Types of Seizures
According to the International League Against Epilepsy (ILAE) 2017 classification, seizures are fundamentally divided into three categories: focal onset, generalized onset, and unknown onset seizures. 1, 2
Focal Seizures
Focal seizures arise within networks of a single cerebral hemisphere and may remain localized or subsequently become more widely distributed 1. They can be further characterized by:
Level of Awareness
- Focal aware seizures (consciousness preserved)
- Focal impaired awareness seizures (consciousness affected)
Onset Manifestations
Motor onset
- Automatisms
- Atonic (sudden loss of muscle tone)
- Clonic (rhythmic jerking)
- Epileptic spasms
- Hyperkinetic (excessive movement)
- Myoclonic (brief muscle jerks)
- Tonic (sustained muscle contraction)
Non-motor onset
- Autonomic (changes in heart rate, blood pressure, etc.)
- Behavior arrest
- Cognitive (language disturbance, thinking problems)
- Emotional (fear, anxiety, etc.)
- Sensory (changes in vision, hearing, taste, smell, etc.)
Focal to bilateral tonic-clonic (previously called "secondarily generalized") 2
Generalized Seizures
Generalized seizures rapidly affect both hemispheres as well as subcortical structures including the thalamus and upper brainstem 1, 3. They are divided into:
Motor
- Tonic-clonic (most common type, accounting for 23% of all seizures) 4
- Clonic
- Tonic
- Myoclonic
- Myoclonic-tonic-clonic
- Myoclonic-atonic
- Atonic
- Epileptic spasms
Non-motor/Absence
- Typical absence
- Atypical absence
- Myoclonic absence
- Eyelid myoclonia 2
Unknown Onset Seizures
When there is insufficient information to categorize a seizure as either focal or generalized, it is classified as unknown onset. These may later be reclassified when more information becomes available.
Important Clinical Considerations
Misdiagnosis risk: Focal and generalized seizures can sometimes be confused. Typical absence seizures may be misinterpreted as focal seizures, especially temporal lobe seizures, and myoclonic seizures can be mistaken for focal clonic seizures 5.
Medication selection: Treatment selection depends on the specific seizure type. For example:
- Carbamazepine is effective for partial seizures and generalized tonic-clonic seizures but not for absence seizures 6
- Levetiracetam has become a first-line treatment for focal seizures due to its favorable side effect profile 3, 7
- Valproate, lamotrigine, and topiramate are effective for generalized seizures 3, 8
Diagnostic approach: MRI is superior to CT for identifying epileptogenic lesions, especially for focal seizures 3. EEG monitoring is essential for detecting epileptic activity and distinguishing epileptic from non-epileptic events 3.
Multiple seizure types: About 40% of patients with epilepsy experience multiple seizure types rather than a single type 4, making accurate classification crucial for proper treatment.
Age considerations: The distribution of seizure types varies with age, which should be considered in diagnosis and treatment planning 4.
Remember that proper classification of seizures is critical for determining appropriate treatment, as different seizure types respond to different medications, and some medications that work for one type may worsen another type.