Focal seizures happen when abnormal electrical activity starts in one specific area of the brain, and they stem from a wide range of causes: structural brain damage, genetic mutations, infections, stroke, and sometimes no identifiable cause at all. About 30% of all epilepsy patients have focal seizures with no known origin despite standard testing. For the rest, pinpointing the cause is often possible and shapes how the condition is treated.
What Happens in the Brain During a Focal Seizure
Your brain relies on a constant balance between two types of chemical signals. One (glutamate) excites nerve cells and makes them fire. The other (GABA) calms them down and keeps firing in check. A focal seizure begins when this balance tips sharply toward excitation in one region of the brain.
Research published in Brain captured this process in real time. Before a seizure starts, calming GABA signals gradually weaken while excitatory glutamate signals grow stronger. At the moment of seizure onset, glutamate activity more than doubles compared to baseline, while GABA signals shrink. This creates a wave of uncontrolled electrical firing that spreads outward from the starting point at roughly 56 millimeters per second. The brain’s calming response moves inward toward the focus at less than half that speed, about 24 millimeters per second, essentially failing to keep up. The result is a runaway burst of electrical activity that can spread to surrounding tissue before the brain’s own braking system catches up.
Structural Brain Abnormalities
Physical changes to brain tissue are one of the most common and best-understood causes of focal seizures. When brain cells are damaged, displaced, or organized abnormally, they can form an irritable focus that generates abnormal electrical signals. The specific abnormality often determines where seizures originate and how they feel.
Some of the most frequently identified structural causes include:
- Focal cortical dysplasia: a patch of brain tissue that didn’t develop normally before birth, with disorganized layers of cells that are prone to misfiring
- Mesial temporal sclerosis: scarring and shrinkage in the inner part of the temporal lobe, the most common finding in adults with temporal lobe epilepsy
- Brain tumors: particularly slow-growing types that irritate surrounding brain tissue over months or years
- Vascular malformations: tangles of abnormal blood vessels (cavernous malformations) that can leak blood into surrounding tissue and create a seizure-prone zone
- Traumatic brain injury: scar tissue from head injuries can become a seizure focus weeks, months, or even years after the original trauma
Several conditions present from birth also cause structural changes that lead to focal seizures. Tuberous sclerosis complex produces benign growths in the brain called tubers. Polymicrogyria, where the brain’s surface has too many abnormally small folds, and periventricular nodular heterotopia, where clusters of nerve cells get stuck in the wrong location during development, can both create seizure-prone areas. These conditions are typically diagnosed in childhood, though mild forms sometimes go undetected until adulthood.
Stroke and Vascular Injury
Stroke is the leading cause of new-onset focal seizures in adults over 60. When blood flow to part of the brain is cut off, the resulting damage leaves behind a zone of scarred, irritable tissue. Roughly 10% of stroke survivors develop epilepsy afterward, with the risk considerably higher after a hemorrhagic stroke (bleeding into the brain), where 10% to 20% are affected compared to 2% to 14% after a clot-related stroke.
The timeline varies, but 40% to 80% of post-stroke epilepsy cases emerge within the first year. Some people experience a seizure within days of a stroke as swelling and chemical changes peak in the damaged area. Others develop seizures months or years later as scar tissue matures and reshapes the local electrical environment.
Infections That Damage Brain Tissue
Infections that reach the brain can trigger focal seizures both during the acute illness and long after recovery, once scarring has formed. Encephalitis, or inflammation of the brain itself, is one of the more dangerous culprits. Viruses are the most common cause of infectious encephalitis, with herpes simplex virus (both type 1 and type 2) being particularly aggressive in its ability to damage brain tissue. Mosquito- and tick-borne viruses can also cause encephalitis.
Bacterial meningitis, while primarily an infection of the membranes surrounding the brain, can spread to brain tissue and leave lasting damage. Parasitic infections like neurocysticercosis (caused by a tapeworm larva lodging in the brain) are a major cause of focal epilepsy in many parts of the world. In all these cases, the seizures may persist long after the infection itself has been treated, because the underlying brain scarring remains.
Genetic Causes
Focal seizures aren’t always caused by visible brain damage. Some people inherit gene mutations that make their brain cells inherently more excitable. One well-studied example involves a gene called DEPDC5, which helps produce a protein that acts as a brake on a cell-growth pathway called mTOR. When this gene is mutated, the brake doesn’t work properly. The mTOR pathway becomes overactive, changing how nerve cells connect to each other and making them fire too easily.
Mutations in DEPDC5 have been linked to several distinct focal epilepsy syndromes. In familial focal epilepsy with variable foci, different family members carrying the same mutation can have seizures starting in different parts of the brain. The same gene is also associated with familial mesial temporal lobe epilepsy and a sleep-related form of epilepsy where seizures cause sudden, vigorous movements during sleep. In infants, mutations affecting sodium channel genes (particularly KCNT1 and SCN2A) are the most common genetic cause of a severe condition called epilepsy of infancy with migrating focal seizures, where the seizure focus shifts from one brain region to another. About 70% of these infant cases have an identifiable genetic cause, and many arise from new mutations not inherited from either parent.
Causes Specific to Children
Children can develop focal seizures from many of the same causes as adults, but certain causes are especially prominent in early life. Brain injuries during birth, including oxygen deprivation and bleeding, can damage specific regions and create a seizure focus. Congenital brain malformations like hemimegalencephaly (where one half of the brain grows abnormally large) and schizencephaly (where abnormal clefts form in brain tissue) are rare but potent causes of focal seizures in infancy and early childhood.
Focal cortical dysplasia deserves special mention here because it’s the most common structural cause of drug-resistant focal epilepsy in children. The misplaced, disorganized cells fire unpredictably and don’t respond well to the usual calming signals, making seizures from these areas particularly difficult to control with medication alone.
Triggers That Lower the Seizure Threshold
For someone who already has a seizure-prone area in their brain, certain everyday factors can tip the balance and provoke a seizure. These triggers don’t cause epilepsy on their own, but they lower the threshold at which seizures occur.
Sleep deprivation is one of the most potent and well-documented triggers. Getting significantly less than seven to eight hours of sleep on a regular basis increases both the likelihood and the severity of seizures. Poor sleep quality matters too: waking up frequently, having restless sleep, or missing important deep-sleep stages can have the same effect even if total hours in bed seem adequate. Alcohol use can trigger seizures, particularly during withdrawal as the brain rebounds from alcohol’s calming effects. Stopping certain sedative medications abruptly, particularly benzodiazepine-type sleeping pills, can trigger seizures in susceptible people. Caffeine, stress, anxiety, and depression also contribute to seizure risk, often by disrupting sleep quality in a cycle that compounds over time.
When No Cause Is Found
Despite thorough testing, about 30% of people with focal epilepsy receive no clear explanation for their seizures. These cases are called cryptogenic, meaning a cause is presumed to exist but hasn’t been detected. Standard MRI scans may look normal, genetic tests may come back negative, and there’s no history of brain injury or infection. In some of these cases, the abnormality is simply too small or subtle for current imaging to detect. Advances in higher-resolution brain scanning and broader genetic testing have been steadily shrinking this group, reclassifying previously unexplained cases into known categories. For now, treatment focuses on controlling the seizures themselves rather than addressing a root cause.