Diagnosing focal seizures involves a combination of brain wave recordings, imaging scans, blood tests, and a detailed clinical history. No single test confirms the diagnosis on its own. Instead, neurologists piece together evidence from multiple sources to determine that seizures are starting in one specific area of the brain, identify exactly where that area is, and rule out other conditions that can look similar.
What Doctors Look for First
The diagnostic process typically starts with a thorough clinical history. Your neurologist will ask you (and anyone who witnessed the event) to describe exactly what happened, in what order, and how long it lasted. Focal seizures produce symptoms tied to the brain region where they originate. A seizure starting in the visual cortex might cause flashing lights in one visual field, while one starting in the temporal lobe might produce a rising sensation in your stomach, déjà vu, or a sudden emotional shift. The specific sequence of symptoms often gives the first strong clue about where the seizure begins.
Under the most recent classification system from the International League Against Epilepsy, focal seizures are one of four main seizure classes. The updated framework distinguishes seizures by whether consciousness is preserved or impaired, and describes them by the chronological sequence of signs and symptoms rather than just the first symptom. This means the order in which your symptoms unfold matters as much as the symptoms themselves.
EEG: The Core Diagnostic Tool
An electroencephalogram, or EEG, is the most important test for diagnosing focal seizures. It records electrical activity across the brain using electrodes placed on your scalp. In someone with focal epilepsy, the EEG looks for characteristic patterns called spikes and sharp waves, which are the electrical signature of a seizure focus. These brief bursts of abnormal activity appear in a specific brain region rather than across the whole brain, which is what separates focal from generalized seizures.
A routine EEG lasts about 20 to 30 minutes, and its sensitivity is limited. Published figures put it between 25% and 55%, meaning up to half of people with epilepsy can have a completely normal routine EEG. Around 10% of people with epilepsy never show these characteristic discharges on any standard recording. That doesn’t mean the test is useless. When it does show focal abnormalities, it’s highly informative. But a normal result doesn’t rule out focal seizures.
One particularly specific finding in temporal lobe epilepsy is a pattern called temporal intermittent rhythmic delta activity. It shows up in fewer than 1% of EEGs overall, but appears in about 28% of patients with temporal lobe epilepsy, making it a strong indicator when present.
Extended EEG Monitoring
When a routine EEG comes back normal, longer recordings dramatically improve the chances of catching abnormal activity. Sleep-deprived EEGs (recorded after a full night without sleep, lasting 1.5 to 3 hours) and ambulatory EEGs (portable recordings worn for 16 to 24 hours at home) both increase detection rates to roughly 50% to 75%. In direct comparisons, ambulatory EEG had a sensitivity of 63% with a specificity of 95%, while sleep-deprived EEG reached 45% sensitivity with 91% specificity. Both are significant improvements over the routine test, and your neurologist may order one or both if the initial recording is inconclusive.
Sleep is a key ingredient in all these approaches. Epileptiform discharges are more likely to appear during sleep, which is why sleep deprivation and overnight recordings are so effective. You may be asked to stay awake all night before a morning EEG specifically to encourage you to fall asleep during the recording.
Inpatient Video-EEG Monitoring
For cases where the diagnosis remains uncertain, or when surgery is being considered, inpatient video-EEG monitoring is the gold standard. You stay in a hospital epilepsy monitoring unit with continuous EEG recording and video surveillance, sometimes for several days. This setup captures actual seizures on both video and EEG simultaneously, allowing neurologists to correlate what’s happening in your brain with what’s visible in your body. It serves two critical purposes: confirming that events are truly epileptic seizures (rather than fainting, panic attacks, or dissociative episodes) and pinpointing where in the brain the seizures originate. This level of monitoring is recommended for anyone with focal epilepsy that doesn’t respond to medication and who might be a candidate for surgery.
Brain Imaging With MRI
While EEG captures electrical activity, MRI reveals the physical structure of the brain. The goal is to find structural abnormalities that could be causing the seizures: scar tissue, tumors, malformations in brain development, or damage to the hippocampus (a common culprit in temporal lobe epilepsy).
Not all MRI scans are equal for this purpose. The International League Against Epilepsy recommends a dedicated epilepsy protocol called HARNESS-MRI, optimized for 3T (3-Tesla) scanners, which produce higher-resolution images than the standard 1.5T machines found in many hospitals. The protocol includes three key sequences. A high-resolution 3D scan that captures fine structural detail at 1 millimeter resolution with no gaps between slices. A specialized fluid-suppressing scan (also at 1 millimeter resolution) that highlights subtle abnormalities like scarring or swelling that would otherwise blend into surrounding tissue. And a coronal scan angled perpendicular to the hippocampus with submillimeter resolution (as fine as 0.4 millimeters), designed specifically to evaluate hippocampal structure for signs of shrinkage or internal damage.
If you’ve had a standard MRI that came back normal, it’s worth asking whether it was performed with an epilepsy-specific protocol on a 3T scanner. Subtle lesions that are invisible on a routine scan can become apparent with the right sequences and magnet strength.
When MRI Finds Nothing: PET and SPECT Scans
Between 20% and 50% of patients with medication-resistant focal epilepsy have no visible structural abnormality on MRI. For these patients, functional imaging becomes critical, especially if surgery is being considered.
PET scans measure metabolic activity in the brain. A seizure focus typically shows reduced metabolism between seizures. In temporal lobe epilepsy with normal MRI findings, PET correctly identified the side of the seizure focus in 87% of patients. Overlaying PET data onto MRI images improves accuracy further, guiding surgical planning and leading to better outcomes than MRI alone.
SPECT scans measure blood flow in the brain. They’re most useful when performed during or immediately after a seizure (ictal SPECT), because the seizure focus shows a surge in blood flow at that moment. SPECT can detect abnormalities in brain regions that appear structurally normal on MRI, providing another layer of localization when other tests fall short.
Blood Tests and Lab Work
Blood tests don’t diagnose focal seizures directly, but they play an essential role in ruling out other causes. A chemistry panel checks sodium, potassium, and blood sugar levels, because imbalances in any of these can provoke seizures. Low sodium in particular is a well-known seizure trigger. A complete blood count screens for infections or anemia that might be contributing factors. Liver and kidney function tests matter both for identifying underlying causes and for guiding medication choices later, since many anti-seizure drugs are processed by these organs.
Toxicology screening may also be performed to check whether drugs or alcohol played a role. The point of all this lab work is to separate seizures caused by a temporary, fixable problem (like dangerously low blood sugar) from seizures caused by an underlying brain condition that will need ongoing treatment.
Ruling Out Conditions That Mimic Seizures
Several conditions produce episodes that closely resemble focal seizures, and distinguishing between them is one of the trickiest parts of diagnosis. The most common mimic is functional neurological seizures (also called psychogenic nonepileptic seizures or dissociative seizures). These episodes involve changes in movement, sensation, or consciousness that look like seizures but produce no abnormal electrical activity on EEG. Video-EEG monitoring is the primary tool for making this distinction, capturing an event while simultaneously showing a normal brain wave pattern.
Migraine with aura is another condition that overlaps with focal seizures. Migraine auras produce transient neurological symptoms, including visual disturbances, tingling, or speech difficulty, that can look very similar to a focal seizure. The key differences are timing and duration. Migraine auras typically develop gradually over 5 to 60 minutes and are followed by a headache lasting 4 to 72 hours. Focal seizures tend to be shorter (often under 2 minutes), with symptoms that evolve more rapidly. Syncope (fainting) is another common mimic, though it usually involves a clear trigger like standing up quickly and lacks the specific sensory or motor symptoms that characterize focal seizures.
How the Pieces Fit Together
Diagnosis rarely comes from a single test. A typical pathway starts with clinical history and a routine EEG. If the EEG shows focal spikes or sharp waves in a region consistent with your symptoms, the diagnosis is fairly straightforward. An epilepsy-protocol MRI then looks for the underlying cause. If the routine EEG is normal, extended monitoring through sleep-deprived or ambulatory EEG comes next. For medication-resistant cases, inpatient video-EEG monitoring, PET, and SPECT scans build a progressively more detailed map of where seizures start and why.
The process can take weeks or months, especially if seizures are infrequent or subtle. Each layer of testing adds confidence to the diagnosis and precision to the localization, which directly affects treatment decisions, particularly whether surgery might be an option.