What are focal seizures and how serious are they?

What Are Focal Seizures and How Serious Are They?

Focal seizures are abnormal electrical discharges originating in a localized area of one brain hemisphere that can manifest with or without impaired consciousness, and they represent the most common and frequently drug-resistant form of epilepsy, affecting over 50 million people worldwide with significant implications for mortality, cognitive function, and quality of life. 1, 2

Definition and Core Characteristics

Focal seizures arise within neural networks limited to a single cerebral hemisphere, though they may subsequently spread to become more widely distributed or evolve into bilateral tonic-clonic seizures. 3, 4 The key distinguishing feature is their origin in a specific brain region rather than affecting both hemispheres simultaneously from onset. 2

Classification by Awareness Level

• Focal aware seizures: The patient maintains consciousness throughout the episode 3
• Focal impaired awareness seizures: Consciousness is affected, often manifesting as staring spells or unresponsiveness 3, 2
• Focal to bilateral tonic-clonic: Seizures that start focally then propagate to both hemispheres 4, 2

Clinical Manifestations

Motor Symptoms

• Jerking of one extremity or one side of the body 3
• Abnormal facial movements 3
• Small repetitive movements 3
• Hemiparesis as the most frequent focal neurological sign 4

Non-Motor Symptoms

• Epigastric aura: A rising sensation in the abdomen, highly characteristic of focal seizures 3
• Olfactory hallucinations: Unusual unpleasant smells 3
• Staring spells indicating impaired awareness 3
• Autonomic symptoms including sweating, chills, and shivering 5
• Sensory disturbances, visual field defects, aphasia, or psychosis depending on seizure location 4

Temporal Pattern

• Episodes typically last longer than absence seizures (>15-30 seconds) 3
• Post-ictal confusion or sleep commonly occurs, unlike generalized absence seizures where patients return immediately to baseline 3
• Median delay from symptom onset to diagnosis is 7 days, highlighting diagnostic challenges 4

Severity and Prognosis

Recurrence Risk

Focal seizures have a recurrence rate up to 94%, considerably higher than generalized seizures at 72%, making them inherently more serious in terms of chronicity. 3, 4

Drug Resistance

• Approximately 30% of patients with epilepsy do not respond to two anti-seizure medications and are considered drug-resistant, with focal epilepsies being the primary culprit. 1
• In children and adolescents, approximately 20% develop drug resistance with risk for poor long-term cognitive and psychosocial outcomes together with poor quality of life. 1
• Drug-resistant focal epilepsy represents the most serious form, requiring surgical evaluation. 1

Surgical Outcomes

• When epilepsy surgery is performed for drug-resistant focal epilepsy, approximately 65% of patients become seizure-free. 1
• Surgery is overall safe, successful, and cost-effective when the epileptogenic zone is correctly detected and located outside eloquent brain areas. 1

Underlying Causes and Structural Associations

Adults

• Hippocampal sclerosis is the most common cause of temporal lobe focal seizures 6
• Acquired structural lesions including stroke and cerebral infarction 6
• Brain tumors, particularly low-grade epilepsy-associated tumors 6
• Vascular malformations 6
• Infections 6
• Traumatic brain injury 2

Children

• Malformations of cortical development (MCD) are the most common cause of medically refractory focal epilepsy 6
• Focal cortical dysplasia (FCD) is the most frequent MCD subtype 6
• Birth-related lesions including perinatal stroke or hemorrhages 6
• Polymicrogyria and hemimegalencephaly 6

High-Risk Indicators

• Presence of focal neurologic examination findings has 97% correlation with symptomatic seizures 6
• Approximately 4% of children with first-time afebrile focal seizures have urgent intracranial pathology, most commonly infarction, hemorrhage, and thrombosis 4

Diagnostic Approach

Neuroimaging Requirements

• MRI is the imaging study of choice for all patients with focal seizures, with detection rates up to 55% in children and sensitivity of 84% for intractable seizures. 3, 6
• Dedicated epilepsy protocols with 3T scanners using T1-weighted volumetric acquisition with 1mm isotropic voxels and high-resolution coronal slices optimized for hippocampal pathology are recommended. 4
• MRI detected abnormalities not identified by CT in 47% of children with focal seizures. 4
• Non-contrast CT has a role only in emergent acute seizures to rapidly identify hemorrhage, stroke, or tumors requiring immediate intervention. 4

Electrophysiology

• EEG should be obtained, showing ictal discharges originating from one hemisphere in focal seizures 4
• A normal interictal EEG cannot rule out epilepsy and must be interpreted in clinical context 3
• Video EEG monitoring may be necessary to capture ictal events for definitive diagnosis 7

Laboratory Evaluation

• Check glucose in all patients with focal features, as hypoglycemia can present with focal neurologic deficits 4
• Glucose abnormalities and hyponatremia are the most frequent metabolic abnormalities in patients with focal neurologic deficits 4

Treatment Implications

First-Line Pharmacotherapy

• Carbamazepine, phenytoin, or valproic acid are often rated as first-line drugs 7
• Among newer anti-seizure medications, lamotrigine, gabapentin, topiramate, and oxcarbazepine have monotherapy indications 7
• Level A evidence for initial monotherapy is available for seven drugs with no robust data supporting superiority of one over another 8
• Cenobamate represents the latest approved option for focal seizures 8

Surgical Evaluation Pathway

• Patients with focal epilepsy who become drug-resistant after failing two appropriate anti-seizure medications should promptly undergo assessment in an epilepsy center 2
• Presurgical evaluation includes scalp video/EEG telemetry, structural MRI, neuropsychological assessment, functional MRI, interictal [18F]FDG PET, and ictal perfusion SPECT 1
• These functional imaging tools are especially useful in non-lesional epilepsy or multifocal structural abnormalities to localize the seizure onset for tailored resection 1

Critical Pitfalls to Avoid

• Do not dismiss epigastric sensations as "just anxiety" or gastrointestinal symptoms, as this is a characteristic aura of focal seizures 3
• Do not confuse brief post-ictal sleep with normal tiredness after a stressful event, as this indicates a seizure occurred 3
• Do not assume a normal interictal EEG rules out epilepsy; clinical diagnosis takes precedence 3
• Do not rely solely on CT imaging, as it misses small cortical lesions and has much lower sensitivity than MRI for epileptogenic foci 6
• The diagnosis of focal status epilepticus is often delayed or missed and should be considered after strokes or clinical seizures when patients do not stabilize or improve as expected 9