Endoscopic epilepsy surgery: Emergence of a new procedure
Background: The use of minimally invasive endoscopic surgery is fast emerging in many subspecialties of neurosurgery as an effective alternative to the open procedures.
Keywords: Commissurotomy; corpus callosotomy; endoscopic assistance; endoscopy; hemispherotomy; hypothalamic hamartoma
The use of endoscopy is now accepted in a number of neurosurgical procedures like pituitary surgery, skull base surgery, disc prolapse, etc. Endoscopic assistance is also a frequent accompaniment of many of the micro-neurosurgical procedures. Improved technology like the use of high definition cameras, three-dimensional visualization systems and better optics has further helped to amalgamate this tool into the neurosurgical armamentorium.
The use of an endoscope in epilepsy surgery is currently limited and not frequently practiced. ,,,,,,, This could be due to several reasons: (1) Most of the epilepsy surgeries are "parenchymal" surgeries not involving an empty space, ventricle or cistern; (2) most of the epilepsy surgeons, during their residency or fellowship, have either limited or no training in endoscopy; (3) most of the epilepsy surgeries utilize extensive brain mapping prior to surgical resection and hence, surgeons do not find any relevance of endoscopic intervention, especially for neocortical epilepsies.
The utilization of an endoscope for epilepsy surgery was initially used for disconnecting or resecting a hypothalamic hamartoma (HH). This was possible for Type II, III, and some of Type IV hamartomas as these lesions projected into the ventricle. In an earlier study, we, for the first time, described the use of an endoscope for performing a hemispherotomy using an inter-hemispheric trans-callosal approach. 
Before this publication, only a conceptual procedure had been reported in a small cadaveric study by Bahuleyan et al.  They described a 2-port endoscopic technique in order to perform a lateral hemispheric disconnection. However, this technique has not been clinically used till date. The possible reason could be that the technique involves a route through the brain parenchyma. While this technique may be possible in a patient with an "atrophic" (e.g., patients with epilepsy presenting as a post infarct sequel) brain, it would be very difficult, if not impossible, to conduct this procedure in patients with hemimegalencephaly where the ventricles will usually be slit-like. Following this, we described a minimally invasive endoscopic assisted procedure for performing a complete corpus callosotomy with commissurotomy (CCWC), for the first time in the literature (under publication). We had also earlier published our initial experience with the use of an endoscope for resecting HHs.  Utilizing this experience, this may be the appropriate time to coin, in this review article, the term "endoscopic epilepsy surgery" to indicate a new subspecialty of epilepsy surgery that is now emerging as an exciting field in its own right.
All the patients subjected to surgery were diagnosed to be having "drug-resistant epilepsy" by the neurologist, that is, the patient should have failed at least 2 anti-epileptic medications given to him/her in an optimal dosage and combination. The duration of the waiting period, while the patients remained on anti-epileptic medication prior to surgical intervention, as recommended by the International League Against Epilepsy (ILAE), ,,,, was usually 2 years. In some children, however, the waiting period was reduced significantly to even weeks to months depending on the severity of epilepsy, the underlying substrate, and the onset of epileptic encephalopathy. ,,,,,,,,
The preoperative investigations usually included a interictal electroencephalography (EEG), a video EEG (VEEG) recording at least 3 habitual seizures, a magnetic resonance imaging (MRI; at least 1.5T) using an epilepsy protocol with thin sections passing perpendicular to the hippocampus. Further investigations were required depending on the MRI findings. Most of the cases requiring a hemispherotomy and having a pathology like a post infarct sequel or hemimegalencephaly, do not require any further investigations. Patients with a disease like Rasmussen's syndrome usually benefited by the performance of a positron emission tomography (PET) scan as it often showed the areas of hyper-metabolism (this investigation was, however, optional). The presence of HHs also did not require any additional investigations. However, patients undergoing a corpus callosotomy required a detailed investigational work-up as this was a procedure performed in patients disabled with drop attacks with no localizing focus/networks. The patients undergoing a corpus callosotomy in our set up, therefore, further underwent a PET scan, an ictal subtracted single photon emission computed tomography, and a magnetoencephalography. It was also important to discuss these cases in an epilepsy surgery conference where the surgical strategy was planned out. In MRI negative cases (especially if the MRI was performed in another center or without a proper epilepsy protocol), a repeat MRI was always performed especially on a 3 Tesla (T) scanner. This is because it may often pick up subtle substrates like Type I cortical dysplasias. , ,5[,26],,,,,,,33]
Assessment and surgical planning were performed in the epilepsy surgery conference. Patients without any definitive localization on all investigations ,,,, and having bi-hemispheric seizure activity were considered for CCWC. A detailed counseling and informed consent were taken as per the institute protocol for epilepsy surgery patients.
The inclusion criteria for an endoscopic hemispherotomy (EH) included: ,,
The inclusion criteria for a CCWC included:
In general, we preferred performing CCWC in cases with a severe Lennox-Gastaut syndrome (LGS) with severe to profound mental retardation especially in the pediatric population. For HHs, the Delande's grading was used to choose our patients for surgery. , The endoscopic approach was preferred for a small hamartoma that was situated either on the floor or on the wall of the ventricle (Type II); or, a hamartoma on the floor projecting inferiorly (Type III). For a Type IV hamartoma, we preferred an endoscopic-assisted trans-callosal approach.
Endoscopic inter-hemispheric trans-callosal hemispherotomy
The patient is placed in a supine position with the head slightly flexed and in a neutral position. A transverse skin incision is marked over the coronal suture, and a 4 cm × 3 cm flap is raised just lateral to the midline with the medial border just over the lateral part of the sagittal sinus. The sagittal sinus is just exposed by 1-2 mm. Neuronavigation is utilized in all the cases to mark the exact position of the bone flap and to avoid a major vein draining the region. Mannitol infusion is started just prior to the skin incision to provide a lax brain. The craniotomy is performed using a high-speed drill. The dura is opened in a C-shaped manner with the base over the sinus. The medial margin of the hemisphere is retracted using a brain retractor. A rigid 0° high-definition pituitary endoscope (Karl Storz) is then brought in, and rest of the surgery is carried out under its visualization [Figure 1]. The author prefers to hold the endoscope with the left hand as a free hand tool. The endoscope may also be supported by the assistant or held with a holding device. A self-irrigating bayonetted bipolar was used with the right hand. This technique serves to facilitate both dissection as well as haemostasis. The irrigation from the bipolar forceps aids in the general irrigation as well. The medial part of the hemisphere is dissected from the falx under endoscopic guidance, and the corpus callosum (CC) is exposed.
The entire surgery was carried out in 3 basic steps [Figure 2]: (1) Complete corpus callosotomy; (2) anterior and middle disconnection; and, (3) posterior disconnection.
First, a corpus callosotomy is performed. This is performed by exposing the CC by gently retracting the hemisphere by a few centimeters [Figure 2]a-e. It is important to ensure using the endoscope that no bridging veins are getting stretched due to the retraction, both in front and behind the craniotomy. Next, using a fine dissector and also micro-scissors, the hemisphere is separated from the falx. The CC may be identified at the depth as it appears white and pale when compared to rest of the cortex [Figure 2]. We prefer to expose the CC first from the anterior to the posterior aspect and then start the disconnection in postero-anterior direction [Figure 2]b and c. It is important to realize that corpus callosotomy performed for a hemispherotomy is different from that performed as a stand-alone procedure. In the former, the surgeon has to ensure that he/she opens the CC until the body of the ventricle on the affected side is reached, as this procedure will allow him/her to perform further hemispheric disconnection. On the other hand, in stand-alone corpus callosotomy, the surgeon should ensure that he/she is in perfect midline so that the cavum may be opened. Once the CC is exposed, the corpus callosotomy is performed with the aid of bipolar forceps and micro-scissors. The posterior part including the splenium is divided first, followed by the genu up to the anterior commissure. Compared to the microsurgical approach, we have discovered that the endoscopic-assisted approach provides better visualization. This advantage is particularly evident while dividing the terminal part of the splenium.
Following a complete corpus callosotomy, the anterior and middle disconnection was carried out. The anterior and middle disconnection, starts at the beginning of the genu of the CC and passes on to the floor of the anterior skull base at the level of the lesser wing of the sphenoid and the planum, first remaining anterior to the head of the caudate nucleus, then curving lateral to it and passing posteriorly, where it joins the body of the lateral ventricle to the temporal horn and then terminates at the atrium, just lateral to the thalamus. The anterior disconnection starts at the genu. Disconnection is carried from the surface until the anterior skull base over the planum is reached. Neuronavigation is used to reach this bony landmark. Once reached, the resection is carried out to the posterior part of the gyrus rectus, similar to that done in the standard vertical hemispherotomy approach. At this stage, the anterior cerebral arteries (ACAs) and the distal part of the optic nerve may be visualized through the arachnoid. The disconnection next proceeds laterally from just anterior to the caudate head to the lateral part of the lesser wing of the sphenoid, and then turns posteriorly to reach the sphenoid ridge. The middle cerebral artery (MCA) was visualized at the level of the sphenoid ridge. The anterior disconnection is completed at the level of the MCA thereby disconnecting the frontal lobe. The middle disconnection is started at the sphenoid ridge. The disconnection next continues posteriorly. The bulk of the basal nuclei lie here. The middle disconnection is completed by dividing the hemisphere lateral to the thalamus and the choroidal fissure, till the atrium is reached. Division is carried out at the level of the atrium both superiorly and posteriorly until it is completed, and the temporal horn is connected with the body of the lateral ventricle. This procedure disconnects the amygdala, hippocampus, and anterior temporal connections.
The posterior disconnection [Figure 2]i involves division of a short segment of tissue consisting of the posterior part of the fornix, which mostly lies between the choroid plexus at the atrium and the posterior-most part of the splenium. The division was performed up to the underlying arachnoid. Caution must be exercised, as the Galenic veins lie just underneath. There is usually a small piece of tissue present under the choroid plexus that may be easily missed. The endoscope is particularly useful in visualizing and dividing this portion. The posterior division completes the disconnection of rest of the temporal lobe.
The postoperative MRI also provides the surgeon with feedback regarding the feasibility of the surgical procedure using the endoscope [Figure 3] and [Figure 4]. We have used an intra-operative MRI in all these cases. We do agree that this is an expensive tool. However, if it is not available, a neuronavigation is mandatory. In addition, reaching proper anatomical landmarks will ensure that the disconnection is complete.
Following the surgery, the dura is closed primarily. An intra-ventricular drain is left inside for the next 24-48 h to drain out the blood stained cerebrospinal fluid (CSF). The drain is removed earlier if the CSF clears completely, or is delayed up to 72 h, if the CSF remains blood stained.
Endoscope-assisted corpus callosotomy with commissurotomy
The craniotomy and dural opening are as described for EH. The inter-hemispheric fissure is accessed, and the cisternal CSF is released to make the brain lax [Figure 5]. The hemisphere is retracted to one side, and the CC was reached. The ACAs are separated. First, the CC is exposed from the anterior to posterior end. Following this, the disconnection is started from the splenial part and then is extended anteriorly. Division of the splenial part is performed till the arachnoidal sleeve containing the internal cerebral veins is visualized. Once the CC is sectioned completely, the septae on either side of the cavum are separated. The anterior commissure is then divided. This was followed by division of the posterior commissure after entering the third ventricle through its roof [Figure 5]. All our patients were placed on elective ventilation for at least 24 h after surgery. The patients underwent routine evaluations on surgical rounds for any possible complications. A thorough neurological evaluation was performed for persistent neurological deficits before discharge.
Endoscopic surgery for a HH should be carefully planned. ,,,,,,,,,,, The use of neuronavigation is mandatory as the ventricles are usually small. The use of rod lens obturator scopes is also useful to access the ventricles [Figure 6]. The approach is such that the line of disconnection is along the axis of the trajectory. A burr hole is made usually at the level of the coronal suture. The exact position is again planned using neuronavigation so that any draining veins can be avoided. The dura is opened widely and cauterized to avoid stripping and causing a possible epidural hematoma. The obturator with the sheath is introduced into the ventricle. Once the lateral ventricle is accessed, a 4 mm diagnostic scope is introduced. The scope is advanced into the third ventricle. The direction of approach cannot be overemphasized. For example, if the hamartoma is arising from the floor and the left lateral wall, the direction of the approach is from the right side. The hamartoma is visualized as a bulge on the floor and lateral wall of the ventricles. The margins of the lesion are again confirmed using neuronavigation by moving the scope inferiorly and laterally. Next, a depth electrode is introduced into the hamartoma, and an EEG recording is performed. A hamartoma is quite active electrically and will produce multiple polyspikes (grade 5).  Following this, a biopsy is taken. If the hamartoma is small, it is removed using a biopsy forceps. The tissue is initially held, rotated to avulse it from the wall and then removed piecemeal. The lesion is usually avascular or very mildly vascular, hence there is usually no problem with hemostasis. In larger lesions, a disconnection from the surrounding area is also performed. The disconnection is usually performed laterally and anteriorly but not posteriorly as here the lesion merges with midbrain structures. ,,,,
Seizure outcome was assessed using the Engel classification.  The postoperative neuropsychological assessment was performed as described earlier.
Thirty four patients (from January 2010-March 2015) underwent endoscopic procedures for epilepsy. These included endoscope-assisted inter-hemispheric trans-callosal hemispherotomy (EH), (n = 11), endoscope-assisted CCWC (n = 16), and endoscopic disconnection/excision for a HH (n = 7).
Endoscopic hemispherotomy group
Of the 11 patients, who underwent EH (8 males), 9 underwent a right sided procedure. The mean age of seizure onset was 1.52 ± 0.99 years (range 0.8-2.9 years). The mean age of surgery was 9.4 ± 6.1 years (range 0.4-18 years). The mean frequency of seizures was 17.25 ± 16.1/day, excluding 1 patient who presented with status epilepticus. The pathologies included post infarct epilepsy (5), Rasmussen's syndrome (3), and hemimegalencephaly (3).
Two patients had prolonged fever (1-2 weeks) after surgery. The CSF counts and cultures were negative. All the investigations for blood and urine were negative. Following 1 week of antibiotics, the fever was treated symptomatically with paracetamol and cold sponging. The mean blood loss was 85 ± 48.4 cc and mean operating time was 210 ± 42 min. None of the patients required any blood transfusions. The mean hospital stay was 8.5 days.
The follow-up ranged from 5 to 16 months with a mean of 8.4 ± 4.1 months. 9 patients had Class I Engel's outcome and 2 (with hemimegalencephaly) had Class II outcome. The mean Stanford-Binet Kamat Test score was 62 ± 12.64 before surgery and 64.25 ± 4.99 at follow-up (normal range 85-100). The improvement was not significant (P = 0.05).
Endoscopic corpus callosotomy with anterior and posterior commissurotomy group
Sixteen patients underwent CCWC. The mean age was 10 ± 5.9 years (range: 2-15 years; 6 males). Seizure onset occurred within 1 month after birth in 3 patients. In others, it ranged from birth to 5.5 years (mean 24.37 ± 34.76 months). The mean duration of epilepsy was 9.2 ± 5.2 years. The mean seizure frequency was 21.2 ± 17.3/day (range 1-45 days). Etiological causes included previous hypoxic insult in 10 patients (due to forceps delivery), meconium aspiration, hypoglycemia, and low birth weight with breech presentation. Changes in the type of seizures were encountered in 3 patients. A delayed development was seen in all patients. On admission, profound mental retardation with an intelligence quotient (IQ) less than 20 was encountered in 7 patients, severe mental retardation with an IQ between 20-34 was seen in 7 patients and moderate mental retardation with an IQ between 35-49 was seen in 2 patients.
The mean follow-up was 18 ± 4.7 months (range 16-27 months). There was complete improvement in drop attacks in all patients. A significant decrease (>90%) in seizure frequency was reported in 11 patients, moderate reduction (>50%) in 5 patients, while an increased seizure frequency was noted in one patient. The decrease in frequency was observed in all types of seizures in these patients including tonic, tonic-clonic, absence, and myoclonic seizures.
Aggression in their behavior for an initial period of 3 months was reported in 8 patients, which gradually reduced in 6-9 months in 3 patients. The mean preoperative IQ was 25.23 ± 10.71. This improved to a mean score of 26.43 ± 11.41 in 6 months and to 26.87 ± 11.95 in 1 year. Behavioural improvement, in particular, in the domains of social contact, attention span, and learning were reported in 6 patients.
Hypothalamic hamartoma group
Four patients underwent a pure endoscopic approach (Type II: 2; Type III: 2). Three patients underwent an endoscopic-assisted trans-callosal approach. The follow-up ranged from 9.2 ± 1.46 months (range 6-24 months). Five of them had Engel IA outcome, 1 had grade II outcome, and 1 had a poor outcome (Type III, patient with grade IV HH). Three patients showed improvement in learning and behavior. Schooling was started for them. Their rage and aggression showed a significant improvement. EEG recordings in various stages of follow-up showed variable normalization of background activities. One patient, who had precocious puberty, had significant improvement. His testosterone level normalized from a value of 711 ug/ml. He was, in addition, also given chemotherapy [Figure 6].
All the patients were continued on their respective anticonvulsants. A repeat MRI was scheduled after 3 months.
Hemispherotomy has an excellent outcome when it is performed with proper surgical indications. ,,,,,,, In the literature, the incidence of freedom from seizures as a result of the hemispheric resection and disconnective procedures is reported to vary from 54% to 89%. ,,,,,,,,,, Better success rates are seen when the insula is also disconnected. These procedures have been mainly been indicated in children having severe catastrophic epilepsy in the presence of either a congenital or an acquired hemispheric pathology. ,,,,,,,,,,,, When first introduced, hemispherectomy was associated with significant short and long term complications. Specially noteworthy was the occurrence of hemosiderosis, which occurs due to the presence of the dead space produced by removal of the entire hemisphere. ,, In 1983, Rasmussen introduced a functional hemispherectomy based on the partial excision of certain areas and disconnection of the major lobes.  This lead to the usage of the term "hemispherotomy" instead of "hemispherectomy," which was first suggested by Olivier Delalande in 1992. , Further evolution lead to the development of two basic procedures. A vertical approach was suggested by Delalande ,, and a peri-insular approach was suggested by Villemure et al. ,,,, Most of the modifications are based on these two procedures. ,,,,,,,, Delande et al., felt that their technique was better as it involved a cortical pathway that avoided major blood vessels and was performed through a trajectory that included landmarks easily identifiable by surgeons. Overall, it is important to realize that a complete hemispheric and insular disconnection , is required to achieve the best possible seizure outcome. Interestingly, when Delande et al. developed this procedure, they started an inter-hemispheric approach but gave it up to prefer a trans-cortical approach due to the issue of the bridging veins.  Development of neuronavigation has solved this problem. Thus, key hole-sized craniotomies may be effectively planned to avoid the bridging veins. If the surgeon does not have access to neuronavigation, we suggest that he/she should use the preoperative MRI to plan the site of craniotomy.
The authors present a novel pilot technique consisting of an endoscopic-assisted approach utilizing a small craniotomy (4 cm × 3 cm). The approach involves a route through the inter-hemispheric trans-callosal corridor to achieve a hemispheric disconnection.  Till date, there has been only one study of endoscopic-assisted hemispherotomy described in the literature, and this was a cadaveric concept study.  Our technique is the first of its kind to be described in the literature in terms of both concept and clinical application. However, a word of caution is that an endoscopic procedure should be initially performed using a larger bone flap. The initial cases should preferably be done on patients suffering from post- infarct sequel, and the surgeon should not shy away from using the microscope in combination with the endoscope.
Hemispherotomy can achieve excellent outcomes when performed in optimally indicated patients. ,,,,,,,,,,,,,,,,,,,,, Since the introduction of functional hemispherectomy by Rasmussen, its morbidity and mortality have steadily reduced. ,,,,,,,,,,, However, this procedure still cannot be considered as trivial surgery, as most of the surgeries are performed in children who cannot tolerate blood loss and are prone to other perioperative morbidities such as hypothermia, electrolyte disturbances, and other problems associated with operating on pediatric patients.
Following an examination of these issues, the existing literature, and also our own experience, , we decided that the best option for EH would be an "endoscope-assisted surgery" utilizing the inter-hemispheric route through a small craniotomy.
An inter-hemispheric endoscope-assisted hemispherotomy has the advantage of providing a "cisternal to ventricular access." This is unlike the technique described by Delalande et al., , which consists of a parenchymal-to-ventricular access. This is also in contrast to the concept described by Bahuleyan et al.,  in which ventricular access would be difficult to obtain in the presence of small ventricles.
A drop attack is a postural seizure (mostly due to atonia) caused by a rapid generalization of epileptiform activity to contralateral hemisphere mostly through the CC (that is considered as the largest inter-hemispheric propagation pathway). Complete callosal sectioning is a very effective "palliative" procedure for breaking secondary bilateral synchrony and alleviating the drop attacks with more than 90% improvement in the drop attacks with a reasonable long-term remission. ,,,,, Most of these above-mentioned authors, however, report the presence of 10-20% of primary non responders; and, close to 30% of patients further relapse in the next few years with the outcome thereafter remaining stable. , The common reasons cited for the failure of callosal sectioning in alleviating the drop attacks or their recurrence is the possibility of transmission of epileptiform activity through other inter-hemispheric pathways like the anterior, posterior, and habenular commissures. ,,, Adam  has demonstrated the role of the anterior commissure in inter-temporal lobe communication. Plausibly, these otherwise nonfunctional commissures attain propagating ability over time in the absence of CC and may continue the spread of epileptiform activity to the contralateral hemisphere causing a recurrence of drop attacks over a time course. Harbaugh et al.  in early eighties reported multiple forebrain commissurotomies including that of CC, anterior commissure, and posterior commissure, mainly for akinetic seizures and reported a good outcome in the magnitude of 80% with the procedure. Although we observed acute disconnection syndromes mostly manifesting as prolonged confusional states in kids and nondominant facio-brachial apraxia, it did not alter the functional status of our patients. While all our patients had severe to profound mental retardation with severe epilepsy, there was a definite improvement at follow-up in their cognitive functions. On the contrary, relief of disabling seizures was perceived as the biggest factor responsible for the postoperative improvement as perceived by the parents.
Our study is the first of its kind to demonstrate the utility and safety of this approach for CC and commissurotomy. We believe that a "key hole" endoscopic-assisted approach helps in minimizing unnecessary brain exposure and reduces the blood loss.
Neurological complications in all major series have an incidence of 2-5% with a permanent sequel in 5% of patients. ,, We did not encounter any motor deficits. However, postoperative akinetic state, apathy and sometimes aggression, buccal apraxia manifesting as drooling of saliva, and memory deficits were common. These resolved completely over a period of time.
Endoscopic approach to hamartoma
A gelastic seizure is the hallmark presentation of these tumors, which has now been proven to be originating in the hamartoma itself and spreads to the adjoining cingulate gyrus through the mammillothalamic tracts. Hence, attachments to mammillary bodies are essential for epileptogenesis in HH. We also demonstrated that depth electrode recordings from the hamartoma show continuous epileptiform discharges. This forms the anatomical and electrophysiological basis of disconnection surgery in HH, which actually aims at removing the epileptogenic lesion from the epileptic networks. ,,,,, In 1998, Delalande et al.  described this novel technique of simple disconnection of the hamartoma from the hypothalamus and reported complete seizure freedom in 3 of the five patients with more than 90% reduction in seizure burden in the other 2 patients. In 2003, Choi et al.  also described a good outcome of seizure reduction after successful endoscopic disconnection of a HH. This was also demonstrated in other studies. ,,
We believe that the trans-callosal approach gives a direct entry into the third ventricle and provides an unobstructed view of the disconnection line thereby decreasing the manipulation of hypothalamus and obviates any need for resection of the hamartoma. In addition, the use of endoscopic assistance further reduces the size of the craniotomy. In our study, we have used trans-callosal endoscopic assistance in Type IV; and, a pure endoscopic approach in Type II and III cases.
In the current paper, the authors have described 2 novel techniques that utilize an endoscope for performing a hemispherotomy and corpus callosotomy. In addition, they have described for the first time, a combination of corpus callosotomy combined with commissurotomy for better control of seizures in patients with LGS with severe to profound mental retardation. They have also described their experience with the use of an endoscope for HHs. The utilization of the endoscope for all the three approaches has led to their coining the term "endoscopic epilepsy surgery" to denote the emergence of a new subspeciality of epilepsy surgery.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]