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 Normal Subjects

The EEG in the normal awake child and adult is well known and needs no detailed description. The following are points of emphasis:

  1. Alpha rhythm in the two hemispheres is very similar in frequency. A consistent difference of even 0.5 to 1.0 cps on the two sides is significant; the side showing a slower frequency may have a hemispheric dysfunction. Amplitude asymmetry is of relatively less significance, unless the asymmetry is prominent. In general, the alpha rhythm is higher in amplitude over the right hemisphere. If the amplitude of the alpha rhythm on the right side is more than 1 1/2 times that on the left side, the asymmetry is usually regarded as significant. When the alpha rhythm is over 25% higher in amplitude on the left side than the right side, this constitutes a significant asymmetry.[1]

  2. Significant theta activity (4 to 7 Hz) is present in the EEG of children and adolescents. Delta activity in the awake tracing is rarely seen after the age of 5 years. A common EEG pattern in adolescents is the presence of intermittent delta waves intermixed with alpha rhythm over the posterior head regions, the so-called "slow waves of youth."

  3. The EEG during non-rapid eye movement (NREM) sleep in children shows very prominent spikelike vertex sharp transients, which are often mistaken for epileptiform activity by EEG interpreters inexperienced with children's EEGs (Fig. 1). Similarly, positive occipital sharp transients (POSTs), when high in amplitude and sharp in configuration, can be easily misinterpreted as abnormal spikes, especially in linkages where occipital electrodes are connected to input terminal 2 (grid 2) of the amplifier (e.g., "double banana run").

  4. In a small proportion of normal adult subjects, clearly identifiable and countable alpha rhythm may be entirely absent. The background may consist of irregular mixtures of low amplitude (<20 µV) activities, mostly from 5.0 to 30.0 cps without a dominant frequency. Such low-voltage EEGs have been studied in detail.[2] The EEG is reactive to various physiologic stimuli such as sleep, drugs, and pathologic processes. In over half of the patients with low-voltage EEGs, hyperventilation may bring out an alpha rhythm. During sleep, normal activities such as vertex sharp transients and sleep spindles may be generated. It is essential that low-voltage tracings be clearly distinguished from EEGs showing electrocerebral inactivity, which have a grave prognosis. These EEGs lack reactivity and lability, and with increased instrumental sensitivities show no electrical activity of cerebral origin. Low-voltage EEGs are generally considered to be a normal variant occurring in 7 to 10% of normal subjects over the age of 20 years. The low-voltage EEG does not correlate with neurologic or psychiatric disease.

  5. Changes in the EEG during normal senescence has been described in detail.[3,4,5] The most frequent change is the slowing of the alpha frequency. By the age of 70 years, the mean alpha frequency decreases to 9.0 to 9.5 cps and decreases further to 8.5 to 9.0 cps beyond the age of 80 years. In healthy elderly subjects, even at or over the age of 100 years, the frequency of the alpha rhythm remains well above 8.0 cps.[6,7] Therefore, an average alpha frequency of less than 8.0 cps measured with the patient fully alert must be considered abnormal in elderly patients at all ages.

Figure 1.

EEG of a 2-year-old child with very prominent spikelike vertex sharp transients.

Another EEG finding is the presence of isolated transients of irregular focal slowing in the theta-delta frequency range over the anterior temporal region, reported in 40% of healthy elderly subjects.[4,5,8] They are most frequent over the left temporal area particularly during drowsiness (Fig. 2). Sometimes poorly defined sharp waves are interspersed with focal slow components. The left-sided accentuation of this activity remains unexplained. Such intermittent slow activity, with or without sharp components over the temporal region, has no correlation with intellectual or cognitive functioning or presence of a seizure disorder. More recent investigations suggest that the temporal slowing in the awake tracing may, in fact, not be the inevitable consequence of advancing age. In neurologically and psychologically normal septuagenarians, Katz and Horowitz[9] found that the focal slow activity was seen in only 17% of records and when present occupied less than 1% of the tracing. Hence, intermittent temporal theta-delta activity occupying only a small proportion of the wake tracing should be considered as a normal aging phenomena. When the temporal slow activity comprises more delta than theta slow waves, which either recur frequently or occur in long runs and are widespread in distribution, a dementing process or focal lesion has to be seriously considered. Diffuse theta-delta activity in elderly subjects are likely to occur in those with intellectual impairment.[5]

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