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E. Liebenthal and T. Singhal
3.2.4 Presurgical Electrophysiological Mapping
EEG is central to the diagnosis and management of patients with seizure disorders.
EEG is available in all clinical centres and incurs relatively low costs. EEG is
particularly useful if epileptic discharges are captured during the EEG recording
session, as this form of abnormal EEG activity is highly correlated with seizure
disorders and is of clinical use. EEG can help determine the seizure type (focal
or generalised) and epilepsy syndrome and thereby guide the choice of antiepileptic
medication and the prognosis. In patients with medically refractory epilepsy who are
candidates for epilepsy surgery, long-term EEG monitoring may be recommended
to characterise and quantify the frequency and type of seizures and to localise the
sources of the epileptogenic activity.
Like EEG, the primary clinical application of MEG is for presurgical localisation and characterisation of epileptic sources, particularly in cases when MRI is
inconclusive with regard to the seizure focus. The location, orientation, and spatial
extent of epileptic discharges with respect to other spontaneous brain activities all
contribute to the sensitivity of MEG and EEG, making it difficult to predict which
method is best suited for each patient. Most electrical discharges can be detected
by both EEG and MEG, but, MEG is more effective for the detection of epileptic
activity arising from the superficial cortical convexity, such as in lateral neocortical
epilepsy and cortical dysplasia [10, 24, 54, 118].
Scalp EEG can be recorded simultaneously with fMRI to map changes in the
BOLD signal that are associated with epileptic discharges [34, 37, 104]. The
advantage of this multimodal method is that it informs the source modelling of the
epileptic activity, particularly in complicated cases when multiple epileptic sources
are suspected, or when no underlying structural pathology has been identified [36,
130]. Intracranial EEG is recommended when deep spike sources such as in the
hippocampus are suspected [1, 111].
3.2.5 Presurgical PET
In epilepsy, asymmetric hypometabolism in the medial temporal lobe, as assessed
by FDG-PET scans, may indicate the seizure focus in refractory epilepsy that occurs
due to mesial temporal sclerosis. Unilateral temporal hypometabolism is a marker
of more favourable surgical outcomes in such cases. Hypometabolic foci noted on
FDG-PET may serve as regions for intra-operative electrode placement to further
identify clinically relevant seizure foci using electrophysiological methods. On the
other hand, multifocal hypometabolism may predict poor surgical outcomes in
refractory epilepsy [19, 61, 113]. Metabolic information provided by FDG-PET can
be complimentary to brain perfusion changes noted on ictal and interictal SPECT.
A combination of these techniques can enhance localisation of seizure focus in
refractory epilepsy patients [113].
E. Liebenthal and T. Singhal
3.2.4 Presurgical Electrophysiological Mapping
EEG is central to the diagnosis and management of patients with seizure disorders.
EEG is available in all clinical centres and incurs relatively low costs. EEG is
particularly useful if epileptic discharges are captured during the EEG recording
session, as this form of abnormal EEG activity is highly correlated with seizure
disorders and is of clinical use. EEG can help determine the seizure type (focal
or generalised) and epilepsy syndrome and thereby guide the choice of antiepileptic
medication and the prognosis. In patients with medically refractory epilepsy who are
candidates for epilepsy surgery, long-term EEG monitoring may be recommended
to characterise and quantify the frequency and type of seizures and to localise the
sources of the epileptogenic activity.
Like EEG, the primary clinical application of MEG is for presurgical localisation and characterisation of epileptic sources, particularly in cases when MRI is
inconclusive with regard to the seizure focus. The location, orientation, and spatial
extent of epileptic discharges with respect to other spontaneous brain activities all
contribute to the sensitivity of MEG and EEG, making it difficult to predict which
method is best suited for each patient. Most electrical discharges can be detected
by both EEG and MEG, but, MEG is more effective for the detection of epileptic
activity arising from the superficial cortical convexity, such as in lateral neocortical
epilepsy and cortical dysplasia [10, 24, 54, 118].
Scalp EEG can be recorded simultaneously with fMRI to map changes in the
BOLD signal that are associated with epileptic discharges [34, 37, 104]. The
advantage of this multimodal method is that it informs the source modelling of the
epileptic activity, particularly in complicated cases when multiple epileptic sources
are suspected, or when no underlying structural pathology has been identified [36,
130]. Intracranial EEG is recommended when deep spike sources such as in the
hippocampus are suspected [1, 111].
3.2.5 Presurgical PET
In epilepsy, asymmetric hypometabolism in the medial temporal lobe, as assessed
by FDG-PET scans, may indicate the seizure focus in refractory epilepsy that occurs
due to mesial temporal sclerosis. Unilateral temporal hypometabolism is a marker
of more favourable surgical outcomes in such cases. Hypometabolic foci noted on
FDG-PET may serve as regions for intra-operative electrode placement to further
identify clinically relevant seizure foci using electrophysiological methods. On the
other hand, multifocal hypometabolism may predict poor surgical outcomes in
refractory epilepsy [19, 61, 113]. Metabolic information provided by FDG-PET can
be complimentary to brain perfusion changes noted on ictal and interictal SPECT.
A combination of these techniques can enhance localisation of seizure focus in
refractory epilepsy patients [113].