treatment with a beta-blocker, angiotensin-converting–enzyme inhibitor, or both. Anaphylaxis can present with a spectrum of signs and symptoms affecting multiple organ systems, including the skin, gastrointestinal tract, cardiovascular system, nervous system, and both the upper and lower respiratory tracts; hallmarks of anaphylaxis are the development of hypotension or the involvement of more than one organ system. The treatment of anaphylaxis in the emergency department should include epinephrine for any patient who has more than cutaneous symptoms; epinephrine should also be considered in adults with urticaria alone. H1-antihistamines can help relieve cutaneous signs and symptoms. For respiratory symptoms, supplemental oxygen and inhaled beta2-agonists should be used. For patients with hypotension, volume resuscitation is indicated, with 1 to 2 liters of 0.9% (isotonic) saline infused rapidly (e.g., a dose of 5 to 10 ml per kilogram in the first 5 to 10 minutes in an adult, and 10 ml per kilogram in a child).

Who should receive venom immunotherapy?
Subcutaneous immunotherapy should be considered in all patients who have had a systemic allergic reaction to an insect sting and who have a positive skin test or a positive result on an in vitro test for venom-specific IgE antibodies. Children 16 years of age or younger who have had isolated cutaneous systemic reactions to insect stings have a very low risk of subsequent reactions and do not require venom immunotherapy. Venom immunotherapy is also generally not necessary in patients who have had only a large local reaction, because their risk of subsequent systemic reactions is relatively low. However, patients with unavoidable or frequent exposures (e.g., beekeepers) may benefit, because observational data indicate that, after immunotherapy, local reactions are reduced in size and duration.

Key points to remember:
- Hymenoptera stings CAN kill, so take them seriously, specially those with prior severe reactions and honeybee stings.
- It is an IgE hypersensitivity reaction, therefore it is fast in most cases, but don't forget it can be biphasic.
- High risk are patients on beta blockers, ACEI and cardiovascular disease, the very young and old.
- Anaphylaxis is hypotension plus one other system involved: Skin, GI, respiratory or neurologic. Don't miss it !
- If it looks bad... IS bad. Get epinephrine IM mas rapido and quickly start an epinephrine drip and ensure good volume resuscitation. Antihistaminics, steroids and beta agonist are secondary agents.
- Arrange for follow up in all severe cases for possible immunotherapy, let the PCP and allergist make that call.
- Don't forget your dog... consult the vet right away! '',