Epilepsy
Introduction
Epilepsy is the commonest neurological condition affecting the general
population. In the UK, 300 000 people have active epilepsy, 100000 of whom are
below 15 years of age. Despite the fact that epilepsy is such a common
neurological condition, the attention given to the surgical management has been
relatively small. Following Hotsley’s initiative however, great interest was
shown in the surgical managernent of epilepsy but it was for the most part confined to
post-traumatic cases and cases with obvious neoplasms (Fig.
35.43). With the
invention and development of the EEG in the 1930s, this was integrated into the
preoperative evaluation and supplemented by the pioneering work of Jasper and
Penfield in Montreal. Their series of patients undergoing surgery for complex
partial seizures identified that, where abnormal tissue was found in the
resected specimen, the outcome in terms of seizure control was generally good.
With the development of effective anticonvulsant medication, epilepsy was
generally confined to specialist centres or to those in whom neoplasms could be
identified. Indeed, oral anticonvulsant medication is still the first-line
management of seizures. However, approximately 20 per cent of patients do not
have their seizures adequately controlled on oral anticonvulsants and it is
these patients who may benefit from surgery. Surgery is directed towards a focal
lesion, the resection of which will abolish seizures. Functional operations
may also be carried out to modify seizure spread and therefore ameliorate the
patient’s condition.
Preoperative
evaluation
History
and examination
A careful history of the onset and nature of the seizures should be made
as well as the way that the seizures have changed with time, by either their
nature or their frequency. Note should be made of previous medications in
addition to current anticonvulsant medication. It is important that the patient
should have received an adequate therapeutic trial of the first-choice
anticonvulsant drugs before consideration for surgery. The examination in
patients with epilepsy is frequently entirely normal. However, when focal signs
do occur, they can be useful in lateralising the seizure focus. Pathology under
these circumstances is commonly neoplastic or the result of previous vascular
episodes. The history of the circumstances of the patient’s birth,
particularly whether there were any complications in the antenatal or perinatal
period, is important, while the history of a prolonged febrile convulsion in
childhood is frequently found in patients who have hippocampal sclerosis and
associated complex partial seizures.. Any associated Todd’s paralysis with the
febrile convulsion may be a very useful lateralising sign. The history should
include the impact that the seizures have on the patient’s activities of daily
living; notably, its effect on education, employment and personal relationships.
Electroencephalography
Interictal scalp recordings may be useful in excluding a patient for
surgery on the grounds of widespread abnormal activity or if a recognisable
electrographic syndrome can be identified. More commonly the interictal
recordings can give broad guidance in terms of latetalisation and localisation
of the seizure focus as identified by slow-wave activity or interictal spikes.
The information obtained from the scalp recording may be supplemented by more
invasive electrodes such as sphenoidal electrodes or alternatively it may be
necessary to perform video telemetry and record a seizure on video with a
time-locked EEG recording. This allows analysis of the semiology of the seizure
as well as the EEG changes associated with the clinical manifestations.
Neuro
psychology and neuro psychiatry
Careful neuropsychological testing may reveal fixed functional
deficits that lateralise and localise the site of cerebral dysfunction. An
evaluation of verbal and performance IQ as well as memory function are also
vital parts in preoperative evaluation of seizures arising from the rnesial
temporal structures. To confirm laterality of language and integrity of memory
function on each side, it may be necessary to carry out a Wada test during which
sodium amytal is injected under angiographic control into the internal carotid
artery, effectively putting the ipsilateral cerebral hemisphere to sleep.
Patients are carefully tested before injection, immediately following
injection and during the recovery phase to evaluate motor, speech and memory
function.
Psychiatric
evaluation is an essential part of the preoperative strategy, in that
psychiatric morbidity is common both in relation to seizures and in the
postoperative period of seizure surgery. Established psychosis is usually deemed
a contraindication for surgery for epilepsy, whereas a postictal psychosis is
not. Integral to the role of the psychiatrist is the counselling that is
necessary before seizure surgery, as often the seizures themselves ate only a
portion of the patient’s morbidity and it is essential that patients and their
partners and relatives understand this.
Imaging
Preoperative imaging in patients considered for epilepsy surgery has
been revolutionised by the advent of MRI and by an increasing understanding of
the necessary sequences and planes of acquisition requited to demonstrate the
pathological entities responsible for epilepsy. It is becoming increasingly
apparent that with more sophisticated imaging techniques, pathology can be
demonstrated preoperatively in an ever-increasing proportion of surgical
candidates. In patients with seizures emanating from the temporal lobe, the
commonest pathological substrate is hippocampal sclerosis. This can be
identified on preoperative scans by careful examination of the mesial temporal
structures. The volume of the affected hippocampus is reduced while the anatomy
may be clearly abnormal, and both the Ti and 12 signals altered. Neoplasrns
lying within the mesial temporal lobe may also be readily identified and their
anatomical boundaries defined, allowing some idea of the feasibility of the
extent of tumour resection possible at surgery. In cases of extraternporal
epilepsy, tumours may likewise be clearly identified as well as areas of
cortical damage or congenital malformations of cortical architecture.
Surgical
procedures
Penfield and Falconer clearly identified that, where pathological
tissue was found in the resected specimens of patients suffering with epilepsy,
then the likelihood of surgical success was high. Further studies have more
recently demonstrated that the more complete the resection of pathological
tissue, the higher the likelihood of a good surgical outcome. As MRI is now such
a powerful investigatory tool, pathology is frequently visualised preoperatively
and thus surgery for focal epilepsy is becoming more and more lesional. Thus,
when neoplasms are identified, the surgical objective is to excise the lesion in
its entirety, with any surrounding abnormal tissue. Where the lesion is small
and circumscribed, this may be best achieved using stereotactic or minimally
invasive techniques with or without intraoperative EEG.
Approximately
60 per cent of patients being treated for epilepsy suffer from complex partial
seizures and their seizure focus is localised to the mesial temporal structures.
Therefore resections of the temporal lobe, particularly the amygdala and the
hippocarnpus, are the most frequently performed operations for the surgical
management of epilepsy. The extent of the neocottical resection is dependent on
whether the lesion is on the dominant or nondorninant side and may be further
guided by intraoperative EEG. In order that careful dissection of the mesial
temporal structures may be safely performed, this part of the procedure requires
an operating microscope. Exttatemporal resections are dependent on the
pathological entity responsible and also upon the eloquence of the brain in
which the lesion is situated. Where the lesion is close to or within eloquent
areas it may be necessary to carry out the surgery under local anaesthesia with
cortical stimulation, so as to minimise the risk of a postoperative neurological
deficit.
When
an extensive area of unilateral hemisphere abnormality exists, as a result of
either a congenital or an acquired lesion, consideration may be given to a
multilobar or hemisphere resection. Hemispherectomy, or more correctly
hemidecortication, is perhaps the most effective operation for treating
epilepsy, with a near 80 per cent seizure-free rate postoperatively (Fig.
35.44). However, the inevitable neurological deficits mean that its use is
limited and should be considered carefully.
The
most commonly performed functional operation is section of the corpus callosum.
The aim of surgery is to improve rather than eradicate the seizures. This
surgical approach was initially described by Dandy in the management of
tumours of the third ventricle but was applied to the management of epilepsy in
the 1940s. The indications for callosal resection are far from clear but
patients suffering with atonic drop attacks appear to have the best outcome. In
the first instance the anterior two-thirds of the corpus callosum are usually
divided to minimise the chances of a longstanding disconnection syndrome. If
anterior section does not result in improvement in seizures, then the resection
may be completed. The rate for extending resection is limited to approximately
10 per cent. More recently, vagal nerve stimulation is being used as an
alternative to corpus callosum resection with similar success rates.
Outcome
Surgery for well-circumscribed lesions such as benign tumours and
cavernomas produces seizure-free rates as high as 70—8 0 pet cent. In the
presence of hippocampal sclerosis, resection of the mesial temporal structures
and temporal neocortex will result in an approximately 70 per cent seizure-free
rate. Extratemporal resections have a less favourable outcome with a
seizure-free rate of between 40 and 50 per cent. This is often the result of a
more diffuse pathological process, such as post-traumatic gliosis or neutonal
migration defects that underlie the extratemporal epilepsy. Equally important in
the postoperative evaluation is an appraisal of neurological, psychological and
psychiatric status, which may then b~ compared with preoperative status to
assess the dynamic impact of the seizure surgery.