Dr.R.R.Singh,
New Delhi

EEG provides a unique way of monitoring cerebral function in the intensive care unit (ICU) and it is useful both for recording short and long acting events. Typically, epileptic spikes may last for a fraction of seconds, but recordings for many hours, for example during sleep cycles may also give important information. Recordings lasting for many days may also be useful, to reflect slow trend changes in the EEG.

One basic requirement for an EEG monitor in the ICU is that it has to present the original EEG on a monitor display, and also has to provide a paper printout. Another additional benefit would be to have the system continuously store signals. This would allow access to previously stored EEGs for inspection if unexpected, rapid changes, such as seizures, should occur.

It would be beneficial for the ICU clinician’s daily work to understand the basics of EEG. It would be even better if the clinicians were able to identify typical EEG patterns such as physiological activity, pathologically slowed rhythms, seizure activity in its various forms, periodic patterns, and burst suppression. It discusses the utilization of the EEG in monitoring comatose patients in the ICU. The selected aspects include reactivity, periodic patterns, and burst suppression that may characterize an EEG in the ICU. Recording of an EEG should be utilized more in the monitoring of comatose ICU patients. The EEG recording, repeated at intervals, can help with broad diagnostic categorization. In special clinical situations, for example in monitoring of the effectiveness of status epilepticus treatment, continuous monitoring of EEG could be deemed obligatory. In the assessment of the clinical importance of EEG patterns, clinical picture, age, etiology, acuity, and the integrity of brainstem reflexes must be taken into consideration. Another factor is that special EEG patterns, like triphasic waves, may suggest metabolic disturbances, which could be caused by liver dysfunction or toxic substances. From clinical and neurophysiological point of view, it is important to test the reactivity of EEG. In general, reactivity can be considered as a feature of the lighter stages of coma. However, even during deep anesthesia, with EEG at the burst suppression level, reactions to minor somatosensory, auditory, or visual stimuli can sometimes be seen.

Concerning the patient’s prognosis, reactivity (including the reactive burst suppression pattern) is a good sign. One can assume that certain arousal functions and sensory systems are working. In such situations, when the brain’s seizure activity and any cardiac problems are under control, there is good potential for brain function recovery.

Periodic EEG patterns refer to distinct waveforms repeating with regular intervals (from 0.5 sec to many seconds). Periodic patterns characterized with sharp waves or spikes can be called epileptiform. In spite of its name, this pattern would not necessarily reflect epileptic brain mechanisms. Periodic epileptiform discharges (PED) can often be seen after generalized epileptic seizures. In fact, in an unconscious patient, they may prevail for a long time after the seizure. In status epilepticus, an ominous sign is if the PED is refractory to treatment. If that pattern would be local or restricted to one hemisphere, it would be called periodic lateralized epileptiform discharge (PLED). Such a change may be multifocal, occurring independently at two or three different locations. Many diseases causing focal damage can produce this, but the reason is often a vascular attack. Sometimes, it may be a late sign of a focal epileptic seizure. Therefore, the total clinical picture is important. The patient’s history must be taken into account, particularly if anti-epileptic medications are planned.

There are two other periodic pattern types, which call for attention, and an EEG may provide a diagnosis. In Herpes simplex encephalitis and Creutzfeld Jacob disease, single sharp waves occur, and the initial EEG may mistakenly suggest an epileptic disorder. Late, the EEG may develop periodic patterns, which then suggests the underlying disease. Repeated EEG recordings may confirm the diagnosis, and continuous EEG monitoring may also prove to be very helpful.

High amplitude EEG patterns, with intervening low amplitude activity, are known as burst suppression. The duration of bursts and suppressions can vary from less than half a second to several seconds. The starting and end points of burst activity are often unpredictable, and the duration of bursts and suppressions may constantly vary. These finding are in contrast with periodic patterns.

It must be emphasized that EEG during suppression is not isoelectric. Hence, the use of this expression should be discouraged. During suppression, some low amplitude mixed frequency activity may be visible. In propofol-induced suppression of the EEG, spindles with a frequency of 13-15 Hz can be found.

Burst suppression in the EEG can be caused by many general anesthetics. In addition, the ischemic brain damage and hypothermia can cause similar EEG changes. These causative effects are, in a way, additive and hypothermia, for example, can further emphasize the suppression effect of general anesthesia. Particularly in isoflurane anesthesia, there may be "benign" bursts, which mainly consist of delta and theta activity. Epileptiform patterns, such as spikes, can occur particularly in barbiturate anesthesia. In enflurane anesthesia, these bursts may even turn into epileptic seizure activity, or the bursts may contain PED activity. Rapid induction with the new inhalation agent sevoflurane produces a variety of epileptiform activities, which may also include PED and burst suppression with epileptic spikes.

These bursts are usually generalized and synchronized in hemispheres. Most likely, the corpus callosum plays a role, as the bursts may become asynchronous if it has been damaged.

Today, diagnostic EEG recording is a routine in the ICU. Additional benefit can be obtained by using continuous EEG monitoring. Continuous EEG, usually monitored on four channels, can display slow trends, reactivity, and sleep cycles. These factors are important for assessing the prognosis of the patient. The EEG can give new diagnostic clues. New developing waveforms (for example periodic patterns or triphasic waves) can be revealed early. It displays new information about epileptiform patterns and seizure activity, which can be non-convulsive, yet require treatment. It makes it easy to follow the effect of treatment with anti-epileptic drugs and general anesthesia in status epilepticus.

Last updated: 1-01-2005

Singh RR. EEG MONITORING IN CRITICAL CARE- PRACTICAL OBSERVATIONS. Pediatric Oncall [serial online] 2005 [cited 2005 January 1];2. Available from:
http://www.pediatriconcall.com/fordoctor/viewersChoice/ecg.asp