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A 54 year old right-handed female underwent pre-surgical evaluation for drug-resistant seizures. A high-resolution brain MRI was normal. PET demonstrated left temporal hypometabolism. Interictal scalp EEG revealed left> right bitemporal epileptiform discharges (75:25 ratio). Independent left (n=2) and right (n=2) temporal onsets were noted on scalp EEG (scEEG). The semiology demonstrated a sudden blank stare, impaired consciousness, and manual automatisms for 40 seconds following which she was agitated and “couldn’t talk” (left temporal onset on scEEG) in addition to a second semiology (infrequent by report) manifest as a blank stare, impaired consciousness, and left arm posturing. Is she a surgical candidate?
Answer:
The patient underwent placement of intracranial EEG electrodes.
Figure: (A) iEEG with left regional ictal fast activity in the depth at seizure onset (B) periodic complexes in the right depth and fast activity in the strips at termination. L/RST=left/right superior temporal; L/RTA=left/right temporal anterior; L/RTI=left/right temporal inferior; L/RTP=left/right temporal parietal; L/RTD=left/right temporal depths. Sensitivity 50 uv; filter settings of 1-70 Hz.
Answer: Discordant Scalp v Invasive EEG
Invasive EEG is frequently used when insufficient localizing information is present from the non-invasive evaluation. Depth and subdural strip or grid electrodes are often combined (figure) to record smaller pools of neurons that are unable to generate a signal adequate for scalp detection. Common neocortical-based low-voltage fast activity composed of beta and gamma frequencies are most attenuated and may not be detectable by scEEG to identify seizure onset. Invasive electrodes may detect seizures earlier than scalp electrode (see above) though imply a greater degree of complexity and potentially more limited expectation of becoming seizure free after surgery. Our patient underwent invasive EEG recording with bitempoal depth electrodes and temporal subdural strips. All seizures (n=8) demonstrated left temporal onset (A) with a “switch” to the right strips/depths (B) noted in 2 seizures associated with left sided dystonic posturing. Seizures commonly propagate to other brain regions. This “switch” from one hemisphere to another may alter the semiology and falsely lateralize the scEEG to suggest bitemporal epilepsy as in our patient. Propagated frequencies are often slower than ictal onset frequencies (1) facilitating false scEEG detection with a propagated semiology. Electrographic patterns during seizures termination are poorly predictive of the surgical outcome (2). Our patient was counseled on our lowered expectation of a seizure free outcome. She underwent a left temporal lobectomy and is seizure free at 6 months.