Virginia Thornley, M.D. Neurologist, Epileptologist
March 27, 2018
Temporal lobe epilepsy is one of the most common types of seizures. The most common cause and one of the most successfully treated causes of temporal lobe epilepsy treated through surgery is mesial temporal sclerosis. This article focuses on mesial temporal sclerosis and does not include discussions of other types of temporal lobe epilepsy due to other causes such as tumors, cystic lesions or head injury or non-lesional temporal lobe epilepsy. In order to identify a patient, the symptoms are generally stereotypical which suggest localizing towards one focus. An early age of identification may portend a better outcome since frequent temporal lobe seizures may cause the development of circuitry to the opposite side causing another focus to develop on the opposite temporal lobe. In addition, it is important to control temporal lobe epilepsy because of the location of the seizures are in the hippocampus which is important in memory. Many patients complain of poor memory which will continue to progress should seizures remain poorly controlled. Epilepsy surgery is the definitive treatment for temporal lobe epilepsy in mesial temporal sclerosis.
To identify an appropriate candidate for surgery, the patient should have stereotypical seizures which localize towards one focus. While the focus may cause contralateral clinical symptoms, automatisms of the limb are generally ipsilateral to the focus. Once a patient has been identified, further diagnostics tests are needed in order to confirm this focus including a routine electroencephalogram and an ambulatory 48-72 hour EEG which can be performed out-patient. The only downfall with an ambulatory EEG is that it is subject to the artifact, since the electrodes may be displaced causing poor adherence of the electrode to the scalp causing resistance manifested as artifact and a poor recording. However, it is still a good screening test to determine whether there may be a single focus versus multiple regions affected. Temporal lobe epilepsy may be seen with high voltage epileptiform spike and wave. It may be accompanied by focal delta slowing within the temporal lobe, suggesting temporal lobe dysfunction due to recurring seizures. If a patient is deemed an appropriate candidate, a referral may be made to an epilepsy center where more in-depth investigations are performed.
Admission to an epilepsy center
Expect to stay at least 1 week or more in order to allow the capture of typical seizures and to obtain an adequate sampling of ictal periods and pinter-ictal periods during wakefulness and sleep. A team of specialists is involved with the work-up including a clinical epileptologist who manages the medications and clinical aspect, a clinical neurophysiologist who interprets the video EEG monitoring and correlates it with the clinical symptoms, a neuropsychologist who performs the WADA testing and a slew of clinical EEG technicians who ensure that the electrodes are properly attached throughout the hospital stay. In-depth conferences are held to review the studies of the patients and evaluate which patients are suitable epilepsy candidates. Sometimes, multiple admissions are necessary before seizures can be captured.
During admission, seizures are captured and correlated with the electroencephalographic recordings to determine the focus. More than one focus correlates with a poor outcome, a single focus is necessary. The clinician may provoke seizures by tapering medications safely in the hospital setting. Other techniques include sleep deprivation and encouraging any triggers. The full spectrum of clinical seizures must be captured in order to ensure adequate localization. Bitemporal foci portend a poor outcome.
A high-quality MRI of the brain using epilepsy protocol with thin cuts through the temporal lobes of 1.5mm to 2mm is essential. Coronal views are the best way to visualize the hippocampi to evaluate for hippocampal sclerosis which characterizes temporal lobe epilepsy. Usually, the hippocampus affected is much smaller than the contralateral one with hyperintensity on T2. As a result of excessive seizures, burning off of the cells in the hippocampus occurs so that is it is now atrophic. Although an MRI of the brain may have already been obtained pre-work-up, a higher resolution and exceptional quality brain MRI is likely to be repeated. This will serve as the visual point on which the neurosurgeon operates. Seeing a sclerotic hippocampus gives a high correlation with mesial temporal sclerosis.
Spectroscopy is obtained in-house, where hexamethylpropylenamine oxime (HMPAO) injection is done 30 minutes before an ictus. When the patient has a seizure, the HMPAO perfuses to the area of interest showing where the seizure localizes. Images are obtained. This test has an added value of further localizing the focus. The drawbacks, however, include not being able to predict when a seizure is about to occur and missing the ictus. It is not unusual for this test to be repeated for it to be meaningful. In addition, it can only be done during office hours so that nocturnal seizure will be missed due to lack of adequate staff.
This is a costly examination which may not be available in some epilepsy centers. It uses a 3-dimensional modality for localizing the focus. The MEG dipoles are superimposed on the MRI images.
A neuropsychologist examines the patient’s memory and language by temporarily putting the opposite side of the brain to sleep through injection of amobarbital into the internal carotid artery. Short-term memory and language are examined. The neuropsychologist must determine that there is adequate memory on the contralateral temporal lobe for temporal lobe surgery to be successful. If both temporal lobes are impaired in terms of memory, the patient will suffer from poor memory following the surgery. Other tests are done by the neuropsychologist to check for cognition, any personality disorders and assess for evidence of mood disorders.
This is one of the final steps in the investigation where the cranium over the temporal lobe of interest is removed and electrodes are placed directly on top of the cerebrum. Depth electrodes are placed in order to capture epileptiform discharges buried deep inside the hippocampus which cannot be adequately detected by electrodes laying on top of the temporal lobe. The seizures are recorded and a more accurate mapping of the seizure focus is obtained.
Once all the appropriate investigations are obtained, if all the data points towards a single focus then the patient is deemed an appropriate candidate. Epilepsy conferences are usually held and reviewed by all the specialists involved in the care. Some patients may proceed directly into surgery after mapping. Others may need to go home and return back for another admission to undergo epilepsy surgery. A patient who is still questionable may need to return for more in-depth recording, this may occur in non-lesional epilepsy where the information is not strong enough to justify surgery. The goal of epilepsy surgery is to resect the dysfunctional epileptogenic zone while preserving the functioning surrounding cortex.
Once the surgery is performed, the patient will need to be on anti-epileptic agents for at least 2 years of seizure freedom. In appropriately investigated patients, a favorable outcome of seizure freedom may reach as high as 60%.