Pearls, Perils, and Pitfalls In the Use of the Electroencephalogram

Omkar N. Markand, MD, FRCPC

Disclosures

Semin Neurol. 2003;23(1) 

In This Article

EEG in Status Epilepticus

Status epilepticus (SE) is usually defined as continuous seizure activity persisting for more than 30 minutes or more than one sequential seizure without full recovery of consciousness between seizures. A very common reason for ordering an emergency EEG is for the diagnosis and management of SE. A simplified classification of SE includes: (1) generalized convulsive status, characterized by motor seizures with loss of consciousness; (2) simple partial or focal status, characterized by focal motor seizures repeating frequently or epilepsia partialis continua with the patient remaining fully conscious; and (3) nonconvulsive status (NCSE) characterized by a variable alteration of consciousness with minimal or no motor activity.

NCSE poses many challenging nosologic, diagnostic, and therapeutic problems. NCSE includes: (1) absence status, occurring in the setting of generalized epilepsy (idiopathic or symptomatic) and (2) complex partial status associated with focal or partial epilepsy of frontal or temporal onset. In both types, the patient may present with mental status alteration (e.g., slowness in behavior and mentation, confusion, and, rarely, stupor or coma). Then, there are patients who after treatment of generalized convulsive status continue to be obtunded or comatose and show epileptiform discharges in their EEG. These patients are often designated as having "subtle" SE or lumped under NCSE.

It is relatively easy to diagnose NCSE associated with focal epilepsy when there are frequent electrographic focal seizures with an ictal EEG pattern that evolves over time with change in the amplitude, frequency, and spatial distribution. However, it is quite common for the ictal EEG pattern associated with complex partial status associated with focal epilepsy to be generalized spikes or sharp waves repeating at 1 to 6 Hz frequency. Such a generalized EEG pattern is similar to that seen in typical absence status associated with idiopathic generalized epilepsy (absence epilepsy) and atypical absence status in children with secondary generalized epilepsy of the Lennox-Gastaut type. To complicate the situation even further, patients with Lennox-Gastaut syndrome interictally have generalized 1.0 to 3.0 cps spike wave discharges that may be very frequent, and one needs to decide if they represent an ictal pattern (hence atypical absence status) or simply represent a prominent interictal pattern. Some waxing or waning of such generalized epileptiform discharges may not help in the distinction because this may be simply related to state changes.

Some helpful criteria are proposed by Young et al[122] in patients who show almost continuously occurring generalized, nonevolving epileptiform discharges in their EEGs, including repetitive generalized or focal epileptiform discharges (spikes, sharp waves, and spike waves) that repeat at a rate faster than three per second, very likely represent an ictal pattern. Such repetitive discharges at a frequency slower than three per second are likely to be ictal if significant clinical and/or EEG improvement is demonstrated following small doses of intravenous lorazepam or diazepam. Rhythmic sinusoidal waves of any frequency (ranging from to frequency) may represent an ictal pattern if there is an evolving pattern at the onset (increasing amplitude and/or decreasing frequency) or a decrement pattern at the termination (decremental amplitude or frequency) or postdischarge slowing or voltage attenuation.

In a patient with obtundation or mental status change of recent onset, an EEG is indicated to rule out NCSE. If repetitive generalized epileptiform discharges are recorded in the EEG, 1 to 2 mg of lorazepam or 5 to 10 mg of diazepam are injected intravenously while the EEG is running. A marked clinical improvement of obtundation and disappearance of generalized paroxysmal activity in the EEG would strongly support the diagnosis of NCSE (Figs. 36 and 37). Such a rewarding experience is most common in typical absence status and less common in other forms of NCSE.

Figure 36.

EEG of a 53-year-old man with one day history of acute confusion and slowness of motor responses, showing almost continuous generalized spike wave activity.

Figure 37.

EEG of the same patient seen in Figure 36 following 4 mg of intravenous lorazepam, showing disappearance of all paroxysmal activity and mental clearing, highly suggestive of nonconvulsive status epilepticus.

Reviewing the previous EEG and obtaining follow-up EEG studies also provide a helpful distinction between ictal and interictal basis for the repetitive generalized spike wave discharges seen in children with Lennox-Gastaut syndrome. A period of frequent repetitive generalized spike wave discharges associated with clinical deterioration of mental status is more likely an episode of atypical absence status, particularly if the previous EEGs or follow-up EEGs display dramatically fewer epileptiform abnormalities.

"Subtle" SE commonly includes patients who had a known episode of convulsive or generalized tonic-clonic status, brought under control by intravenous antiepileptic therapy (e.g., phenytoin, lorazepam, and barbiturates), but continue to remain obtunded or comatose without significant motor activity. EEG of such patients often show repetitive discharges, which may include lateralized periodic discharges (e.g., PLEDs or BiIPLEDS) or generalized periodic discharges (PEDs). Some epileptologists[123] are of the opinion that progressive sequential EEG changes occur during generalized convulsive SE with an "intermediary" pattern of PEDs and PLEDs (unilateral or bilateral) before disappearance of all paroxysmal EEG activities, and that the presence of these "intermediary" EEG patterns necessitate further aggressive therapeutic measures (e.g., inducing pentobarbital coma, etc.). Such views are not universal. Many, on the other hand, consider PED and PLED patterns observed during the course of convulsive status not an ictal pattern but suggestive of a severe epileptic encephalopathy reflective of a neuronal dysfunction from underlying brain damage.[124]

Refractory SE is usually treated by continuous intravenous anesthesia maintained by pentobarbital, propofol, or medazolam. The dose is regulated such as to control all clear-cut clinical or electrographic seizures and to maintain a suppression-burst pattern in the EEG. Therefore, continuous bedside EEG is monitored. There is no consensus as to the duration of "burst" and "flat" periods for optimal dosing. Most consider that establishing and maintaining any degree of suppression-burst pattern is adequate.

One final note of caution is that focal motor seizures including epilepsia partialis continua may not show ictal changes in the EEG because of the limited size of neuronal tissue involved during the epileptic seizure or because the ictal pattern in the EEG may be obscured by artifacts. Careful review of the EEG using different montages (especially a transverse bipolar montage going through the midline electrodes) and use of appropriate muscle filters may reveal a low-amplitude ictal pattern.

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