SEIZURE PATHOPHYSIOLOGY
A seizure is a transient episode of abnormal neurological function caused by excessive, synchronous electrical activity within the brain. Rather than representing a single disease, seizures reflect a failure of normal mechanisms that regulate neuronal excitability and network stability. These events may be focal or generalised and can arise from a wide range of structural, metabolic or functional disturbances within the nervous system.
Understanding seizure pathophysiology requires an appreciation of how neurons normally generate and control electrical signals. Seizures occur when the balance between excitation and inhibition is disrupted, allowing uncontrolled neuronal firing to propagate through neural networks.
What You Need to Know
Normal brain function relies on tightly regulated electrical signalling between neurons. Neuronal communication occurs through action potentials that are generated and propagated by ion channels and shaped by synaptic neurotransmitters. Under normal conditions, excitatory signals, largely mediated by glutamate, are balanced by inhibitory signals, primarily mediated by gamma-aminobutyric acid (GABA). This balance ensures that neuronal firing remains controlled and prevents excessive or sustained electrical activity.
Seizures occur when this balance between excitation and inhibition is disrupted. Neurons become abnormally excitable due to increased excitatory input, reduced inhibitory control, or changes in membrane stability that lower the threshold for firing. Once a critical number of neurons begin firing in an excessive and synchronous manner, normal regulatory mechanisms are overwhelmed and a seizure is initiated.
At a broad level, seizure generation involves several overlapping mechanisms:
Excessive excitatory neurotransmission or reduced inhibitory neurotransmission
Altered ion channel function that increases neuronal firing
Synchronous firing of neuronal networks that allows abnormal activity to spread
After initiation, abnormal electrical activity may remain localised to a specific region of the brain or propagate through neural pathways to involve both hemispheres. The pattern and extent of this spread determine the clinical features of a seizure, including changes in consciousness, motor activity, sensation, or behaviour.
Beyond the Basics
Neuronal excitability and membrane instability
Neuronal firing depends on the regulated movement of ions across the cell membrane. Voltage-gated sodium, potassium, and calcium channels, which control how electrical charge enters and leaves the neuron, determine how easily an action potential is generated and propagated. In seizure-prone tissue, changes in ion channel function lower the threshold for depolarisation, meaning neurons require less stimulus to become electrically active.
When membrane stability is reduced, neurons become hyperexcitable, meaning they fire more easily and more often than normal. Instead of responding briefly and then returning to a resting state, these neurons may fire repeatedly or in bursts. This abnormal responsiveness allows stimuli that would normally be insufficient, such as routine sensory input, to trigger sustained electrical activity.
Excitatory and inhibitory neurotransmitter imbalance
Normal cortical activity relies on a balance between excitatory and inhibitory neurotransmission. Glutamate, the primary excitatory neurotransmitter, increases the likelihood that neurons will fire, while gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter, suppresses neuronal firing and stabilises neural networks.
Seizures may develop when excitation outweighs inhibition. This can occur through increased glutamate release, impaired glutamate reuptake, or reduced GABA-mediated inhibition, meaning the brain’s normal braking system is weakened. When inhibitory control is lost, excitatory signals dominate, allowing neurons to fire continuously and in a coordinated manner rather than in isolated, controlled bursts.
Paroxysmal depolarisation shifts and synchronisation
At the cellular level, seizures are associated with paroxysmal depolarisation shifts, which are sudden, prolonged depolarisations that trigger bursts of action potentials instead of a single electrical impulse. These shifts reflect a failure of normal repolarisation, where the neuron is unable to reset its electrical state after firing.
When paroxysmal depolarisation shifts occur in clusters of neighbouring neurons, synchronisation becomes more likely. Local circuits begin firing together rather than independently, amplifying the abnormal signal. As additional neurons are recruited, coordinated firing spreads, allowing seizure activity to extend beyond its initial focus.
Propagation of seizure activity
The spread of seizure activity depends on anatomical connectivity and network organisation within the brain. In focal seizures, abnormal electrical activity remains confined to a specific cortical region, producing symptoms related to the function of that area, such as motor, sensory, or language disturbance.
In generalised seizures, abnormal activity rapidly involves both cerebral hemispheres, often through thalamocortical circuits, which act as relay pathways connecting deep brain structures to the cortex. Structural abnormalities such as scarring, tumours, malformations, or areas of prior injury can act as epileptogenic foci, meaning regions where seizures are more likely to originate and spread.
Metabolic and systemic contributors
Neuronal excitability is highly sensitive to metabolic conditions. Hypoxia, hypoglycaemia, electrolyte disturbances, and acid–base imbalance can all lower seizure threshold by impairing energy-dependent ion pumps, which normally maintain stable electrical gradients across the neuron membrane.
Systemic illness, infection, or withdrawal from central nervous system depressants can further destabilise neuronal networks. In these situations, seizures may occur even in individuals without epilepsy, because the brain’s normal mechanisms for controlling electrical activity are temporarily compromised.
Postictal suppression and neural exhaustion
Following a seizure, neurons often enter a period of reduced activity known as the postictal state. This phase reflects temporary depletion of excitatory neurotransmitters, accumulation of inhibitory mediators, and activation of protective mechanisms that suppress further firing while the brain recovers.
The postictal state corresponds to confusion, drowsiness, headache, or focal neurological deficits such as weakness or speech disturbance. These features gradually resolve as neurotransmitter balance is restored and neurons regain their normal electrical stability.
Clinical Connections
Seizures present with a wide range of clinical features, including motor activity, sensory disturbance, altered awareness, or behavioural change, depending on which brain regions are involved. Motor manifestations such as tonic or clonic movements arise from involvement of motor cortex, while sensory symptoms reflect activation of somatosensory regions. Altered consciousness or automatisms occur when seizure activity disrupts networks responsible for awareness, memory, or behaviour. This variability means that seizures do not follow a single pattern and may be subtle or dramatic depending on the underlying neuroanatomy.
The diversity of presentation is explained by differences in seizure onset and spread.
Key clinical patterns include:
Focal seizures, where abnormal electrical activity originates in a specific cortical region and produces localised symptoms
Generalised seizures, where activity rapidly involves both hemispheres, leading to widespread motor and consciousness changes
Focal seizures with impaired awareness, where spread to networks involved in consciousness alters responsiveness and behaviour
Electroencephalography supports diagnosis by demonstrating abnormal synchronous firing patterns, either during a seizure or as interictal epileptiform discharges between events. EEG findings also assist in localising seizure onset and classifying seizure type, which guides further investigation and treatment planning. Identifying underlying causes is essential, as seizures may be provoked by structural brain lesions, metabolic disturbances such as hypoglycaemia or electrolyte imbalance, infection, or acute brain injury. Addressing reversible contributors and defining the seizure mechanism reduces recurrence risk and informs long-term management.
Concept Check
Why does loss of inhibitory control increase seizure risk?
How do ion channel abnormalities contribute to neuronal hyperexcitability?
What role does glutamate play in seizure propagation?
Why can metabolic disturbances precipitate seizures?
What physiological mechanisms contribute to the postictal state?