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Technical information of the surgical treatment of epilepsy
Possible surgical candidates were evaluated according to the following presurgical evaluation protocol:
1) Prolonged video-EEG recording for ictal events
Prolonged Video-EEG recording complements standard interictal scalp recordings, as it is a “continuous record ”, usually employing additional and special electrodes and consisting of multiple awake and sleep samples along a period of 1-2 weeks. The aim of prolonged Video-EEG monitoring is to exclude a generalized epilepsy syndrome and pseudoseizures, to give a clear picture of seizure type and semiology and to lateralize and/or localize seizure onset. Localization and lateralization of seizures may be difficult and focal lesions may demonstrate diffuse EEG changes, while bilateral changes may be seen with apparent unilateral pathology.
Three special dedicated monitoring rooms has been set up in the Neurology Unit located on the 2nd floor of the Meyer Hospital. They are 4 multichannel (32 and 128 channels) digital EEG recording machines linked to a digital infrared video camera. An EEG technologist and some nurses are present round the clock in this unit. This ensures patient safety and prompt therapy when seizures occur during telemetric recordings. The recording machine is located by the patient’s bedside. However, the patient’s freedom to move around in the room, watch television, and interact with family members is unhampered. This unit is linked via a fast local area network to a review station located in the Neurophysiology department on the ground floor.
2) Structural neuroimaging
Structural imaging is the most reliable tool for identifying pathology causative of epilepsy, and an increasing percentage of patients undergoing surgery have structural lesions on their preoperative MRI. Localization and complete excision of lesion is associated with the best surgical outcome and all imaging techniques should be directed on the basis of the seizure history, the semiology and the neurophysiological findings.
Structural imaging is almost entirely MRI based and CT scan is now only used when information related to calcifications is required, as in Sturge-Weber syndrome.
In our department 1.5 and 3T MRI is available. The epilepsy MRI protocol includes: transverse spin-echo (SE) DP-T2W images of the whole brain, T1 and T2 sequences with particular reference to the perpendicular axis of the hippocampus, axial and coronal TSE fluid-attenuated inversion-recovery (FLAIR), axial and coronal TSE inversion recovery (IR). In most patients, 3D volume Fast Field Echo (FFE) T1-W images are also obtained.
3) Functional Imaging (PET, fMRI, SPET)
a) Positron emission tomography (PET) and single-photon emission computerized tomography (SPECT) are the two functional imaging techniques that are more commonly employed in presurgical evaluation. [18F]fluoro-2-deoxyglucose (FDG) PET assesses interictal glucose metabolism and single regions of hypometabolism are highly associated with the region that can be resected to control seizures in partial epilepsies. In temporal lobe epilepsy interictal FDG scans detect unilateral hypometabolism or asymmetrical bitemporal hypometabolism in 70 to 90% of cases, whereas extratemporal epilepsies are less often associated with severe regional hypometabolisms.
SPECT measures blood flow and provides indirect information about brain activity. It has the advantage of allowing the tracer to be injected during video-EEG monitoring and to preserve in time the blood flow pattern until imaging can be performed, up to hours later. Comparing interical and ictal SPECT studies, it is possible to evaluate the relative increase of certain cerebral regions during the ictal phase with respect to the interictal period.
In our department SPECT and PET are performed in selected patients, in particular in nonlesional cases.
b) Precise localization of eloquent regions is critical to achieve maximum excision of epileptogenic zone. Blood oxygen level dependent (BOLD) functional MRI (fMRI)is able to detect blood flow changes related to neuronal activity and has been applied in presurgical localization of motor cortex and language areas.
In our department fMRI is performed in selected patients, if epileptogenic zone is located in proximity of eloquent areas.
When localizing data from conventional methods do not allow us to generate a hypothesis about seizure onset zone, patients undergo invasive recordings . Prior to an implantation taking place, a clear strategy is formulated as to the aim of implantation and in particular, the likely surgical procedures that will result, should resective surgery be possible.
Two different invasive recording techniques are available:
a) Subdural grids can cover an extensive cortical surface and are particularly helpful for mapping eloquent areas. They consist of stainless steel or platinum contacts that are embedded in a thin matrix of biologically inert but flexible material. The shape and size of the subdural electrodes varies from simple strips of a single row of 4 or 8 electrodes to rectangular arrays of 16 to 64 electrodes.
The area where more likely seizure origin, is determined by means of non-invasive presurgical evaluation. Then a craniotomy is done which not only permits insertion of the subdural electrodes but also gives easily access to the cortex that probably will be resected. Subdural grids are usually inserted for 5 to 10 days to perform Video-EEG recordings and obtain ictal events. Cortical stimulations are performed to test for higher cortical functions and identify eloquent areas
b) In stereoelectroencephalography (SEEG), electrical activity is recorded by intracerebral elelctrodes, implanted by a stereotactic technique in pre-identified cortical and subcortical structures. It may be more sensitive for exploring deep-buried structures such as hippocampus and insula and allows to analyzing the dynamics of epileptic phenomena in humans.
In our department invasive recordings are performed by means of subdural grids. In selected cases, we use a combined approach employing both grids and depth electrodes.