Surgery for Temporal Lobe Epilepsy

Introduction

The International League Against Epilepsy defines an epileptic seizure as “a transient occurrence of signs or symptoms due to abnormal excessive or synchronous neuronal activity in the brain.” There are many types of epileptic seizures, but the diagnosis of epilepsy entails a persistent epileptogenic abnormality that is intrinsic to the brain, spontaneously generating such paroxysmal activity. A “practical” characterization of epilepsy defines it as any of the following:

• 1.

  At least two unprovoked (or reflex) seizures occurring >24 hours apart

• 2.

  One unprovoked (or reflex) seizure and a probability of further seizures similar to the general recurrence risk (at least 60%) after two unprovoked seizures, occurring over the next 10 years

• 3.

  Diagnosis of an epilepsy syndrome

Epidemiologic studies suggest that more than 68 million people are affected by epilepsy, representing a significant global burden of disease, roughly equivalent to lung cancer in men and breast cancer in women. A large portion of patients presenting with new seizures achieve seizure freedom with monotherapy; however, approximately one in three to four treated patients are refractory to medical management, despite exposure to multiple treatment regimes. Temporal lobe epilepsy is the most common cause of medically refractory epilepsy, and it is one of the most common types of epilepsy referred for surgical treatment. Importantly, uncontrolled temporal lobe epilepsy is associated with progressive cognitive decline, and patients with high seizure frequency have more than four times the relative risk of mortality compared with those who are seizure free, often from sudden, unexplained death.

Classically, temporal lobe epilepsy (TLE) is seen as an acquired, drug-resistant disease associated with hippocampal sclerosis. Resective surgery is considered in patients with drug-resistant, disabling focal epilepsy if the region of origin can be removed with a lower risk of disabling cognitive or neurologic dysfunction. Fortunately, temporal lobe epilepsy can be very responsive to surgical treatment, with two randomized controlled trials showing that surgical treatment with anterior temporal lobectomy (ATL) is superior to the best medical management. These studies show that rates of seizure freedom and quality of life are improved by resection of mesial temporal structures, including the hippocampus, parahippocampal gyrus (PHG), and portions of the amygdala, and that much of the disability and increased mortality of chronic epilepsy will be mitigated by successful neurosurgical treatment.

Common Temporal Lobe Epilepsy Syndromes

Mesial temporal lobe epilepsy is the most common surgically treated epilepsy syndrome, with efficacy demonstrated in randomized controlled trials. Seizures often begin with an aura that may include smells, fear, déjà vu, or jamais vu and progress to impairment of consciousness, orofacial automatisms, ipsilateral hand movements, and contralateral posturing. The most common underlying lesion is hippocampal sclerosis, pathologically characterized by neuronal loss in CA1 and CA3 and the dentate gyrus. Structural alterations can include loss of volume and alterations of internal architecture that may be visible on high-field magnetic resonance imaging (MRI) ( Fig. 55.1 ).

Lateral, neocortical temporal lobe epilepsy tends to be associated with semiology distinct from mesial temporal sclerosis. Potential lesions include tumors, such as meningiomas, gangliogliomas, dysplastic neuroepithelial tumors, and gliomas. Developmental anomalies such as type I or type II focal cortical dysplasias can be associated with epilepsy. Atrophic lesions, resulting from stroke or trauma, are also significant causes of epilepsy. Patients may also have mesial temporal sclerosis, leading to “dual-pathology.” If a lesion and mesial temporal sclerosis (MTS) are present on neuroimaging, resection of both the lesion and the atrophic hippocampus leads to a more favorable seizure outcome.

Presurgical Evaluation

Epilepsy is considered drug-resistant when two appropriately chosen antiepileptic drugs (AEDs) are used, tolerated, and fail to achieve seizure freedom. The definition of “seizure freedom” depends on two considerations. The first is the frequency of observed seizures prior to treatment. For example, if a patient has had only one seizure in the year prior to intervention, remaining seizure free for 6 months after a new treatment may not be unexpected. The “rule of three” is commonly used to establish confidence intervals for evaluation of response to therapy: a seizure-free duration that is at least three times longer than the longest seizure-free period prior to intervention is a reliable indicator of a positive treatment response.

Another consideration is observation of a sustained response to treatment that is clinically meaningful; consensus appears to be that 12 months of seizure freedom is associated with meaningful improvement in measures of quality of life, such as anxiety, depression, and employment rates. Thus in general, the definition of seizure freedom is taken to be three times the longest preintervention interseizure interval or 12 months, whichever is longer.

Patients with drug-resistant epilepsy should be evaluated at an epilepsy center to confirm the diagnosis and to determine the epilepsy syndrome and possible candidacy for neurosurgical intervention.