Surgery (II):Related Issues

Turkis/i Neiirosurgery 10: 80 - 89, 2000

Pediatric EpilepsySurgical Strategies and

Çntnltepe: Pedintric Epilq'sy Surgery (Il)

Pediatrik Epilepsi Cerrahisi (II): Cerrahi Yaklasim Stratejilerive ilgili Hususlar

OGUZ ÇAT ALTEPE

Institute for Neurology and Neurosurgery, Department of NeurosurgeryBeth Israel University Hospital, New York, NY, U.s.A.

INTRODUCTION

Children with intractable epilepsy constitute aspecial subgroup among epilepsy patients becauseof distinct challenges characteristic for this age. Theseage-specific characteristics, pre-surgical assessmenttechniques and selection criteria of surgicalcandidates were discussed in the first part of thisreview. Here, we will review special aspects ofsurgical approaches in this patient group. There aresome critical questions which should be addressedby medica1/surgical team and the patient's familybefore proceed for a surgical intervention: what isthe chance for seizure control after surgery? what arethe risks of surgery? what are the risks of not havingsurgery? what is the natural course of the underlyingdisease? what are the other medical managementoptions? what are the surgical options? and finaly,what is the goal of the surgery? Sometimes, there areno straightforward answers to some of thesequestions. However, these questions should bediscussed with the patient and family using allavailable data.

We strongly believe that pre-surgical andsurgical approaches to children with intractableepilepsy should be tailored based on individual cases.

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Even among children, indications and surgicaltechniques are often different in infants/youngchildren, preadolescent and adolescent groups.Therefore, this review is not a search for a generalguideline for surgical treatment of epilepsy inchildren, but an effort to document and to discusscurrently available approaches and techniques.

STRA TEGICAL ASPECTS OFSURGICAL APPROACHES

One of the main challenges in epilepsy surgeryarises from overlapping eloquent cortex andepileptogenic zone (7,8,14,40). The goal in surgery isto obtain optimal resection of epileptogenic tissuewhile preserving eloquent cortex. Although, there isno perfect solution for this problem, epilepsysurgeons frequently need to address this issue. Thereare two main options: removing epileptogenic zonealong with a significant amount of eloquent corticaltissue or preserving whole eloquent cortex whileremoving much less epileptogenic tissue. The priceof the first approach is causing new neurologicaldeficits while having satisfactory seizure control, andthe price of second approach is causing no newneurological deficit but no satisfactory seizurecontrol neither. This theraupatic dilernma is not rare

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especially in pedia tric epilepsy surgery and challengeis to find optimal balance.

First step in surgical planning is to definesharper boundaries of epileptogenic zone andeloquent cortex. This is only possible by obtainingextensive data using aii available cartical mappingtools. All da ta should be assessed and discussed

throughly by an experienced pediatric epilepsysurgery team. The goal is to create a sharper corticalmap of fundionally important areas as well as safeareas for resection and to obtain a sharper map ofepileptogenic zone (34). Epilepsy surgery team mustalso try to determine potential risks of surgicalresedion, to predict long-term recovery eliance ofpossible functional loss and potential contributionof brain plasticity to recovery. In addition, othersurgical techniques sucli as disconnection techniquesand multiple subpial transedion procedure shouldbe considered based on available data. After reachingan agreement about surgical plan, pediatric epilepsysurgery team should discuss available data andsurgical plan, including all potential risks andbenefits of surgical approach, with the familyandthe patient, if he/she is mature enough. All othermanagement options from most radical resediOlIstrategies to non-surgical approach should alsodiscussed with them, and decision for a surgicalintervention should be made together. The patient'sfamily should be informed about available dataregarding mortality and morbidity risks of plannedsurgical procedure. Although, perioperativemortality is not a high risk (1.2%-2) in this patientgroup, it stilI is most serious issue to be discussed(38,66). Pediatric epilepsy cases, especially infants,are surgicaiiy more vulnerable patients because ofsmall blood volume, risk of hypothermia,anesthesiology complications and relatively morefrequent indications for extensive surgicalinterventions such as multilobar resection and

hemispheredomy (8,19,20,37).

Surgical approaches in epilepsy can bereviewed roughly in two groups: resedive surgeriesand other techniques, including corpus caiiosotomy,multiple subpial transection and vagal nervestimulation. Here, we will briefly review eachsurgical technique and related complications.

Cortical Resedion:

Temporal Lobe Resection: As I emphasizedabove, temporallobe epilepsy has been encounteredless frequently in children than adults and especiaiiy

Çiila/tepe: Pedialric Epilcl'sy Surgay (Il)

mesial temporal selerosis is very uncomrnon in earlychildhood. Even temporallobe lesions of children areless frequently localized to mesial temporal regioncomparing to neocortical involvement nO,18,23,26,32,63). The plan for temporal lobe resection isdetermined individuaiiy for each patient based onEEC and neuroimaging results. Surgical approachmight be tailored or standard anterior temporallobectomy with / withou tamygdalohypocampectomy;selective amygdalohippocampectomy; purelesionectomy or lesionectomy plus removal ofepileptogenic area. The most frequent complicationof temporal lobedomy is homonymous superiorquadrantanopia. This complication is predidable tosome extent if the length of the resediOlI from thetemporal pole is taken into account. Speechdifficulties and memory dysfundion mayaiso occuraf ter temporal lobectomy on dominant side.Contralateral hemiparesis or hemiplegia is a rarecomplication and most likely to arise from a choroidalartery related vascular damage (15,28,42). Availabledata is variable about exact incidences of these

complications in childhood. There is very raremortality in temporal epilepsy surgery series. Polkeyreported hemiparesis in 4.2% of patients withsignificant recovery (54). Exad percentage of visualfield defeds is not certain because of difficulty tojudge in children, however superior quadrant defedswere reported in 20-40% of the patients(18,26,35,45,55,57,63). it has been noted that schoolperformances of children typicaiiy improve af tertemporallobe resediOlI (18).

Extratemporal Resection: Extra tem poralseizures are much comOlon in infants and youngerchildren and they Olay cause devastating, veryfrequent and intractable seizures. Therefore,extratemporal resections constitute most comOlonepilepsy surgery procedure among infants and veryyoung children. Extratemporal epilepsycharaderized with ear1ier onset and high frequencydaily seizures. Developmental abnormalities,especiaiiy cortical dysplasia, are very comOlonpathologic findings in pediatric epilepsy patients.This also presents a significant diagnostic andsurgical elia11enge because of understatedneuroimaging findings of these cases. Locating aseizure focus in an extratemporal region is one ofthe most cha11enging tasks for a elinicalneurophysiologist. In addition, large cortical surfacearea, very widespread and functional neuralpathways permitting very fast spread of dischargeswithin and outside of a lobe complicates thechaiienge more. Therefore, extratemporal epilepsy

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cases more frequently need invasive EEC monitoring,functional neuroimaging studies and extensive work­up for better definition of borders of epileptogeniczone as well as eloquent cortex. The complicationsof extratemporal epilepsy surgery are mostly relatedto functional characteristics of resection area. There

are a few pediatric epilepsy surgery series regardingthe complications of extratemporal epilepsy surgerycases (1,2,24-26,37,46,57).Unfortunately, these seriesmostly report combine complication rates in bothextratemporal and temporal cases. The complicationrates are between 12 and 50% in these series and varybased on resection site. We reviewed our pediatricextratemporal surgery cases (n:51) which wereoperated at the Cleveland Clinic between 1991 and1996(8).The neurological complications in this seriesinclude increased or new motor deficit in 8 patients,sensory deficit in 3 patients and visual field cuts in 6patients. In general, the type of extratemporalcomplications closely related to resected anatomicalaraes such as paracentral cortical area,supplementary sensorimotor area (SSMA),remaining frontal lobe, parietal lobe and occipitallobe resections. Resection of hand sensory-motorcortex cause severe, permanent fine motor deficit andabnormal position sense, whereas resection of motorleg area leads to flaccid leg paralysis with areasonable chance of incomplete recovery. Resectionof non-dominant primary face area does not causeany significant deficit, because of the bilateralinnervations of facial musc1es. However, thefeasibility of extensive resection of dominant facemotor cortex is controversial. Some authors reportedpostoperative dysarthria in these cases, especially ifadjacent opercular area was removed (27,40,44,48).Resection of supplementary sensorimotor area leadsto transient contralateral neglect andi or weakness,mutism in various severity without cognitiveimpairment, diminished spontaneous movementsand apraxia. Post-operative findings resolve withindays or weeks with no gross deficit at long-termfollow-up. The intensity of deficits are much less inchildren after the surgical intervention in SSMA(8,14,27,33,40-42,44,52,56). Resection of Broca's areais only feasible using slow, staged removal ofpremotor opereular frontal cortex and with frequentintra-operative language testing under localanesthesia (4). Transient motor dysphasia may beseen if resection is extensive (8). Non-dominantparietal lobe may be removed extensively withoutany clinical1y significant sensorimotor deficit excepta smaIl risk (0.5%) of hemiparesis (44,48). However,Olivier recommends that parietal resections shouldnot exceed intraparietal su1cus as inferior limit of the

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resection (53). Angular gyrus (Wernicke's area)should be respected to avoid postoperative aphasiaand alexia (30,44,48,50,56). if resections are carriedinto the underlying white matter of parietaloperculum, contralateral lower quadrantic orhemianopic visual field defect can be seen. Occipitalresections may produce contralateral homonymoushemianopsia and language disorders in the dominan themisphere (8).

Hemispherectomy:

Hemispherectomy is the procedure of clioice forpatients with Rasmussen's encephalitis withprogressive hemiparesis, Sturge-Weber syndrome,hemimegalencephaly, hemispheric atrophy withinfantil spasms and infantile hemiplegia syndromes(16,22,29,30,49,50,58,61,62).These patients general1yhave frequent, severely debilitating, medicallyintractable seizures arising from a structurallyabnormal hemisphere and originating from multipleepileptogenic foci which are too manyandi or tooextensive for focal/lobar excision. Therefore thesepatients are not candidates for a more restrictedsurgical resection. The typical surgical candidate isa patient who aiready has hemiparesis with absentfine finger and foot tapping movements, but withpreserved shoulder, elbow, wrist, and some fingerflexion, and with ability to walk. In addition otherhemisphere should be structurally and functionallynormaL. In good selected patients, hemispherectomyis very gratifying procedure by yielding complete ornear complete seizure control (up to 85%) with nonew neurological deficits. it should be noted thatalthough seizures are decreased significantly,developmental gains are relatively poorer inhemimegalencephaly cases. This is most likelybecause of dysfunctional non-megalencephalichemisphere. Hemispherectomy modalities areanatomical or functional hemispherectomy andhemidecortica tion. Origi na i ana tomica ihemispherectomy included en blo c removal of theentire hemisphere down to basal ganglia. However,serious earlyand Iate onset complications werereported in the 705,such as superficial hemosiderosiscaused by numerous acute/ chronic hemorrhagesfrom the fragile capilleries within a subduralmembrane, obstructed hydrocephalus, bleeding intothe hemispherectomy cavity, or brain stern shift withprogressive neurological deterioration. Severalprevious studies with long term follow-ups (up to20 years) documented the incidence of thesecomplications as 17-35% with high mortality rates.However, much better complication rates were

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reported at more recent series (16,22,29,30,50,56,58­62).Functional hemispherectomy was developed byRasmussen to overcome these long-termcomplications. Functional hemispherectomy is ananatomically limited, but functionally completehemispherectomy which aims complete functionaldisconnection of damaged hemisphere fromremaining brain. Functional hemispherectomyinc1udes removal of pre- and post-central corticalregion s, transection of all frontal, parieto-occipitalwhite matter and corpus callosum, and disconnectionor resection of the temporallobe inc1uding amydalaand hippocampus (62,49,59,60). Villemure et.al.reported 54 functional hemispherectomy cases withearly hydrocephalus in 7.4% of the patients andmortality rate of 3.7 percent (60). Hemidecorticationconsists of removing whole cortical tissue of thehemisphere while preserving the white matter andnot entering ventride. Several series reportedhydrocephalus in 20-33% of the cases and severe per­operative haemodynamic disturbances in these cases(16,50,59,60).

Lesionectomy:

Some patients have distinct lesions which causeepilepsy such as tumor and vascular malformation.In these cases, surgical decision must be based onnot only concem of seizure control, but also potentialbehavior and natural history of the lesion. it shouldbe confirmed that epileptogenic discharges originatefrom the lesion, and gold standart for this assessmentis long-term non-invasive or invasive EEC-Videomonitoring depend on the characteristics of the case.Main controversy is extent of the resection in thesecases: removing just lesi on without resectingsurrounding brain tissue or extending resection areabeyond the boundaries of the lesion. In general,accessibility of the lesi on and its relation witheloquent cortex determine the extent of the resection.Available data shows better outcome in

extratemporal cases with strict lesionectomy.However striet lesion resection is less effective in

temporal lobe lesions. if there is any tumorinvolvement, active epileptogenic activity in mesialstructures or hippocampal atropy, best strategy isresecting mesial temporal structures along the lesion.However, mesial structures may be preserved in apatient with pure lateral neocorticallesions. if tumorborders were not wel1defined, or epileptogenic areais much larger than tumor, temporal lobectomy isrecommended. Borders of resection site may betailored based on electrophysiological, neuroimagingstudies or histopathologic criteria. Extended

çata1tt'l)e: Pediatric Epilel'sy Siirgay (LL)

lesionectomy, which is resection of lesion withsurrounding epileptogenic area, if functionallyfeasible, is our preference as in most epilepsy surgerycenters (7,8,9,21,40,41).

Corpus Callosotomy:

Corpus cal1osotomy is not a definitive surgicalintervention. The procedure does not aim to eliminateseizure focus. The purpose of surgery is to alter thepropagation pathways of epileptic discharges and toprevent the spread of discharges to the oppositehemisphere. The surgical candidates typical1y havesymptomatic generalized epilepsy arising frommultiple areas bilateral1y and present with multipleseizure types (atypical absence, myodonic, tonic,atonic, tonic-donic). The procedure is usuallyreserved for patients who are not candidates forresective surgery but severly disable with veryfrequent seizures, particularly" drop" attacks. Thereis no firm criteria that can be used to predict goodoutcome after corpus cal1osotomy. However, it hasbeen believed that normal intel1igence, focal EECabnormalities, focal radiological abnormalities, thepresence of generalized tonic-c1onic, tonic and atonicseizures, and hemiparesis are good predictors forbetter outcome. Pre-surgical MRI should be assessedto see the presence, length and thickness of corpuscal1osum. The procedure can be performed withdifferent techniques such as sectioning en tire corpuscallosum with or without sectioning anterIorcommisure and fomices, two stages complete corpuscallostomy with/without cutting the anteriorcommissure, and one stage anterior one-half or twothirds corpus callosotomy. Although surgicalapproach should be decided individual1y, partialcal1osotomy, if it is effectiye in that group of patients,frequent1y preferred to one stage sectioning of wholecorpus callosum. Several complications weredescribed after corpus cal1osotomy. The patients withacute disconnection syndrome exhibit decreasedspontaneity of speech which may range from simpleinitiating problems to complete mutism, variabledegrees of paresis, forced grasping on the non­dominant side, incontinence and antagonism in basicmanual tasks because of persistent antagonismbetween two hemispheres. Failure in recent memoryfundion has also been noted in minority ofcal1osotomy patients. Split-brain syndrome is relatedto the extent of cal1osotomy. Sectioning of spleniumleads to deficits of tactile and visual transfer. Speechdeficit usually improves, however permanentlanguage disturbances mayaiso occur (3,6,11,42,67)Cendes et.a!. reported persistent postoperative

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dysarthria, diminished speech output, and gaitdyspraxia in 13 out of 34 callosotomy patients (11).

Multiple Subpial Transedion:

MST is a surgical alternatiye for managementof seizures arising from epileptogenic zonesoverlapping the eloquent cortex. The idea behind thissurgical technique is to prevent lateral spread of theepileptogenic discharges over the cortex in aneloquent area while the columnar circuitry andcentral projections of the cortex, consequentIy normalphysiological function of the brain tissue, arepreserved. Although, this technique is not as efficientas resectiye surgeryon seizure control, it has beenused successfully in weii selected and carefullymapped cases. In general, candidates for thisprocedure are the patients with Landau-Kleffnersyndrome, epilepsia partialis continua originatingfrom eloquent cortex, focal sensory-motor seizuressecondary focal cortical dysplasia on eloquent cortex.Appropriate cortical areas for MST are pre-and post­central gyrus, Broca's and Wernicke's areas. Theparticular difficulty to choose appropriate surgicaltechnique (cortical resection vs MST) in childrenarises from excellent synaptic plasticity andfunctional recovery potential of children frequentlyseen after resective surgery. The complications of thisprocedure are related to hemorrhage and directinjury of neuronal structures. The incidence ofpermanent morbidity, inc1uding hemiparesis,aphasia and visual field defects, was reported about7% in addition to transient deficits in 7% of thechildren (36,42).

Vagal Stimulation:

Vagal nerve stimulation is relatively a newtechnique. it has been used with a reasonable degreeof seizure relief in patients who are not goodcandidates for resectiye surgery. Although it is notcertain, the technique probably works by globaiiyreducing epileptogenicity in brain and locus seruleusplays a crucial role in this. Most commoncomplications are persistent cough (15%), voicealteration (55%), dyspnea (13%), pa in, paresthesiaand headache. There were 2 deaths among 195patients (17).

SURGICAL OUTCOME

Available data regarding overall surgicaloutcome of children with epilepsy shows comparableseizure control with adults. it appears that young age

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does not adversely effect good surgical outcome. Oneof the biggest series in children came from MontrealNeurological Institute and showed favorableoutcome in 56/118 patients with amedian foiiow upduration of 15 years (23). it showed that temporallobe cases had better seizure free outcome after

temporal lobe resection than extratemporal cases asin adults. Several recent studies provided mu chdetailed overall outcome data which shows muchbetter results in children (12,20,38,65,66). These recentseries documented that 60 to 65% of infants hadseizure free outcome while another 13 to 20% of

patients had rare seizures. These rates are 59 to 67%and 11 to 19% respectively among preadolescentchildren and 69% and 20% among adolescents. Theserates are variable secondary to type of epilepticsyndrome and type of surgical procedure. Fish et. aL.reported 73 pediatric temporal surgery patients fromMNI with seizure free outcome in 34 patients (23).Another study from Oslo, Norway reportedsignificant reduction (more than 95%) of seizurefrequency in 60% of the children with temporal lobeepilepsy (26). Both studies also agree that lesionalcases had better outcome than non-lesional cases in

this group. Tuxhorn et.a!. published anothertemporal epilepsy surgery series from Bethel,Germany (55). This series inc1udes 36 patients withthe mean age of 8. Seventy five percent of the patientshad seizure free outcome and 11% had significantseizure reduction (>90%).

We reviewed extratemporal epilepsy surgeryand hemispherectomy cases distributed from the ageof 3 months to 18 years (mean 11.3 year old) (n: 66)in Cleveland Clinic (8). Fifteen of the patientsunderwent functional hemispherectomy and 51.

patient underwent various extratemporal resectionoperations. In extratemporal resection group, 69.1 %

of the patients had Engel Class I outcome, 3.8% ClassII, 11.5% Class III and 15.3% Class IV outcome. Inhemispherectomy group, 75% of the patients wereseizurefree after the operation (Class lA), 12.5% wereClass IIB and 12.5 % Class IllA. Villemure et.a!.

reported similar results with 75% seizure freeoutcome rate in 54 hemispherectomy patients (60).

Blume reviewed several corpus callosotomyseries (total 513 patients) and documented thatoutcome varies based on seizure types (6). Favorablesurgical outcome was reported in 70% of childrenwith drop attacks whereas this rate droped to 63%in tonic-c1onic seizures and to 54% in absence and

complex partial seizures. Morreii reported surgicaloutcome results in 99 patients who underwent

Turkish Neurosiirgery 10: 80 - 89, 2000

multiple multiple subpial transection (36). Fifty twopercent of the patients were seizure free, 11% of thehad rare seizures and 21% of the patients hadseizured reduction more than 90% after MST. One

of the largest long-term prospective studiesregarding vagal nerve stimulation (n:195 cases)showed more than 50% seizure reduction in 35% of

the patients and more than 75% seizure reduction in20% of the patients at 12 months (17).

There is a limited data regarding epilepsysurgery in infants, however the results of this datashows that epilepsy surgery in infants may begratifying in a group of well selected patients. TheCleveland Clinic reported 12 cases of infants (2.5-24months of age) with focal cortical dysplasia, Sturge­Weber, hemimegalencephaly, and neoplastic lesions(64).They documented that six patients were seizurefree (Engel Class 1), three had rare seizures (EngelClass II) and two patients had worthwhileimprovement (Engel Class III). No patient had a newneurological deficit after surgery. Several patientshad marked "catch-up" developmental progress. Inanother series from UCLA, 15 patients were reported.Sixty five percent of the patients were seizure free,13%had seizure reduction more than 90% in seizure

frequency (Engel Class II (64,66). Chugani et.al alsoreported 23 patients with intractable infantile spasms(13). These patients underwent cortical resection(n:15) and hemispherectomy (n:8) and 15 patientsbecame seizure free, 3 has 90% seizure control andone had 75% seizure control. Four patient did nothave any benefit. Another important data from thesestudies show lower seizure free outcome in children

with cortical dysplasia (52%) comparing to childrenwith low grade tumors (82%) (6.5).

SURGICAL TECHNIQUEAND CONSIDERATIONS

Extensive pre-surgical assessment, right andwell planned surgical approach, and appropriatesurgical technique constitute keystones of goodoutcome in epilepsy surgery. Appropriate epilepsysurgery techniques for adults are also relevant forchildren. In addition, pediatric neurosurgerytechniques and principles should also be followedstrictly during epilepsy surgery procedures inchildren. Here, we also would like to emphasize someimportant technical points in pediatric epilepsysurgery cases.

it is recommended to make a large craniotomyin resectiye surgeries to have better exposure and

çata/tepe: Pediatric Epilel'sy Siirgery (Il)

access to critical anatomical landmarks and

functionally important areas such as post-centralgyrus and parieto-temporallanguage areas. This willhelp to assess satisfactorily the borders of resectionsite and to apply electrical stimulation and mappingtechniques if necessary (27,41/44,48,50,56).

Intraoperative ultrasonography and especiallyframeless stereotaxy became part of the standardarmamentarium of pediatric epilepsy surgery cases.Frameless stereotaxy is especially importan t inco rp us callosotomy cases for pre-operativelocalization of bridging veins and determining fullextent of resection during surgery. Framelessstereotaxy is also very helpful for resectiye surgeryin paracentral and central cortical areas. Finally, itsplace in hemispherectomy cases is particularlyimportant if surgery has been performed throughnarrow exposures. Frameless stereotaxy especiallycritical in hemimegalencephaly cases which havevery distorted anatomy and smaIl ventricles.

Subpial resection technique is the main surgicaltechnique to avoid the complications in the epilepsysurgery. This technique helps to recognize theanatomicallocation, extend and depth of the varioussuIci and gyri in the resection area at every stage ofthe surgery. it also provides excellent anatomicalguidance to remove pre-determined cortical areawithout disturbing the vascular supply of adjacentcortex. The endopial resection technique providesexcellent hemostasis while resecting the tissue andprevents development of meningo-cerehral scartissue and distal ischemic insult. The surgeonselectively protect the pia by peeling it from cortexandi or aspirating cortical tissue without perforatingor disturbing the pia and pial vessels. Thanks toendopial resection technique, all arteries, veins,sinuses and cranial nerves around the resection area

are kept subpially throughout the surgery and leftintact. The most helpful surgical to ol in subpialdissection of the cortex is a low setting aspirator orultrasonic dissector set at a very low parameter ofsuction and vibration (8/15).

Stripping the brain and pia-arachnoid fromsinuses, especially during parietal and occipitalresections, can cause postoperative bleeding andhydrocephalus secondary to decreased CSFabsorbtion. We use strictly suppial resectiontechnique in these cases and leave the pia-arachnoidmembranes attached to sinuses. Protecting allvascular structures and vascular supply ofsurrounding cortex is another important surgical

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principle. Especially, the watershed territory betweenthe anterior and middle cerebral arteries should be

protected from any surgical damage as much aspossible and surgeon should not leave anydevascularized tissue behind in this area to preventpostoperative swelling which may be catastrophic.

Another critical issue for a successful surgicalresection is determining the borders of functionalareas. Best tools for this purpose are using intra- orextra-operative cortical stimulation techniques,evoked potential responses, functional neuroimagingstudies and stepwise resection of eloquent cortex inan awake patient by continuously testing sensory­motor functions during resection. Recognizingcentral artery when it exit from the sylvian fissurehas central importance during surgery of this region.it can be recognized easily by its typicallocalization.The central suIcal artery exits from sylvian fissureand makes a loop over the central operculum beforepenetrating deep into the lower part of the centralsulcus. Utmost care must be taken to protect centralartery and all other vascular structures in Rolandic,precentral and postcentral suIci during the surgery.

Resection of eloquent cortex is a challengingtask. Beforehand knowledge of potential deficits isimportant. Therefore, meticulous mapping hascritical importance for resection on and around motorcortex, especially mapping of tongue, thumb and lipareas. One of the biggest challenges duringparacentral resection in children is identification ofcentral region. Classical anatomical landmarks,although helpful, may not be always reliable.Frequently, tubers and dysplastic lesions distortparacentral area. In these cases, using SSEP/MEPtechniques has critical importance to determinecentral suIcus by phase reversal signal and to evokespecific motor responses. Postoperative languagedysfunction can be seen if adjacent pial banks,underiying white matter or ascending vascularsupply are manipulated or if opercular areas areremoved in this region. (24,50,56). The surgeon alsoshould not extend the resection boundaries at motor

cortex to the point of 2-3 mm below the lowest elicitedthumb response, if the patient has no arm weakness(50). 5timulation studies and mapping of speechareas may be challenging. At times, producing speechinterference with stimulation in Broca area may bevery difficult. Olivier (40) as well as Rasmussen (48)emphasized that speech interference with stimulationin the Broca area is often negatiye with currents wellabove sensory and motor thresholds and a negatiyeresponse does not necessarily indicate absence of

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function. Therefore, the posterior 2.5cm of inferiorfrontal gyrus or at least three opercular gyri in frontof the lower end of the precentral gyrus must bepreserved to avoid speech disturbances (40,48). Inaddition to resection of this area, venous occlusionof ascending frontal veins draining into the superiorlongitudinal sinus in the dominant hemisphere maycause transient motor dysphasia as well. Childrenhave significant advantage for recovery af terresection of eloquent cortex because of neuralplasticity in the developing brain. The samemechanism is also important in tumoral cases. if thetumoral mass is in opercular region in small children,it can be expected having an aiready moved speecharea. This should be kept in mind during mappingstudies.

Even if cortical tissue is preserved,manipulation or removal of underlying white mattermay produce significant deficits. Therefore, utmostcare not to compromise functionally critical axonaltracts should be taken during the resection of corticaltissue.

As we emphasized before, children withepilepsy have some specific patologies such as5turge-Weber, hemimegalencephaly, infantilehemiplegic syndromes. These patologies have somecharacteristics which should be aware of to avoid

trouble during surgery. The cortex of a 5turge-Weberpatient generally has abundant tortuous capilleriesand small veins. These vascular structures may bevery thick at times, and may constitute severallayersdeep in subarachnoid space and may penetrate intocortex and subcortical white matter withaccompaniment of calcifications. In addition, somelarge, tortuous veins frequently in unusuallocationscan be seen in these patients. Non-filling anteriorsagittal sinus, thrombosed branches of middlecerebral artery or simply an area of avascularity isalso not uncommon in these cases. The venous

drainage may radiate toward the supependymalvenous network, especially toward the superolateralventricular edges. From surgical point of view, thisextensively abnormal vascular anatomy can causeserious problems during surgery (4,30,47). Therefore,it is an absolute necessity to do adequate preoperativevascular evaIuation and to use appropriate resectiontechniques in these cases. .

Another peculiar and rare developmentalpathology is hemimegalencephaly characterized bycongenital hypertrophy with thickened, abnormalcortex of one hemisphere (53,54).

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Hemimegalencephalic brains had poor nutrition,abnormal vascular supply and AYM-like vesselswhich can cause heavy bleeding problem s andcardiac faI1ure during surgery. Unusual anatomy andabsence of anatomicallandmarks might also causeserious orientation problems during these operations.Adequate pre-operative vascular work-up anddetailed structural neuroimaging studies should beobtained and a meticulous surgical planning shouldbe done based on this data. Frameless stereotaxytechnique provide great help during surgery.

CONCLUSION

Epilepsy surgery in childhood is a safe andeffective therapy with well selected cases andadequate surgical technique by an experiencedsurgical team. The outcome in seizure control issimilar to that obtained in the adults. it may be adefinitive treatment not just for controlling seizuresbut also for preventing their long-term harmfuleffects. Focal resectiye surgery is the procedure ofchoice if there is a focal zone of epileptogenic onsetandi or zone of cortical abnormality concordant withseizure semiology and if epileptogenic zone isamenable for resection. Hemispherectomy is theprocedure of choice if there is multifocal orgeneralized seizure onset limited onlyonehemisphere with associated significant contralateralhemiparesis. if resective surgeries are notappropriate, then corpus callosotomy, multiplesubpial transection and vagal nerve stimulationshould be considered. Satisfactory outeome can onlybe obtained using a team approach by an experiencedpediatric epilepsy surgery group. Extensive andthroughly assessed pre-surgical evaIuation data, wellplanned surgical approach and strict adherence toappropriate epilepsy surgery and pediatricneurosurgery technics and principles are criticaldeterminants of success in pediatric epilepsy surgery.

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