Absence seizures are one of several kinds of seizures. These seizures are sometimes
referred to as petit mal seizures. Absence seizures are characterized by a brief loss
and return of consciousness, generally not followed by a period of lethargy.
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, he or
she stands 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.
Absence with impairment of consciousness only as per the above description
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.
Absence with atonic components. Here there may be a diminution in tone of muscles subserving
posture as well as in the limbs leading to drooping of the head, occasionally slumping
of the trunk, dropping of the arms, and relaxation of the grip. Rarely tone is sufficiently diminished
to cause this person to fall. 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. The head may tonically draw to one or another side.
Absence with automatisms. Purposeful or quasipurposeful 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.
Absence with autonomic components. These may be pallor, and less frequently flushing, sweating,
dilatation 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. Precipitating factors
Typical absences are easily induced by hyperventilation in more than 90% of the patients. This is
a reliable test for the diagnosis of absence seizures: a patient suspected of typical absences
should be asked to overbreathe for 3 min, counting his or her breaths. 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 seizure. Valproic acid and gabapentin have multiple mechanisms of action including blockade
of T-type Ca++ channels and are useful in treating multiple seizure types.
Diagnosis The only diagnostic test for absence seizures
is EEG. However, brain scans such as by an MRI can help rule out other diseases, such
as a stroke or a brain tumor. During electroencephalography, 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 4–12
years old. 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 given a proper diagnosis. There are no known before or after
effects of absence seizures. They have two essential components:
clinically, the impairment of consciousness Electroencephalography 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 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 suffer
from frequent seizures of other types, such as atonic, tonic and myoclonic.
onset and termination is not so abrupt and changes in tone are more pronounced.
ictal 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. Syndromes
These are childhood absence epilepsy, epilepsy with myoclonic absences and juvenile myoclonic
epilepsy. Other proposed syndromes are Jeavons syndrome and genetic generalised epilepsy
with phantom absences. These types of seizures are also known to
occur to patients suffering with porphyria and can be triggered by stress or other porphyrin-inducing
factors. Treatment
Treatment of patients with absence seizures only is mainly with sodium valproate or ethosuximide,
which are of equal efficacy controlling absences in around 75% of patients. Lamotrigine monotherapy
is less effective, with nearly half of the patients becoming seizure free. This view
has been recently confirmed by Glauser et al., who performed a double-blind, randomized,
controlled clinical trial to compare the efficacy, tolerability, and neuropsychological effects
of ethosuximide, valproic acid, and lamotrigine in children with newly diagnosed childhood
absence epilepsy. Drug dosages were incrementally increased until the child was free of seizures,
the maximal allowable or highest tolerable dosage was reached, or a criterion indicating
treatment failure was met. The primary outcome was freedom from treatment failure after 16
weeks of therapy; the secondary outcome was attentional dysfunction. Differential drug
effects were determined by means of pairwise comparisons. The 453 children who were randomly
assigned to treatment with ethosuximide, lamotrigine, or valproic acid were similar with respect
to their demographic characteristics. After 16 weeks of therapy, the freedom-from-failure
rates for ethosuximide and valproic acid were similar and were higher than the rate for
lamotrigine. There were no significant differences between the three drugs with regard to discontinuation
because of adverse events. Attentional dysfunction was more common with valproic acid than with
ethosuximide. If monotherapy fails or unacceptable adverse reactions appear, replacement of one
by another of the three antiepileptic drugs is the alternative. Adding small doses of
lamotrigine to sodium valproate may be the best combination in resistant cases.
While ethosuximide is effective in treating only absence seizures, valproate is effective
in treating multiple seizure types including tonic-clonic seizure, as such it may be a
better choice if a patient is exhibiting both types of seizures. Clonazepam is effective
in the short term but is not generally recommended for treatment of absence seizure because of
the rapid development of tolerance and high frequency of side effects.
Contraindicated drugs Carbamazepine, vigabatrin, and tiagabine are
contraindicated in the treatment of absence seizures, irrespective of cause and severity.
This is based on clinical and experimental evidence. In particular, the GABA agonists
vigabatrin and tiagabine are used to induce, not to treat, absence seizures and absence
status epilepticus. Similarly, phenytoin, phenobarbital, gabapentin, and pregabalin
should not be used in the treatment of absence seizures.
Data limitations In the treatment of absence seizures there
is often insufficient evidence for which of the available medications has the best combination
of safety and efficacy for a particular patient. Nor is it easily known how long a medication
must be continued before an off-medication trial should be conducted to determine whether
the patient has outgrown the absence seizures, as is often the case in children. To date
there have been no published results of any large, double-blind, placebo-controlled studies
comparing the efficacy and safety of these or any other medications for absence seizures.
The studies that exist have been small and not produced clear conclusions.
References
External links (Video of Absence Seizure)
Mechanisms of absence seizures Thalamocortical oscillations
Absence