Absence seizure


Absence seizures are one of several kinds of generalized seizures. Absence seizures are characterized by a brief loss and return of consciousness, generally not followed by a period of lethargy. Absence seizures are most common in children. They affect both sides of the brain.
In the past, absence epilepsy was referred to as "pyknolepsy," a term derived from the Greek word "pyknos," signifying "extremely frequent" or "grouped". These seizures are sometimes referred to as petit mal seizures ; however, usage of this terminology is no longer recommended.
Childhood absence epilepsy represents a significant portion, accounting for approximately 10 to 17%, of all cases of childhood-onset epilepsy, establishing it as the most common form of pediatric epilepsy. This syndrome is characterized by daily occurrences of frequent but brief episodes of staring spells. These episodes typically commence between the ages of 4 and 8 years and manifest in otherwise seemingly healthy children. On classic electroencephalograms, distinct patterns emerge, featuring generalized spike-wave bursts occurring at a frequency of 3 Hz, accompanied by normal background brain activity. Despite sometimes being mistakenly perceived as a benign type of epilepsy, childhood absence epilepsy is associated with varying rates of remission. Children affected by this condition often experience cognitive deficits and encounter enduring psychosocial challenges in the long term.

Epidemiology

The incidence of absence seizures in the United States is 1.9–8 cases per 100,000 population. The morbidity from typical absence seizures is related to the frequency and duration of the seizures, as well as to the patient's activities; effective treatment ameliorates these factors. Educational and behavioral problems often result from frequent, unrecognized seizures. No deaths result directly from absence seizures. However, if an individual suffers an absence seizure while driving or operating dangerous machinery, a fatal accident may occur.
Absence seizures affect between 0.7 and 4.6 per 100,000 in the general population and 6 to 8 per 100,000 in children younger than 15 years. Childhood absence seizures account for 10% to 17% of all absence seizures. Onset is between 4 and 10 years and peaks at 5 to 7 years. It is more common in girls than in boys.

Cause

An absence seizure is specifically caused by multifactorial inheritance. The voltage-gated T-type calcium channel is regulated by Gamma-aminobutyric acid receptor subunit gamma-2, GABRG3, and CACNA1A2 genes. Inheritance of these genes is involved in the etiology of absence seizure. The commonly held belief is that the genetic factor is the primary cause of childhood absence epilepsy. Furthermore, patients with childhood absence epilepsy have also been reported to exhibit certain copy number variations, such as 15q11.2, 15q13.3, and 16p13.11 microdeletions. Almost 25% of children suffering from absence seizure has a relative that suffers from seizures. Some specific anticonvulsant drugs such as phenytoin, carbamazepine, and vigabatrin have been identified to raise the chances of experiencing absence seizures.

Signs and symptoms

The clinical manifestations of absence seizures vary significantly among patients. Impairment of consciousness is the essential symptom, and may be the only clinical symptom, but this can be combined with other manifestations. The hallmark of the absence seizures is abrupt and sudden-onset impairment of consciousness, interruption of ongoing activities, a blank stare, possibly a brief upward rotation of the eyes. If the patient is speaking, speech is slowed or interrupted; if walking, they stand transfixed; if eating, the food will stop on its way to the mouth. Usually, the patient will be unresponsive when addressed. In some cases, attacks are aborted when the patient is called. The attack lasts from a few seconds to half a minute and evaporates as rapidly as it commenced. Absence seizures generally are not followed by a period of disorientation or lethargy, in contrast to the majority of seizure disorders. If the patient has jerking gestures during the seizure this might be the indication of another type of seizure occurring onward with the absence seizure.
  1. Absence with impairment of consciousness only as per the above description.
  2. Absence with mild clonic components. Here the onset of the attack is indistinguishable from the above, but clonic components may occur in the eyelids, at the corner of the mouth, or in other muscle groups which may vary in severity from almost imperceptible movements to generalised myoclonic jerks. Objects held in the hand may be dropped.
  3. Absence with atonic components. Here there may be a diminution in tone of muscles subserving posture as well as in the limbs leading to dropping of the head, occasional slumping of the trunk, dropping of the arms, and relaxation of the grip. Rarely tone is sufficiently diminished to cause this person to fall.
  4. Absence with tonic components. Here during the attack tonic muscular contraction may occur, leading to increase in muscle tone which may affect the extensor muscles or the flexor muscles symmetrically or asymmetrically. If the patient is standing, the head may be drawn backward and the trunk may arch. This may lead to retropulsion, which may cause eyelids to twitch rapidly; eyes may jerk upwards or the patients head may rock back and forth slowly, as if nodding. The head may tonically draw to one or another side.
  5. Absence with automatisms. Purposeful or quasi-purposeful movements occurring in the absence of awareness during an absence attack are frequent and may range from lip licking and swallowing to clothes fumbling or aimless walking. If spoken to, the patient may grunt, and when touched or tickled may rub the site. Automatisms are quite elaborate and may consist of combinations of the above described movements or may be so simple as to be missed by casual observation.
  6. Absence with autonomic components. These may be pallor, and less frequently flushing, sweating, dilation of pupils and incontinence of urine.
Mixed forms of absence frequently occur. These seizures can happen a few times a day or in some cases, hundreds of times a day, to the point that the person cannot concentrate in school or in other situations requiring sustained, concentrated attention.

Risk factors

Typical absences are easily induced by hyperventilation in more than 90% of people with typical absences. This is a reliable test for the diagnosis of absence seizures: a patient suspected of typical absences should be asked to hyperventilate for three minutes, counting breaths. During hyperventilation, the oxygen and carbon dioxide level will become abnormal. This results in weakening of electrical signal which leads to a reduction in the seizure threshold. Intermittent photic stimulation may precipitate or facilitate absence seizures; eyelid myoclonia is a common clinical feature.
A specific mechanism difference exists in absence seizures in that T-type Ca++ channels are believed to be involved. Ethosuximide is specific for these channels and thus it is not effective for treating other types of seizures. Valproate and gabapentin have multiple mechanisms of action including blockade of T-type Ca++ channels, and are useful in treating multiple seizure types. Gabapentin can aggravate absence seizures.

Pathophysiology

The corticothalamic cortical circuit plays an important role in the pathophysiology of absence seizure. Some of the neurons are important in their occurrence. They are
  • Cortical glutamatergic neurons
  • Thalamic relay neurons
  • Neurons of thalamic nucleus reticularis
Abnormal oscillatory rhythms develop in the thalamic nucleus reticularis. This causes inhibition of GABAergic neurotransmission and excitation of glutamate neurotransmission. Abnormal oscillatory spikes are produced by the low threshold T-type calcium channel. This explains how inheritance of gene code for T-type calcium channel leads to an absence seizure. Antiepileptic drugs such as Gabapentin, Tiagabine and Vigabatrin cause inhibition of GABA resulting in exacerbation of absence seizures.

Diagnosis

The primary diagnostic test for absence seizures is electroencephalography. However, brain scans such as by an MRI can help rule out other diseases, such as a stroke or a brain tumor.
During EEG, hyperventilation can be used to provoke these seizures. Ambulatory EEG monitoring over 24 hours can quantify the number of seizures per day and their most likely times of occurrence.
Absence seizures are brief generalized epileptic seizures of sudden onset and termination. When someone experiences an absence seizure they are often unaware of their episode. Those most susceptible to this are children, and the first episode usually occurs between 4 and 14 years old. In the case of JAE, the typical age at which it begins is traditionally within the range of 10 to 19 years, with the highest occurrence observed around the age of 15. Unlike CAE, seizures in JAE are not as frequent but tend to have a longer duration. It is very rare that someone older will experience their first absence seizure. Episodes of absence seizures can often be mistaken for inattentiveness when misdiagnosed, and can occur 50–100 times a day. They can be so difficult to detect that some people may go months or years before being given a proper diagnosis. The majority of children experiencing typical absence seizures have an overall normal health condition. However, these absence seizures can disrupt the learning process and hinder concentration in a school environment. This underscores the crucial significance of treatment There are no known before or after effects of absence seizures.
Absence seizures have two essential components:
  • Clinical the impairment of consciousness
  • EEG the EEG shows generalized spike-and-slow wave discharges
Absence seizures are broadly divided into typical and atypical types:
  • Typical absence seizures usually occur in the context of idiopathic generalised epilepsies and an EEG shows fast >2.5 Hz generalised spike-wave discharges. The prefix "typical" is to differentiate them from atypical absences rather than to characterise them as "classical" or characteristic of any particular syndrome.
  • Atypical absence seizures:
  • * Occur only in the context of mainly severe symptomatic or cryptogenic epilepsies of children with learning difficulties who also have frequent seizures of other types, such as atonic, tonic and myoclonic.
  • * Have slower onset and termination and changes in tone are more pronounced.
  • * Have particular ictal characteristics: EEG is of slow spike and slow wave. The discharge is heterogeneous, often asymmetrical and may include irregular spike and slow wave complexes, fast and other paroxysmal activity. Background interictal EEG is usually abnormal.