Based on location and the structure of the epileptogenic
abnormalities a variety of surgical interventions are perfor-
med under local or general anesthesia.The main disadvan-
tages using local anesthetics are the limitation in head fi-
xation the extra time involved and the availability of an
experienced neuroanesthesiologist.
But local anesthesia remains a useful technique when ap-
proaching an epileptogenic lesion in crucial zones such as
the motor and speech areas.When preoperative anatomic
and electrophysiologic studies have clearly demarcated
the epileptogenic zone in the non crucial area,general
anesthesia can be used quite satisfactorily .
Most resection are performed nowadays under general anesthesia.
General anesthesia for resection of epileptogenic structures Preanesthetic preparation :
As a rule patients arrive in the operating room unpremedi
cated.
If neither ECoG or cortical mapping is planned the patients
usual antiepileptic medication is administered early in the
morning prior to surgery and anxiety is controlled with a
short acting benzodiazepine.
The intraoperative use of eletrophysiological diagnostic
techniques including activation precludes the using of any
sedative with anticonvulsant properties with the exception
of patients presenting with hyperactive epileptic features
who would be at risk otherwise,the AEDs are withdrawn
for at least 48 hr.Forty eight hours to surgery all patients
are started on a coticosteroid regiment of hydrocortisone
prednisone or dexamethesone to minimize brain swelling.
Epilepsy patients are likely to convulse at anytime particularly when they are off medication,a potent anti epileptic agent that can be used at immediately to abort a tonic clonic phenomenon in the short period preceding the
induction. Methohexital are prefered but sodium thiopental,
propofol or midazolam are acceptable alternatives.
Monitoring during the procedure is kept simple as the ma-
jority of patients coming for surgery are healthy except
for their neurologic dysfunction.In recent years,elevation
of the head to improve acess to deep mesial structures has
resulted in the utilization of precordial Doppler monitor
for observation of potential air embolism.But the use of
a central venous catheter for management of air embolism
is controversial because patients usually supine or have mi-
nimal head elevation during surgery and venous air emboli
(VAE) has not been reported as a significant problem.
A CVC may be inserted but only after careful consideration
of its risk in a given patient.
ECG automatic sphygmanometer,oxygen saturation and end tidal CO2 are standard and apnoe monitor is useful.
Positioning requires breaking the table at various angles to
improve comfort and padding exposed bodypart and atten-
tion to details(eg.eye protection ) are important.
Straps and retainers to limit movements of freeparts should
the patient awaken or develop a seizure during the procedu
re but a small amount of movement should be allowed to maintain comfort throughout this long procedure.
An unobstructed view of the patient is essential if cortical
stimulation of the motor area is planned.
A heating-cooling blanket is wrapped around the patient.
Placement of a large bore (gauge 16) iv catheter is perfor-
med in the unpremedicated awake patient while introduc-
tion of an intra arterial catheter is usually postphoned un-
til after induction.A bladder catheter is always inserted.
INDUCTION OF ANETHESIA :
Ultra short acting thio barbiturate are usually recomended
for induction.If ECoG is planned either ethomidate or metho
hexital is suitable alternative.If ECoG is not planned an an-
ticonvulsant anethetic maintenance regiment such isoflu-
rane with or without N20 or moderate dose of opioid should
be used.Consideration in anethetic management should be
given to the effect of long term anticonvulsant therapy that
increases the dosage requirement for opioid and muscle re-
laxant.Intubation is facilitated by the administration of neuromuscular blocking drugs. Fluctuation of heart rate and BP in response to intubation are prevented by the additional injection of iv lidocaine and opioid and/or esmolol.
After endotracheal intubation controlled ventilation with
normocapnia because hypocapnia can produce or potentiate
seizure activity by the decrease in mesencephalic reticular
function activity unless needed for surgical exposure,then
initiated in a conventional way with a N20:O2 mixture and
isoflurane with concentration below 0,5 MAC are known to
interfere minimally with electrical brain activity.
When the time comes for recording of interictal electrical
activity and brain mapping the concentration can be rapidly reduced.At the some centers if ECoG is planned the pa-
tients should be maintain with N20-based or enflurane base
technique with added methohexital as indicated and both
isoflurane and halothane should be avoided.
When used judiciuosly,enflurane may effectively synchroni-
ze and activate an epileptogenic focus making easier to
identify. Methohexital and alfentanil have also been used as
activating agents.Methohexital has been used preoperative-
ly and intraoperatively and was observed in 87% cases to
cause selective activation of epileptogenic focus during acute ECoG. As little as 25 mg of methohexital was needed to induce activation.In a study comparing alfentanil and methohexital use in patients with epilepsy,alfentanil more riably provoked epileptiform discharge.
From the outset analgesia is provided using any
one of the potent narcotic agent such fentanyl. Alfentanil
sufentanil appears are effectively during anesthesia.
Continuous infusion has the potential for maintaining analgesia at constant level throughout the procedure and is
taught to optimize any total iv anesthesia technique.
Intermittent administration of drugs on the contrary will result in variable plasma drug levels overtime with larger to total doses utilized ultimately.
Indeed,however as observed during procedures under local
anesthesia the largest part of the surgical procedure is painless with painful episodes restricted to the incision of the extracranial tissue,the exposure of brain and the final sutures at the end of the procedure.
For the first 30 minutes the total sufentanil dose average 5.0 to 7,5 mcg/kg,if moderate hypotension and bradycardia observed may be compensated by the rapid administration of iv fluids,vagolytic drugs (atropine or glycopyrolate)
and vasopressors (ephedrine or phenylephrine).
An alternative technique for maintaining an adequate level of anesthesia is continuous propofol administration with
or without N20.Whatever the methode that elected consideration should be given to the effect of all agents on the ECoG and the spontaneous or activated epileptogenic patterns. If cortical stimulation and mapping are planned only intermediate or short acting of muscle relaxants are used during the first part of the procedure and are deliberately held to allow for observation of the peripheral muscle responses.A return of better than 90% of the strenght of the musculus adductor policis as estimated with
a TOF technique is usually needed.
Intraoperatively the prerolandic motor area is identified by
direct stimulation of the non paralyzed patients.
The absence of muscular paralyses deemed necessary to optimize the motor response to cortical stimulation of motor strip combined with a very light level of anesthesia to avoid interference with the ECoG represent of a state of inadequate anesthesia. As a result patients then wake up and at times attempt to move. In sometimes verbal reassurance may succeed in controlling the patients but it is often necessary to resort at once to pharmaclogic means of control.
A few patients may,as a response to stimulation procced
with focal seizures that may progress to full tonic clonic
convulsions. The early administration of an appropriate do-
se of barbiturate (methohexital or penthotal) or propofol
will help abort a full fledged seizure.
As reported that non depolarizing muscle relaxant and steroids which frequently used at the time of neurosurgical procedures will also shorten the duration of action of muscle relaxant.
Craniotomy for epilepsy surgery are long procedure and re-
quire a large exposure of the brain and these factors should
result in higher than average incidence of bleeding.
A cautious and unhurried approach throughout the procedure will minimize blood loss.As an additional benefit the vasoconstricting resulting from infiltration with 1:200.000 epinephrine solution limits the volume blood loss at opening.
More commonly patients experience nausea or vomiting
may be exacerbated by surgical stimulation, such as stripping of dura or temporal lobe or meningeal vessel manipulation or by unsufficient analgesia combined with hypovolemia.
Dropridol is used for its antiemetic properties.
In the small amount the drug has no effect on ECoG.While
the control of vomiting and nausea extends over many hours
Other anti emetic agents are not used although ondansetron
appears to be a valid alternative.Metoclopramide is contra indicated in epilepsies.
After the complete removal of the epileptogenic tissue,the
AEDs level is restored.Phenytoin 15mg/kg is administered
intraoperatively.
Emergence after craniotomy for seizure surgery involves
prevention of post operative hypertension ,facilitation of
rapid emergences and prevention of excessive coughing.
In planning for the completion of the procedure the some
times dramatic antiepileptic induce decrease in the duration of neuromuscular blocking agents and opioid drugs should be anticipated.
POST ANESTHETIC MANAGEMENT :
In the PARU vital signs and neulogic responses are closely
monitored for the next 12 h.Hypotension is corrected with
fluid administration and if indicated (Ht below 25%) with
blood replacement.Ephedrine and phenylephrine are rarely
indicated,Severe hypertension and tachycardia are treated
witha beta blocker(esmolol) or combined alpha and beta
blocker(labetalol). Moderate hypertension is frequently related to agitation and sometimes to pain and appropriate
measures should be considered for correction.
Bradycardia is managed with vagolytic agents.
At times abnormal movements of dystonic or myoclonic nature,uni or bilateral,as well as other abnormal neurologic
symptoms (hypo or hyperreflexia,disconjugated eye movement,tremor) or shivering may be confused with post surgical seizure activity.Small amount of propofol,large enough to give sedation without interference with spontaneous respiration.With doses 25 mcg/kg per min,patient remain alert and answer questions,they become sedated.
Corticosteroid treatment initiated prior to surgery is pursued for a few days.Histamine H2 antagonist are commenly given along with the steroid.
As the occurence of early post operative seizures may be re-
lated to variations in the AEDs level,it look wise to follow
plasma levels closely during the first week.
to be continued:
to be continued
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