A seizure that starts on one side of the brain is called a focal seizure. Previously known as a “partial seizure,” this is the most common seizure type, accounting for roughly 60% of adult-onset epilepsy. Focal seizures are classified based on where exactly they begin in the brain and whether you remain aware during the episode.
How Focal Seizures Are Classified
The International League Against Epilepsy defines a focal seizure as one that originates in a single hemisphere of the brain, as opposed to a generalized seizure, which appears to start in both hemispheres simultaneously. Within the focal category, there are two main types based on your level of consciousness during the event.
Focal aware seizures (formerly called simple partial seizures) do not cause any loss of awareness. You can sometimes talk during one and remember it afterward. These seizures may produce a sudden feeling of déjà vu, an unusual sensation in the stomach, or brief abnormal movements on one side of the body.
Focal impaired awareness seizures (formerly called complex partial seizures) do cause a loss of awareness. During one of these episodes, you might appear confused or dazed, pick at your clothes, smack your lips, or be unable to respond to questions or follow directions for several minutes. You typically won’t remember the seizure once it ends.
How Symptoms Depend on Brain Location
Because focal seizures originate in a specific area, the symptoms you experience directly reflect what that part of the brain normally does. This is one of the key features that distinguishes focal seizures from generalized ones, where symptoms tend to affect the whole body at once.
Seizures starting in the temporal lobe, which is involved in memory and emotion, often produce feelings of unreality (derealization), vivid memory flashbacks, or intense emotional surges. These are among the most common focal seizures in adults. Seizures originating in the frontal lobe, responsible for movement and planning, can cause “forced thinking” (an intrusive, repetitive thought you can’t control), unusual postures, or jerking movements in one arm or leg. Parietal lobe seizures may produce strange sensory experiences like tingling, numbness, or a distorted sense of your own body. Occipital lobe seizures, at the back of the brain, typically cause visual disturbances like flashing lights or partial vision loss on one side.
When a Focal Seizure Spreads
A focal seizure doesn’t always stay on one side of the brain. It can spread through wider brain networks and develop into a full-body convulsive event called a focal to bilateral tonic-clonic seizure (previously known as a “secondary generalized seizure”). When this happens, you lose consciousness and experience stiffening followed by rhythmic jerking on both sides of the body.
Sometimes the spread happens so quickly that no preceding focal symptoms are noticed at all. This can make it look like a generalized seizure from the start, which is one reason doctors rely on brain-wave testing rather than eyewitness descriptions alone to determine where seizures actually begin.
How Focal Seizures Are Identified
An electroencephalogram (EEG), which records electrical activity across the scalp, is the primary tool for confirming that a seizure starts on one side. Focal seizures produce characteristic patterns: rhythmic wave activity or repetitive electrical spikes that evolve in speed and intensity, concentrated over one region of the brain.
The specific EEG pattern also helps pinpoint the location. Rhythmic slow-wave activity at the start of a seizure is seen almost exclusively in temporal lobe seizures, while rapid repetitive spikes are far more common in seizures originating on the outer surface of the frontal lobe. Seizures from deeper brain structures tend to show slower rhythmic patterns, while those from the brain’s outer surface show sharper, more distinct spikes. These differences matter because knowing the exact starting point shapes treatment decisions, especially if surgery becomes an option.
What to Do If Someone Has a Focal Seizure
Focal aware seizures rarely require emergency action since the person remains conscious throughout. They may just need a moment to collect themselves afterward. Focal impaired awareness seizures call for more attention because the person is not fully conscious and may wander or make repetitive movements without realizing it.
If you’re with someone having a seizure of any kind, stay calm and stay with them. Move anything nearby that could cause injury. If they’re lying down, gently roll them onto their side with their mouth pointing toward the ground to keep the airway clear. Time the seizure. If it lasts longer than five minutes, call 911.
A few things to avoid: don’t hold the person down or restrain their movements, don’t put anything in their mouth, and don’t offer food or water until they’re fully alert. Once the seizure ends and they’re conscious, help them sit somewhere safe, explain what happened, and offer to contact someone who can help them get home.
You should also call for emergency help if a second seizure follows closely after the first, if the person has trouble breathing or waking up, if they’re injured during the episode, or if it’s their first seizure ever.