Epilepsy, Epilepsy
Surgery
Definitions
 Epileptic seizures are
the clinical manifestations (signs and symptoms) of
excessive and/or hypersynchronous abnormal activity
of neurons in the cerebral cortex. This activity is
usually self-limited. The features of the seizure
reflect the functions of the cortical areas from
which the abnormal activity originates and to which
it spreads. Epileptic seizures have
electrophysiologic correlates that are recorded on a
scalp electroencephalogram (EEG).
Epilepsy is a chronic
disorder caused by a variety of pathologic processes
in the brain and is characterized by epileptic
seizures. The incidence of epilepsy ranges from 0.5%
to 2% of the total population; 25% to 30% of persons
who have epilepsy experience more than one seizure a
month.
Classification of epileptic seizures
Partial seizures have an onset that is localized or
focal within the brain.
1. Simple partial. Alteration in consciousness does not
occur during these seizures. They are classified
according to symptoms: motor, sensory, autonomic,
and psychic. Auras are the sensory, autonomic, or
psychic symptoms that precede a progression to
impaired consciousness or motor seizure.
2. Complex partial. These seizures spread into multiple
areas of the brain and alter consciousness; they are
also called psychomotor or temporal lobe seizures. A
simple partial seizure can progress to become
complex.
3. Convulsive. These seizures have a partial onset but
then spread to involve most areas of the brain and
brain stem. They are not easily distinguishable from
generalized seizures.
Generalized seizures when the EEG shows
simultaneous involvement of both cerebral
hemispheres and consciousness is impaired. These
seizures are the following:
1.
Inhibitory or nonconvulsive, such as atonic or
absence seizures (petit mal)
2.
Excitatory or convulsive, which produce myoclonic,
tonic, or clonic seizures
Unclassified seizures
Mechanisms of epilepsy
are diverse and include
abnormalities in the regulation of neural circuits
and the balance of neural excitation and inhibition.
Factors that influence the appearance of epilepsy
can be genetic, environmental, or physiologic.
Associated medical problems
include the following:
Psychiatric disorders
Rare syndromes: tuberous sclerosis,
neurofibromatosis, multiple endocrine adenomatosis
History of trauma
Sleep deprivation
Treatment of epilepsy
Medical therapy
involves the following:
1.
Various antiepileptic drugs are used: phenytoin,
phenobarbital, primidone, carbamazepine, clonazepam,
valproic acid, and diazepam. Some of the newer drugs
are gabapentin, lamotrigine, and topiramate.
2.
Treatment consists of either a single medication or
multiple drug therapy.
3.
The choice depends on considerations of the
pharmacokinetics, clinical toxicity, efficacy, and
type of epilepsy.
Adverse effects of antiepileptic drugs are dose
dependent and are usually associated with long-term
therapy. Newer drugs claim to have fewer side
effects.
1.
Many drugs have neurologic side effects including
sedation, confusion, learning impairment, and ataxia
as well as gastrointestinal problems such as nausea
and vomiting.
2.
Most anticonvulsants are metabolized by the liver.
Therefore, long-term usage may cause induction of
liver enzymes, which increases the rate of
metabolism of other drugs, particularly anesthetics.
3,
Long-term therapy with phenytoin causes gingival
hyperplasia with poor dentition and, potentially,
difficulties with airway management.
4.
Carbamazepine can depress the hemopoietic system
and, in rare cases, causes cardiac toxicity.
5.
Valproic acid may occasionally lead to
thrombocytopenia and platelet dysfunction.
Surgical treatment. Epilepsy is deemed refractory if
unacceptable side effects associated with
antiepileptic drugs preclude adequate seizure
control. This occurs in 5% to 30% of patients.
Approximately 15% to 20% of patients who have
intractable epilepsy are candidates for surgical
resection of the epileptogenic focus.
Status epilepticus
Status epilepticus is defined as epileptic seizures
that are so frequently repeated or so long in
duration that they create a fixed and lasting
epileptic condition, either convulsive or
nonconvulsive. This is considered a neurologic
emergency.
Treatment.
To prevent brain damage, seizures must be
stopped as quickly as possible. Approaches for
treatment are as follows:
1.
Secure the airway, provide oxygen, and maintain
circulation.
2.
Protect the patient from traumatic injury secondary
to involuntary motor movements.
3.
If hypoglycemia is present or cannot be ruled out,
50% glucose, 50 mL intravenously (i.v.), and
thiamine, 100 mg i.v., should be given.
4.
There are different approaches, but the initial drug
choices usually include phenobarbital, phenytoin,
and benzodiazepines; an example is diazepam, 0.2
mg/kg i.v., or lorazepam, 0.1 mg/kg i.v., followed
by phenytoin, 15 to 20 mg/kg, given slowly at a rate
of no >50 mg/minute.
5.
Seizures that continue to be refractory might
require barbiturate coma titrated to EEG effect.
6.
Other anesthetic drugs that have been used include
etomidate, ketamine, propofol, halothane, enflurane,
isoflurane, and desflurane.
Pro- and anticonvulsant effects of anesthetic drugs.
Numerous reports describe how anesthetic agents can
paradoxically exhibit convulsant and anticonvulsant
properties with different doses, under different
physiologic situations, and with different species.
The inhalation drugs isoflurane and desflurane are
effective anticonvulsants. Although controversial,
sevoflurane has also been shown to produce
epileptiform activity. Nitrous oxide (N2O) does not
have any anticonvulsant properties, nor does it
produce seizure activity on EEG.
Barbiturates are anticonvulsants, but when given in
small doses, thiopental and methohexital activate
the epileptiform activity from a seizure focus, as
indicated by EEG monitoring. Etomidate and ketamine
can activate the epileptogenic focus and have also
been used to treat status epilepticus.
Benzodiazepines are effective anticonvulsants.
Propofol is an anticonvulsant but there have been
controversial reports of seizure and seizure-like
activity after its use in patients who have and do
not have epilepsy.
Opioids (e.g., fentanyl, alfentanil, and
remifentanil) can activate the epileptiform activity
from a seizure focus in patients who have epilepsy.
Local anesthetic drugs are anticonvulsant in low
doses but, at higher serum concentrations, can
produce central nervous system excitation.
Interaction between anesthetic and antiepileptic
drugs
The requirements for muscle relaxants, opioids, and
barbiturates increase in patients taking most
anticonvulsants, particularly phenytoin and
phenobarbital, on a long-term basis owing to the
enhanced activity of hepatic microsomal enzymes,
which accelerates hepatic biotransformation.
Interactions with endogenous neurotransmitters and
changes in the number of receptors, including
opioid, may occur.
Anesthetic management of an epileptic patient for
nonepilepsy surgery
Preoperative assessment focuses on the following:
a. General assessment and preparation
b.
Specific concerns with an epileptic patient
(1) Medical problems including psychiatric disorders
associated with epilepsy
(2) Complications from anticonvulsant therapy
(3) Continuation of anticonvulsant therapy
Anesthetic management and monitoring depend on the
needs of the patient and the procedure.
1.
Drugs that potentiate seizure activity should not be
used.
2.
The requirement for anesthetic drugs may increase.
3.
Consideration should be given to the administration
of additional doses of antiepileptic drugs during
prolonged procedures.
4.
Hyperventilation might potentiate seizure activity
and should be avoided unless necessary for surgery.
5.
Seizures can occur postoperatively because
anesthetic drugs and changes in body physiology
during the operation can significantly affect blood
levels of anticonvulsants.
II. Epilepsy surgery
Procedure
Surgery for partial seizure disorders involves the
resection of a specific epileptogenic focus that may
show either sclerosis or gliosis. This is frequently
accomplished by some form of a temporal lobectomy.
Generalized seizures are treated by interrupting the
seizure circuits by a corpus callosotomy or a
hemispherectomy.
A patient who either remains seizure free or has a
significant reduction in seizure frequency is
considered a surgical cure. This occurs in 50% to
80% of patients.
Cognitive improvement also results because the doses
of anticonvulsive drugs are either reduced or
eliminated.
Patient suitability for epilepsy surgery.
A complete
multidisciplinary evaluation is needed to assess
whether the patient is a candidate for epilepsy
surgery. Invasive and noninvasive investigations are
needed to identify the origin of seizure activity
and to evaluate the feasibility of performing
surgery safely with minimal risk of neurologic and
cognitive injury. Advances in neuroimaging
techniques have reduced the need for invasive
evaluation.
Noninvasive evaluation includes medical history;
assessment of the frequency, severity, and type of
seizures; physical examination; and psychosocial and
neuropsychiatric testing. Surface-electrode
monitoring of EEG activity may also be combined with
video-camera monitoring of the seizures.
Radiologic imaging
can supplement EEG data. Computed
tomographic (CT) scanning and magnetic resonance
imaging (MRI) can help identify areas of sclerosis
and low-grade intracranial neoplasms.
Functional imaging
is accomplished with positron
emission tomography, single-photon emission CT scan,
and functional MRI and spectroscopy to assess brain
activity, cerebral blood flow, and the metabolic
effects of resection of the seizure focus.
Thiopental testing
may be performed to assist in EEG
localization of the seizure focus. The technique is
accomplished by producing a gradual increase in the
blood level of thiopental during EEG recording. This
causes an increase in beta activity in normally
functioning neural tissue but not in the seizure
focus.
Intracarotid sodium amytal injection (Wada test) is
used to test for lateralization of language and
memory.
Invasive evaluation
is accomplished by the insertion
of intracranial electrodes. Epidural electrodes are
inserted through multiple burr holes; subdural grids
or strip electrodes are inserted through a full
craniotomy. Stereotactic techniques can also be
used. These electrodes are inserted several weeks
before the definitive operation to monitor the
patient for an adequate period of time. The
patient's behavior and EEG are continuously recorded
and displayed on a television monitor in specialized
units.
Placement of intracranial electrodes or grids is
usually performed under general anesthesia. The
anesthetic plan should consider the concerns of a
patient who has epilepsy and the precautions that
apply to any craniotomy. Routine noninvasive
monitoring is required with the addition of
intra-arterial blood pressure measurement
as indicated. The anesthetic drugs used are not
specific because there is no EEG recording.
Electrode plates and large grids are quite bulky and
might require brain shrinkage through the use of
mannitol and hyperventilation. These patients may
develop postoperative problems with brain edema and
require urgent removal of the grid because of the
development of intracranial hypertension.
Intraoperative localization of epileptogenic focus
Electrocorticography (ECoG)
is performed during
surgery after opening of the dura by placing
electrodes directly on the cortex over the area
predetermined to be epileptogenic as well as on
adjacent cortex. Additional recordings can be
obtained from microelectrodes inserted into the
cortex or depth electrodes into the amygdala and
hippocampal gyrus.
Stimulation of epileptogenic focus is possible
pharmacologically, if insufficient information is
obtained to define the seizure focus adequately
during routine ECoG. Drugs used in adults include a
small dose of methohexital, 10 to 50 mg; thiopental,
25 to 50 mg; propofol, 10 to 20 mg; or etomidate, 2
to 4 mg. If the patient is under general anesthesia,
other drugs such as alfentanil, 20 to 50 mcg/kg, and
enflurane can be used with or without hypocarbia.
Direct electrical stimulation of the cortex
delineates eloquent areas of brain function, such as
speech, memory, and sensory and motor function. This
allows these areas to be preserved during resection
of the seizure focus. Only motor testing can be done
when the patient is under general anesthesia.
Preoperative preparation for epilepsy surgery.
Communication among all members of the team,
including the neurologist, neurosurgeon, and
anesthesiologist, is vital to the successful
management of the patient throughout the
perioperative period.
Routine and specific epilepsy assessment is carried
out.
Appropriate preparation of the patient for the
anesthetic technique selected is carried out.
Anticonvulsant agents are administered before
surgery in consultation with the neurologist and
surgeon.
Premedication for the purpose of sedation is rarely
required because these patients are usually well
informed; all drugs that might influence EEG, such
as benzodiazepines, should be avoided.
Techniques of anesthesia. Historically, epilepsy
surgery was performed with the patient awake for at
least some part of the procedure. These procedures
are now performed with the use of either conscious
sedation (neuroleptanesthesia) or general
anesthesia. The neurosurgeon usually makes the
decision, which depends on the location of the
seizure focus, the need for testing of eloquent
function, and the patient's ability to withstand an
awake procedure.
Conscious sedation/neuroleptanesthesia
A.
The reasons for having an awake patient are as
follows:
(1) Better ECoG localization of the seizure focus
without the influence of general anesthetic drugs
(2) Availability of immediate responses from the
patient to direct electrical stimulation of the
cerebral cortex to delineate eloquent areas of brain
function to preserve them during surgical resection
(3) Continuous clinical neurologic monitoring of the
patient throughout the procedure
B.
The challenge is to have the patient comfortable
enough to remain immobile through a long procedure
but sufficiently alert and cooperative to comply
with testing. The analgesic and sedative drugs
employed must have minimal interference with ECoG
and stimulation testing.
C.
Specific preoperative preparation
(1) The patient is prepared psychologically and
informed about the complexities and demands of an
awake craniotomy.
(2) The establishment of good rapport between the
anesthesiologist and the patient is absolutely
essential.
(3) The anesthesiologist should be aware of the
signs and symptoms that may indicate that the
patient is experiencing the onset of a seizure.
D.
Preparation of the operating room. An awake
craniotomy adds additional stress to the patient and
the entire team. All preparations should be complete
before the patient arrives in the room so that the
patient can receive the full attention of all team
members.
(1) Anesthetic drugs and equipment for conscious
sedation, induction of general anesthesia, and the
treatment of complications are available.
(2) Routine monitoring equipment is ready to connect
to the patient.
(3) Extra pillows, soft mattress, and soft headrest
or fixed head frame are available for positioning
the patient.
(4) Room environment is at normal room temperature
with a quiet, reassuring atmosphere. It is essential
to prevent unnecessary traffic by placing a sign on
the door advising people of the procedure within.
E.
Patient management
(1) Positioning is usually in the lateral decubitus,
which is most comfortable for the patient and allows
better access to the patient.
(a) Pillows are placed behind patient's back,
between the legs, and under the arms.
(b) Extra blankets may be needed at the beginning.
(c) Patients should be positioned in such a way as
to have some freedom of movement of the extremities.
(d) The patient's head is positioned on a pillow of
appropriate size and shape. However, neuronavigation
for imaging is now frequently used, which
necessitates the placement of the patient's head in
a rigid skull pin-fixation system. Pins are inserted
with the use of local anesthesia under conscious
sedation.
(e) The placement of the surgical drapes should
allow for maximum visibility of the patient's face
by the anesthesiologist and for the patient to see
the anesthesiologist continuously.
(2) The neurosurgeon usually performs scalp block.
(a) Long-acting local anesthetic agents, such as
bupivacaine with the addition of epinephrine, are
used.
(b) Lidocaine, which has a fast onset, may be added
and used to infiltrate areas that are still painful
during the procedure, such as dura.
(c) The maximum dose for bupivacaine is 3 mg/kg and
for lidocaine, 5 to 7 mg/kg.
(d) The scalp block is painful. The patient might
need analgesia and sedation.
(3) Monitors
(a) Electrocardiogram (ECG), noninvasive blood
pressure cuff, pulse oximeter, and end-tidal carbon
dioxide (CO2) via nasal prongs used to deliver
supplemental oxygen.
Invasive monitoring is not routinely required for
all patients.
(b) The intravenous catheter should be inserted in
the arm not involved in seizure activity.
(c) Fluids should be kept to a minimum; therefore, a
urinary catheter is not routinely needed.
(d) Dextrose-containing fluids should be avoided.
(4) Anesthetic drugs
(a) The techniques of drug administration and dosage
requirements vary greatly and need to be titrated to
each patient.
(b) The drugs may be administered by intermittent
bolus, continuous infusion, target-controlled
infusion, patient-controlled analgesia, or a
combination.
(c) Short-acting anesthetic drugs provide good
conditions and ensure that the patient is alert for
assessments.
(d) Traditionally, intermittent boluses of fentanyl
and droperidol were used. Now the most common
combination is an infusion of propofol (25 to 100
mcg/kg/minute) with either intermittent boluses of
fentanyl (0.5 to 1 mcg/kg) or an infusion of
remifentanil (starting at 0.0125 mcg/kg/minute).
Other opioids (e.g., sufentanil and alfentanil) have
also been used.
(e) The infusion of propofol has to be discontinued
at least 20 minutes before the start of ECoG
recording.
(f) Dexmedetomidine, a new alpha2-adrenoreceptor
agonist, has been used as an adjunct for sedation
and analgesia with minimal risk of respiratory
depression.
(g) Antiemetic drugs including dimenhydrinate,
prochlorperazine, metoclopramide, odansetron,
dolasetron, and granisetron, may also be needed and
do not affect the ECoG.
(5) Nonpharmacologic measures are very useful to
help the patient through the procedure. These
include frequent reassurance, allowing the patient
to move intermittently, warning the patient in
advance about loud noises (drilling and rongeuring
bone) and painful interventions, providing ice chips
and a cold cloth
to the face, and just holding the patient's hand.
Intraoperative complications
(1) Pain/discomfort. At certain times, patients
might feel either pain or discomfort and should be
warned about this (e.g., the scalp block) in
advance. Patients may also experience pain during
the bone work if dural vessels come in contact with
instruments and during the manipulation of the dura
mater and major vessels within brain tissue. The
loud noises of drills and rongeurs can be
frightening if not actually painful.
(2) Nausea/vomiting. Many factors may be responsible
for the high incidence of nausea and vomiting
including anxiety, medications, and surgical
stimulation, especially the stripping of the dura
and manipulation of the temporal lobe and meningeal
vessels.
(3) Seizures can occur at any time.
(a) Short, mild seizures may not require any
treatment. Convulsive or generalized seizures need
to be treated immediately.
(b) The patient should be protected from injury.
(c) A patent airway, adequate oxygenation, and
circulatory stability must be ensured.
(d) Before ECoG recording, seizures can be treated
with a small dose of either thiopental, 25 to 50 mg,
or propofol, 10 to 20 mg.
(e) After all recordings have been completed,
benzodiazepines may be used.
(f) If repeated treatments are required, the patient
may become very drowsy and need airway support.
(4) Respiratory. Oxygen desaturation and airway
obstruction may result from oversedation, seizures,
mechanical obstruction, or loss of consciousness
from an intracranial event. Treatment needs to be
immediate and includes decreasing sedation and jaw
thrust, or the insertion of an oral airway,
laryngeal mask, or endotracheal tube.
(5) Induction of anesthesia. If a patient either
becomes uncooperative or complications such as
hemorrhage or continuous seizures develop, the
induction of general anesthesia may be required. To
do
this safely a plan of action is necessary. Airway
assessment determines the best approach.
(a) The laryngeal mask airway may be used
temporarily or for completion of the procedure.
(b) Occasionally, endotracheal intubation will be
required. This can be accomplished with the patient
either on his or her side or supine. With adequate
assistance, the anesthesiologist comes to the
patient's head while the surgeon protects the
sterile brain field. After preoxygenation,
anesthesia can be induced if necessary with a small
dose of propofol (with or without opioids and muscle
relaxant). Intubation may be accomplished with any
airway device with which the anesthesiologist is
comfortable, such as direct or fiberoptic
laryngoscopy or the intubating laryngeal mask.
(c) If any difficulty in securing the airway is
anticipated, an awake intubation with local
anesthesia should be performed.
(6) Other less common complications include
excessive blood loss and a tight brain.
Closure.
During closure of the wound, the patient
may be sedated with other drugs, such as
benzodiazepines, that were not used up to this
point.
Recovery of the patient
takes place in an intensive
care or specialized observation unit.
Postoperative
complications are the same as for any patient after
a craniotomy. Seizures might still occur and may
require treatment.
Asleep-awake-asleep
is a modified technique of
conscious sedation that may also be used.
General anesthesia is used for the craniotomy and
closure. Either inhalation or intravenous anesthetic
drugs with or without controlled ventilation may be
used.
Appropriate airway devices include endotracheal
tube, special oral airway, or, most commonly, the
laryngeal mask airway.
The advantages of the laryngeal mask airway are
easier placement and decreased coughing and
laryngospasm.
The patient is awakened completely and the airway
device removed for the period of intraoperative
neurologic evaluation.
For resection of the lesion and for closure, general
anesthesia is again induced with reinsertion of the
airway device.
This technique requires complex intraoperative
airway manipulation after neurologic testing while
the head is fixed.
Advantages include increased patient comfort and
tolerance during craniotomy and a secured airway
with ability to control ventilation and prevent
hypercapnia.
General anesthesia
The reason for choosing general anesthesia depends
on the preference of the neurosurgeon, or the
patient's inability to tolerate an awake craniotomy,
or both.
The advances in preoperative neuroimaging,
functional testing, and the use of frameless
stereotactic surgery for localization of the
epileptic focus have lessened the need for the
patient to be awake during the procedure.
The challenge to general anesthesia, if
intraoperative localization is needed, is to provide
good conditions for EEG, ECoG, and motor testing.
The influence of the anesthetic drugs needs to be
kept at a minimum while avoiding long periods of
potential awareness on the part of the patient.
Specific preoperative preparation involves informing
patients of the possibility that awareness might
occur at the time of ECoG recording and testing but
reassuring them that it will be brief and painless.
Preparation of the operating room, anesthetic
equipment and drugs, and positioning supplies are as
for any craniotomy. In addition to routine monitors,
intra-arterial and urinary catheters are frequently
used.
Anesthetic management
(1) Specific concerns include the increased dosage
requirements for opioids and neuromuscular blocking
drugs effected by long-term anticonvulsant therapy.
(2) N2O. If the patient has had a recent craniotomy
or burr holes for electrode placement, intracranial
air might still be present. N2O should be avoided to
prevent complications from an expanding
pneumocephalus.
(3) Anesthetic drugs
(a) Drugs should be short acting with minimal
influence on EEG and ECoG and nonseizure-producing
activity.
(b) A balanced technique may be used with opioids,
muscle relaxant, N2O,
and low concentrations of inhalation drugs.
(c) Total intravenous anesthesia with propofol may
also be used.
(d) Inhalation drugs and propofol must be eliminated
at least 20 minutes before ECoG recording. N2O may
also have to be eliminated.
(4) Motor testing is possible during general
anesthesia by discontinuing all inhalation drugs and
propofol and either reversing or allowing the muscle
paralysis to wear off. This testing demands very
careful planning and care of the patient. Either
additional opioids or lidocaine might decrease the
chance of the patient's coughing.
Complications
(1) Craniotomy-related complications
(2) The possibility of awareness
(3) Movement from seizures, especially during any
stimulation testing
Recovery is the same as for awake patients
Pediatric surgery.
The considerations for epilepsy
surgery in pediatric patients are similar to those
for adults, except that most children are not able
to tolerate an awake craniotomy.
General anesthesia is used for most procedures.
Awake craniotomy may be tolerated by an older child.
Asleep-awake-asleep technique with the use of a
laryngeal mask airway is an alternative.
Coexisting conditions with multiple organ system
involvement and significant psychological and
behavior problems may be present.
Parents may be very actively involved in the
patient's management and also require consideration
and education.
Cerebral hemispherectomy and corpus callosotomy.
Treatment of diffuse generalized seizures might
require either resection of substantial portions of
the entire cerebral hemisphere or section of the
corpus callosum. These procedures are usually
performed under general anesthesia because they
involve a large craniotomy and most of the patients
are children. The major concern with these lengthy
procedures is the possibility of extensive blood
loss and air emboli because the surgical site is
close to major vessels and sinuses. |
