One must be assessed the ability of an individual patient to tolerate the procedure awake.There is significant associated discomfort and possibly pain which must be explained very clearly to the patient.
In ability to comprehend this (e.g.in mentally retarded patients or children ) is a relative contra indication to awake craniotomy.
The upper airway should be assessed;a patient in whom
difficulty with tracheal intubation is predicted may not be
suitable candidate for awake craniotomy because intuba-
tion sometimes is needed emergently even with the patient
in the akward position required for surgery.
Another factor is the anesthesiologist experience;local anesthesia with concious sedation should not be used without proper training.
The patient must be concious and alert while electrical stimulation is being carried out. On the day of surgery only steroid are given prior to admission to the operating suite Anxiety should be dealt with without use of pharmalogic means. Unneccessary verbal comments and noise should be kept to a minimum.The temperature in the room should be comfortable for the patient. Anesthesia equipment and medication for control of a seizure episode should be at hand. After transfer to the table patient's position them selves on the operating table so that they are as comfortable as possible.
Although they are need to be restrained, a small amount of movement should be allowed to maintain comfort throughout this long procedure.
The lateral position is preferred because freedom of the airway is easier to maintain.
Careful attention is paid to the protection of pressure points which should be padded.
At this time an iv lines is secured at once,an a saline infusion started at a low flowrate which will be maintained through the procedure.
To limit the discomfort of a full bladder the drip rate is a adjusted to 1 ml/kg per hour. An indwelling urinary catheter is inserted although some prefer to limit the amount of iv fluids and withhold the catheter.
Oxygen is delivered through nasal prongs while capnography can be performed via an 18-20 g Teflon catheter in one nasal prong.
Before surgical incision local anesthesia is injected subcutaneously into the scalp.Some patients dont require futher sedation at incision.
However,the painful parts of the procedure should be anticipated. They mostly occur early during surgical exposure.Using a neurolept analgesia technique has demonstrated that adequate ventilation could be maintain in such conditions.
An infusion of propofol (8mg/ml)mix with alfentanil
(50 mcg/ml) and lidocaine (2 t0 4mg/ml)is begun at
100 to 200mcg/kg/min,reading the propofol dosage
(alfentanil is simultaneously administered at a ratio
of 1:160 (i.e.when propofol is 80 mcg/kg,alfentanil
is 0,5 mcg/kg/min).
This dose is occasionally supplemented with bolus doses through the infusion pump to achieve sedation.
The patient has little or no response to these manouvers yet ventilated and oxygenated well.
The infusion is typically decreased to 50 to 80 mcg per kg per min propofol while the patient is unstimulated.
Trap has described in detail about concious sedation analgesia. Before the surgeon starts skin preparation and aplication of the drapes fentanyl 0,5-0,75 mcg/kg and dropridol 0,15 mg/kg are given intravenously.Immediately before the episodes the patients need reassurance and
possibly,supplementation of sedation analgesia with fentanyl 25 to 100 mcg/kg per min,titrated to maintain a comfortable level of analgesia and respiratory rate of 12 breaths/min.
More recently Rosa et all and Silbelger et all have reported similar view using propofol. A bolus of propofol (0,3 to 1 mg/kg) is used followed by a continuous infusion at 75 to 100 mcg/kg per min.
In case of insufficient sedation,supplementary boluses at 0,5 mg/kg are added and the infusion rate is gradually increased to 125 mcg/kg/min.
Small increments of fentanyl (1 mcg/kg) are injected at 3 min intervals. A minute amount of dropridol (0,015 mcg/kg) is given for prevention of nausea and vomiting.
An appropriate level of sedation is achieved on the average within 10 minutes.
Most patients tolerate the procedure especially when the anesthesiologist has been able to established good communication and is able to predict and control painful stimulation.
Possible complication of awake craniotomy include rest-
lessness and agitation, with a good rapport with the patient in the preoperative period and at times a change in the level of sedation will resolve this complications.
A three pin Mayfield head holder is applied after infiltration of the sites of pin placement with local anesthesia.
Not only are patients able to tolerate the head holder well, but the head remains secure even during seizure.
Craniectomy and dural stimulation sometimes require additional bolus dosing of iv anesthesia.
A fairly large craniotomy incision is usually needed, extending down to the zygoma for maximum visualization of the tip of the temporal lobe and the inferior temporal circumvolution.
Local anesthesia is achieved with a mixture of bupivacaine 0,25% and lidocaine 1,0% both with 1:200.000 epinephrine.
Optimal points of injection for scalp anesthesia include the origin of the greater and lesser occipital nerves below superior nuchal line,the auriculo temporal nerve just in front of the ear and the supraorbital nerve above the eye brow. Finally an injection made into the subctaneous tissue of the anterior temporal region will join the zygomatic are with the lateral part of the superior orbital ridge.
To optimize the homeostasis the injections must be made into the subcutaneous tissue of the scalp and not exclusively into the subgaleal region.
At least 10 to 15 ml of solution must be injected into the deep part of the temporalis muscle extending from the supraorbital ridge through to the posterior part of the zygoma.
Intracranial structures that are painful to touch and traction include the dura and meningeal vessels.
Once the dura is exposed pain sensation is blocked by intradural injection of small amounts of local anesthetic on each side of the middle meningeal artery and its mayor branches.
The patients are usually awakened before opening the dura.
The propol infusion was titrated down from its original rate so as to maintain a grade 3 to 4 sedation score is now set to zero.
Sedation scores according Wilson et al :
Grades Degree of sedation
1. Fully awake and oriented
2. Drowsy
3. Eyes closeed but rousable to
command
4. Eyes closed but rousable to mild
physical stimulation.
5. Eyes closed and unrousable to mild
physical stimulation.
Patients wake up and within 5 to 10 minutes, are able to speak and answer commands.
At least 30 minutes before ECoG the propofol is stopped although some anesthesiologist will wait until tha last
few minutes.
The patients usually awaken abruptly and demonstrated no lingering confusion.
Spontaneous seizures sometimes have occured in severe epileptic asleep with propofol,methohexital may be
needed to control the seizure.
With the patient awake and the brain exposed direct cortical readings are performed.The goal of ECoG recording is to delineate the full extent of interictal epileptiform activity, Draps must be placed in a way that allows access to the face and also allows the anesthesiologist to converse with the patient.
Face and arm must be clearly visible and permit monitoring the peripheral response to cortical stimulation.
Following recording of spontaneous electrical activity the exposed cortex is stimulated to map out important neurologic areas.
Depending on the degree of surgical confidence and on the location of the planned resection only topographyc landmarks are used for identification of the central sulcus and the central area.
Stimulation of motor cortex results involuntary movements of the face and extrimities.The area representing the face and the mouth is at the inferior portion of the precentral gyrus (motor cortex) just above temporal lobe.
Although the patient is usually aware of the involuntary movements direct observation is important.
Stimulation of the post central gyrus (sensory cortex) may result in paresthesia. Tingling or prickly sensation in tongue,gums,face and extrimities. It is important to instruct the patient prior to the procedure to report any unusual feeling.
The patient may also be asked to perform certain task during stimulation.Usually the effect of stimulation on speech is evaluated during counting or reciting such familiar concepts as days of the week or months of the year.
Stimulation of speech area is signaled by an abrupt interuption in speech, which will resume immediately upon cessation of the stimulation. If localization of the seizure focus is still in doubt after cortical recording depth electrodes are positioned toward deeper structures in the vicinity of the amygdala and the hypocampus.Spontaneous activity is again recorded followed by stimulation.
Activation of epileptic activity is attempted by adminis-
tration of a 0,5mg/kg of methohexital and its effect on the ECoG is observed.
Resection is initiated after the epileptogenic focus is identified and functional areas of the exposed brain have been mapped out.
To relief anxiety and pain with unwanted side effects for instance tachycardia and hypertension,movements,the profol infusion is reinitiated and additional amounts of fentanyl administered especially when there is traction or coagulation blood vessels.
The continuous infusion of propofol during resection of an epileptogenic focus under local anesthesia has considerably facilitated the anesthetic management of procedure. If the propofol is not available,neurolept analgesia has proved to be a safe technique. Intra operative nausea and vomiting is rare with the use of propofol.
However the potential pharmacologic interference with the intraoperative ECoG tracings recently has been demonstra-
ted such as beta activity triggered by propofol.
Seizure control is occasionally necessary;methohexital (1 mg/kg) or benzodiazepine (midazolam) is effective.
At the completion of the surgery all patients are taken to the recovery room awake and responsive.
MONITORING:(3)
Non invasive monitoring is used as a rule for concious sedation analgesia. The oxygenation status of the patient is monitored with a pulse oximeter. Et CO2 is continuously sampled at the nose and the information is utilized essentially as a means to monitor respiration rate.
Non invasive blood pressure measurements are displayed every 5 minutes.
ECG tracing displayed continuously on the video screen to monitor hemodynamic parameter.
Blood loss is estimated and corrected when it approaches 20% of circulating blood volume.
COMPLICATION OF EPILEPSY SURGERY:(1,3)
General complication include acute post operative hemor-
rhage,infection and hydrocephalus.
Spesific complication may be subdivided in transient and permanent deficits which relate directly to the nature the original lesion,the location of the epileptogenic focus and the extent of the surgical removal.
During concious sedative analgesia the intraoperative problems included convulsions,nausea and vomiting,exces-
sive sedation,tight brain and local anesrthetic toxicity.
However as stated by Girvin,the most frequent and promi-
nent risk of epilepsy surgery is the failure to achieve the goal of the investigation and treatment.
SUMMARY :
Epilepsy is a common disease that has major effect both on individuals carrying the diagnosis and society as a whole.
Approximately 10% of the epilepsy patient have medically intractable seizure resulting in progressive neurologic impairment.
These patients may benefit from surgical therapy which should be carried out in a medical center with well organized program consisting neurologist,neurosurgeon and anesthesiologist and experienced electro encephlographer necessary to perform prolonged electroencephaloraphy
studies and resultant neurologic procedures.
One of the most important toll for the investigation of seizures and epilepsy is the EEG.
One of the more important of these concerns is the most appropriate anesthetic approach for surgery that's wether to use general anesthesia or concious analgesia with local anesthesia.
Other disputed area include the optimal anesthetic technique, wether anesthetic induced seizure activity can cause brain damage and the appropriateness of inducing a seizure with an anesthetic drug during ECoG.
Concious sedation analgesia remains a useful technique when approaching an epiloptogenic 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.
REFERENCES :
1.Johnson O Joel: Anesthesia for epilepsy surgery;Stone
JD,Sperry JR:The Neuroanesthesia Handbook,Mosby Year
Book Company,Newyork inc,1989,pp 201-9.
2.Kofke Andrew W,Tempelhoff Rene,Dasheeff Richard:
Anesthesia for epileptic patients and for epilepsy
surgery;Cottrel E James,Smith S David: Anesthesia and
Neurosurgery ;4th edition,Mosby Company,St Louis
London,Philadelphia, Sydney,Toronto.2001 pp.474-90
3.Dure S Lean :Seizures and epilepsy;Albin S Maurice,
Textbook of Neuroanesthesia with Neurosurgical and
Neuroscience perspectives; McGraw Hill,USA,1997.
pp.613-17
4.Stoelting K Robert,Dierdarf F Stephes: Epilepsy Aneshesia
and coexisting disease,3rd edit,Churchill Livingstone
Newyork,Melbourne,1993,pp.232-4.
5.Mc Namara Brian,& Boniface J Simon:Electrophysiological
Monitoring of The Central Nervous System; Matta F Basil,
Menon K David,Turner M John:Textbook of Neuroanesthe
sia and Critical Care,Greenwich Medical Media,1st edition
London,2000.pp.69-83
Ltd.,London,2000.pp.69-76.
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