EEG Patterns in Stupor and Coma

Chapter 12 EEG Patterns in Stupor and Coma


The term coma refers to a state in which a person is unaware of self and surroundings, even if stimulated from the outside. Between consciousness and deep coma, there is a continuum of possible levels of responsiveness and awareness. Encephalopathy is a broad term that may be used to indicate a decrease in awareness; a patient who develops confusion and decreased awareness can be said to be “encephalopathic.” Because many of the nuances of the neurologic examination are lost in the comatose patient, the EEG plays a special role in ascertaining the depth of coma. In patients who have been pharmacologically paralyzed, a common practice in intensive care units (ICUs), the neurologic examination yields limited information. In such patients the EEG may be the principle source of information regarding the patient’s neurologic state.


Broadly speaking, the EEG may contribute information in the setting of coma in three ways. First, the pattern seen on a single EEG “snapshot” may suggest the depth and severity of the coma. Second, trends seen in repeat or serial EEGs can be a useful indicator of improvement or deterioration in a patient’s status. The specific EEG parameters used to follow such trends and their implications are discussed in this chapter. Third, in a minority of cases the EEG pattern seen in coma can suggest its specific cause, such as the association of triphasic waves with hepatic and other metabolic encephalopathies or the unexpected discovery of continuous subclinical seizure activity.



INDIVIDUAL PARAMETERS OF THE EEG IN COMA: Voltage, Frequency, Reactivity, and the Presence of Normal Sleep Elements


There is a general correspondence between EEG coma patterns and the depth and severity of the coma. A variety of EEG attributes can be followed on serial testing to track a patient’s progress in the comatose state. In patients who have a deteriorating neurological status, a parallel deterioration in the EEG is expected. Likewise, in patients with progressive neurologic improvement, a concomitant improvement in the EEG is expected. Thus the EEG can serve as a useful adjunct to the clinical examination.



Slow-Wave Voltage


Low-voltage slow waves intermixed with the patient’s baseline background activity may be the first EEG sign of encephalopathic change (see Figure 12-1). An increase in the amount or amplitude of slow-wave activity suggests an increase in the severity of the encephalopathy. With deepening coma, slow-wave amplitude may continue to increase, and very high-voltage slow-wave patterns may be seen. Rather than intermixing with the background activity, the high-voltage slow-wave activity becomes the background. As cerebral function is increasingly affected, however, slow-wave amplitude can only increase to a certain point. With yet more severe cortical dysfunction, cortical rhythms begin to decrease in amplitude. With the most severe neurological processes cortical function becomes depressed and the brain becomes less able to maintain slow-wave voltages, resulting in diminished background activity and voltage. Thus, very low-voltage patterns in coma (voltage depression) are considered more ominous than high-voltage slow-wave patterns. The EEG patterns associated with the most severe degrees of cortical dysfunction show marked suppression of voltages or even electrocerebral inactivity.


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Figure 12-1 Low- to medium-voltage delta activity is seen superimposed on an otherwise unremarkable background in a stuporous 12-year-old boy with meningitis.


Given this described sequence of initially increasing, then decreasing slow-wave amplitude with increasingly severe encephalopathy, a linear relationship between slow-wave amplitude and severity of encephalopathy cannot be assumed. When amplitudes are seen to decrease, this could represent either a trend toward normalization or signal a trend toward voltage depression and increasing dysfunction. In such cases, other EEG features (discussed later) such as frequency and reactivity of the background may help clarify the meaning of the change (see Figures 12-2 and 12-3).


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Figure 12-2 A typical slow-wave pattern in coma is shown with high-voltage semirhythmic delta waves. A small amount of intermixed theta activity is also seen, particularly near the vertex and in the occipital areas. Compare to Figure 12-3.


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Figure 12-3 The EEG of the same patient seen in Figure 12-2 recorded 48 hours later. As discussed in the text, a decrease in slow-wave voltage in coma can signal either an improvement or a deterioration in the patient’s state. In this tracing, the appearance of faster rhythms accompanies the decrease in amplitudes, clarifying that the drop in voltages represents a trend toward improvement.


The evolution of slow-wave activity during the improvement phase of a neurologic process may be less tightly linked to the patient’s neurologic status. The clearing of slow-wave activity often lags behind the patient’s clinical improvement. In a patient who is recovering from a dramatic encephalopathy, EEG slow-wave activity may still be present even as the patient wakes up, sits up, and begins talking. The persistence of slow-wave activity in the face of an improving neurologic picture is not necessarily a poor neurologic sign as long as there is a trend toward EEG improvement. Likewise, the slow-wave activity that follows a seizure (postictal slowing) may persist well past the point that patients report feeling back to their preseizure baseline. Slow-wave activity may persist after a seizure for hours, commonly a few days, but occasionally for as long as 3 to 4 weeks depending on the type of seizure, the duration of the seizure, and the general neurologic health of the individual.



Slow-Wave Frequency


The relationship of slow-wave frequency to coma severity is more straightforward than it is for slow-wave amplitude. In general, decreasing slow-wave frequencies suggest increasing severity of encephalopathy. A decrease in slow-wave amplitude can be associated with either improvement or deterioration in neurologic status as described earlier. Counting wave frequency is a useful tool for distinguishing between the two possibilities. If background frequency is increasing, this is a good sign; slower slow waves suggest deterioration. A similar approach is taken when comparing two hemispheres with slow-wave activity—one with higher voltages than the other. Higher voltage slowing may mark the more affected hemisphere, but it may be that the opposite hemisphere manifests lower voltages because it is the more abnormal side. In such cases, comparing the frequencies generated by each side may clarify which is the relatively “healthier” hemisphere, identified by its higher frequency.



Reactivity


EEG reactivity is an additional useful feature in assessing the depth coma. The EEG is monitored for change when the patient is stimulated. The stimulus may be as simple as calling the patient’s name or could include purposeful noxious tactile stimulation. Intensive care unit procedures such as endotracheal tube suctioning or venipunctures also provide an opportunity to observe EEG reactivity. An unreactive EEG is one that shows no change in response to stimulation. Reactive EEGs show a change with stimulation, such as an increase in amplitude and rhythmicity in low-voltage tracings or a relative flattening of the background in higher voltage tracings.



Presence of Normal Sleep Elements


The presence of identifiable sleep elements in the EEG in coma is felt to be associated with a relatively better neurologic prognosis. The presence of sleep features implies that there is enough cerebral structure intact to generate these elements. Sleep spindles are the most commonly identified sleep feature in this setting. In rare cases, the higher centers that generate sleep elements are intact, but there has been a severe injury at lower levels of the central nervous system, resulting in a poor outcome despite the persistence of sleep elements.



SPECIFIC EEG PATTERNS IN COMA AND NEUROLOGIC PROGNOSIS


The prognostic impact of the EEG patterns discussed here must always be interpreted in the context of the coma’s underlying etiology. Although various coma patterns have different reputations in terms of the severity of the encephalopathic state that they imply, even the most severe patterns can have a good final outcome if the etiology of the coma is inherently reversible. A good example of a reversible process is drug overdose. Patients with drug overdose may show, at least for a period of time, otherwise ominous EEG patterns such as burst suppression, voltage depression, or even “flat” EEG patterns. After the drug effect has cleared, assuming no permanent brain injury, the patient (and the EEG) may recover completely. This stands in contrast to the patient who shows a burst-suppression pattern or voltage depression after a prolonged cardiac arrest, a type of injury that is less likely to be reversible. In this group of patients, these EEG patterns have a more ominous significance.


Some of the most useful studies that have examined the prognostic impact of different EEG patterns in coma have limited the study group to patients with anoxic insults, such as those caused by cardiac arrest. This approach has the advantage of excluding the important variable of coma etiology from long-term outcome; however, the conclusions of these studies should only be extrapolated outside this etiologic group studied with caution. It is no surprise that two patients with the same EEG pattern in coma, such as a drug overdose patient and a patient with a malignant brain tumor, may have very different neurologic outcomes but similar EEG findings. Because EEG patterns are dictated more by the function of the cerebrum than the brainstem, the minority of patients with devastating brainstem injuries but relative sparing of the cerebrum may have misleadingly benign EEG findings. The order that specific coma patterns are listed in the following subsections should not imply a strict ranking, although they are generally described in order of increasing severity.



Intermittent Rhythmic Delta Activity


Among EEG findings in encephalopathy, intermittent rhythmic delta activity (IRDA) is considered to lie at the milder end of the spectrum of encephalopathic EEG patterns. IRDA may appear in patients who are awake or who are mildly lethargic or stuporous; IRDA patterns are not associated with deeply comatose states. IRDA tends to occur in the frontal regions in adults (frontal intermittent rhythmic delta activity, or FIRDA) and in the occipital regions in younger children (occipital intermittent rhythmic delta activity, or OIRDA; see Figure 12-4). When encephalopathic states become more severe, IRDA patterns may be replaced by continuous slow-wave patterns. Various types of IRDA are discussed in more detail in Chapter 9, “The Abnormal EEG.”


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Figure 12-4 Occipital intermittent rhythmic delta activity (OIRDA) and excess fast activity are seen in a 9-year-old intensive care unit patient with postictal confusion (arrows). The increased fast activity is the result of treatment with lorazepam. Intermittent rhythmic delta activity is usually associated with mild encephalopathies.

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Default ThumbnailThe Structure and Philosophy of the EEG Report Introduction The Electroencephalogram of the Newborn The Abnormal EEG Filters in the Electroencephalogram Introduction to Commonly Used Terms in Electroencephalography

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Mar 12, 2017 | Posted by in NEUROLOGY | Comments Off on EEG Patterns in Stupor and Coma

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