Acute Electrocorticography (EcoG) During Mesial Temporal Lobe Epilepsy Surgery-Review
Sajeesh Parameswaran* and Anu Mohan
Department of Neurosciences, Ananthapuri Hospitals and Research Institute, India
Submission: January 19, 2018; Published: May 24, 2018
*Corresponding author: Sajeesh Parameswaran, M.Sc DNT, Electroencephalographer (Certified by Asian Epilepsy Academy (ASEPA), ASEAN Neurological Association (ASNA) and Indian Epilepsy Society (IES) Department of Neuroscience, Ananthapuri Hospitals and Research Institute, Trivandrum, Kerala, India, 695024, Email: email@example.com
How to cite this article: Sajeesh P, Anu M. Acute Electrocorticography (EcoG) During Mesial Temporal Lobe Epilepsy Surgery-Review. Open Access J
Neurol Neurosurg. 2018; 7(5): 555723. DOI: 10.19080/OAJNN.2018.07.555723.
Temporal lobe epilepsy is the commonest type of partial epilepsy and surgery for intractable temporal lobe epilepsy is the most common form of epilepsy surgery performed world over, with consistently goods results. Mesial temporal lobe epilepsy (MTLE) constitutes around 80-90% of all temporal lobe epilepsies. The surgical procedures commonly carried out for MTLE include standard anterior temporal lobectomy with amygdalohippocampectomy (ATL+AH), electrocorticographically guided tailored resections of the amygdale, hippocampus and the neocortical structures, and selective amygdalohippocampectomy. The role of intra-operative acute electrocorticography (ECoG) during temporal lobe surgery remains controversial despite being in use for over 6 decades.
Temporal lobe epilepsy (TLE) is the commonest type of partial epilepsy, and surgery for intractable TLE is the most common form of epilepsy surgery performed world over, with consistently good results [1,2]. Mesial temporal lobe epilepsy (MTLE) constitutes around 80-90% of all temporal lobe epilepsies, while lateral temporal neocortical epilepsy accounts for the remaining 10-20% . The commonest pathological substrate
for MTLE is mesial temporal sclerosis (MTS) . The surgical procedures commonly carried out for MTLE include standard anterior temporal lobectomy with amygdalohippocampectomy (ATL+AH), electrocorticographically guided tailored resections of the amygdala, hippocampus and the neocortical structures (tailored resection) and selective amygdalohippocampectomy (Figure 1-3).
The role of intra-operative acute electrocorticography
(ECoG) during temporal lobe surgery remains controversial
despite being in use for over 6 decades. While some centers
perform ECoG routinely, others do not depend on ECoG results.
In a survey in 1993 among 42 epilepsy centers world over, 19%
centers never performed ECoG during surgery for MTLE .
Several questions need answers: do all spiking regions need
to be resected? What is the significance of continued spiking
posterior to the resection margin? While many studies have
shown that it is important to remove as much of the spiking
regions as possible without producing neurological deficits
for good seizure outcome [6-10], others do not agree with this
concept [11-14]. Several authors have shown that ECoG can
guide the amount of mesial temporal and lateral neocortical
tissue to be resected, thereby preventing unnecessary removal
of uninvolved tissue, with good results, the so-called tailored
Usually patients undergo standard anterior temporal
lobectomy and amygdalo-hippocampectomy (ATL+AH)
for intractable mesial temporal lobe epilepsy. Few
patients only undergo tailored resection or selective
amygdalohippocampectomy. Acute ECoG usually records after
discontinuing isoflurane anesthesia. Simultaneous recording
of all the temporal neo-cortical and mesial structures are
recommended. However; the following the recording strategy
can also be done.
A 4-contact electrode strip then sequentially places on the
superior temporal gyrus (STG), inferior temporal gyrus (ITG)
and sub temporal region (ST). For the STG and ITG, contact 1
of the strip becomes anterior and contact 4 turn out to be
posterior (Figure 1). For sub temporal region, contacts 1 and
2 lay close to the para hippocampus gyrus and contacts 3 and
4 lay laterally, on the under surface of the ITG. Through the
middle temporal gyrus (MTG), a 4-contact depth electrode
inserts with the presumed head of hippocampus, contacts 1 and
2 presumably inside the heads of hippocampus and contacts 3
and 4 within the MTG. For each position, 3 minutes of recording
recommended. After neocortical temporal resection, which
consisted of maximum 4 cm on the left side and 5cm on the right
side, amygdala and hippocampus were exposed. Two contacts
of the 4cm depth electrode then inserts sequentially into the
amygdala, head, body and tail of the hippocampus. Recording
for at least 2 minutes at each site. However, many centers doing
simultaneous recording from the mesial and lateral structures.
After resection of these mesial structures, post resection ECoG
recording for 3 minutes, by placing the 4 contact strip over the
resection margin with contact 4 overlying the ITG and contact 1
over the STG. Epileptiform abnormalities consist of spikes, sharp
waves, rhythmic spikes and electrographic seizure patterns
(Figure 2 & 3). The polarity is mostly negative but amygdale
and hippocampus spikes frequently have a positive polarity of
varying amplitudes. At each site, the spikes and number spikes/
minute can be calculated to determine the maximal spiking zone
for each patient [17,18].
The maximal spiking zone in the mesial temporal structures
in the majority of the patients with amygdale being the most
frequently spiking zone followed by the head and the body of the
hippocampus when compare to lateral neo-cortex. This pattern
of involvement was found in many studies [19-21].
Alarcon et al.  have shown the most common pattern
of spiking is from the mesial to lateral sub temporal neo-cortex.
Spikes seen in the lateral neo-cortex in the mesial group probably
represents propagation from mesial structures and removal of
the mesial structures would result in significant reduction or
abolition of neocortical spikes. Utrero et al.  studied the
pathophysiological relationship between mesial and lateral
neocortical spikes in 33 patients with nonlesional TLE and
demonstrated that the commonest pattern was spikes located
only in the mesial temporal structures, followed by mesial spikes
propagated to lateral neo-cortex.
Gomez Utero et al. , who also shown that when preresection
spiking was confined to the mesial structures, patients
had god postoperative outcome, while those with predominantly
lateral temporal neocortical spiking had a poorer outcome.
McBride et al.  found that persistence of more than 50%
spikes at the end of the resection resulted a poor outcome.
However, other studies have not found a correlation between
residual spiking and seizure outcome. Tran et al carried out
standard ATL+AH in 47 patients regardless of the ECoG findings
and found no correlation between residual spiking and the
seizure freedom . Similarly, Chatrian et al. , also found
no correlation between residual spiking and the seizure freedom
among 72 patients, who underwent tailored resection.
These conflicting results may be related to the manner in
which ECoG data are analyzed. Alarcon et al.  demonstrated
leading regions within the mesial as well as the anterior
temporal and sub temporal structures by performing online
analysis using a computer program. Removal of the leading
regions resulted good outcome whereas non removal resulted
poor outcome. Therefore, it appears that identification and
the removal of all leading regions is more important than the
removal of all spikes. Many studies reveal that spikes in acute
ECoG are not helpful for identifying the borders of epileptogenic
anterior temporal resection . However, the importance and
practice of intra operative ECoG is considered as a useful tool
. Most common cause of surgical failure in TLE is thought
to be insufficient hippocampal resection [27,28]. Intra operative
hippocampal ECoG can predict how much hippocampus should
be resected to achieve maximal seizure-free outcome, thus
allowing to spare the functionally important hippocampus .
The Relationship between the extent of resection of mesial
structures and postsurgical seizure outcome have been well
studied by different centers [30-33]. Newer studies published
regarding prognostic significance of inter ictal spikes and highfrequency
oscillations in ECoG and MRI-guided laser-interstitial
have been published [34,35]. Luther N et al. have correlated
acute intraoperative ECoG findings with ictal onset zone in
chronic ECoG recording in patients with magnetic resonance
imaging (MRI) normal TLE. They found that acute ECoG in MRInegative
TLE may helpful to categorize a subset of patients
who could proceed straightforwardly to standard anteromesial
resection without undergoing chronic invasive recording .
The usefulness of acute ECoG during standardized mesial
temporal lobe epilepsy surgery is limited. However, acute ECoG
may be helpful in tailored temporal lobectomy; especially to
identify the extent of epileptogenic area. Post resection residual
spikes on ECoG may associate with a poor seizure control.