The skull’s effect on the EEG’s appearance becomes clinically relevant when a portion of the skull is absent, and understanding this effect is necessary to avoid misinterpretation of the EEG recording. When a breach in the skull is present, EEG activity is higher in amplitude because of the reduced electrical barrier, but it is also sharper in appearance. The amplitude increase usually is less than two times the amplitude of the surrounding area, but it may be as much as five times. This results in the region over the breach receiving more of the reader’s attention than the surrounding background activity, and the sharper appearance may result in normal rhythms seeming either arciform or epileptiform (Brigo et al., 2011; Cobb et al., 1979; Jaffe and Jacobs, 1970). During brief, normal increases in amplitude, sharp transients may stand out from the rhythm, and this can lead to misidentification of normal activity as epileptic.

Sometimes the breach effect does not have an increase in amplitude and the effect is manifested as only the appearance of an increase in faster frequencies. This may be due to the size of the skull breach or the abnormality of the underlying cerebrum. Identifying the breach effect is more difficult in such circumstances and misinterpretation of the EEG is more likely. The best way to avoid misinterpretation is for the patient to be specifically asked about head injuries and brain surgery, and for the technologist applying electrodes to actively observe for surgical scars on the scalp and skull contour abnormalities. Questioning about skull abnormality is better than scalp palpation because an artificial material that does not have the same electrical properties as bone may have replaced the skull. Knowing about a skull defect should raise suspicion that focal differences in amplitude or frequency are not due to brain abnormality.

Breach effects occur only over the skull defect, abruptly diminish beyond the margins of the defect, and they rarely extend beyond two electrodes (Lee et al., 2010). Therefore, breach effects are best identified with bipolar montages because of their better spatial resolution. Small skull defects, such as from a burr hole, do not produce a breach effect. This is presumably because the field detected by each electrode is larger than the skull defect.

The increased amplitude produced by the breach effect is caused by bone’s considerable electrical resistivity, and a subsequent increase in current between the cerebral cortex and the recording electrode. The skull’s resistivity is 40 to 80 times that of the scalp (Haueisen et al., 1997; Remond, 1977). However, the basis for the appearance of a sharper contour is not likely due to bone having high frequency filtering properties. Measurements of live skull’s conductance over EEG’s frequency range have not identified differences in conductance over the EEG frequency range (Akhtari et al., 2002, 2003; Gabriel et al., 1996; Stinstra and Peters, 1998; Tang et al., 2008

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