Focal epilepsy starts in one specific area of the brain, and its causes range from structural brain abnormalities and head injuries to infections, immune system problems, and genetic factors. It is the most common type of epilepsy, outnumbering generalized epilepsy by roughly two to six times in population studies. In many cases, a clear cause can be identified on brain imaging or through other testing, but for some people the origin remains unknown.
How Focal Epilepsy Differs From Other Types
Epilepsy is classified based on where seizure activity begins in the brain. In focal epilepsy, electrical misfiring starts in a specific region of one hemisphere. This is different from generalized epilepsy, where abnormal activity involves both sides of the brain from the start. That distinction matters because focal epilepsy often points to a localized problem, whether it’s a scar, a small tumor, a malformation, or damage from an old injury, and that localized cause can sometimes be treated directly.
Focal seizures can look very different depending on which part of the brain is involved. Some cause brief jerking of one hand or a strange rising sensation in the stomach. Others impair awareness, leaving a person staring blankly or making repetitive movements for a minute or two. A focal seizure can also spread to become a full-body convulsion, which is why some people with focal epilepsy experience what appears to be a generalized tonic-clonic seizure even though it started in one spot.
Brain Malformations Present From Birth
One of the most important causes of focal epilepsy, especially in children and young adults, is a category of structural problems called focal cortical dysplasia. During fetal brain development, neurons sometimes migrate to the wrong place or organize themselves abnormally, leaving a patch of cortex that is wired incorrectly. These malformations are invisible to the naked eye on the surface of the brain but show up on high-resolution MRI scans.
Focal cortical dysplasia comes in several forms. Type I involves abnormal layering of the cortex and can appear in one or more lobes. Type II features both disorganized layers and abnormally shaped neurons, sometimes with distinctive “balloon cells” that pathologists can identify under a microscope. Type III describes cortical dysplasia that exists alongside another problem: scarring of the hippocampus, a small brain tumor, a blood vessel malformation, or damage from an early-life injury like a stroke. Each type can generate seizures because the malformed tissue is inherently more excitable than normal brain, firing in patterns that recruit surrounding neurons into seizure activity.
Hippocampal Sclerosis and Temporal Lobe Epilepsy
The single most common structural finding in adults with drug-resistant focal epilepsy is hippocampal sclerosis, a pattern of scarring and cell loss in the hippocampus deep within the temporal lobe. This condition underlies mesial temporal lobe epilepsy, which typically causes focal seizures with impaired awareness. People experiencing these seizures often describe an aura, such as a wave of fear or déjà vu, followed by a period of blank staring and automatic movements like lip smacking or fumbling with their hands.
The exact trigger for hippocampal sclerosis is still debated, but it is strongly linked to a history of prolonged or complicated febrile seizures in early childhood, as well as other early-life brain insults such as infections and head trauma. Ongoing inflammation appears to play a role, with certain immune signals and antibodies potentially contributing to progressive damage in the hippocampus over years. Many people with this condition have a gap of several years between their initial childhood injury and the onset of habitual seizures in adolescence or early adulthood.
Head Injury and Post-Traumatic Epilepsy
Traumatic brain injury is a well-established cause of focal epilepsy, particularly after penetrating injuries, skull fractures, brain bleeds, or prolonged loss of consciousness. The process from injury to epilepsy unfolds over weeks, months, or even years in a chain of biological events that researchers call epileptogenesis.
At the moment of impact, the brain experiences a massive release of the excitatory chemical glutamate, triggering a flood of calcium into neurons that can poison and kill them. Blood vessels tear, the protective blood-brain barrier breaks down, and both neurons and supporting brain cells swell. In the weeks and months that follow, the brain attempts to repair itself, but the reorganization often goes wrong. Inhibitory neurons, which normally act as brakes on brain activity, are damaged or lost. Meanwhile, surviving excitatory neurons sprout new connections to each other, creating circuits that are prone to runaway electrical activity. The scar tissue and rewired networks left behind become the focal point where seizures originate.
Stroke and Vascular Problems
Stroke is one of the leading causes of new-onset focal epilepsy in older adults. When a blood vessel in the brain becomes blocked or ruptures, the resulting area of dead or damaged tissue can become an ongoing source of seizure activity. A large prospective study from the South London Stroke Register found that about 6.4% of stroke survivors developed epilepsy, with the cumulative risk climbing to roughly 12% over ten years. Most cases emerge within the first year, though new seizures can appear years later.
The risk is higher after hemorrhagic strokes (bleeding into the brain) than after ischemic strokes (blocked blood flow), and larger strokes involving the cortex carry greater risk than small, deep strokes. Blood vessel malformations present from birth, such as arteriovenous malformations or cavernous angiomas, can also cause focal seizures at any age, sometimes as the very first sign that something is wrong.
Low-Grade Brain Tumors
Certain slow-growing brain tumors are so closely linked to seizures that they are classified as “epilepsy-associated.” These include gangliogliomas and dysembryoplastic neuroepithelial tumors, both of which tend to appear in the temporal lobe (roughly 65 to 80% of cases) and often cause seizures beginning in childhood or early adulthood. Because these tumors grow slowly, a person may have seizures for years or even decades before the tumor is discovered.
These tumors trigger seizures through at least two mechanisms. The tumor cells themselves carry genetic mutations that make them electrically hyperactive. At the same time, the brain tissue immediately surrounding the tumor undergoes chemical changes: the balance between excitatory and inhibitory signaling shifts, creating a zone around the tumor that is primed to seize. This is why surgical removal of the tumor alone does not always stop seizures. The surrounding irritated cortex sometimes needs to be removed as well.
Infections That Affect the Brain
Brain infections are a major cause of focal epilepsy worldwide, though the specific infections vary by region. In many parts of Latin America, sub-Saharan Africa, and Southeast Asia, neurocysticercosis is one of the leading causes. This condition develops when a person ingests eggs from the pork tapeworm Taenia solium, typically through contaminated food or water rather than from eating pork directly. The larvae travel to the brain, where they form cysts that provoke inflammation and scarring as they eventually die. In endemic regions of Peru, brain imaging revealed cysticercosis-compatible findings in 39% of people with epilepsy, and about 40% of people with epilepsy had antibodies indicating past infection.
Other infections that can cause focal epilepsy include bacterial meningitis, viral encephalitis (particularly herpes simplex encephalitis, which has a strong predilection for the temporal lobes), tuberculosis affecting the brain, and in some regions, malaria and toxoplasmosis. The seizures may begin during the acute infection or develop months to years later as the brain heals with scar tissue.
Autoimmune Causes
Over the past two decades, autoimmune encephalitis has emerged as an increasingly recognized cause of focal epilepsy. In these conditions, the immune system produces antibodies that attack specific proteins on the surface of brain cells, disrupting normal signaling and causing inflammation. The result is often a combination of seizures, memory problems, and behavioral changes that develop over days to weeks.
Several antibody types are particularly associated with seizures. Antibodies targeting a protein called LGI1 cause a distinctive syndrome that includes brief, frequent seizures involving the face and arm on one side, known as faciobrachial dystonic seizures. These episodes are considered essentially diagnostic for this condition. Antibodies against GABA-B receptors frequently lead to severe, hard-to-control seizures, with 62% of patients in one study developing status epilepticus, a dangerous state of prolonged seizure activity. NMDA receptor antibodies, more commonly discussed in the context of psychiatric symptoms, also cause seizures in a large proportion of affected patients. Importantly, standard anti-seizure medications often fail to control these seizures. Treatment needs to target the underlying immune attack to be effective.
Genetic Factors
Focal epilepsy has traditionally been viewed as an acquired condition caused by brain lesions, but genetic contributions are now well recognized. Mutations in several genes can cause focal epilepsy even when brain imaging appears normal. The gene DEPDC5, which helps regulate cell growth signaling in the brain, is one of the most frequently identified. Mutations in this gene have been found in families where multiple members develop focal seizures, often originating from different brain regions in different family members.
A targeted sequencing study of people with focal epilepsy and normal or near-normal cognitive function found potentially disease-causing genetic variants in about 6% of patients, with DEPDC5 being the most commonly affected gene. Other genes implicated include SCN1A, which controls sodium channels critical for how neurons fire, and PCDH19, which affects how brain cells connect to one another. In many cases, a person with a genetic predisposition may never develop epilepsy unless a second factor, like a minor brain injury or developmental abnormality, tips the balance.
When No Cause Is Found
Despite advances in brain imaging, genetic testing, and antibody panels, a significant number of people with focal epilepsy receive no definitive explanation. This is sometimes called “unknown etiology” focal epilepsy. It does not mean the brain is normal. It means that current technology cannot detect the abnormality. As MRI scanners improve in resolution and genetic testing becomes more comprehensive, some of these cases are being reclassified. Subtle cortical malformations that were invisible on older scanners, for instance, are now being detected on ultra-high-field MRI. For the person living with focal epilepsy of unknown cause, treatment with anti-seizure medications follows the same principles as it does for cases with a known cause, and many people achieve good seizure control regardless of whether the underlying reason has been identified.