, Ali T. Ghouse2 and Raghav Govindarajan3
(1)
Parkinson’s Clinic of Eastern Toronto and Movement Disorders Centre, Toronto, ON, Canada
(2)
McMaster University Department of Medicine, Hamilton, ON, Canada
(3)
Department of Neurology, University of Missouri, Columbia, MO, USA
Patients having EEG studies for a seizure disorder should be studied when awake as well as while asleep in the same record, since interictal epileptiform activity is increased during sleep and when drowsy . Patients are routinely asked to arrive for the examination after having been sleep-deprived the night before to allow for easy transition to drowsiness.
If the routine recordings are normal and suspicion for epilepsy is high, then prolonged monitoring with sleep deprivation should be done. Patients are usually kept awake for 24 h before the EEG. The transition from drowsiness to light sleep may be very short, and therefore must be examined closely for the detection of abnormal epileptiform activity. Sometimes sharply contoured slow waves may be misinterpreted as spikes or sharp wave activity, particularly in young children. False-positive EEG interpretations are to be avoided. Most EEG phenomena noticed by beginners are artifacts.
True epileptiform spikes are usually stereotypical and stand out from the background with a fast rising phase. They may be followed by a slow wave and have a potential field of activity. A sharp transient is any wave of any duration that has a pointed peak at standard recording speeds. As such, any sharply contoured waveforms that do not meet the criteria of being epileptiform are called sharp transients. These sharp transients are often variable in morphology; their rising phase may be slower than the falling phase, and the rising phase is usually not followed by slow waves. Often they do not have a potential field; there is no change with sleep see Table 3.1 below for more details.
Table 3.1
Differences between spike and nonspike potentials
Spike potential (epileptiform) | Non-spike potential (sharp transient) |
|---|---|
Stereotypical in appearance Usually present consistently | Variable in morphology Nonconsistent presence |
Rising phase is fast | Rising phase is slower than the falling phase |
Usually followed by a slow wave | Usually not followed by a slow wave |
Stands out from background | Does not stand out from background |
Activated in sleep state | No change with sleep state |
Defined potential field | Does not have a defined field, may be a single electrode |
Spikes and sharp waves are abnormal in most conditions, except when they are in the form of vertex sharp waves and positive occipital sharp transients of sleep (POSTS) in stage I sleep, 14- and 6-Hz positive spikes, wicket spikes, occipital lambda waves, or six-per-second phantom spike and wave discharges. Spikes are between 20 and 70 ms in duration and sharp waves are 70–200 ms in duration. Spike and wave discharges correlate more with the likelihood of epilepsy than a single spike. Generalized spike and wave discharges are of the following types see Table 3.2:
- 1.
Three-per-second spike and wave
- 2.
Slow spike and wave
- 3.
Fast spike and wave
Absence epilepsy (Fig. 3.1) has classic three-per-second spike and wave complexes, which increase with hyperventilation with a normal background. Absence epilepsy is an example of a primary generalized epilepsy.
Figure 3.1
3-Hz spike and wave activity (absence epilepsy)
Juvenile myoclonic epilepsy (JME) is another example of a primary (i.e., idiopathic) generalized epilepsy syndrome, in which the background EEG is normal, but in which faster (>3.5 Hz) generalized polyspike discharges with slow waves occur, more prominent in the frontocentral regions. The condition is characterized as myoclonic jerks in the early morning, with the possibility of generalized tonic-clonic seizures. This condition typically requires lifelong therapy.
Generalized tonic or tonic/clonic seizures (GTCS; Fig. 3.2) are characterized by a progressively higher amplitude and lower frequency discharge pattern observed simultaneously in both cortical hemispheres, reaching a maximum of 10 Hz.
