Absence seizures may occur to patients suffering from idiopathic or symptomatic generalized epilepsy. They imply brief unawareness of one's surroundings and are less common in adults than in children.
An absence seizure typically consists of brief lapses of awareness, i.e. the patient will suddenly interrupt his or her doings, stare blankly into space and will not react to their surroundings. Sometimes rapid blinking of the eyelids or uncontrolled movement and jerking of an arm or a leg may be observed, too .
These lapses of awareness generally last a few seconds. They are associated with amnesia, as the patient does not remember what happened during the seizure. Frequently, the patient just continues doing what he or she has been doing before the seizure. Since absence seizures generally do not imply physical harm nor confusion, they are not followed by a recovery period.
Particularly in case of pyknoleptic CAE, the high frequency of absence seizures may hinder the child in his or her day-to-day activities, in paying attention to class, to sports, to games. Conversations with the child may frequently be interrupted by the child's attention "wandering". It is not uncommon that the child's teacher is the first person to notice something wrong. Either parents or teacher may complain about the child's lack of attention or about the child daydreaming because they are usually not familiar with absence seizures.
Since absence seizures are characterized by lapses of awareness, by the patient staring off into space, differential diagnoses have to be excluded. Those may include metabolic disorders and intoxications. A detailed medical history may give initial indications regarding the nature of those staring spells. If metabolic anomalies or an intoxication are presumed, laboratory tests may be required to confirm or rule out this possibility.
In case of absence seizures, an EEG may be diagnostic and usually shows features characteristic of certain types of epilepsy. In order to produce these features, it may be helpful to provoke an absence seizure. Therefore, the patient may be stimulated with flickering lights or just asked to breathe rapidly. A thorough analysis of the EEG should be carried out to identify the precise type of epilepsy and maybe even its cause.
If an EEG does not yield conclusive results or if organic anomalies are suspected, a brain scan may be indicated. Magnetic resonance imaging (MRI) is the method of choice to detect malformations, neoplasias and other pathological structures that may cause epilepsy .
Anti-epileptic treatment should aim at finding the appropiate dose for each individual requiring therapy. Here, the appropiate dose referes to a dose as high as necessary and as low as possible. Usually, treatment starts with a low dose that progressively increases until seizures are under control.
It is recommended to continue treatment until long after the last seizure, because relapses are frequent and often include tonic-clonic seizures. A seizure-free period of two years may justify therapy discontinuation, but decisions have to be taken on a case by case basis.
Absence seizures may be treated with the following drugs:
The distinct forms of absence epilepsy are associated with different prognoses. In general, young age at onset, physiological EEG background signals and satisfying response to therapy are good prognostic signals.
Patients suffering from CAE generally respond well to treatment and remission is possible. However, up to 40% of children diagnosed with CAE do develop tonic-clonic seizures. Should that be the case, chances for complete remission decrease.
Although the probability of ocurrence of tonic-clonic seizures is high in adolescents diagnosed with JME, they usually respond well to anti-epileptic treatment. Only few cases may be resolved by lifestyle adaptation (abstinence from alcohol, well controlled sleep regimen) and the majority of patients requires treatment for life because the relapse rate is >90% .
Epileptic seizures are triggered by abnormal or excessive electrical activity in the brain that is, in turn, caused by disorders in electrolyte and neurotransmitter balance . Sometimes this abnormal electrical acitivity is compared to cerebral short circuits. In an EEG, seizures are often characterized by abnormal, repetitive patterns.
Generally, no direct triggers can be identified. It has been reported that hyperventilation may provoke an absence seizure. Also, genetic predisposition seems to play an important role, especially in children . However, the majority of children suffering from absence seizures does overcome them when growing up. Of note, some children with absence seizures may also develop tonic-clonic seizures.
Epidemiological studies regarding absence seizures and those forms of epilepsy that may involve them are complicated by the existence of distinct classification systems.
Thus, the following data should merely provide orientation regarding the incidence of different forms of absence epilepsy. For children and adolescents younger than 18 years, studies conducted in the US showed that 4,000 cases of individuals with absence epilepsy and 1,500 individuals with JME were diagnosed each year .
Studies regarding the incidence of idiopathic generalized epilepsies have been carried out. In this context, an incidence of up to 8 per 100,000 inhabitants has been estimated for CAE . Prevalence of CAE is highest in children aged 2 to 9 years and is of up to 0.7 per 1,000 persons. Girls are affected significantly more often than boys. With regards to JAE, less data is available. The disease is usually diagnosed in adolescents aged 10 to 17 years and its prevalence has been estimated to be about 0.1 per 100,000 persons . JME is also diagnosed in adolescents, mainly in those aged 12 to 18 years, and the incidence is of approximately 1 per 100,000 inhabitants . Thus, JME is diagnosed more frequently than JAE, but less often than CAE. Similar to CAE, females seem to be affected more often. The prevalence has been estimated to be up to 0.2 per 2,000 persons. It has been speculated that JME does occur less frequently in developing countries.
With regard to symptomatic generalized epilepsies, few data is available regarding epilepsies with myoclonic absences and the Lennox-Gastaut syndrome . Interestingly, a male preponderance has been reported for epilepsies with myoclonic absences and the Lennox-Gastaut syndrome . The incidence of the Lennox-Gastaut syndrome has been estimated to be about 2 per 100,000 persons.
Current knowledge regarding pathophysiologic processes behind absence seizures is mainly derived from animal models. These animal models are based on a reverberating circuit generating abnormal, oscillating rhtyhms between cortex and thalamus. In an EEG, characteristic spike and wave complexes can be seen.
The reticulothalamic nucleus of the thalamus contains inhibitory gabaergic interneurons. Prolonged opening of the GABA-receptor is presumably associated with an increased influx of chloride ions and leads to hyperpolarization. Low voltage-gated calcium channels are active at such low membrane potentials, may cause rebound depolarization and trigger absence seizures. Indeed, absence seizures may be treated with ethosuximide, a drug blocking such calcium channels.
Because genetic predisposition does play an important role in the development of absence seizures, several studies have been carried out to identify those genes that contribute to idiopathic generalized epilepsies such as CAE, JAE and JME. Not surprisingly, genes that were proven to affect the incidence of idiopathic generalized epilepsies, encode for ion channels. Polymorphisms affecting the expression of GABA receptors and voltage-gated calcium channels may be responsible for CAE and JME. Furthermore, mutations in genes encoding voltage-gated chloride channels have been associated with CAE, JAE and JME. Certain types of absence epilepsies have been related to mutations affecting the expression of the neuronal nicotinic acetylcholine receptor and the GLUT1 glucose transporter .
No preventive strategies have been identified.
Patients suffering from in idiopathic or symptomatic generalized epilepsy may have absence seizures .
Childhood absence epilepsy (CAE), juvenile absence epilepsy (JAE) and juvenile myoclonic epilepsy (JME) are idiopathic generalized epilepsies that may involve typical absence seizures. In electroencephalography (EEG), generalized 3-4 Hz spike and slow wave complexes indicate an absence seizure. Seizures are pyknoleptic in childhood absence epilepsy, i.e. they occur frequently up to several times a day, but only last a few seconds. On the other hand, in epilepsies with later onset, absence seizures usually occur less often but last longer. They are therefore of non-pyknoleptic or spanioleptic nature. Myoclonic and tonic-clonic seizures may be associated with these idiopathic generalized epilepsies, but are seldom observed in forms with an early onset.
Contrary to idiopathic generalized epilepsies, absence seizures in symptomatic generalized epilepsies are represented by 1.5-2-5 Hz slow spike and wave complexes . They are therefore termed atypical absence seizures. Since these absence seizures may involve decrease or increase of muscle tone, patients may fall during a seizure.
According to the recommendations of the International League Against Epilepsy, absence seizures are now classified as typical, atypical or special, whereby absence seizures with special features include myoclonus .
Absence seizures are associated with certain forms of epilepsy. They generally last only a few seconds and do involve brief periods of time when an indicidual lapses out of awareness. They are most frequently observed in children.
While some genetic variants have been identified that contribute to the development of absence seizures, no direct causes could be determined so far. It has been proposed that flickering lights and rapid breathing may trigger an absence seizure, but that may be limited to genetically predisposed persons.
Some types of epilepsy associated with absence seizures mainly affect young children, whereas others are more frequently observed in adolescents. Depending on the exact type of epilepsy, further symptoms may be present.
The person suffering an absence seizure usually interrupts what they are doing, stares into space and resumes his or her activities after some seconds. During the seizure, the patient neither speaks nor responds to their surroundings. In some cases, the affected person blinks rapidly.
Absence seizures are typically not associated with changes in muscle tone, although these may be the case in certain forms of epilepsy. Also, absence seizures may be accompanied by myoclonic or tonic-clonic seizures that involve twitching and jerking.
Because absence seizures mainly affect children, they are sometimes simply referred to as staring spells or wandering attention. It is not uncommon for parents or teachers to attribute them to child behavior rather than to a medical problem like epilepsy.
A detailed medical history may therefore be given if a physician is consulted. While blood tests may be conducted to rule out other causes for the observed brief losses of awareness, an EEG is usually diagnostic for epilepsy. In an EEG, the brain's electrical activity is recorded. In cases of epilepsy, these recordings show specific alterations. If needed, a brain scan may be carried out for further diagnostics.
Because absence seizures may impair the patient's ability to partake in day-to-day activities, school or even work, they should be treated with anti-epileptic drugs. Ethosuximide, valproate and lamotrigine are only some of the currently available alternatives. Most people suffering from absence seizures respond well to treatment. It may be necessary to take more than one drug if other types of seizures are experienced.
An anti-epileptic therapy is usually a long-term treatment that should be continued even if the patient has been seizure-free for some time. The decision to discontinue treatment should be taken by the physician, not by the patient.