Red Alert 4

Aug 05, 2025

This young adult is asleep at approximately 02h00. The differential diagnosis is of epilepsy and non-epileptic seizures/functional seizures. 

 

 

Using the criteria of morphology and field, you might call this a left prefrontal spike and slow wave (interictal epileptiform discharge). 

 

In the page above, the high frequency filter has been changed from 30 Hz to 70 Hz. The morphology of the wave, especially at FP2, now contains a jagged upslope, as well as an extremely sharp peak, atypical of a spike. This is typical of an environmental artefact.  There is another environmental artefact adjacent to the first red arrow. The vertical red line corresponds to two sequential vertex waves. This page is normal.

 

Bottom line? I see far too many EEG recordings where the high frequency filter has been set at 35, 30 or even 15 Hz. There are multiple problems with this, including the conversion of artefacts into waveforms that resemble sharp waves and spikes. Technologists should simply perform the recording at 70 Hz; failure to do so obscures early recognition of electrode artifacts that need to be corrected immediately and runs the risk that benign waves are interpreted as pathological discharges. If filters need to be applied, this can be done retrospectively and judiciously. Hence, there is no advantage to perform the EEG with the high frequency filter set lower than 70 Hz, while there are substantial disadvantages. 

 

PS: in the days of paper EEGs, it was important for the technologist to try to ensure that the electroencephalographer would have a decent look at cortical rhythms without these being completely obscured by artefact. Even then, it was important to perform the EEG with the high frequency filter initially set at 70 Hz and for the technologist then to do everything possible to remedy the artefacts. If these attempts failed, it was acceptable to change the high frequency filters while annotating EEG, to draw the reader's attention to this change. The electroencephalographer would then be forewarned to be disproportionately careful about the interpretation of apiculate waves. In principle, this has not changed; try to fix the artefacts but do not set the high frequency filter lower than 70 Hz. That can be left to the reader.

 

Here is another example from the same patient: