What Are Partial Onset Seizures? Types & Causes

Partial onset seizures are seizures that begin in one specific area of the brain rather than across the entire brain at once. You may also hear them called focal seizures, which is the preferred medical term today. They are the most common seizure type in adults with epilepsy, and they range enormously in severity, from a brief wave of tingling or déjà vu that lasts a few seconds to events that impair consciousness and progress into full-body convulsions. What makes them “focal” is their starting point: abnormal electrical activity fires up in one region of the cortex and either stays local or spreads outward from there.

Why Doctors Stopped Saying “Partial”

If you were diagnosed before 2017, your medical records probably say “partial seizures.” The International League Against Epilepsy (ILAE) officially replaced that language in its 2017 classification overhaul. “Partial” became “focal,” and terms like “simple partial” and “complex partial” were retired. Under the new system, awareness is the main divider: a focal seizure is classified as either “focal aware” (you remain conscious and oriented throughout) or “focal impaired awareness” (your consciousness is altered or lost during the event). The older term “secondarily generalized” was also replaced by “focal to bilateral tonic-clonic seizure,” describing a focal seizure that spreads to involve both hemispheres of the brain.

The name change was not just cosmetic. Reclassifying “complex partial seizures” as “focal seizures with impaired awareness” lets clinicians add useful descriptors about what the seizure actually looks like, such as whether it involves motor symptoms or non-motor symptoms, and what part of the brain it seems to originate from.

In practice, many patients and even some clinicians still use the older terminology, so you will encounter both. When reading older studies or medical records, “simple partial” maps roughly to “focal aware,” and “complex partial” maps to “focal impaired awareness.” The underlying biology has not changed; the labels have just been updated to be clearer and more consistent.

Types of Focal Seizures

Under the current classification, focal seizures are divided first by awareness and then by the dominant symptom. The two broad categories are motor onset and non-motor onset, and each includes several specific subtypes.

Focal Aware Seizures

During a focal aware seizure, you stay fully conscious and remember the event afterward. These were previously called simple partial seizures. They can involve motor symptoms like rhythmic jerking of one hand or one side of the face, or they can involve purely sensory or emotional experiences. A tingling sensation that starts in your fingertips and creeps up your arm, a sudden rush of fear with no obvious trigger, a strange taste or smell, or a brief episode of déjà vu can all be focal aware seizures. Many people experience them as an “aura,” which is actually a focal seizure in its own right rather than a warning sign of something else.

Somatosensory auras, the tingling-numbness-pain variety, are especially well studied. In a review of patients with focal epilepsy, roughly one in eight reported somatosensory auras. The most common sensation was tingling, reported by about three-quarters of those affected, and the hands and arms were the body parts most often involved.

Pain, temperature changes, and a pulling or movement sensation also occur. In general, when the aura affects only one side of the body, the seizure focus tends to be on the opposite side of the brain, which gives neurologists a useful clue about where the seizure originates.

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Focal Impaired Awareness Seizures

These are the events that used to be called complex partial seizures. Your consciousness is clouded or lost during the seizure. You may stare blankly, make repetitive movements such as lip smacking or hand rubbing (called automatisms), or wander aimlessly. After the episode ends, you typically have no memory of it and may feel confused for minutes to hours. Focal impaired awareness seizures most commonly arise from the temporal lobe, though they can originate elsewhere.

Focal to Bilateral Tonic-Clonic Seizures

Sometimes a focal seizure does not stay focal. The abnormal electrical activity can spread from its starting point to both hemispheres, producing a full-body convulsion with stiffening (tonic phase) and rhythmic jerking (clonic phase). Imaging research suggests that people whose focal seizures tend to generalize in this way have more widespread abnormalities in the brain’s structural connections, which may provide a pathway for the seizure to travel.

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This progression is the most dangerous common scenario with focal seizures, since tonic-clonic convulsions carry risks of injury from falling, prolonged oxygen deprivation, and in rare cases, sudden unexpected death in epilepsy (SUDEP).

What Causes Focal Seizures

Focal seizures have many possible causes, and the mix changes with age. In adults, focal epilepsies are often acquired. Stroke, traumatic brain injury, infections, brain tumors, vascular malformations, metabolic disorders, autoimmune conditions, and genetic causes all appear on the list.

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In children, the picture splits into three broad groups: self-limited focal epilepsy syndromes that tend to resolve with age, focal epilepsy of unknown cause, and focal epilepsy with a known structural or genetic origin.

Autoimmune encephalitis has emerged as an increasingly recognized cause. In this condition, the immune system produces antibodies that attack brain tissue, and the result can include seizures alongside behavioral changes, cognitive problems, and movement abnormalities.

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Anti-MOG antibody disease, for instance, can present with focal motor seizures as a primary symptom.

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The Role of Brain Somatic Mutations

One of the more surprising findings from the past decade is that focal epilepsy can be caused by genetic mutations that arise after conception and exist only in certain brain cells, not throughout the body. These somatic mutations, particularly in genes involved in a signaling pathway called mTOR, are a major cause of focal cortical dysplasia, which is the single most common reason children undergo epilepsy surgery. Brain somatic mutations in mTOR pathway genes account for up to about 40% of patients with sporadic focal malformations of cortical development, and even a very low percentage of mutated cells in the affected tissue can be enough to produce drug-resistant seizures.

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Research has broadened this picture further. In a study of surgically removed brain tissue, pathogenic variants in mTOR pathway genes were identified in three-quarters of hemimegalencephaly cases and about 29% of a specific type of focal cortical dysplasia. Other somatic gene variants were found across an even wider range of malformation types, including some genes not previously linked to visible brain malformations at all.

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This means focal epilepsy can have a genetic cause even when standard blood-based genetic testing comes back normal, because the relevant mutation only exists in the brain.

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What Happens in the Brain During a Focal Seizure

At the cellular level, a focal seizure involves groups of neurons firing in abnormally synchronized bursts. One well-studied component of this is called a paroxysmal depolarization shift, a sudden, large electrical discharge in a neuron. These bursts were long considered a marker or side effect of seizures rather than a driver of them. Newer evidence, including recordings from human seizure tissue and computational modeling, suggests they actively contribute to seizure propagation. In particular, when certain inhibitory brain cells experience these depolarization bursts, they temporarily fail at their job of restraining neighboring excitatory activity, and the seizure wavefront advances.

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This interplay between excitation and inhibition is why focal seizures can behave so unpredictably. When the brain’s inhibitory brakes hold, the seizure stays small and localized. When they fail more broadly, the seizure spreads.

Common Triggers

People with focal epilepsy often identify specific circumstances that seem to bring on seizures. In one survey, the most commonly reported triggers were missed medication (about 41% of patients), emotional stress (about 31%), sleep deprivation (about 20%), and fatigue (about 15%). Missing meals, fever, and smoking were also mentioned, though less frequently.

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Sleep deprivation deserves particular attention. It has been used clinically for decades to provoke abnormal brain activity during diagnostic EEG recordings, precisely because it is such a reliable way to coax out epileptic spikes.

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That said, the relationship between sleep and seizures is more nuanced than “less sleep means more seizures.” A diary study that tracked actual sleep hours found that the total sleep time on nights before a seizure was not meaningfully different from seizure-free nights. Instead, the presence of self-reported triggers like stress or anxiety was the stronger predictor of seizure occurrence.

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The takeaway is that sleep deprivation increases vulnerability, but the relationship is not as straightforward as a simple dose-response. Stress and medication adherence may matter more on any given day.

How Focal Seizures Are Diagnosed

The cornerstone of diagnosis is the electroencephalogram (EEG), which records electrical activity across the scalp. For focal epilepsy specifically, the pattern of “spikes” between seizures can point toward the brain region where seizures originate. Research shows that spike rates tend to be higher during sleep, particularly during deep non-rapid-eye-movement sleep, and that incorporating this sleep-wake information into analysis improves the accuracy of pinpointing the seizure-onset zone.

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MRI is the standard imaging tool for finding structural causes like focal cortical dysplasia. But subtle lesions, especially small ones sitting in the folds of the brain, are frequently missed. A multi-center study found that roughly 37% of surgically confirmed focal cortical dysplasias had been reported as MRI-negative at some point, meaning that the radiologist reviewing the scan did not see them. A machine-learning tool trained on brain surface features was able to detect about 69% of these previously missed cases, and in patients with a confirmed subtype of dysplasia who later became seizure-free, its sensitivity reached 85%.

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Specialized MRI sequences and post-processing techniques are increasingly being used to catch these subtle abnormalities that standard scans miss.

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Treatment Options

Medication is the first-line treatment. A systematic review comparing antiepileptic drugs for focal epilepsy found that several newer medications, including levetiracetam, lamotrigine, oxcarbazepine, and topiramate, were as effective as the older standard, carbamazepine, but significantly better tolerated. Lamotrigine stood out for having the best tolerability profile across both children and adults.

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For people whose seizures do not respond to medication, which happens in roughly a third of focal epilepsy cases, surgical evaluation should be considered. If the seizure focus can be precisely identified and does not overlap with critical brain functions like speech or movement, removing that piece of tissue is the most effective option. When surgery is not feasible, neurostimulation devices offer an alternative.

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Three main neurostimulation approaches are in clinical use: vagus nerve stimulation (VNS), responsive neurostimulation (RNS), and deep brain stimulation (DBS). A meta-analysis from the ILAE found that all three reduce seizure frequency substantially over time, with the benefits improving the longer the device is in place. RNS showed a median seizure reduction of about 53% at two years, growing to 75% at nine years. DBS followed a similar trajectory, reaching about 75% reduction at seven years. VNS results were more variable but still showed meaningful improvement. All three approaches also improved quality of life, though side effects like hoarseness with VNS, and headaches or localized pain with RNS and DBS, are common.

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How Focal Seizures Differ in Children and Adults

The underlying causes of focal seizures shift with age, but the basic mechanics of how seizures start and spread are remarkably consistent from childhood through adulthood. After age two, the electrical signatures on EEG, the way seizures look clinically, and the response to medication are similar enough between children and adults that drug regulators now allow efficacy data from adult trials to be applied to pediatric approvals for focal seizures.

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Where children and adults diverge is in the distribution of causes. Children are more likely to have focal epilepsy from developmental brain malformations or self-limited epilepsy syndromes that resolve by adolescence. Adults are more likely to develop focal epilepsy from acquired injuries like stroke, head trauma, or tumors.

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This distinction matters most for prognosis: a child with a self-limited syndrome may outgrow seizures entirely, while an adult whose focal epilepsy follows a stroke may need lifelong management.

The Mental Health and Memory Burden

Focal epilepsy does not just produce seizures. Psychiatric conditions are two to three times more common in people with epilepsy than in the general population, and in one study of patients with drug-resistant focal epilepsy, over half had at least one psychiatric comorbidity.

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Depression and anxiety are the most frequent, and memory problems are widespread, particularly for autobiographical memory, the kind of memory that involves recalling personal experiences and facts about your own life. Research has confirmed that people with epilepsy perform significantly worse on measures of both personal-event memory and general knowledge memory, with high rates of depressive symptoms compounding the problem.

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Cognitive difficulties in focal epilepsy are not fully explained by seizure frequency or medication side effects alone. The abnormal electrical discharges that occur between seizures, called interictal epileptiform discharges, may themselves contribute to memory and attention problems even when no outward seizure is happening.

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This is one reason seizure freedom, not just seizure reduction, is such an important treatment goal. The brain’s electrical environment between seizures matters too.

Wearable Devices and Seizure Monitoring

One of the practical challenges of focal epilepsy is that seizures with impaired awareness often go unrecognized by the person having them. If you black out during a seizure, you may not know it happened unless someone witnessed it, which makes accurate seizure counts difficult for both patients and doctors. Wearable technology is starting to change this.

An ear-worn EEG device tested in a hospital setting allowed epileptologists to identify about 86% of focal seizures when reviewing the ear-EEG recordings, with a very low false alarm rate of roughly one per ten days.

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Wrist-worn devices that track movement, skin conductance, and blood flow are also being studied. For focal seizures with a motor component, machine learning models applied to wristband data have achieved sensitivities ranging from 67% to 100% in personalized assessments, with false alarm rates under one per day.

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Heart-rate-based wearable systems are another avenue, since many focal impaired awareness seizures produce detectable changes in heart rhythm.

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None of these devices is ready to fully replace hospital EEG monitoring, but they are moving steadily closer to giving people with focal epilepsy a reliable way to track seizures in daily life, which in turn makes it easier to judge whether a treatment is actually working.

SUDEP Risk and Focal Seizures

Sudden unexpected death in epilepsy, or SUDEP, is the most feared complication of uncontrolled seizures. A large prospective study found that focal seizures by themselves, and even episodes of low blood oxygen during focal seizures, were not significantly associated with SUDEP risk. What did stand out was the location of the seizure focus: people with extratemporal epilepsies, particularly those originating in frontal or perisylvian regions, had a dramatically higher likelihood of SUDEP compared to those with temporal lobe epilepsy.

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The strongest modifiable risk factor for SUDEP remains having ongoing tonic-clonic seizures, especially those occurring during sleep. This brings the discussion back to focal to bilateral tonic-clonic seizures: preventing that spread, whether through medication, surgery, or neurostimulation, is the single most important step in reducing SUDEP risk. Nighttime seizure detection, supervision, and sleeping position (avoiding facedown sleeping) are practical measures that families and caregivers are advised to consider.