Classification and Mechanisms of Epileptic Seizures

Exam Tip: If a question describes a patient who is awake but experiencing strange smells or "goosebumps," look for Focal Onset Aware symptoms. If they are performing repetitive, purposeless movements like rubbing their hands, think Automatisms in a Focal Impaired Awareness seizure.

Clinical Manifestations and Motor Symptoms

Seizures are often categorized by their physical presentation. The following terms describe common motor patterns:

• Tonic-Clonic Seizures: A severe seizure involving two phases. The tonic phase involves sustained muscle contraction and limb extension, followed by the clonic phase, characterized by rhythmic limb jerking.
• Absence Seizures: Often referred to as "petit mal," these are non-motor seizures characterized by brief staring spells and a sudden loss of awareness.
• Atonic Seizures: Characterized by a sudden loss of muscle tone, causing the patient to become limp or weak.
• Myoclonus: Brief, sudden muscle twitches.
• Epileptic Spasms: Repeated, sudden flexing and extending of the body.

Cellular and Electrophysiological Mechanisms

To understand why seizures occur, we must look at the behavior of individual neurons and the electrical patterns they create.

The Role of Voltage-Gated Sodium Channels (VGSC)

At the single-cell level, the propagation of an action potential is controlled by the transition of voltage-gated sodium channels between three states: resting, open, and inactivated.

When a neuron depolarizes, the VGSC opens rapidly, allowing sodium influx. Shortly after, the channel enters an inactivated state. This inactivation is crucial because it imposes a "time-limit" on how frequently a neuron can fire. If the time between action potentials is too short, the channels cannot recover to the resting state, preventing repetitive firing. Seizures occur when groups of neurons override these limits, firing in a repetitive, synchronized fashion.

Paroxysmal Depolarization Shift (PDS) and EEG Patterns

When clinicians perform an Electroencephalogram (EEG), they look for specific electrical signatures. One critical finding is the Interictal Spike Discharge, which represents the synchronous depolarization of a group of neurons between actual seizure events.

The underlying mechanism for these intense electrical bursts is the Paroxysmal Depolarization Shift (PDS). This is a high-voltage, long-duration depolarization current caused by a massive influx of $Na^+$ and $Ca^{2+}$ ions.

The Mechanism of Absence Seizures

Absence seizures have a very specific electrophysiological signature: a 3-Hz spike-and-wave pattern on an EEG. This pattern is driven by abnormal activity in the thalamus, specifically involving T-type $Ca^{2+}$ channels.

Summary Table: Key Seizure Differences