Classification of Epileptic Seizures

The International League Against Epilepsy (ILAE) created an international recognized system of classification in 1981, that separated epileptic seizures into two broad divisions; partial seizures characterized by the focal cerebral localization of onset and generalized seizures which demonstrate excessive neural discharges within both hemispheres simultaneously. As with many classifications, these do not account for all forms of seizure and many remain unclassifiable. The ILAE is currently looking to create a new system of classification which better meets the broad range of seizure criteria. In 1997 it was proposed classifications should fall under self-limiting and continuous seizures, though this has not been adopted.

The International League Against Epilepsy (ILAE) Classification of Seizures

Partial Seizures

Theses are seizures whose etiologies lie in focal cerebral defects. Due to the site specific nature of these conditions, the symptoms of a seizure can be mapped against maps of brain function (see below), for instances visual disturbances could traced to the visual cortex or association areas. Most commonly the frontal and temporal lobes are affected, though just about any region of the brain can be affected. Partial seizures can occur at any age.

Functional map of the brain; from https://en.wikipedia.org/wiki/File:Brain_2.jpg  under the wikimedia commons liscence

Simple partial seizures are defined as those in which the consciousness of the individual is unaffected. Diagnosis relies on the stereotyped recurrence of somatosensory, motor, autonomic, or psychic disturbances during seizure. Effects usually last from a few seconds to minutes, though partial status epilepticus is known to occur (see below).

Though partial seizures are generated in specific areas of the brain, they are not necessarily constrained to them, consequently numerous symptoms may be experienced. Complex partial seizures involve more than one component of the seizures symptom and therefore more than one cerebral area. In about 60% of cases these arise from the temporal lobe, 30% from the frontal lobe and 10% from other areas. A typical complex partial seizure arising from the frontal or temporal lobe consist of:

• A usually brief psychic manifestation (a.k.a. an aura)
• A following or simultaneous alteration to the state of consciousness, often known as an absence seizure due to the arrest of any motor activity.
• Involuntary motor actions known as automatisms within a period impaired consciousness

Partial seizures have been known to propagate throughout the entirety of the brain, creating secondary generalized seizures. It is possible for simple and complex partial seizures to occur separately within the same individual.

Generalized Seizures

These exhibit excessive hypersynchronised neural activity arising from multiple locations and can often charaterised by the simultaneous discharge of both cerebral hemispheres:

• Absence seizures – Brief episodes of impaired consciousness, during which the effected loses contact with their environment and ceases motor activity, possibly accompanied by minor automatisms such as facial twitches and blinking. After such an attack, activity resumes as before, and in many instances the patient is unaware of the seizure at all. Around 80% of absence seizures (a.k.a. petit mal seizures) last less than 10 seconds and can occur in the region of 100 times a day. Such seizures typically begin in childhood or adolescence, may be provoked by hyperventilation or light stimulation, and occur almost exclusively with idiopathic generalized epilepsies. Absence seizures present a characteristic rhythmic and symmetric, 3hz spike wave paroxysm EEG trace. Atypical absence seizures present different clinical symptoms, EEGs and aetiologies though the general loss of consciousness without dramatic loss of tone is still observed.
• Myoclonic seizures – brief contraction of a, or several, muscle groups, varying from unperceivable twitches to dramatic jerks that may occur once or evolve into clonic seizures. Attacks last less than a second and recovery is fast, consciousness usually being retained. Myoclonic seizures are manifestations of a number of very different epilepsies and may even be experienced in their absence, such as when falling asleep. Epileptic seizures can be differentiated by a characteristic assymetrical spike wave EEG trace. Myoclonic seizures are also experienced in some focal seizures such as focal occipital lobe epilepsy and are one of the three forms of idopathic seizure.
• Clonic seizures – irregular, recurrent jerking muscle contractions in which consciousness may be retained or lost. They most commonly frequent neo nates, infants and young children, and are always symptomatic. EEG show fast activity mixed with large amplitude slow waves. Care should be taken not confuse this with bilateral clonic seizures which arise from partial seizures in the frontal lobe.
• Tonic seizures – sudden extension or flexion of limbs or the contraction of other muscles persisting for several seconds (usually less than 60) accompanied by altered conscious states and possibly a cry followed by cessation of respiration. Seizures are linked with drowsiness with an increased incidence around sleep. They may occur at any age with diffuse cerebral damage and learning disability, and subsequently often associated with other seizure types. EEGs show flattening fast activity (15-25Hz) with increasing amplitude with the progression of the attack though this is seldom typical, being subject to a large level variation and may even evolve into spike-wave patterns. Confusion should be avoided with partial motor seizures showing primarily tonic features and partially treated tonic-clonic seizures.
• Primary generalized tonic-clonic seizures – The stereotyped epileptic “fit” (a.k.a. grand mal seizures), exhibit a number of well defined stages. First a loss of consciousness occurs, then a loss of body tone and a consequent fall. The proceeding tonic phase consists a brief period of tonic flexion is proceeded by a longer period of rigid extension during which eyes are rolled and jaws and fists clenched and respiration ceases. This may last 10 to 30 seconds and is followed by a longer clonic phase lasting usually slightly under a minute. The final phase in which consciousness is regained may last anywhere between 2 and 30 minutes. Sometimes these seizures are preceded by  tora (a partial seizure itself), in which case the seizure is secondarily generalised. Tonic clonic seizures can occur at any age and are encountered with numerous forms of epilepsy, including generalized idiopathic forms. EEGs taken during seizures vary widely in appearance, with bi-lateral characterstics matching tonic and clonic seizures depending on the phase of the attack.
• Atonic seizures – brief loss of postural tone, often resulting in falls, the most severe form of which is known as “drop attacks”. Many forms are milder resulting in localised and/or partial losses of tone, for instance causing sagging at the knees or nodding of the head. Seizures may be progressive exhibiting increasingly severe symptoms as long as it persists. Atonic seizures are known to occur at any age, and are always associated with diffuse cerebral damage learning disabilities and are common in sever symptomatic epilepsies. EEGs are irregular and show, one or a mix of, waveforms.

Up to a third of seizures fall into none of the above classifications, displaying EEG traces that fail to conform to any known characterisations, even on extensive review. 

Seizures are usually self limiting and commonly terminate within 5 minutes. Status Epilepticus may be used to describe any the above seizures when they persist, or the seizure reccurs without the patient regaining consciousness over periods longer than 30 minutes (though there is some debate over this). This may be potentially life threatening in instances where a loss of consciousness is caused or evolves, or in other instances such as in seizures that alter the regulation of the autonomic nervous system. Due an associated progressive resistance to anticonvulsant medication, immediate medical treatment should be sought. There is some evidence that seizures lasting more than 5 minutes  may damage brain cells.