What are the different types of seizures and their characteristics?

Types of Seizures

Seizures are classified into three primary categories based on their origin: focal onset (arising from one hemisphere), generalized onset (rapidly affecting both hemispheres), and unknown onset, with each category further subdivided by motor and non-motor features. 1, 2

Primary Classification Framework

The International League Against Epilepsy (ILAE) 2017 classification system organizes seizures based on their origin in the brain rather than underlying mechanisms, as the fundamental mechanisms remain incompletely understood 1. This classification is crucial for accurate diagnosis, treatment selection, and prognostication 1.

Three Main Categories by Onset

• Focal onset seizures arise within networks of a single cerebral hemisphere and may remain localized or become more widely distributed 1, 2
• Generalized onset seizures rapidly affect both hemispheres and both sides of the body from the very beginning 1, 2
• Unknown onset seizures are classified when the origin cannot be determined 2

Focal Onset Seizures

Key Characteristics

• These seizures originate in networks limited to one hemisphere 1, 2
• They can be further classified as motor or non-motor (previously called "simple partial" when awareness is retained and "complex partial" when awareness is impaired) 2
• Focal seizures may evolve to bilateral tonic-clonic seizures (previously termed "secondary generalization") 1, 2

Clinical Significance

• Patients with focal seizures, particularly those with complex symptomatology (psychomotor, temporal lobe), often show greater improvement with anticonvulsant treatment than those with other seizure types 3
• Carbamazepine is specifically indicated for partial seizures with complex symptomatology 3

Generalized Onset Seizures

Definition and Subtypes

The term "generalized" indicates diffuse bilateral brain involvement from the very onset, fundamentally different from focal seizures that secondarily spread 1. Generalized seizures are subdivided into:

• Tonic-clonic seizures (grand mal) - characterized by stiffening followed by rhythmic jerking 2, 3
• Other motor types including:
  • Myoclonic seizures 2
  • Atonic seizures 2
  • Clonic seizures 2
  • Tonic seizures 2
• Non-motor (absence) seizures - brief lapses in awareness 2

Important Clinical Distinctions

• Absence seizures (petit mal) do not respond to carbamazepine and require different treatment approaches 3
• Primary generalized epilepsy syndromes include juvenile myoclonic epilepsy and childhood absence epilepsy 1
• Many generalized seizures have genetic underpinnings, which is why MRI is rarely indicated in neurologically normal patients presenting with generalized seizures (low yield of positive intracranial findings) 1

Motor vs. Non-Motor Classification

Motor Seizures

All seizure types (focal, generalized, or unknown onset) can manifest with motor features 2. Motor manifestations include:

• Tonic (stiffening) 2
• Clonic (rhythmic jerking) 2
• Myoclonic (brief shock-like jerks) 2
• Atonic (loss of muscle tone) 2
• Epileptic spasms 2

Non-Motor Seizures

Non-motor seizures can also occur with any onset type and include absence seizures and other seizures without prominent motor features 2.

Clinical Context: Provoked vs. Unprovoked

Provoked (Acute Symptomatic) Seizures

• Occur at the time of or within 7 days of an acute insult 1
• Causes include hyponatremia, other electrolyte abnormalities, alcohol or drug withdrawal, toxic ingestions, encephalitis, or acute CNS mass lesions 1
• These do NOT define epilepsy 1

Unprovoked Seizures

• Occur without acute precipitating factors, more than 7 days after any insult 1
• Include idiopathic seizures and remote symptomatic seizures (from prior stroke, traumatic brain injury, or other past events) 1
• Recurrent unprovoked seizures define epilepsy 1

Epilepsy Definition

The ILAE defines epilepsy as having:

• At least two unprovoked seizures occurring more than 24 hours apart, OR 1, 2
• One unprovoked seizure with high probability of recurrence (similar to the general recurrence risk after two unprovoked seizures), OR 1, 2
• Diagnosis of an epilepsy syndrome 1, 2

Common Pitfalls

• Do not confuse focal seizures with secondary generalization with primary generalized seizures - the former start focally and then propagate, while the latter affect both hemispheres from onset 1
• Absence seizures will not respond to carbamazepine - this is a critical treatment consideration 3
• Avoid falsely diagnosing epilepsy when uncertain - the diagnosis has significant treatment and psychosocial implications; it is better to postpone diagnosis than to incorrectly label a patient 2
• Remember that approximately 10% of the population will experience at least one seizure during their lifetime - a single seizure does not equal epilepsy 2