Focal onset seizures (also known as focal seizures) start with abnormal electrical activity in one specific part of the brain, usually limited to one hemisphere. Because they originate in a localized area, the initial symptoms depend on the function of that part of the brain where the seizure begins. After starting in one part of the brain, what the seizure does next can be unpredictable:
- The seizure activity may remain localized to that part of the brain, causing symptoms related to that area only.
- Alternatively, the seizure activity can spread from the initial focal area to other parts of the brain, potentially involving the other hemisphere.
- In some cases, a focal seizure can evolve into a generalized seizure, involving both sides of the brain, leading to convulsions affecting the whole body. This is referred to as a "focal to bilateral tonic-clonic seizure."
Symptoms during a focal seizure vary widely and are often linked to the brain area affected. These can include motor symptoms (such as twitching or jerking in specific muscles), sensory changes (such as tingling or visual disturbances), emotional or cognitive changes, or changes in consciousness. Thus, focal seizures start in a specific brain part, but their progression after that can range from staying localized to spreading widely in the brain, making their effects unpredictable.
