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 (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. 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 being given a proper diagnosis. There are no known before or after effects of absence seizures.Absence seizures have two essential components:
Clinical - the impairment of consciousness (absence)
EEG - the EEG shows generalized spike-and-slow wave dischargesAbsence 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.
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. 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. 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. Risk factors
Typical absences are easily induced by hyperventilation (over-breathing) in more than 90% of people with typical absences.