A seizure is an abnormal surge of electrical activity in the brain that causes involuntary changes in body movement, function, sensation, awareness, or behavior. Seizures have a focal starting point in the brain where they originate. Determining the seizure onset zone allows doctors to identify the brain region responsible for generating seizures. This information guides treatment to stop seizures through surgery or other interventions.
What causes a seizure?
Seizures are caused by abnormal electrical discharges in the brain. Different factors can disrupt the brain’s normal electrical signaling and trigger a seizure, including:
- Head trauma or injury
- Brain tumors
- Stroke
- Drug or alcohol withdrawal
- Infections like meningitis or encephalitis
- Genetic conditions like epilepsy
- Metabolic abnormalities like low blood sugar
In many cases, the specific cause of seizures may not be identified. However, determining where in the brain the abnormal activity begins provides important clues about what triggers a person’s seizures.
How do doctors locate the seizure onset zone?
Doctors use various diagnostic tests to pinpoint the region of the brain where a person’s seizures originate:
- Medical history – Questions about seizure symptoms, frequency, triggers, and family history of epilepsy can provide initial insight.
- Neurological exam – Assessing functions like reflexes, balance, coordination and mental status.
- Blood tests – Helps rule out metabolic causes like low blood sugar or electrolyte imbalances.
- Neuroimaging – CT scans, MRIs and PET scans reveal brain structure, anatomy, lesions or tumors.
- EEG – Records electrical activity in the brain and can detect abnormal discharges.
Combining results from these tests allows doctors to pinpoint the site of abnormal activity driving seizure activity.
Focal onset seizures
Seizures beginning in one area or hemisphere of the brain are called focal onset or partial onset seizures. These account for around 60% of all seizures. Focal seizures arise from specific regions like:
Frontal Lobe Seizures
- Origin: frontal lobes (behind the forehead)
- Symptoms: muscle jerking, head and eye deviation, unusual smells or tastes
Temporal Lobe Seizures
- Origin: temporal lobes (above the ears)
- Symptoms: sensory disturbances, déjà vu, nausea, emotional changes
Parietal Lobe Seizures
- Origin: parietal lobes (top section of the brain)
- Symptoms: sensations of numbness, tingling, or pain
Occipital Lobe Seizures
- Origin: occipital lobes (back of the brain)
- Symptoms: visual hallucinations or illusions
Doctors can identify the lobe or hemisphere where seizures start based on a person’s symptoms and EEG results.
Generalized onset seizures
Generalized seizures rapidly engage neurons in both hemispheres of the brain. They account for about 40% of all seizures and include:
Absence seizures
- Brief loss of consciousness or awareness lasting up to 30 seconds
- Staring spell with mild twitching
- More common in children
Tonic seizures
- Stiffening of the muscles
- Lasts from 10 seconds up to one minute
Atonic seizures
- Loss of muscle control causing collapse or fall
- Lasts less than 15 seconds
Tonic-clonic seizures
- Also called grand mal seizures
- Involves stiffening followed by violent muscle contractions
- Loss of consciousness and confusion afterwards
Determining whether seizures have a focal or generalized onset guides treatment options.
Identifying the seizure focus
In up to 70% of epilepsy cases, testing can identify an epileptogenic lesion, scar, tumor or malformation that is triggering seizure activity. This abnormal area driving seizures is called the epileptogenic zone or seizure focus.
Pinpointing the seizure focus guides surgical treatment to remove this area when possible.
Tests to identify the seizure focus include:
- High-resolution MRI to detect subtle lesions
- PET scans visualizing brain metabolism
- SPECT scans revealing blood flow changes
- Magnetoencephalography (MEG) mapping electrical activity
- Wada testing to see if specific brain regions can be safely removed
- Simultaneous EEG and functional MRI to link seizure activity to specific regions
Identifying and removing the seizure focus provides the best chance of seizure freedom when surgery is an option.
When the seizure onset zone is unknown
In about 30% of epilepsy cases, testing does not reveal an obvious lesion, tumor or malformation that is triggering seizures. The inability to pinpoint where seizures start is more common in these situations:
- Childhood epilepsy syndromes like infantile spasms
- Genetic generalized epilepsy
- Scarring or subtle developmental abnormalities
- Normal MRI and EEG results
When imaging, EEG and other test results are inconclusive, doctors may do prolonged EEG monitoring to record typical seizures. This can provide more information about seizure type, duration, frequency and onset zone.
If the seizure focus remains unclear, treatment focuses on seizure control through medication, diet, devices or surgery when possible. Identifying where seizures originate remains essential for guiding treatment.
Conclusion
Determining the seizure onset zone is key for understanding the type and cause of seizures. Focal onset seizures arise from specific brain regions, while generalized seizures rapidly engage both hemispheres. Up to 70% of epilepsy cases have an identifiable seizure focus that can be removed by surgery when possible. Even when the exact origin is unknown, locating the seizure onset zone provides critical information to guide treatment and improve seizure control.
