Seizure classification answers one practical question: what network is firing, and how widely. The answer drives drug choice and prognosis. The framework taught here is the International League Against Epilepsy (ILAE) 2017 operational classification, used as a stable working vocabulary: it classifies seizures by onset (focal, generalized, or unknown), by awareness (aware versus impaired awareness, for focal seizures), and by motor versus non-motor features. A 2025 ILAE seizure-classification update exists and is not taught here. Older terms such as simple partial and complex partial map onto focal aware and focal impaired awareness; both vocabularies still appear in clinical practice and older texts.
Focal seizures
Focal seizures arise from networks limited to one hemisphere. They are the most common type, roughly 60% of seizures. A focal aware seizure does not impair consciousness. The person stays alert but may show clonic jerking in one region, a sensory march such as tingling spreading across a limb, autonomic symptoms such as rising epigastric discomfort, or visual and auditory phenomena. Because the discharge stays restricted, the symptom maps the firing cortex.
A focal impaired awareness seizure impairs consciousness, often with a vague, confused, disengaged appearance. These arise most often in the temporal lobe. Typical features include deja vu, a rising epigastric sensation, fear or dreaminess, automatisms (repetitive involuntary movements) such as lip-smacking or chewing, and language disturbance when the dominant hemisphere is involved. An aura is the subjective beginning of such a seizure: a private sensation, felt by the patient and invisible to observers, that localises the onset zone. A focal seizure can spread to both hemispheres, becoming a focal to bilateral tonic-clonic seizure (formerly called secondarily generalized).
Generalized seizures
Generalized seizures rapidly engage bilaterally distributed networks from onset and account for roughly a third of seizures. In a generalized tonic-clonic seizure, consciousness is lost suddenly and the patient falls. In the tonic phase (sustained muscle stiffening) agonist and antagonist muscles are held rigid for some 20 to 30 seconds: air forced through constricted airways produces a cry, and incontinence may follow. The clonic phase (rhythmic jerking) follows as fatiguing neurons alternate contraction with relaxation. The whole episode usually lasts 1 to 2 minutes, and respiratory arrest during the tonic phase makes hypoxia (too little oxygen) the central concern.
An absence seizure is mostly a childhood event, roughly 5% of seizures. It is a sudden brief pause of awareness lasting around 15 seconds: unresponsive staring, sometimes with eyelid fluttering, and normal behaviour between episodes. Its characteristic signature on the electroencephalogram (EEG, the scalp recording of brain electrical activity) is a rhythmic spike-and-wave pattern at about 3 Hz.
Other generalized types are named by their motor feature: myoclonic (brief shock-like jerks), tonic (stiffening without clonus), clonic (jerking without a tonic phase), and atonic (sudden tone loss with drop attacks). Epileptic spasms, the flexor or extensor contractions seen mainly in infants, sit under unknown onset.
Etiology
Etiology means cause. The ILAE framework groups causes into six categories, which are not hierarchical: one epilepsy can sit in more than one of them, and the label used depends on the clinical question.
- Genetic: known or presumed mutations, usually in ion channel genes. Most show multifactorial inheritance with age-dependent expression, and only a minority follow Mendelian inheritance.
- Structural: visible CNS lesions such as perinatal injury, cortical malformations, hippocampal sclerosis, tumours, trauma, and cerebrovascular disease. A subtle structural lesion needs epilepsy-protocol MRI.
- Metabolic: disturbed brain biochemistry that lowers the threshold.
- Immune: autoantibodies against neuronal antigens.
- Infectious: including meningitis and encephalitis.
- Unknown: no cause found despite investigation.
Older notes saying symptomatic mean structural, and cryptogenic means unknown; these mappings are vocabulary notes, not current claims. How mutations, lesions, and biochemical disturbances each make cortical neurons fire abnormally is a question of mechanism.