Absence Seizure

Article Author:
Ana Albuja
Article Editor:
Patrick Murphy
Updated:
2/28/2019 10:49:48 PM
PubMed Link:
Absence Seizure

Introduction

Absence seizures are brief seizures characterized by a behavioral arrest that correlates with generalized 3-Hertz spike-and-wave discharges on electroencephalogram (EEG). Absence seizures occur in multiple idiopathic/genetic generalized epilepsies including childhood absence epilepsy (CAE), juvenile absence epilepsy (JAE), and juvenile myoclonic epilepsy. Atypical absence seizures have been reported in up to 60% of patients with Lennox-Gastaut syndrome. 

Historically, absence epilepsy was known as “pyknolepsy.” This term originates from the Greek term pyknos, which means “very frequent” or “grouped.” The term “petit mal” seizure was once used to describe an absence seizure in the past, but it is no longer encouraged. 

The International League Against Epilepsy Classification of 2017 defines absence seizures as “generalized non-motor seizures.” However, this term is not entirely precise, because, as discussed below, motor manifestations of absence epilepsy are frequently seen.[1][2]

Etiology

A genetic component exists for all generalized epilepsies and, specifically, for absence epilepsy. In 1951, Lennox reported that 66% of monozygotic twins showed concordance for the EEG pattern of 3-Hertz spike-and-wave. Doose at al, described 252 patients with absence epilepsy with a 3-Hertz spike-and-wave pattern. He proposed that there was multifactorial inheritance. The voltage-gated T-type calcium channel gene, the GABA-A receptor subunits GABRG2 and GABRG3, and the CACNA1A gene are thought to be involved in the etiology of this epilepsy syndrome. However, the mode of inheritance and the majority of genes implicated in CAE are still unknown.[3]

Epidemiology

The incidence of absence seizures is between 0.7 and 4.6 per 100,000 in the general population and around 6 to 8 per 100,000 in the pediatric population younger than 15 years.

CAE is a common pediatric epilepsy syndrome. Among all cases of epilepsy in school-aged children, 10% to 17% are due to CAE. The age of onset for CAE is typically between 4 and 10 years, with a peak between ages 5 and 7. Girls have CAE more frequently than boys.[4][5]

Pathophysiology

Although some of the pathways involved in the development of absence seizures have been described, their pathophysiological mechanisms are yet to be fully understood. The cortico-thalamic-cortical circuit is considered to play a major role in the pathophysiology of absence seizures. [6][7]

Some of the neurons involved in the cortico-thalamic-cortical system include:  

  • Cortical glutamatergic neurons are originating on cortical layer VI and projecting to the nucleus reticularis of the thalamus. 
  • Thalamic relay neurons that have excitatory projections to cortical pyramidal neurons.
  • Neurons from the thalamic nucleus reticularis containing inhibitory GABA-ergic projections that connect with other neurons from the same nucleus, and with thalamic relay neurons. These neurons do not connect directly with the cortex.

Neurons from the thalamic nucleus reticularis can fire in an oscillatory pattern (for example, rhythmic bursts involved in the generation of sleep spindles) or continuously in single spikes (tonic firing during wakefulness). Shifts between these two firing patterns in the thalamic nucleus reticularis are modulated by spikes present in thalamocortical networks and neurons from the thalamic nucleus reticularis. These are mediated through low-threshold transient calcium channels known as T-type channels. After depolarization, T-type channels briefly allow calcium inflow before becoming inactivated. Reactivation requires a relatively long hyperpolarization facilitated by GABA-B receptors. Therefore, abnormal oscillatory rhythms can originate from T-type channel abnormalities, or from the increased GABA-B activity. 

As explained by the genetics of absence epilepsy, genes coding for T-type calcium channels and for GABA receptors have been associated with the etiopathogenesis of this type of epilepsy. Medications that suppress T-type calcium channels, such as ethosuximide and valproate, are effective anti-absence drugs. Conversely, medications that increase GABA-B activity (e.g., vigabatrin) exacerbate absence seizure activity. In contrast, GABA-A agonists (e.g., benzodiazepines), that preferentially enhance GABA-ergic activity in neurons from the thalamic nucleus reticularis, can suppress absence seizures.

History and Physical

The age of onset of CAE is usually between 4 and 10 years, with a peak between the ages of 5 and 7. The onset of absence epilepsy before age 4 should raise concern for an underlying glucose transporter type 1 (Glut1) deficiency. 

Regarding its clinical presentation, family members and teachers usually describe brief spells in which the patient has a loss of awareness, is unresponsive, and has a behavioral arrest. They describe the patient during these spells as having “a blank stare.” Episodes frequently occur, often 10 to more than 30 times throughout the day. Most children stop their activity completely, but some can continue the activity more slowly or in an unusual manner. Some children have 3-Hertz regular eyelid fluttering. Oral automatisms can also occur, especially with prolonged seizures or during hyperventilation. Children frequently have mild clonic or tonic movements in the first few seconds of the spell.  Pallor is frequently reported.  Urinary incontinence is rare. Spells usually last between 4 and 30 seconds. Hyperventilation, state of arousal, sleep deprivation, and medication use can affect the duration of the seizure. These seizures are not preceded by an aura and do not have a postictal state.

For JAE, the age of onset is classically between 10 and 19 years with a peak at 15 years. Seizures are less frequent than in CAE but tend to last longer. 

On physical examination, hyperventilation can trigger absence seizures. To test this, the examiner asks the child to blow repetitively for more than 2 minutes. It can be helpful to use a pinwheel or a paper because this encourages the child to cooperate more during testing. If hyperventilation is done successfully, the patient will develop seizures that can be seen on EEG. 

Absence status consists of generalized, non-convulsive seizures characterized by impairment of awareness, and intermittently has other manifestations such as automatisms or subtle myoclonic, tonic, atonic, or autonomic phenomena. Patients usually have a previous diagnosis of generalized epilepsy. Absence status presents as a non-convulsive seizure lasting between a half hour and several days. It usually ends spontaneously and suddenly but should be treated with anti-seizure medications at the time of diagnosis.

Evaluation

EEG is the main diagnostic tool for the evaluation of absence epilepsy. In the case of childhood absence seizures, EEG shows bilaterally synchronous and symmetrical 3-Hertz spike-and-wave discharges that start and end abruptly. These discharges can sometimes have maximum frontal amplitude or begin with unilateral focal spikes. In 50% of seizures in CAE, the initial discharge seen has a typical spike-and-wave morphology. The remaining 50% can show a single spike, polyspikes, or an atypical, irregular, generalized spike-and-wave. The background is normal. [8][9]

Atypical absence seizures show a more insidious onset and offset, slower spike-and-wave paroxysms (less than 3 Hertz), and an abnormal interictal background.  

Neuropsychological testing has shown that patients with CAE have cognitive impairment, especially involving attention, executive function, verbal and visuospatial memory. The difficulty with language and reading is also commonly reported. Depression, anxiety, and attention-deficit/hyperactivity disorder also have been reported more frequently in patients with CAE. 

Since CAE is a generalized epilepsy, imaging studies are not performed routinely.

In JAE, EEG demonstrates paroxysms of generalized 3- to 4-Hertz spike-and-wave or polyspike-and-wave discharges. 

In the case of absence status, EEG shows continuous or nearly-continuous generalized spike-and-wave or polyspike-and-wave discharges at 2–4 Hertz.

Treatment / Management

The first-line treatment for absence epilepsy is ethosuximide. A randomized controlled trial performed in 2010 that included 446 children with CAE showed that ethosuximide and valproic acid were superior to lamotrigine. However, this study had a low seizure-free rate with 53% of patients in the ethosuximide group, 58% on the valproic acid group, and 29% of patients taking lamotrigine. The group that received valproic acid had significantly lower scores on attentional measures as compared to the ethosuximide and lamotrigine groups. For this reason, ethosuximide is the preferred agent for treatment of absence epilepsy. [10][11]

The most frequent side effects of ethosuximide are abdominal pain and nausea. For this reason, ethosuximide should be taken with meals. Other medications that can be used for management of CAE include valproate, lamotrigine, and topiramate. Second-line medications that can be used as adjunct therapy include zonisamide and levetiracetam.

It is important to note that some sodium channel blockers like phenytoin, carbamazepine, gabapentin, pregabalin, and vigabatrin can worsen absence seizures. 

Differential Diagnosis

The differential diagnosis for staring spells includes absence epilepsy, focal seizures with alteration of awareness, and non-epileptic paroxysmal events. 

Focal epilepsy with alteration of awareness (complex partial epilepsy) can also present with behavioral arrest and automatisms. However, these seizures are usually less frequent than absence seizures. Patients can have generalized seizures with focal epilepsy. Semiology of automatisms can vary depending on the area of the cerebral cortex where the seizures originate. 

A study that evaluated non-epileptic staring spells using video-EEG monitoring found that these episodes were often characterized by arrest of all activity, vague facial expression, and vision fixed on one point without blinking. When the duration of the events was quantified, staring episodes lasted anywhere between 3 and 74 seconds. In most children, it was difficult to determine the onset and the end of the event. Most parents were not able to regain the child’s attention if they waved their hand in front of the child. Other more energetic measures like hand clapping or other loud sounds were successful at stopping the events in all children. A significant percentage of children (41%) were inactive at the onset of the stare, and 18% of children were watching television when the event began.

A retrospective chart review performed in a tertiary care epilepsy center showed that, among 276 patients in the epilepsy monitoring unit to have their staring spells monitored, only 11% were deemed to have seizures. Therefore, most staring spells are non-epileptic in nature. When patients present with staring spells, practitioners should be careful and not tell parents or other providers that these episodes are “absence seizures” before EEG evaluation is completed. The above-mentioned study developed a tool to determine pretest probability of seizures in children presenting with staring spells. This tool accounts for patient variables such as results of previous EEG, previous use of antiseizure medications or treatments for psychiatric conditions, and duration of the spells. 

Consultations

A general pediatric neurologist or a pediatric epileptologist should be consulted when a patient has staring spells that are suspected to be seizures. Typically, an outpatient evaluation is a reasonable first step.

Deterrence and Patient Education

As explained above, most staring spells are non-epileptic in nature. When patients present with staring spells, practitioners should explain to caregivers that the differential diagnosis includes seizures, but they should avoid giving a specific diagnosis of “absence seizures” before EEG evaluation is completed. 

It can be very useful to request caregivers to take videos of staring spells, as this can help characterize them. Event calendars/logs can assist in understanding the frequency, pattern, and possible triggers. 

Caregivers of children with CAE should know that generalized tonic-clonic seizures are rare. For this reason, rescue medications such as rectal diazepam and intranasal midazolam are not routinely prescribed. Nevertheless, caregivers should be taught what to do if the child has a generalized tonic-clonic seizure. 

Caregivers often assume that, since absence seizures are very brief, they are harmless. On occasions they question the need to treat with medications, arguing that risks could outweigh benefits. In these situations, we recommend explaining that the child is experiencing frequent episodes of altered consciousness that can increase the risk of accidents. Seizures also can interfere with learning and have a negative impact on school performance.  

Pearls and Other Issues

  • Absence epilepsy is a primary generalized epilepsy.
  • It is classified as typical or atypical absence depending on seizure characteristics and EEG pattern. 
  • Absence seizures are characterized by behavioral arrest and EEG showing 3-Hertz spike-and-wave discharges. Episodes usually occur multiple times per day. 
  • Absence seizures are seen in several generalized epilepsies including childhood absence epilepsy, juvenile absence epilepsy, and juvenile myoclonic epilepsy. 
  • Episodes of behavioral arrest should be called “staring spells” until EEG evaluation is performed. A patient can only be diagnosed with absence epilepsy if they have characteristic seizure semiology and EEG findings. 
  • Staring spells can be epileptic or non-epileptic, with the majority being non-epileptic. When episodes are epileptic in nature, they may be due to absence seizures or to focal seizures with impaired awareness (complex partial seizures) 
  • The first-line treatment for absence seizures is ethosuximide. Other therapies include valproate, lamotrigine, and topiramate. Second-line medications that can be used as adjunct therapy include zonisamide and levetiracetam.
  • Carbamazepine, phenytoin, gabapentin, vigabatrin, and other medications with similar mechanisms of action can worsen absence epilepsy. 
  • Approximately 60% of patients with childhood absence epilepsy eventually achieve seizure freedom.
  • Lack of response to ethosuximide and presence of generalized tonic-clonic seizures have been associated with lack of seizure remission and life-long epilepsy. 

Enhancing Healthcare Team Outcomes

Absence seizures are best managed by a neurologist, but the long-term care and monitoring is done by the primary care provider, nurse, and pharmacist. The patient should be educated on the importance of medication compliance. While some diets have been recommended for absence seizures, it is important to speak to a neurologist before starting a ketogenic diet. Patients who are managed on special diets need close monitoring at centers which offer these modified diets. Further, any physical activity that may pose a threat to life should be avoided. For example, swimming or driving alone is not recommended.[12][13]

Typical absence epilepsy occurs in childhood and frequently resolves by adolescence. Seizure freedom is reported in 57% to 74% of patients. Patients who have generalized tonic-clonic seizures or children who do not respond to initial ethosuximide monotherapy usually do not achieve long-term remission. 

The risk of accidental injuries during absence seizures, as compared with controls, is well established. As mentioned previously, CAE patients can have problems involving attention, executive function, and verbal and visuospatial memory. Difficulties with language and reading are commonly reported. Depression, anxiety, and attention-deficit/hyperactivity disorder have also been described frequently in patients with CAE.[14]



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      Contributed by Ana C. Albuja, MD

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