Basic Classification of Seizures
INTRODUCTION
What is the difference between a seizure and epilepsy?
A seizure is a sudden, uncontrolled electrical disturbance in the brain.
Epilepsy is a condition in which a person has a risk of recurrent seizures due to a chronic, underlying process.
Epilepsy is a clinical syndrome with many causes and predisposing factors
Some causes of epilepsy are epilepsy syndromes in which clinical and pathologic characteristics are distinctive and suggest a specific aetiology
CLASSIFICATION OF SEIZURES
Seizures are classified based on the guidelines by International League Against Epilepsy (ILAE) Commission on Classification and Terminology
This classification is based on clinical features and EEG findings
Fundamentally seizures are classified into:
Focal seizures → originate within networks limited to 1 brain region
Generalised seizures → originate and rapidly spread to networks across both cerebral hemispheres
FOCAL ONSET SEIZURES
Focal seizures arise from a neuronal network that is localised to a single region or is wide spread but is still contained within a single hemisphere
Previously focal seizures were sub-classified into simple and complex seizures based on the consciousness. But this terminology has been eliminated
Current classification of focal seizures is based on:
consciousness level → impaired or intact
nature of onset → motor or non motor
EEG
in partial seizures it may be normal or may show brief discharges → epileptiform spikes or sharp waves
focal seizures can arise from the medial temporal lobe or inferior frontal lobe (i.e., regions distant from the scalp), the EEG recorded during the seizure may be nonlocalizing.
Focal Seizures with Intact Awareness
Focal seizures with intact awareness can present as:
Motor - tonic, clonic, myoclonic movements
Non motor - sensory, autonomic, emotional
Features of focal motor seizures are
JACKSONIAN MARCH ⇒
Some seizures start in very restricted regions like the fingers and progress (over seconds to minutes) to include a larger portion of the extremity.
This represents the spread of the seizure activity over a larger region of the motor cortex.
TODD'S PARALYSIS ⇒
localised paresis in the involved area following the seizure → may last for minutes to hours.
EPILEPSIA PARTIALIS CONTINUA ⇒
rarely, seizures continue for hours or days → often is found to be refractory to medical therapy
Non motor focal seizures
Focal seizures with intact awareness may also manifest as changes in
somatic sensation → paresthesias
vision → flashing lights or formed hallucinations
equilibrium → sensation of falling or vertigo
autonomic function → flushing, sweating, piloerection
some patients described internal feelings such as fear, sense of impending change, detachment, depersonalization, deja vu, micropsia, macropsia ⇒ these subjective internal feelings are called AURAS
Focal Seizures with Impaired Awareness
Focal seizures may also present with the patient's inability to maintain normal contact with the environment
Patients are typically unable to respond properly to visual or verbal commands
Frequently, these seizures begin with an Aura → ictal phase starts with a motionless stare
AUTOMATISMS → involuntary, automatic behaviours that have a wide range of manifestations
eg- chewing, lip smacking, swallowing, picking movements
Usually there is some post ictal confusion that is present and complete recovery may take up to few days
Other post ictal complications
anterograde amnesia may be present
transient neurological deficits - aphasia, hemineglect, visual loss
post ictal inhibition of the cortical regions that caused the seizures.
EVOLUTION OF FOCAL SEIZURES TO GENERALISED SEIZURES
Focal seizures can spread to involve both cerebral hemispheres and produce a generalised seizure
This phenomenon is commonly seen in seizures originating in the frontal lobe
It is difficult to differentiate a partial seizure which becomes generalised and a GTCS → mostly because on giving history, the bystanders tend to focus on the tonic clonic movements
EEG studies can distinguish between the two.
Distinction is really important because the further evaluation and treatment of seizures is different for the two types of seizures.
GENERALISED ONSET SEIZURES
These seizures start at some point in the brain but rapidly involve networks in both cerebral hemispheres
The following are a few subtypes of generalised onset seizures.
Typical Absence Seizures
These seizures are characterised by sudden brief lapses of consciousness WITHOUT loss of postural control.
It usually last for only a few seconds → consciousness returns as suddenly as it was lost
There is usually no post ictal confusion
Absence seizures are usually accompanied by subtle, bilateral motor signs such as rapid blinking of the eyelids, chewing movements, or small-amplitude, clonic movements of the hands.
Some of these typical absence seizures are genetically determined
seen in childhood 4-10 years of age or early adolescence
may have 100s of seizures per day → child maybe unaware or unable to convey them
may present as frequent day dreaming and a decline in scholastic performance
EEG
hallmark of absent seizures → generalised symmetric 3Hz spike-and-slow wave discharges that begin and end suddenly
HYPERVENTILATION can provoke these changes and the absence seizure as well → used as a tool in EEG recording
Atypical Absence Seizures
Characterised by lapse or loss of consciousness which is a lot longer and less abrupt in onset and cessation than typical absence seizures
Also associated with more obvious motor signs → may include focal localizing signs
EEG
generalised slow spike and slow wave pattern → frequency of 2.5Hz
Associated with diffuse of multifocal structural abnormalities of the brain
Less responsive to anticonvulsants
Generalized Tonic Clonic Seizures
This is the most common seizure type resulting from metabolic derangements → frequently encountered in many settings
The onset is abrupt and without warning
Some patients have vague premonitory symptoms in the hours before the seizure → AURA
Clinical Phases of GTCS
Tonic contractions
respiratory and laryngeal muscle contraction → respiratory cry
respiration is impaired + secretions pool in the oropharynx → cyanosis develops
jaw muscle contraction → tongue bite
sympathetic tone increase → increase in heart rate, blood pressure and pupillary size
Clonic phase
superimposition of periods of muscle relaxation over the tonic muscle contraction.
the periods of rest in between keep increasing until the seizure activity stops → usually does not last more than 1 minute
Post ictal phase
characterised by unresponsiveness, muscular flaccidity, and excessive salivation → stridorous breathing and partial airway obstruction
bladder or bowel incontinence
consciousness is regained within minutes to hours → but may be associated with postictal confusion
EEG
Tonic phase - progressive increase in generalised low-voltage fast activity, followed by generalised high-amplitude, polyspike discharges
Clonic phase - high-amplitude activity is typically interrupted by slow waves to create a spike-and-slow-wave pattern
Postictal waves - diffuse suppression of all cerebral activity, then slowing that gradually recovers as the patient awakens.
Atonic Seizures
Characterised by sudden loss of postural muscle tone lasting 1-2s.
Consciousness is very briefly impaired and there is usually no post ictal confusion
Clinically it may present as a quick head drop or nodding movement → dangerous because it can cause trauma
EEG - spike and wave discharges followed by slow waves which correlate with muscle tone
Commonly seen in association with epilepsy syndromes
Myoclonic Seizures
Myoclonus → sudden and brief muscle contraction that may involve one part of the body or the entire body
Normal physiologic myoclonus → sudden jerking movements observed while going to sleep
Pathologic myoclonus is seen in:
metabolic disorders
degenerative CNS diseases
anoxic brain injury
Myoclonic seizures → are true epileptic events that are caused by cortical dysfunction
EEG → bilaterally synchronous spike and slow wave discharges immediately prior to the movement
Predominantly a part of juvenile myoclonic epilepsy.
Epileptic Spasms
These are brief sustained flexion or extension of proximal> distal muscles
EEG -
Hypsarrythmias → diffuse giant slow waves with a chaotic background or irregular multifocal spikes
during the episode, there is a decrease in the background waves → Electrodermal response
EMG → rhomboid shape which can help differentiate between muscle spasm and tonic and myoclonic seizures
Occurs predominantly in infants.
Check out more about EEGs in the following articles:
Author: Narendran Sairam (Facebook)
Sources and citations
“Seizures and Epilepsy” Harrison's Principles of Internal Medicine, by J. Larry Jameson et al., 20th ed., vol. 2, McGraw-Hill Education, 2018.