6.1 Seizure Classification, Etiology & Pathophysiology

Seizure Classification, Etiology & Pathophysiology

Introduction to the ILAE 2017 Classification

The International League Against Epilepsy (ILAE) released a revised classification of seizure types in 2017, representing a significant paradigm shift from previous versions. The primary goal of this updated framework is to provide a more accurate, clinically relevant, and transparent system for diagnosing and managing seizures. At the most fundamental level, the ILAE 2017 classification categorizes seizures based on their onset into three main groups: focal onset, generalized onset, and unknown onset.

Focal onset seizures originate within networks limited to one hemisphere. They may be discretely localized or more widely distributed. Generalized onset seizures arise within and rapidly engage bilaterally distributed networks. Unknown onset is a category reserved for when the start of the seizure is not witnessed or cannot be determined with a high degree of confidence.

Focal Seizures and Level of Awareness

A critical modification in the 2017 classification is the emphasis on the level of awareness during focal seizures. Focal seizures are now explicitly subclassified as either "focal aware" (formerly simple partial) or "focal impaired awareness" (formerly complex partial). Awareness is used as a surrogate marker for the degree of network involvement and consciousness. If awareness is impaired at any point during the seizure, it is classified as a focal impaired awareness seizure. Additionally, focal seizures are further described by their first prominent sign or symptom, such as motor (e.g., automatisms, atonic, clonic, epileptic spasms, hyperkinetic, myoclonic, tonic) or non-motor (e.g., autonomic, behavior arrest, cognitive, emotional, sensory).

An important concept retained and clarified in the 2017 update is the "focal to bilateral tonic-clonic" seizure (formerly secondary generalized). This describes a seizure that begins focally and subsequently spreads to engage bilateral networks, resulting in a tonic-clonic manifestation. Recognizing this progression is vital for appropriate antiepileptic drug (AED) selection and determining the potential for epilepsy surgery.

Generalized Seizures

Generalized onset seizures are divided into motor and non-motor (absence) types. Motor types include tonic-clonic, clonic, tonic, myoclonic, myoclonic-tonic-clonic, myoclonic-atonic, atonic, and epileptic spasms. Non-motor absence seizures are subcategorized into typical, atypical, myoclonic, and those with eyelid myoclonia. The distinction between typical and atypical absence is crucial; typical absence seizures typically feature sudden onset and offset, whereas atypical absence seizures have less abrupt onset/offset and are often associated with other cognitive impairments.

Etiology Framework

The ILAE framework also emphasizes the etiology of epilepsy, which has profound implications for treatment, prognosis, and genetic counseling. Etiologies are categorized into six groups: structural, genetic, infectious, metabolic, immune, and unknown.

1. Structural: Involves visible abnormalities on structural neuroimaging (e.g., MRI) that are known to be associated with an increased risk of epilepsy. Examples include mesial temporal sclerosis, malformations of cortical development, tumors, stroke, and traumatic brain injury.
2. Genetic: Epilepsy directly results from a known or presumed genetic defect. This does not necessarily mean the condition is inherited; many are de novo mutations.
3. Infectious: This is the most common etiology worldwide, including neurocysticercosis, tuberculosis, HIV, cerebral malaria, subacute sclerosing panencephalitis (SSPE), and post-infectious encephalitis.
4. Metabolic: Involves a known or presumed metabolic defect causing epilepsy, such as porphyria, uremia, aminoacidopathies, or pyridoxine-dependent epilepsy.
5. Immune: Autoimmune-mediated CNS inflammation, such as anti-NMDA receptor encephalitis or LGI1 encephalitis.
6. Unknown: The cause remains undetermined despite appropriate evaluation.

Pathophysiology and Channelopathies

At a cellular level, epileptogenesis—the process by which a normal brain develops epilepsy—involves a complex interplay of neuronal excitability, network synchronization, and neuroinflammation. The fundamental mechanism of a seizure is an imbalance between excitatory and inhibitory neurotransmission. Glutamate is the primary excitatory neurotransmitter, acting on NMDA, AMPA, and kainate receptors, leading to calcium and sodium influx and subsequent depolarization. Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter, opening chloride channels (GABA-A) or potassium channels (GABA-B) to hyperpolarize the neuron. During a seizure, there is a paroxysmal depolarizing shift (PDS), a sustained depolarization of the neuronal membrane resulting in a burst of action potentials.

Recent advances in neurogenetics have highlighted the critical role of channelopathies in the pathophysiology of genetic epilepsies. Channelopathies are diseases caused by disturbed function of ion channel subunits or the proteins that regulate them.

SCN1A and Dravet Syndrome

The most well-characterized channelopathy is associated with the SCN1A gene, which encodes the alpha subunit of the voltage-gated sodium channel Nav1.1. Mutations in SCN1A are the primary cause of Dravet syndrome (Severe Myoclonic Epilepsy of Infancy). Dravet syndrome typically presents in the first year of life with prolonged, febrile or afebrile, focal or generalized clonic seizures, progressing to multiple seizure types, cognitive regression, and a high risk of Status Epilepticus (SE) and Sudden Unexpected Death in Epilepsy (SUDEP). Pathophysiologically, Nav1.1 is highly expressed on inhibitory GABAergic interneurons. Loss-of-function mutations in SCN1A selectively impair the firing of these inhibitory interneurons, leading to a net increase in cortical excitability and recurrent seizures. This explains why sodium channel blockers (like phenytoin or lamotrigine) can paradoxically exacerbate seizures in Dravet syndrome, as they further inhibit the already compromised inhibitory interneurons.

KCNQ2 and Benign Familial Neonatal Epilepsy

Another critical set of channelopathies involves the KCNQ2 and KCNQ3 genes, which encode voltage-gated potassium channels (Kv7.2 and Kv7.3). These channels mediate the M-current, a slowly activating and non-inactivating potassium current that strongly regulates subthreshold electrical excitability and limits repetitive neuronal firing. Mutations in KCNQ2 are classically associated with Benign Familial Neonatal Epilepsy (BFNE), characterized by frequent, brief focal or generalized seizures starting in the first week of life, which typically remit spontaneously by a few months of age. However, severe KCNQ2 encephalopathies also exist, presenting with neonatal onset intractable seizures and severe developmental delay.

Understanding these genetic channelopathies is not merely academic; it drives precision medicine in epileptology. For instance, in severe KCNQ2 encephalopathy, sodium channel blockers like oxcarbazepine or phenytoin may be highly effective, reflecting a specific pharmacogenomic interaction that counters the underlying potassium channel defect. The ongoing elucidation of the genetic architecture of epilepsy promises further targeted therapies and improved outcomes for patients with these complex neurological disorders.