A focal onset seizure is a seizure that starts in one specific area on one side of the brain, rather than across the entire brain at once. These are the most common seizure type, accounting for roughly 60% of adult-onset epilepsy. What a focal seizure looks and feels like depends heavily on where in the brain the abnormal electrical activity begins, which is why two people with focal epilepsy can have very different experiences.
Aware vs. Impaired Awareness
Focal onset seizures fall into two main categories based on whether you remain conscious during the event. In a focal aware seizure, you stay alert and know what’s happening around you, even though something unusual is clearly going on in your body or perception. You might feel a wave of déjà vu, notice a strange taste or smell, get a rising sensation in your stomach, or have involuntary twitching on one side of your body. These episodes were previously called “simple partial seizures,” and many people experience them as an aura, a brief warning that something is off.
In a focal impaired awareness seizure (formerly called a “complex partial seizure”), consciousness is disrupted at some point during the episode. You may stare blankly, appear dazed, or be unable to respond to questions or directions for several minutes. Repetitive, automatic movements are common: lip smacking, chewing, swallowing, or picking at clothing. Afterward, you typically have no memory of what happened and may feel confused or very sleepy for a period of time.
How Symptoms Vary by Brain Region
The brain has four lobes on each side, and the location where a focal seizure originates shapes the symptoms you experience. The temporal lobe, which processes memory, sound, and emotion, is the most common starting point. Temporal lobe seizures often produce intense feelings of fear or joy, déjà vu, odd smells or tastes, and that roller-coaster sensation in the stomach. When awareness is impaired, lip smacking, repeated swallowing, and picking motions with the fingers are characteristic.
Seizures starting in the frontal lobe, which governs voluntary movement and decision-making, tend to produce jerking or stiffening on one side of the body, head turning, or unusual postures. These can happen during sleep and are often brief but dramatic. Parietal lobe seizures may cause tingling, numbness, or a distorted sense of body position, while occipital lobe seizures typically involve visual disturbances like flashing lights, blind spots, or patterns in your field of vision.
Recognizing which symptoms appear first can help pinpoint the seizure’s origin, which becomes critical if medication doesn’t control the seizures and surgery is being considered.
Spreading to the Whole Brain
A focal seizure doesn’t always stay focal. Sometimes the abnormal electrical activity spreads from its starting point to both sides of the brain, producing a full-body convulsive seizure with stiffening and rhythmic jerking. This is called a focal to bilateral tonic-clonic seizure (older terminology: “secondary generalization”).
The thalamus, a deep brain structure that relays signals between different brain regions, plays a key role in this spread. It acts as a kind of relay station that can synchronize abnormal electrical discharges across both hemispheres. Other deep structures, including the basal ganglia, may actually work to suppress this spread in some cases. Whether a focal seizure generalizes depends partly on the integrity of the nerve fiber pathways connecting brain regions. This is one reason why the same person may have some seizures that stay focal and others that spread.
When a focal seizure does generalize, it looks identical to a seizure that started across the whole brain from the beginning. The distinction matters for treatment, because the underlying cause and the best medication choices can differ.
Common Causes
Focal seizures happen because something in a specific brain area creates abnormal electrical activity. The list of possible causes is long, but some stand out. Structural abnormalities present from birth, such as focal cortical dysplasia (a patch of brain tissue that didn’t develop normally), are a leading cause, particularly in younger patients. These abnormalities can sometimes be subtle enough to escape detection on a standard MRI, requiring advanced imaging techniques like PET or SPECT scans to identify.
Other common causes include scarring in the temporal lobe (often from a previous injury or infection), brain tumors, stroke, traumatic brain injury, and infections that affect the brain. In many cases, especially in adults, no clear structural cause is found, and the epilepsy is attributed to genetic factors or remains unexplained.
How Focal Seizures Are Diagnosed
Diagnosis starts with a detailed description of what happens before, during, and after each episode. Because many people with impaired awareness seizures can’t recall their own symptoms, accounts from family members or witnesses are extremely valuable.
An electroencephalogram (EEG) records electrical activity across the brain and is the primary tool for confirming focal epilepsy. Between seizures, the EEG may show localized spikes or sharp waves in the region where seizures originate. During a seizure, distinctive patterns of rhythmic activity appear, and their location helps confirm the diagnosis. Different underlying causes produce recognizably different EEG signatures, which can guide further testing.
MRI is used to look for structural causes. Some abnormalities like focal cortical dysplasia can be difficult to detect, especially in young children whose brains haven’t fully matured. In those cases, imaging may need to be repeated later, or specialized scans that measure brain metabolism or blood flow may be needed.
Treatment Options
Anti-seizure medications are the first line of treatment. Several medications have strong evidence for controlling focal onset seizures, and modern options tend to have fewer side effects than older drugs. Your doctor will typically start with a single medication and adjust based on seizure control and how well you tolerate it. If the first medication doesn’t work, switching to a different one or combining medications is the next step.
For people whose seizures don’t respond to medication (roughly one-third of people with focal epilepsy), surgery can be highly effective, particularly when imaging and EEG pinpoint a clear seizure focus. Removing or disconnecting the small area of brain tissue responsible can, in some cases, eliminate seizures entirely. This is why accurate localization matters so much.
Other options for medication-resistant focal epilepsy include nerve stimulation devices and specialized diets, though these typically reduce seizure frequency rather than stopping seizures completely.
What to Do During a Focal Seizure
If you’re with someone having a focal seizure, the most important things are to stay calm and stay with them. Remove nearby objects that could cause injury, and avoid restraining them or putting anything in their mouth. 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. Once the seizure ends, help the person sit in a safe place and, once they’re alert, calmly explain what happened. They may be disoriented and need a few minutes, or longer, to recover. Offering to call someone to help them get home safely is a practical step many people overlook. Don’t offer food or water until they’re fully alert, as this poses a choking risk.