Patterns of cortical discharges and their relation to routine scalp electroencephalography.

The object of the present study was that of determining the relationship between an electrical event recorded at the cortical level and its scalp correlate. Taking into consideration the limitations implicit in this study, as well as the numerous possible sources of error, the following observations were made: 1. 1. A considerable number of epileptiform discharges present at a cortical level will not be recorded with the routine (overlying) scalp electrodes. 2. 2. A certain number of paroxysmal discharges will, instead, also be recorded with routine scalp electrodes, and their amplitude appears to be directly proportional to that of the cortical discharge. 3. 3. Another group of discharges will also be recorded with the routine scalp electrodes, but their amplitude is definitely not related to that of the original discharge. 4. 4. The ratio of the amplitudes cortical-scalp, is highly variable and values may oscillate from as high as 58 to 1 to as low as 2 to 1. 5. 5. The method employed for scalp recording (bipolar or monopolar) and the position of the “indifferent” electrode, are important factors in determining the value of the ratios. 6. 6. The size of the cortical area involved in the discharge also appears important in determining presence and amplitude of the scalp discharge. 7. 7. Amplitude of the “original” discharge and extent of the area involved by the discharge itself are not the only factors responsible for presence or absence of spread to the scalp; however, the morphology of the discharge shows no characteristic of constant features to suggest its role in the spread. 8. 8. Cortical spikes do not seem to spread more easily or more constantly than cortical paroxysmal waves, and the duration of the spike itself is immaterial in the process of spread. 9. 9. Regardless of the amplitude of the “original” cortical discharge the voltage variations of the event at the scalp level are very comparable to those of the event simultaneously “projected” to deeply located structures. 10. 10. Preferential spread to the different scalp electrodes has been described and analyzed. Some facts of practical significance result from this study. It is an accepted notion in the field of clinical EEG that a rather considerable number of proven epileptics may present repeatedly a negative, if not completely normal, record. It would seem logical to explain this on a functional basis, or, in other words, by taking for granted that the epileptic tissue may be “quiet” for more or less long periods of time, and becoming activated for some known or unknown reasons only sporadically, in analogy to the clinical behavior, which is in fact characteristic of the epileptic syndrome. It is dangerous to generalize, however, from our observations one would take a slightly different view on this problem. In their interseizure periods, all of our patients would show a very high number of paroxysmal discharges (from 1 up to 20 or more every 10 sec.) and yet only a small percentage, and in some cases, an exceedingly small percentage of these discharges would spread and become recordable with the routine scalp electrodes. In view of this fact, one would be justified in considering a negative routine EEG in a proven epileptic, not as the expression of temporary “quietness” of the epileptogenic process, but only as an attenuation in absolute amplitude and in cortical distribution of its discharges, these factors being, for the moment at least, the only two which seem to play a role in determining the appearance of the discharges on the scalp. In analogy, the interpretation of the so-called “electrically silent” seizures becomes clearer. Following our observations, one not only has to consider the lack of electrical correlates during a clinical attack as only apparent, but should not conclude that because of this negativity of the routine scalp EEG the seizure itself must “originate” and remain localized in the depth, as suggested by many authors. The activation could well be mainly or exclusively cortical, and yet have no correlate on the scalp. On the other hand, the presence of a “positive” scalp EEG provides only some information of what is going on in the cortex. Thus, to a given discharge recorded from one scalp electrode could correspond a similar discharge at the cortical level with an amplitude, however, which could be larger from two up to about sixty times. The absolute voltage of the scalp transient would, in addition, reflect the extent of the cortical area involved in a given discharge and, according to which method is used in scalp EEG, voltage variations could indicate either an increase or a decrease in the extent of this area. In the specific case of temporal lobe epilepsy, our observations have provided some useful indications on the methodology and on the interpretation of EEG records, pointing to the limitations of some bipolar linkages, and confirming the inadequacy of monopolar recording with reference on the ipsilateral ear in comparison to similar recordings with a relatively “more truly” indifferent electrode. From a more theoretical standpoint, and in particular, in regard to the mechanism of spread from cortex to scalp, further study is obviously needed. Our analysis has proved rather negative. Although at times the amplitude as well as the extent of the cortical distribution of the “original” discharge seems to play a role, the morphology itself did not correlate consistently. This fact makes it rather difficult to explain the extreme variability of ratio between cortical and scalp discharge. In fact, if one assumes that the size of the field depends on factors such as amplitude, distribution and form of a discharge, one would expect that when these three parameters are constant, the resulting scalp event would also be constant. As this is definitely not the case, one has to consider other factors which presently escape our analysis such as variability in the impedance of the volume conductor, variability in the orientation of the dipole or even occasional presence of a second, “destructive” field. On the other hand, the cortical distribution of the discharge could not be checked extensively in view of the limited number of electrodes employed, and it might as well be that its extent at the cortical level varies continously in a fashion similar to that described for the spikes component of the 3/sec. spike-and-wave complexes of petit mal epilepsy (Fischgold et al. 1955). If such were the case, the discrepancy between cortical and scalp recording would only be apparent and could probably be understood. In conclusion, we have the impression that such type of study is justified and that it should be carried on in more detail. The new field of depth electrography has definitely enlarged the possibilities of research in the field of EEG and in particular in the field of the epilepsies; however, we strongly feel that answers to too many questions are necessary before this method can be accepted, its reliability established, and the wealth of information it provides, profitably used.