An allergic reaction is an overblown immune response to a normally harmless substance (allergen). A seizure is a brief episode of abnormal electrical activity in the brain causing involuntary changes in movement, sensation, or awareness. Although uncommon, a severe, life-threatening allergic reaction can trigger a seizure as a secondary complication of the body’s systemic response.
The Direct Link Between Severe Allergy and Seizures
Seizures are almost exclusively associated with the most severe type of allergic reaction, known as anaphylaxis. Mild or localized allergic responses, such as hay fever symptoms or a small patch of hives, do not have a known causal link to seizure activity. The systemic nature of anaphylaxis is the condition that creates the risk for a neurological event.
Anaphylaxis is commonly triggered by substances like certain foods, insect venom, or specific medications. The seizure is not a direct response to the allergen itself, but rather a symptom of the systemic disruption occurring in the body during the severe allergic response. This widespread physiological disruption causes the conditions necessary for a neurological event to occur.
Physiological Causes of Neurological Events
The seizure in anaphylaxis is primarily driven by insufficient blood and oxygen delivery to the brain. During a severe allergic reaction, the body releases inflammatory mediators, such as histamine, which cause widespread vasodilation (widening of blood vessels). This expansion of the vascular system leads to a significant drop in blood pressure, known as hypotension.
The resulting hypotension causes insufficient blood flow to the brain, a state called cerebral hypoperfusion. Simultaneously, the inflammatory cascade can lead to swelling and constriction of the airways, which severely limits the body’s ability to take in oxygen, resulting in hypoxia. The combination of cerebral hypoperfusion and hypoxia deprives the brain’s neurons of the oxygen and energy they need to function, which is a potent trigger for a seizure.
Inflammatory mediators may also play a direct role. Pro-inflammatory substances, including cytokines and mast cell products, are released during anaphylaxis. Evidence suggests these mediators can cross the blood-brain barrier, increasing neuronal excitability and contributing to seizure development.
Immediate Medical Response
If a seizure occurs during an allergic reaction, the immediate priority is treating the underlying anaphylaxis, which addresses the root cause. The first action is the administration of an epinephrine auto-injector. Epinephrine works rapidly to constrict blood vessels, increasing blood pressure, and to relax airway muscles, improving breathing and oxygen delivery.
After epinephrine administration, emergency medical services should be called immediately (911 or local equivalent). The patient must be positioned safely during the seizure, protecting them from injury and turning them onto their side if possible to prevent choking. Do not attempt to restrain the person or place anything in their mouth.
The patient requires immediate transport to an emergency room, even if the seizure stops or symptoms improve after the epinephrine injection. Medical professionals will stabilize and monitor the patient, typically for several hours, to prevent a potential second wave of the reaction. Follow-up with specialists is necessary, including an allergist to manage the underlying allergy and possibly a neurologist to determine if the seizure was purely situational.