Biphasic anaphylaxis is unpredictable, but there are some evidence-based predictors indicating which children are more at risk.
A biphasic, or two-phase anaphylactic reaction occurs when anaphylaxis is successfully treated, but after an asymptomatic period the symptoms return—without re-exposure to the allergen.
The second part of a biphasic reaction can be less, equal to or more severe than the initial set of symptoms, and tends to occur four to ten hours after the first reaction. However, the second phase can happen within a couple hours, and up to 72 hours later.
Biphasic High Risk Indicators
Because it is unpredictable, researchers and medical personnel do not always agree on how long children (or adults) should be kept at an emergency department for observation following an anaphylactic reaction.
Observations of four to six hours are typical, but longer observation periods may be indicated for those at higher risk of biphasic reactions. The question is: who is at higher risk?
A study completed at Children’s Hospital of Eastern Ontario (CHEO) Research Institute revealed five predictors of biphasic reactions in children:
- A delay in getting emergency medical treatment, or a delay in ephinephrine administration, of more than 90 minutes after the start of initial symptoms.
- Having a wide pulse pressure (changes in blood pressure during the heart's contractions) at triage.
- Treatment of the initial symptoms requires more than one dose of epinephrine.
- The patient’s respiratory distress requires the administration of inhaled salbutamol in the emergency department.
- The child is 6 to 9 years old.
“It’s clear that children with severe initial reactions would benefit from a prolonged period of observation in the Emergency Department,” said researcher Dr. Waleed Alquarashi.
“On the flip side, knowing what to look for helps to better utilize resources so that children with mild allergic reaction, who do not match any of the identified predictors can go home faster.”
Crucial: Awareness and Epinephrine
The CHEO research outcomes mesh with the knowledge and observations of Dr. Anne K. Ellis, Director of Allergy Research Unit, Kingston General Hospital.
Dr. Ellis states the highest risk of biphasic reactions is in patients whose initial symptoms require more than one epinephrine dose, are severe or life-threatening, and take longer to stabilize. With these patients, observation periods of up to 24 hours may be indicated.
What is crucial - for anaphylaxis patients and caretakers - is awareness of the possibility of a second reaction, to have an epinephrine auto-injector (or two), know how to use it properly, and to call 911 when necessary.