Focal and Generalized Rhythm Abnormalities

Understanding the background of the EEG is much like understanding a foreign language. Recognizing particular waveforms such as a spike or a K complex is like knowing a word or phrase. Fluency comes with the understanding of the background. Particular waveforms gain their meaning in the context of the background rhythms.

There is not a single unique normal background. On the contrary, the background will be different in patients of different ages, and will change with the state of the patient as described in the preceding chapters. A “normal” appearing background can be abnormal in the wrong context, such as an alpha rhythm found in a comatose patient.

Analysis of the background activity requires an evaluation of symmetry and synchrony. This cannot be determined just from a given page of the study but requires consideration of the study as a whole. This concept also holds true for the analysis of slowing and transients.

ASYMMETRY

The symmetry of the background rhythms should be evaluated in each of the recorded states since it is possible for the symmetry to change from state to state. It is normal for there to be some asymmetry in amplitude of the alpha rhythm from side to side. The normal variability may be related to skull thickness, vascular anatomy, and the underlying variability of the signal.

The normal amplitude of the alpha rhythm is 15-50 µV. In the normal patient, the amplitude may be up to 50% lower in amplitude on the left (see Fig. 8.1).¹ The signal may be up to 35% lower in amplitude on the right; the allowable difference is lower here since amplitude is typically higher on the right due to thinner skull thickness and more prominent alpha activity over nondominant cortex. It must be remembered that the alpha rhythm has an inherent variability over time with a sine wave envelope, previously mentioned as modulation of the amplitude. Therefore, the amplitude comparison should be made between corresponding elements of the alpha rhythm within that envelope of modulation, as well as at homologous derivations and the same state.

A focal lesion in one hemisphere that involves the optic tracts can cause failure of the normal attenuation of the alpha rhythm on that side with eye opening, due to blocking of the visual input to the occipital lobe. This is known as Bancaud phenomenon. In the “bad old days” before CT and MRI imaging, this was a way to localize brain tumors.

A number of disorders can be associated with asymmetry, either by interfering with normal cortical function or by increasing the separation between brain and scalp electrodes. Space-occupying lesions such as tumors, hematomas, CSF collections, and unilateral scalp edema are common causes of background asymmetry. Infectious processes, trauma, and infarction may also result in asymmetry. It may be difficult to distinguish between lesions that alter brain function (intraaxial lesions or extraaxial masses that press on the brain) from those that do not (scalp hematoma or edema) on the basis of amplitude asymmetry alone. However, brain injury may be detected due to loss of faster frequencies, slowing or absence of the posterior background rhythm, or other abnormalities.

FIGURE 8.1. Asymmetry of the background rhythms with decreased alpha activity on the left compared to the right following a left thalamic infarction. (From Blume WT, Kaibara M. Atlas of Adult Electroencephalography. New York, NY: Raven Press; 1995:401, with permission.)

ASYNCHRONY

The posterior dominant rhythm frequency is 8.5-13 Hz in the normal individual. There may be up to a 1-Hz difference in frequency between hemispheres.² A side-to-side difference in the background frequency >1 Hz is considered abnormal asynchrony. Technically, asynchrony means that the waveforms of one hemisphere do not occur at the same time as those over the other, implying different phase relationships between waves on either side, but this is rarely noticed unless there is a difference in the underlying frequency between hemispheres. The analysis of this difference may be complicated by the presence of sub- or supraharmonic frequencies, which can lead to an apparent (but not real) difference in the background. An example is slow alpha variant, which is a subharmonic at half of the true alpha frequency. While it is usually best developed with eyes closed, there may be a slower less well-developed alpha activity with eyes open, especially during drowsiness. In this case, as in many other instances, good notations from the technologist are invaluable.

The disorders associated with asynchrony are similar to those associated with asymmetry. Space-occupying lesions such as tumors, hematomas, and CSF collections as well as infectious processes, trauma, and infarction may result in asymmetry. Unilateral thalamic lesions can also cause asynchrony of the background.

In more severe cases, asynchrony may be seen as delays in the appearance of transient waveforms (eg, K complexes) between hemispheres, or even independence of such waveforms between hemispheres, which can be quite marked in severe encephalopathies such as burst suppression (see below) in which bursts may occur at different times over each hemisphere.

FAST ACTIVITY

Normal Fast Activity

Beta activity (>13 Hz) can occur as a normal component of the EEG or can be abnormal, depending on the location, persistence, and amplitude. In normal adults, beta amplitude is typically quite low, usually <25 µV. Frontocentral beta is common in adults, particularly in drowsiness (stage 1 sleep) and REM sleep. Anxiety can enhance this normal beta activity. Beta activity is also induced or increased by many centrally acting medications, especially alcohol and benzodiazepines.

Sleep Spindles

Sleep spindles are waxing, waning sinusoidal activity occurring most frequently during stage 2 sleep and much less frequently during stage 3 sleep. They have a frequency of 12-14 Hz and a duration of at least 0.5 seconds (see Fig. 8.2).³ Mature sleep spindles are present by age 3 years. They may be seen asymmetrically over one hemisphere or the other at any given time, but the total amount of spindle activity should be equal between sides.

EXCESSIVE BETA ACTIVITY

Drugs

Beta activity is commonly induced by drugs such as barbiturates and benzodiazepines (see Fig. 8.3). This high-frequency activity is often relatively high in amplitude and seen throughout the record, either superimposed over normal background activity or in bursts, with a frontal predominance. Beta-enhancing medications tend to increase the normal bilateral frontal beta activity and do not represent a significant abnormality. This finding should be mentioned in the report but correlated with the history of exposure to medications that cause increased beta activity.

Intracranial Lesions

Focal high-amplitude beta can be seen in ipsi- or contralateral lesions. These lesions include space-occupying lesions such as tumors, as well as vascular malformations and ischemia.

Encephalopathy

Prominent beta activity may be seen in static or progressive encephalopathy, even in the absence of an offending medication. This beta activity is often frontally predominant but can be diffusely distributed and relatively high in amplitude.

Breach Rhythm

A breach rhythm results from a skull defect,⁴ which decreases the high-frequency filtering of the signal by the skull, allowing more fast activity and sharper-appearing cortical activity to be recorded. The skull defect may be due to a brain

surgery or a prior skull fracture, which could have gone undetected from an old head injury. The breach rhythm often has a mu-like pattern, with frequencies of 6-11 Hz and intermixed faster components with sharp negative phases (see Fig. 8.4). Just as with the normal mu rhythm, if more central it may be inhibited by contralateral movement; if temporal it is not. Despite the sharply contoured nature of this fast activity, there is no increased risk of seizures associated with a breach rhythm. Indeed, spike activity that occurs in the context of a breach rhythm must be extremely convincing due to the “sharpening” of normal activities by the skull defect.

FIGURE 8.2. Sleep spindles. The spindles are asymmetric. (From Geyer JD, Payne TA, Carney PR, Aldrich MS. Atlas of Digital Polysomnography. Philadelphia, PA: Lippincott Williams & Wilkins; 2000:38, with permission.)

FIGURE 8.3. Excessive beta activity due to a medication effect.

FIGURE 8.4. Breach rhythm. Prominent mu rhythm with skull defect. Age 30 years. Eye opening in the fourth second blocked the alpha rhythm revealing a more ample left mu rhythm (breach rhythm) than that of the right as a consequence of the skull defect. Wiggling the right thumb (asterisk) abolished this mu rhythm. Calibration signal 1 sec, 50 µV. (From Blume WT, Kaibara M. Atlas of Adult Electroencephalography. New York: Raven Press; 1995:431, Figure 7-31, with permission).

Generalized Paroxysmal Fast Activity

The interictal EEG associated with Lennox-Gastaut syndrome (usually involving slow generalized spike-and-wave or multifocal independent spike-and-wave discharges on a slow/disorganized background) may exhibit generalized paroxysmal fast activity (GPFA) ranging in frequency from 10 to 25 Hz and lasting several seconds during slow-wave sleep.⁵ These bursts are usually frontally predominant and vary significantly in amplitude. GPFA during sleep is not usually associated with a clinical ictal event. GPFA is also associated with the clinical tonic seizures seen in Lennox-Gastaut syndrome when the discharge lasts longer than 6 seconds. This discharge is often preceded by generalized attenuation or slow (<2.5 Hz) spike-and-wave activity, as described in Chapter 9 on epileptiform activity. Atypical absence seizures in Lennox-Gastaut patients can be associated with 7-20 Hz activity similar to GPFA.

REVIEW

8.3: Which of the following is associated with generalized paroxysmal fast activity (GPFA)?

a. Frequency ranging from 15 to 20 Hz

b. Associated with clinical partial seizures

c. Interictal EEG associated with Lennox-Gastaut syndrome

d. Interictal EEG associated with West syndrome

View Answer

8.3: c. GPFA is seen as an interictal epileptiform finding in patients with Lennox-Gastaut syndrome, and can be ictal associated with tonic seizures (if the discharge lasts longer than 6 seconds) or atypical absence seizures. The frequency range is from 10 to 25 Hz. GPFA-like activity is sometimes seen associated with infantile spasms in West syndrome, but the main EEG correlate is the electrodecremental response.

Attenuation of Beta Activity

Focal attenuation of beta activity is abnormal when there is a 35% or greater difference in the amplitude of the beta activity in homologous contralateral brain regions (see Fig. 8.5).⁶ Interestingly, the same conditions that cause focal enhanced beta activity can also cause focal attenuation of the beta activity. These include spaceoccupying lesions such as tumors, cortical dysplasia, abscesses, vascular lesions, and ischemia. The use of barbiturates or benzodiazepines can enhance this finding by increasing beta in unaffected brain regions.

SLOW WAVES

Normal Slowing

The normal EEG is rife with slow activity. Whether slow activity is normal or abnormal depends on the patient’s state. For example, centrally predominant theta activity is a normal finding that progresses with drowsiness. In contrast, delta activity is rarely normal in the awake adult.

Temporal Delta in the Elderly

One exception to the rule that delta is abnormal in awake adults is that intermittent temporal delta activity can be normal in the elderly patient as long as it represents <1% of the record.⁷ Such slowing, when present, is more common on the left.

FIGURE 8.5. Attenuation of beta activity with concomitant attenuation of the alpha activity. (From Blume WT, Kaibara M. Atlas of Adult Electroencephalography. New York, NY: Raven Press; 1995:414, with permission.)

FIGURE 8.6. Rhythmic midtemporal bursts of drowsiness with depictions of symmetric and asymmetric theta activity. (From Blume WT, Kaibara M. Atlas of Adult Electroencephalography. New York, NY: Raven Press; 1995:141, with permission.)

Rhythmic Midtemporal Theta of Drowsiness

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