Abstracts

Rationale: Clinical use of dense array EEG has not been done on a routine basis until user-friendly convenient scalp cap system was commercially available. Inter-electrode distance in 128 channel dense array system is 2.25 cm while in the standard 10-20 system is 6 cm. The utilization of dense packed array EEG is expected to salvage non-redundant information in the 6 cm distance and improve spatial and temporal resolution. Methods: 128 channel dense array high-resolution EEG (HR-EEG) recording system (manufactured by Electrical Geodesics, Inc. Eugene, OR USA) has been used in our outpatient EEG Lab since September 2013. All EEGs have been recorded in the way which is analyzed in preset 128 channel net scalp cap montage based on standard 10-20 system as shown in the reference figure. EEGs were ordered to evaluate encephalopathy, episodic mental status change, and seizure disorders. Three EEG cases that demonstrated multifocal electrographic seizure (EGS) patterns are presented. Results: Case 1: 69 year old Veteran with episodic amnesia of normal MRI. Interictal HR-EEG detected evolving EGS starting from electrode #91 with 3Hz and #101 with 10Hz in right posterior temporal lobe, escaping T6 in 10-20 electrodes. Each EGS focus had small independent electrical fields. The findings suggest neuro-network communication between two EGS foci which was not detected in 10-20 system. Case 2: 39 year old Veteran with TIA-like episodic left sided weakness and severe migraine headaches. EEG demonstrated multifocal EGS preceding in the right dorsofrontal neocortex (#103) which provoked contralateral homotopical location in left (#45). Subsequent multifocal activation of EGSs were noted with variable firing patterns and network connections (Electrode #103-#45 #45-#36, #64-#50) and independent EGS (#10, #39, #59). The findings suggest inter-foci connection network transimitting seizure propagations. Case 3: 64 year old Veteran with intermittent episodic confusion. 10-20 EEG showed EGS of 11Hz at C4 (#104) followed by accelerating-decelerating EGS at P4 and #91. It also provoked prolonged EGS at #98 in evolving discharge frequency between 6 to 14 Hz lasting for 3 minutes with overriding HFO presenting series of damped oscillatory waves. Another EGS involving #97 in 15 Hz, driving #93 in 14Hz and immediate next lead #86 in 9 Hz with phase delay. Traditional 10-20 montage failed to capture any of electrographic seizure pattern. These findings suggests detectable interictal EGS by dense array HR-EEG. Conclusions: Interictal multifocal EGS by dense array HR-EEG were unexpected finding in our routine outpatient EEG practice. The phenomena were not detected in the standard 10-20 system in which inter-electrode distance was 6 cm, but 128 channel system captured them. It is therefore legitimate assumption that a standard scalp 10-20 system easily could have missed these surface findings. More studies are required to determine the clinical significances.