Hyperventilation is rapid, deep breathing that causes the body to expel an excessive amount of carbon dioxide (\(\text{CO}_2\)). A seizure is a transient event resulting from abnormal electrical activity in the brain. Hyperventilation can cause seizures, but only under specific circumstances, usually in individuals who have an underlying susceptibility. Understanding the physiological cascade linking altered breathing to neuronal activity explains this phenomenon.
The Physiological Mechanism of Hyperventilation
Over-breathing quickly decreases the level of carbon dioxide (\(\text{CO}_2\)) in the blood, a condition known as hypocapnia. This decrease causes a subsequent rise in the blood’s pH level, leading to respiratory alkalosis. The brain’s blood vessels are highly sensitive to these chemical changes, and this alkaline shift causes the cerebral arterioles to narrow, a process called cerebral vasoconstriction.
This narrowing significantly reduces the flow of blood to the brain tissue. A decrease of just \(1 \text{ mm Hg}\) in the arterial partial pressure of \(\text{CO}_2\) can result in a \(2.5\%\) to \(4\%\) reduction in cerebral blood flow. This reduction decreases the delivery of oxygen and glucose, the brain’s primary energy sources, to its neurons.
The combination of reduced blood flow and altered pH levels destabilizes nerve cell membranes. This physiological stress increases neuronal excitability and lowers the seizure threshold, making the brain more prone to abnormal electrical discharges. In a susceptible individual, the vascular and chemical changes initiated by hyperventilation can trigger a seizure.
Hyperventilation as a Diagnostic Tool
Neurologists leverage this physiological effect to aid in the diagnosis of epilepsy during an electroencephalogram (EEG). Hyperventilation is a standard activation technique used to provoke subclinical or latent seizure activity that might not appear during a resting EEG. Patients are asked to breathe deeply and quickly for three to five minutes while their brain activity is monitored.
This controlled provocation is most effective in identifying generalized epilepsy syndromes, particularly absence seizures. In children with absence epilepsy, hyperventilation can trigger characteristic spike-wave discharges in over \(90\%\) of cases. Although the procedure is designed to activate latent brain activity, the risk of inducing a prolonged or severe convulsive seizure is low. Clinicians manage any event, ensuring the diagnostic benefits of capturing electrical patterns outweigh the minimal risks.
Susceptibility and Risk Factors
A seizure triggered by hyperventilation is rare in a healthy individual with no history of neurological disorder. The phenomenon primarily affects people who already have a pre-existing or latent epileptic condition. Those with a genetic predisposition to certain types of epilepsy are vulnerable to this trigger.
Individuals with genetic generalized epilepsies are the most susceptible, as hyperventilation can trigger clinical seizures in up to \(50\%\) of patients. The technique can also activate epileptiform activity in some individuals with focal epilepsies, such as those originating in the temporal lobe. Spontaneous hyperventilation outside of a clinical setting is often triggered by emotional states, such as anxiety or a panic attack.
Distinguishing Seizures from Other Events
Hyperventilation frequently causes symptoms that may be mistaken for a seizure but are non-epileptic events. Physiological changes from over-breathing can cause lightheadedness, dizziness, and paresthesia (a tingling sensation) in the extremities or around the mouth. Reduced cerebral blood flow can also lead to syncope, a temporary loss of consciousness commonly called fainting.
Some individuals experience psychogenic non-epileptic seizures (PNES), which are paroxysmal events resembling epileptic seizures but are not caused by abnormal electrical discharges. The gold standard for differentiation is an EEG, which shows the characteristic electrical storm of a true seizure but remains normal during a PNES event. PNES are often related to psychological distress and may involve rhythmic movements or twitching that appear less organized than a typical epileptic event.