What are the different types of seizures and their classifications?

Seizure Classification

Seizures are fundamentally classified by their origin in the brain into three primary categories: focal onset (arising from one hemisphere), generalized onset (engaging both hemispheres from the start), and unknown onset. 1, 2

Primary Classification Framework

The International League Against Epilepsy (ILAE) 2017 classification represents the current standard for categorizing seizures based on their origin and clinical manifestations, not on underlying mechanisms which remain incompletely understood. 1, 3

Three Main Categories by Onset

• Focal onset seizures arise within networks of a single cerebral hemisphere and may remain localized or become more widely distributed throughout that hemisphere. 1, 2

• Generalized onset seizures rapidly affect both hemispheres and both sides of the body from the very beginning of the seizure. 1, 2

• Unknown onset seizures are those where the beginning of the seizure was not witnessed or observed, making classification of origin impossible initially. 3, 4

Focal Onset Seizures - Detailed Classification

Focal seizures can be further subclassified based on two key features: level of awareness and motor versus non-motor manifestations. 3, 5

Classification by Awareness

• Focal aware seizures occur when consciousness and awareness remain intact throughout the entire seizure episode. 3, 5

• Focal impaired awareness seizures involve any degree of impaired awareness during any segment of the seizure (previously called complex partial seizures). 3, 5

Motor Manifestations of Focal Seizures

• Automatisms - repetitive, purposeless movements. 3, 5
• Atonic - loss of muscle tone. 3, 5
• Clonic - rhythmic jerking movements. 3, 5
• Epileptic spasms - sudden flexion or extension movements. 3, 5
• Hyperkinetic - excessive, often thrashing movements. 3, 5
• Myoclonic - brief, shock-like jerks. 3, 5
• Tonic - sustained muscle stiffening. 3, 5

Non-Motor Manifestations of Focal Seizures

• Autonomic - changes in heart rate, blood pressure, sweating, or other autonomic functions. 3, 5
• Behavior arrest - cessation of ongoing activity and freezing. 3, 5
• Cognitive - alterations in language, memory, or other cognitive functions. 3, 5
• Emotional - fear, anxiety, or other emotional experiences. 3, 5
• Sensory - visual, auditory, olfactory, gustatory, or somatosensory symptoms. 3, 5

Focal to Bilateral Tonic-Clonic

• Focal seizures can evolve and spread to engage both hemispheres, resulting in a bilateral tonic-clonic seizure (previously termed "secondary generalized"). 3, 5

Generalized Onset Seizures - Detailed Classification

Generalized seizures engage bilateral brain networks from onset and are divided into motor and non-motor types. 3, 4

Generalized Motor Seizures

• Tonic-clonic (grand mal) - stiffening followed by rhythmic jerking affecting both sides of the body. 3, 5
• Clonic - rhythmic jerking without initial stiffening phase. 3, 5
• Tonic - sustained muscle stiffening without clonic phase. 3, 5
• Myoclonic - brief, shock-like jerks affecting both sides. 3, 5
• Myoclonic-tonic-clonic - myoclonic jerks followed by tonic-clonic activity. 3, 5
• Myoclonic-atonic - myoclonic jerks followed by loss of muscle tone. 3, 5
• Atonic - sudden loss of muscle tone causing falls (drop attacks). 3, 5
• Epileptic spasms - sudden flexion or extension movements affecting both sides. 3, 5

Generalized Non-Motor Seizures (Absence Seizures)

• Typical absence (petit mal) - brief staring spells with abrupt onset and offset, usually lasting seconds. 3, 5
• Atypical absence - similar to typical but with more gradual onset/offset and often longer duration. 3, 5
• Myoclonic absence - absence seizures with prominent myoclonic jerking. 3, 5
• Absence with eyelid myoclonia - brief absences accompanied by rapid eyelid fluttering. 3, 5

Unknown Onset Seizures

When the beginning of a seizure is not observed, it can still be classified by observable features. 4, 5

• Motor - any motor manifestations without known onset. 4, 5
• Non-motor - behavioral or sensory changes without known onset. 4, 5
• Tonic-clonic - generalized convulsion where onset was not witnessed. 4, 5
• Epileptic spasms - spasms where onset classification is unclear. 4, 5
• Behavior arrest - cessation of activity where onset is unknown. 4, 5

Clinical Context: Provoked vs Unprovoked Seizures

Beyond the seizure type classification, seizures are categorized by their relationship to precipitating factors. 6

Provoked (Acute Symptomatic) Seizures

• Occur at the time of or within 7 days of an acute neurologic, systemic, metabolic, or toxic insult. 6
• Examples include seizures from hyponatremia, other electrolyte abnormalities, alcohol or drug withdrawal, toxic ingestions, encephalitis, or acute CNS mass lesions. 6

Unprovoked Seizures

• Occur without acute precipitating factors, more than 7 days after any insult. 6
• Include idiopathic seizures and remote symptomatic seizures (from prior stroke, traumatic brain injury, or other past events). 6
• Recurrent unprovoked seizures define epilepsy. 6

Common Pitfalls in Classification

• Absence seizures do not respond to carbamazepine - this is a critical distinction as carbamazepine is effective for focal and generalized tonic-clonic seizures but not absence seizures. 7

• The term "generalized" implies diffuse bilateral involvement from onset - this differs fundamentally from focal seizures with secondary generalization (now called focal to bilateral tonic-clonic), which start focally then propagate. 1

• Many generalized seizures have genetic underpinnings - neuroimaging is rarely indicated in neurologically normal patients with generalized seizures, as MRI and CT have low yield given the genetic basis. 1

• Correct seizure classification is crucial - it directly impacts treatment selection, prognosis, and diagnosis, making accurate identification essential before initiating therapy. 1, 2