Keywords
epilepsy surgery, complications, temporal lobe surgery, selective amygdalohippocampectomy, hemispherectomy, grid placement, radiofrequency ablation, vagus nerve stimulator
Highlights
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The safety and efficacy of epilepsy surgery have been demonstrated in several randomized trials; however, it is critical to reiterate potential complications arising from epilepsy surgery to ensure best practices of care.
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The association of vascular injury, particularly with the anterior choroidal artery during medial temporal resections, the most commonly performed epilepsy surgery, can have deleterious effects ranging from clinically silent infarctions to hemiplegia, hemianopsia, and more.
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Visual deficits from direct injury to the Meyer loop are potentially avoidable.
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Prevention of injury through adequate visualization, aided by the surgeon’s anatomic knowledge and neuronavigation, is a key to successful surgery.
Introduction
Epilepsy is a well-known chronic neurologic disorder affecting an estimated 1% of the general population. Despite advancement in therapeutic management, approximately one-third of cases are refractory to medical therapy alone. Patients with drug-resistant epilepsies (DRE) are at increased propensity for developing serious morbidities including cognitive disorders, depression, and sudden death in epilepsy. In selective patients with DRE, including adults and the pediatric population, epilepsy surgery is widely regarded as a gold standard therapeutic modality for seizure remission, thereby increasing quality of life. Patient selection is critical because not all patients with DRE are surgical candidates. The safety and efficacy of epilepsy surgery compared with medical therapy alone in patients with temporal lobe epilepsy has been demonstrated in several randomized controlled trials. Temporal lobe epilepsy emanating from a focal lesion with a unilateral onset is the most common diagnosis associated with surgical resectability. Despite the utility of surgical therapy in treating DRE, it is often underutilized, with a meager 3.6% of temporal lobe DRE undergoing surgical management. In patients with mesial temporal sclerosis or other focal temporal lobe epilepsies, the response rates range from 67% to 82%.
With improvements in surgical technique including preoperative planning, neuronavigation and, most importantly, appropriate patient selection, algorithms have dramatically reduced the rates of morbidity and mortality from this therapy over the years. A study comparing adverse events across various epilepsy surgeries between 1980–1996 and 1996–2012 noted a pronounced decline in neurologic complications from 42% to 5%, whereas permanent neurologic deficits decreased from 10% to 1%. Although rare, these permanent complications are of sufficient impact to outweigh much, if not all, of the benefit that even Engel class 1A seizure control would provide; as such, all efforts to avoid these rare but potentially devastating complications should be made. In this chapter we provide a comprehensive overview of the surgical modalities available for epilepsy, discussing anatomic insights and considerations and the potential complications associated with surgical epilepsy procedures.
Epilepsy Surgery: Anatomic Considerations
Most complications after epilepsy surgery occur as a direct result of severance to the local vasculature, particularly the anterior choroidal artery (AChA), and the visual pathways. Anatomic insights on these pertinent structures can help mitigate life-threatening complications; hence a review of these critical structures is necessary.
The AChA, the terminal branch of the internal carotid artery before bifurcation into the middle and anterior cerebral arteries, courses posteriorly between the lateral diencephalic and medial telencephalic structures. Intraoperative severance to the AChA can cause symptomatology related to the affected structures supplied by the artery, such as the anterior perforated substance, optic tract and optic radiations, uncus, cerebral peduncle, temporal horn, choroid plexus of the temporal horn, lateral geniculate body, posterior two-thirds of the posterior limb of the internal capsule, and globus pallidus. Rarely, the head of the caudate, the pyriform cortex, posteromedial amygdala, substantia nigra, subthalamic nucleus, red nucleus, caudate tail, the hypothalamus, and the superficial part of the ventrolateral nucleus of the thalamus can also be supplied by the AChA. It is interesting to note the developmental equilibrium between the posterior communicating artery (PComm) and the AChA; a robust PComm results in a diminished field for the AChA, whereas a diminutive PComm is associated with a greater territory for the AChA. Overlap with the areas supplied by the posterior choroidal artery and the posterior cerebral artery occurs with a similar balance, and often there are anastomoses between the AChA and these posterior circulation vessels. Interruption of the AChA supply to these various components can be predicted to potentially produce visual loss, speech deficits, and sensory and motor deficits described in the epilepsy surgery literature, and it is commonly associated with injury to the AChA itself.
The visual system’s importance to our day-to-day function is underscored by its extensive and highly organized network of white matter tracts and gray matter connections. The Meyer loop, a white matter tract, carries visual information about the contralateral superior quadrant from the lateral geniculate body to the inferior primary visual cortex. As it leaves the lateral geniculate body, it passes through the temporal stem and sweeps forward over the lateral roof of the temporal horn, eventually turning back toward the inferior occipital pole and visual cortex. Its anterior extent is on average less than a centimeter posterior to the tip of the temporal horn, with the left side often slightly more anterior than the right. The fibers have a visuotopic arrangement, with anterior fibers corresponding to the medial visual field, whereas the posterior fibers carry the lateral field. However, there exists significant variability from patient to patient; additionally, inability to distinguish these fibers from other white matter intraoperatively can likely account for high estimated injury risk to visual pathways.
Epilepsy Surgery: Indications and Complications
Surgical intervention for epilepsy is often age independent. Patients with persistent, frequent seizures that adversely impact quality of life, including those with impaired cognition and psychosocial development despite being on a dose-adjusted medical regimen, are ideal candidates. However, the choice of surgical selection is often tailored based upon a comprehensive epilepsy evaluation including tracing of the epileptogenic focus, determination of the extent of resection needed, seizure semiology, and frequency and severity as well as patient tolerance to surgery. Components of presurgical evaluation usually include clinical examination, neuropsychological testing, scalp and video electroencephalography (EEG), high-resolution magnetic resonance imaging (MRI), and other magnetic resonance techniques including assessment of functional and structural integrity, and positron emission tomography/single-photon emission computed tomography (PET/SPECT) scans as needed. Routinely performed surgical procedures ( Table 34.1 ) for seizure remission are as follows:
| Epilepsy Surgery | Indications | Common Complications |
|---|---|---|
| Hippocampal sclerosis |
|
| Lesional focus | ||
| Nonlesional | ||
| Lesional |
|
| Nonlesional | ||
|
| |
|
| |
| Refractory epilepsy |
|
Temporal Lobe Surgery
Temporal lobectomy, the most common “resective” surgery or lesionectomy, is a highly successful seizure control procedure where the epileptogenic focus is known to be localized from a distinct area of the brain (temporal lobe). The rationale is simply to remove a portion of the lesion (defect), such as a tumor or malformed vessel, known to induce seizure. It involves removal of a portion of the lobe, usually the size of a golf ball. In most cases, complete remission is achieved while the risk of permanent brain damage is mitigated.
Permanent deficits include contralateral superior quadrantanopsia (most common), hemianopsia, hemiparesis (more common in extratemporal resections), stroke/cerebrovascular accident, and aphasia. Although on the face of things these varied complications may seem to be unrelated, there is a common thread connecting them all: the AChA. The reported incidence of quadrantanopsia varies substantially, but reasonable estimates suggest rates between 18% and 26% for anterior temporal lobectomy (ATL). Quadrantanopsia is frequently unnoticed by the patient unless it is severe, and it is markedly underreported. To distinguish partial or full quadrantanopsia from hemianopsia, injury localization is valuable. AChA injury causes hemianopsia by ischemic damage to the optic tract, whereas superior quadrantanopsia is produced as a resultant direct injury to the Meyer loop.
Selective Amygdalohippocampectomy
As an alternative to ATL, selective amygdalohippocampectomy has emerged as a viable option for surgical seizure control. In contrast to the traditional temporal lobectomy that involves en bloc resection of approximately 3 to 6 cm of the temporal neocortex to ensure permissible access to the mesial structures, SAH is a more targeted approach for mesial temporal resections that spares the temporal lobe neocortex. Commonly employed approaches for SAH include transsylvian, transsucal, transgyral, and subtemporal corridors ( Fig. 34.1A ). A small craniotomy is adequate for a 1- to 2-cm permissible corridor to provide access to mesial structures ( Fig. 34.1B ). However, consideration of patient selection is critical for SAH. Patients with well-defined mesial temporal onset seizures, including bitemporal onset seizures, are at increased risk for memory impairment and thus should be excluded. Likewise, patients with a dominant seizure focus in the temporal lobe are at risk for postsurgical functional decline, including speech abnormalities. Patients with extratemporal focal epilepsy or with seizure focus in the temporal neocortex, along with those with primary idiopathic generalized epilepsies and those with psychogenic nonepileptic seizures (PNES), are not suitable candidates for SAH. Potential surgical complications include, but are not limited to, hemorrhage, infarction (lacunar strokes), infections, incomplete resection, neurocognitive impairments including speech disorders and memory loss, and mood disorders.
Whether performing an SAH to remove only the mesial structures or carrying out a full ATL, the key element of the surgery is maximal safe resection of the hippocampus, uncus, and parahippocampus. Relevant knowledge of the AChA course can minimize morbidity. The course of the AChA carries it directly into the field of resection during temporal lobe epilepsy resection. After passing through the crural cistern, the AChA pierces the choroidal fissure superior to the uncus, usually around the posterior half of the uncus, and enters the temporal horn of the lateral ventricle to supply the choroid plexus. The choroid plexus of the temporal horn lies immediately superior and medial to the hippocampus and its fimbria.
A 40-year-old woman presented with a long history of drug-resistant complex partial seizures. An EEG localized the epileptic focus to the left temporal region. MRI shows mesial temporal sclerosis in the ipsilateral hippocampus. Neuropsychological testing reveals a full-scale intelligence quotient (FSIQ) of 95 with specific verbal memory deficits and minimal visual perceptual deficits. The patient underwent left-side selective microsurgical amygdalohippocampectomy without intraoperative complication. While in the recovery room, the patient was noted to have right hemiparesis and a left homonymous hemianopsia. MRI shows a diffusion-weighted imaging (DWI) 2-cm irregular defect in the left anterior internal capsule and the region of the left optic tract ( Fig. 34.2 ). After transfer to inpatient rehabilitation, a slow improvement in hemiparesis over the course of 6 weeks and a reduction of her visual field deficit to a right superior quadrantanopsia were noted. Five years after surgery, she remains Engel class I on a single anticonvulsant. Mild right-hand weakness and a right superior quadrantanopsia persist.
