What is focal epilepsy?

What is Focal Epilepsy

Focal epilepsy is a seizure disorder in which abnormal electrical activity originates within networks limited to one cerebral hemisphere, manifesting as seizures that may remain localized or spread more widely, including potential progression to bilateral tonic-clonic seizures. 1, 2

Core Definition and Pathophysiology

• Focal seizures arise from abnormal epileptic firing of brain cells in a localized area or areas of the brain, distinguishing them from generalized seizures that rapidly affect both hemispheres simultaneously 3, 1
• These seizures originate within networks of a single cerebral hemisphere and may remain localized or become more widely distributed 4, 5
• Focal epilepsies account for the majority of epilepsy cases in both children and adults, making them the most common epilepsy type encountered in clinical practice 2, 6

Clinical Classification

Focal seizures are subdivided into three primary categories based on level of awareness and spread 2:

• Focal aware seizures (previously called simple partial seizures) - consciousness is retained throughout the event 4
• Focal impaired awareness seizures (formerly complex partial seizures) - consciousness is altered or lost during the seizure 4, 2
• Focal to bilateral tonic-clonic seizures (previously called secondarily generalized seizures) - focal seizures that propagate to involve both hemispheres 2, 7

Etiologic Categories

In Adults

The International League Against Epilepsy categorizes focal epilepsy etiologies into structural, genetic, infectious, metabolic, immune, and unknown causes 5:

• Structural causes include hippocampal sclerosis (the most common cause of temporal lobe focal seizures), acquired lesions from stroke and cerebral infarction, brain tumors, vascular malformations, and traumatic brain injury 1, 2
• Infectious causes such as encephalitis or CNS infections 1
• Metabolic, autoimmune, and genetic causes may also underlie focal epilepsy in adults 2

In Children

Pediatric focal epilepsy has a multifactorial etiology with distinct patterns 4:

• Malformations of cortical development (MCD) are the most common cause of medically refractory focal epilepsy in children, with focal cortical dysplasia (FCD) being the most frequent MCD subtype 4, 1
• Low-grade epilepsy-associated brain tumors (LEAT) are an important structural cause 4
• Birth-related lesions including perinatal stroke or hemorrhages 4, 1
• Other developmental abnormalities such as polymicrogyria (excessive small gyri with cortical overfolding) and hemimegalencephaly (congenital unilateral cortical malformation) 4, 1

Diagnostic Yield and Clinical Significance

• Neuroimaging positivity rates are considerably higher in focal seizures compared to generalized seizures - approximately 50% positive findings in focal seizures versus only 6% in generalized seizures 4
• The presence of any focal feature to a seizure is independently associated with clinically relevant abnormalities on neuroimaging 4
• Focal seizures have a recurrence frequency up to 94%, considerably greater than generalized seizures at 72% 4

Special Syndromes

• Certain focal epilepsy syndromes are sufficiently characteristic to be diagnosed clinically or through specific EEG patterns and usually do not require imaging, including benign rolandic epilepsy and benign occipital epilepsy with classic EEG findings 4, 8
• The term "benign" in these syndromes implies easily treated or untreated seizures with remission before adulthood, though mild neuropsychological impairment may occur even in typical cases 8

Clinical Pitfall

A critical distinction exists between true primary generalized seizures and focal seizures with secondary generalization (focal to bilateral tonic-clonic) - the latter begin focally and then propagate to both hemispheres, requiring different diagnostic and treatment approaches than primary generalized epilepsy 7, 2