出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/11/14 16:15:20」(JST)
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| Absence seizure | |
|---|---|
| Classification and external resources | |
| ICD-10 | G40.3 |
| ICD-9 | 345.0 |
| DiseasesDB | 32994 |
| MedlinePlus | 000696 |
| eMedicine | neuro/3 |
| MeSH | D004832 |
Absence seizures are one of several kinds of seizures. These seizures are sometimes referred to as petit mal seizures (from the French for "little illness", a term dating from the late 18th century).[1] Absence seizures are characterized by a brief loss and return of consciousness, generally not followed by a period of lethargy (without a notable postictal state).
The clinical manifestations of absence seizures vary significantly among patients.[2][3][4] 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 (post-ictal state), in contrast to the majority of seizure disorders.[citation needed]
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.[citation needed]
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.[citation needed]
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 (among others) have multiple mechanisms of action including blockade of T-type Ca++ channels and are useful in treating multiple seizure types.[citation needed]
The only diagnostic test for absence seizures is EEG.[5] However, brain scans such as by an MRI can help rule out other diseases, such as a stroke or a brain tumor.[6]
During electroencephalography, hyperventilation can be used to provoke these seizures.[5] Ambulatory EEG monitoring over 24 hours can quantify the number of seizures per day and their most likely times of occurrence.[5]
Absence seizures are brief (usually less than 20 seconds), generalized epileptic seizures of sudden onset and termination. When someone experiences an absence seizure they are often unaware of their episode.[7] 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.[citation needed] 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.[8] They have two essential components:[2][3][4]
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:
These are childhood absence epilepsy, epilepsy with myoclonic absences and juvenile myoclonic epilepsy. Other proposed syndromes are Jeavons syndrome (eyelid myoclonia with absences) 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 of patients with absence seizures only is mainly with valproic acid 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. (2010),[9] who studied the 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 dose 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. 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, valproic acid is effective in treating multiple seizure types including tonic-clonic seizure and partial seizure, as such it may be a better choice if a patient is exhibiting multiple types of seizures.[10] Similarly, lamotrigine treats multiple seizure types including partial seizures and generalized seizures, therefore it is also an option for patients with multiple seizure types.[11] Clonazepam (Klonopin, Rivotril) 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.[12]
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.[4] In particular, the GABA agonists vigabatrin and tiagabine are used to induce, not to treat, absence seizures and absence status epilepticus.[13] Similarly, oxcarbazepine, phenytoin, phenobarbital, gabapentin, and pregabalin should not be used in the treatment of absence seizures because these medications may worsen absence seizures. [11]
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.[14] 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.[citation needed] The studies that exist have been small and not produced clear conclusions.[14][15]
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| 関連記事 | 「absence」「seizure」 |