Focal seizures start from abnormal electrical activity in one specific area of the brain, and they have a wide range of causes, from structural damage like strokes and tumors to metabolic imbalances, infections, genetic mutations, and autoimmune conditions. They account for roughly two to six times more epilepsy cases than generalized seizures, making them the most common seizure type overall. Understanding the cause matters because it shapes treatment and outlook.
How Focal Seizures Start in the Brain
Every focal seizure begins with a cluster of neurons in one hemisphere firing abnormally. What makes this happen at the cellular level is more surprising than you might expect. Rather than being purely a problem of overexcited brain cells, inhibitory signaling (the system that normally calms neurons down) plays a central role in triggering the seizure. When inhibitory cells fire excessively, they cause a buildup of chloride inside nearby neurons and potassium outside them. That chemical shift flips the effect of inhibition from calming to excitatory, creating a feedback loop that spreads electrical activity through the local area.
This is why focal seizures produce symptoms tied to one brain region. A seizure starting in the area that controls your right hand might cause involuntary twitching there, while one in the temporal lobe might produce a strange taste, a wave of déjà vu, or a rising feeling in your stomach. During a focal aware seizure, you remain conscious through the whole event. During a focal impaired awareness seizure, consciousness is disrupted at some point.
Stroke and Blood Vessel Problems
Stroke is one of the most common causes of focal seizures in adults over 60. When blood flow to part of the brain is cut off, the damaged tissue can become a source of abnormal electrical activity. Seizures after a stroke fall into two categories based on timing. Early seizures happen within the first week, and 73% of those occur in the first 24 hours, driven by the acute swelling and chemical disruption around the injured tissue. Late seizures appear weeks to years afterward, once scar tissue has formed and permanently altered the local brain circuitry.
The distinction matters because early seizures don’t necessarily mean you’ll develop ongoing epilepsy, while late seizures are more likely to recur. Hemorrhagic strokes (bleeding into the brain) carry a higher seizure risk than ischemic strokes (blocked blood flow), though both can cause them.
Traumatic Brain Injury
Head injuries are a leading cause of focal seizures, and the risk scales with severity. A large nationwide study found that the overall 10-year risk of developing epilepsy after any traumatic brain injury was 4.0%, compared to 0.9% in the general population. But the numbers diverge sharply by injury type: mild TBI carried a 2.6% risk, skull fractures 2.8%, and focal brain injuries (where a specific area of brain tissue is damaged) reached 12.9%. People with focal brain injuries had 16 times the risk of epilepsy compared to the general population.
Seizures can begin days after the injury or emerge years later. As with stroke, early post-injury seizures are driven by acute inflammation and swelling, while later seizures reflect permanent changes in brain structure.
Brain Tumors
Tumors are a particularly common cause of focal seizures because they physically disrupt the brain tissue around them. Counterintuitively, slower-growing tumors are more likely to cause seizures than aggressive ones. Up to 85% of people with low-grade gliomas develop seizures, and in the smallest, most slow-growing subgroup, the rate reaches 95%. By comparison, seizures occur in roughly 49% of people with glioblastoma, the most aggressive type. Oligodendrogliomas, another slow-growing type, cause seizures in about 72% of patients.
The likely explanation is that slow-growing tumors sit within functional brain tissue for years, gradually irritating surrounding neurons without destroying them outright. Fast-growing tumors tend to destroy tissue rather than irritate it. For many people with low-grade brain tumors, a focal seizure is the very first symptom that leads to diagnosis.
Abnormal Brain Development
Some people are born with small areas of brain tissue that didn’t form correctly during fetal development. The most well-known of these is focal cortical dysplasia, where a patch of the brain’s outer layer has disorganized, abnormally shaped neurons. These malformations are a leading cause of drug-resistant focal epilepsy in children and young adults.
Focal cortical dysplasia results from genetic mutations that occur in a small number of cells during brain development, rather than being inherited from a parent. These mutations affect a growth-signaling pathway in cells, causing neurons in the affected area to grow too large and wire themselves incorrectly. The severity of seizures tends to correlate with how overactive this growth pathway is. Because the abnormality is confined to one area, surgical removal of the affected tissue can sometimes eliminate seizures entirely.
Genetic Causes
Several inherited gene mutations increase the risk of focal epilepsy. In one study of patients with focal epilepsy and normal or near-normal intellectual function, targeted genetic testing found disease-related mutations in about 8% of cases. The most commonly affected genes were DEPDC5 (which regulates the same cellular growth pathway involved in focal cortical dysplasia), SCN1A (which affects how sodium moves through nerve cells), and PCDH19 (involved in how brain cells connect to each other).
Familial focal epilepsy often runs in families with variable severity. One person might have frequent seizures starting in childhood, while a relative with the same mutation has rare episodes or none at all. Genetic testing is becoming increasingly useful for identifying these causes, particularly when brain imaging looks normal.
Autoimmune and Inflammatory Causes
The immune system can sometimes produce antibodies that attack proteins on the surface of brain cells, triggering inflammation and focal seizures. Three antibody types are most commonly involved, and each produces distinct seizure patterns.
Antibodies against LGI1 (a protein involved in nerve signaling) cause a characteristic seizure type called faciobrachial dystonic seizures, brief episodes of jerking in the face and arm that occur dozens of times per day in some patients. About 29% of patients with these antibodies experience them. These antibodies also cause a distinctive “pilomotor seizure,” where goosebumps appear on one side of the body, seen in about 15% of cases.
Antibodies against GAD (an enzyme involved in making the brain’s main inhibitory chemical) tend to produce seizures with repetitive automatic movements and a strong sense of déjà vu, reported in 35% of patients. Antibodies against the NMDA receptor, more common in younger women, cause seizures that are often part of a broader syndrome including psychiatric symptoms, movement problems, and reduced consciousness.
Autoimmune epilepsy is important to identify because it responds to immune-suppressing treatment rather than standard seizure medications.
Infections
Brain infections, both current and past, are a major cause of focal seizures worldwide. Any infection that inflames brain tissue (encephalitis) or its surrounding membranes (meningitis) can trigger seizures during the acute illness and leave lasting scarring that causes epilepsy afterward.
Globally, the parasitic infection neurocysticercosis is one of the leading causes of acquired epilepsy. It occurs when the larval form of a pork tapeworm lodges in the brain. The immune response and inflammation around the cyst cause seizures, which are often the first symptom. In sub-Saharan Africa alone, an estimated 2.5 million people have epilepsy caused by neurocysticercosis, with up to 8.2 million people carrying undiagnosed brain cysts. Other infections that commonly cause focal seizures include herpes simplex encephalitis, brain abscesses, HIV-related brain infections, and tuberculosis of the brain.
Metabolic and Blood Chemistry Imbalances
Disruptions in blood sugar, sodium, calcium, and other electrolytes can provoke focal seizures even in people without epilepsy. These are called acute symptomatic seizures, and they typically resolve once the underlying imbalance is corrected.
- Low blood sugar (hypoglycemia): Seizures can occur when blood glucose drops below 50 mg/dL with symptoms, or below 40 mg/dL even without. These seizures can be focal or generalized.
- High blood sugar: A condition called non-ketotic hyperosmolar hyperglycemia is particularly seizure-prone. About 25% of patients with this condition develop seizures, and focal motor seizures are the most common type, especially when blood sugar rises between 600 and 800 mg/dL.
- Low sodium (hyponatremia): Seizures typically appear when sodium drops below 115 mEq/L, though a rapid decline can trigger them at higher levels.
- High sodium (hypernatremia): Neurological symptoms generally develop above 160 mEq/L, with faster changes producing worse symptoms.
- Low calcium (hypocalcemia): Seizures occur at calcium levels between 5 and 6 mg/dL. About 20% of hypocalcemic seizures are focal rather than generalized.
- Low magnesium: Seizures appear when magnesium falls below 0.8 mEq/L and can be focal or generalized.
Alcohol withdrawal is another common metabolic trigger. The brain adapts to chronic alcohol exposure by increasing its baseline excitability, so when alcohol is suddenly removed, that unchecked excitability can produce seizures.
Hippocampal Sclerosis
One of the most common causes of temporal lobe focal seizures is scarring and shrinkage of the hippocampus, a structure deep in the brain involved in memory. This condition, called hippocampal sclerosis, creates a self-reinforcing cycle: an initial injury (often a prolonged febrile seizure in childhood, a head injury, or an infection) damages the hippocampus, and the resulting scar tissue becomes a persistent source of seizures. Repeated seizures then cause further changes, including swelling of surrounding structures like the amygdala and thickening of specific hippocampal layers.
Mesial temporal lobe epilepsy with hippocampal sclerosis often responds poorly to medication but is one of the most surgically treatable forms of epilepsy. Removing the scarred hippocampus eliminates seizures in a significant proportion of patients.