Types of Epilepsy Surgery: Lobectomy, Disconnection, VNS

There are three types of epilepsy surgery: resection, which removes the tissue generating the seizures, as in temporal lobectomy with amygdalohippocampectomy for mesial temporal sclerosis; disconnection, which stops seizures spreading, as in corpus callosotomy and hemispherectomy; and neuromodulation, which reduces seizure activity through vagal nerve stimulation or deep brain stimulation. Each suits a different patient.

Dr. Dinesh Rattnani, Senior Director - Neurosurgery, Neuro Critical Care & Spine Surgery, Max Super Speciality Hospital, Noida, India, operates on patients whose epilepsy has resisted at least two drugs at full dose. His view of the operating theatre is anatomical and exacting: the operation is expected to be fully successful, and most complications are avoided by respecting a small number of surgical planes. This article describes each procedure, how it is performed, and the risks it carries. How patients are selected for surgery is covered in drug-resistant epilepsy: workup, EEG and MRI concordance and when to refer, and the overall pathway in the complete guide to epilepsy diagnosis and management.

Resective surgery: the first type of epilepsy surgery

Resective surgery removes abnormal tissue that is generating seizures. Dr. Rattnani considers it the simpler form of epilepsy surgery when a specific abnormality is present. The targets include:

• a sclerosed mesial temporal lobe (mesial temporal sclerosis)
• tumours such as gliomas and meningiomas
• a depressed bone fragment or scar tissue after head injury
• neurocysticercosis cysts
• a temporal, frontal or parietal lobe damaged by hypoxic injury, trauma or malignancy

Lesionectomy removes the lesion itself; lobectomy removes part of the affected lobe. In these cases, removal can give a seizure-free outcome.

Temporal lobectomy and amygdalohippocampectomy for mesial temporal sclerosis

Mesial temporal sclerosis, also called hippocampal sclerosis, is scarring of the deep, inner part of the temporal lobe, which contains the hippocampus and amygdala. It is common, and the sclerosed mesial temporal lobe is responsible for the seizure activity. It also offers the best-case scenario for surgery: when the MRI shows the sclerosis and the EEG localises the seizures to the same temporal lobe, the patient can proceed to temporal lobectomy with hippocampectomy or amygdalohippocampectomy, removal of the hippocampus and amygdala.

The resection is planned before the operation. Dr. Rattnani predetermines how much of the temporal lobe will be removed, taking roughly 4 cm from the temporal pole, and then resects the mesial structures.

The critical step is the arachnoid plane, the thin membrane separating the mesial temporal lobe from the structures beside it. That plane must be maintained, because every vital structure lies medial to it. Beyond the medial temporal lobe and the tentorial notch lie the brainstem, the third cranial nerve (the oculomotor nerve) as it emerges from the brainstem, the posterior communicating artery and the anterior choroidal artery. The early part of the resection is easy. The last part of the dissection is the crucial one, and the surgeon has to be very selective and cautious in removing the final portion of the cortex.

The second structure to protect is the vein of Labbé, a large vein draining the temporal lobe. Dr. Rattnani regards its preservation as a very important aspect of temporal lobectomy. The surgeon should neither sacrifice the vein of Labbé nor go medial to the tentorium.

Electrocorticography: knowing when to stop

Electrocorticography (ECoG) records electrical activity directly from the surface of the brain during the operation. It matters most when the MRI and EEG are discordant and the exact source of seizures is uncertain.

A grid of electrodes is placed over the exposed cortical surface, and the team watches for bursts of epileptiform discharge. The resection follows those areas, and tracing is repeated during the operation. Tissue is removed until the epileptiform discharges disappear completely; at the end of the operation, the recording should show no epileptiform discharge. Smaller electrode strips can be passed into the depth or around the exposed area. If the seizures appear to extend beyond the exposure, the craniotomy (the opening made in the skull) may need to be enlarged so that the remaining focus can be removed.

There is one limit. If the discharging tissue is vital cortex, the surgeon may have to leave some of it. Abnormal brain that is not of vital importance can be removed.

Disconnection surgery: stopping seizures spreading

Disconnection surgery works on a different principle. Epileptic activity starts at one point and then spreads to the whole brain. If the surgeon creates barriers so that it cannot spread, seizures can be controlled even though the source remains.

• Corpus callosotomy divides the corpus callosum, the band of fibres connecting the two hemispheres. Dr. Rattnani reserves it for atonic seizures, the sudden loss of muscle tone that causes drop attacks, where it stops the spread to the other side. It is mostly not done.
• Multiple subpial transection cuts horizontal connections in the cortex to limit spread while preserving function. It is not very common today.
• Hemispherectomy disconnects large parts of one hemisphere.

Hemispherectomy: the last resort

Hemispherectomy is used for the most resistant epilepsy, including Rasmussen's encephalitis, a rare chronic inflammatory disease affecting one hemisphere of the brain. These patients are often already on four or five drugs and still having seizures, and their lives are miserable.

The name overstates what is removed. The brain is not taken out entirely, and the operation does not remove the entire frontal, parietal or temporal lobe. Instead, the frontal lobe is disconnected from the temporal and parietal lobes, certain areas are disconnected from the occipital lobe, and only a limited part of a lobe that is not of great importance is removed, so that seizures cannot spread.

The question of paralysis on the opposite side of the body is answered by patient selection. Most patients selected for hemispherectomy already have weakness on the opposite side of the body, and fine hand movement on that side is usually lost after the operation, although most patients can walk.

Dr. Rattnani is candid about the limits. Hemispherectomy carries the most complications of any epilepsy operation, although published series report seizure freedom in a majority of well-selected patients. It does not necessarily take the patient off antiepileptic drugs; rather, with the drugs, seizures that were previously uncontrolled may then be controlled. It is not a boon for the patient, but when there is no other option it is worth trying.

Neuromodulation for epilepsy: vagal nerve stimulation and deep brain stimulation

In neuromodulation, seizure activity is still generated, but devices modulate the activity of the neurons. It is the usual surgical route for patients with a normal MRI in whom no single focus can be localised for resection.

Vagal nerve stimulation

Vagal nerve stimulation (VNS) places a stimulating electrode on the vagus nerve in the neck, connected to a pulse generator. It is always placed on the left vagus nerve, because the right vagus supplies the sinoatrial node, the heart's natural pacemaker.

Nobody fully understands how VNS works. The proposed mechanism runs through the nerve's afferent fibres: stimulating the vagus sends signals back to the brainstem and on to wider brain networks, and the electrical activity there is reduced. About half of patients achieve at least a 50% reduction in seizure frequency within two years of implantation. In published trials, success is usually defined as a reduction in seizure frequency rather than seizure freedom, and complete freedom from seizures with VNS alone is uncommon.

Deep brain stimulation for epilepsy

Deep brain stimulation (DBS) places very thin electrodes into specific nuclei deep in the brain and delivers continuous electrical stimulation that modulates abnormal activity. The US Food and Drug Administration approves DBS for essential tremor, Parkinson's disease and epilepsy, and permits it for dystonia and obsessive-compulsive disorder under humanitarian device exemptions.

For epilepsy, the target is a nucleus in the thalamus known as the anterior thalamic nucleus (ANT). Stimulating the ANT reduces the spread of epileptic activity.

Dr. Rattnani's experience with DBS spans movement disorders as well as epilepsy. He described a patient with Parkinson's disease, treated by stimulating the subthalamic nucleus, whose hands were steadier after surgery, and a patient with essential tremor whose continuous tremor had made drawing impossible and who could draw a circle afterwards. The target differs with the condition, including the anterior thalamic nucleus, the subthalamic nucleus and the globus pallidus, and in his experience the results are excellent.

Risks of epilepsy surgery and how they are avoided

For resective surgery and neuromodulation, Dr. Rattnani puts the complication rate at 5 to 10%, varying with the site of surgery. The complications fall into three groups:

1. General surgical complications: infection, bleeding and meningitis.
2. Neurological complications: weakness, hemiparesis and hemiplegia (partial or complete paralysis of one side of the body) and memory loss. These are the most important, because a complication in this setting can reduce a patient's quality of life rather than improve it.
3. Failure of the surgery: seizure frequency is not reduced. Many patients come to surgery saying they are otherwise well and only the seizures remain, so failure matters a great deal to them. Not achieving the expected result can occur in about 5% of cases.

Dr. Rattnani recalled one patient, operated on before navigation was available to him, who needed a second operation to remove the most medial part of the temporal lobe; the patient improved afterwards. Residual tissue is the specific worry for the epilepsy surgeon: if some tissue is left behind, seizures may continue.

His rules for prevention are few and strict:

• Do not breach the arachnoid plane. If the plane is preserved, complications are very unlikely. They arise when bleeding is profuse and a surgeon, under pressure, goes beyond the arachnoid; once it is breached, the anatomy becomes very difficult to read.
• Use navigation, image guidance that tracks instruments against the patient's scans, in as many cases as possible.
• Be very selective in the final stages of resection.
• Use electrocorticography so the work is complete without harming the patient.
• Preserve the vein of Labbé and stay lateral to the tentorial edge.

Every decision to operate is taken with the neurologists, the neuropsychiatrist and the psychologist, and the drugs are tapered gradually afterwards, as described in the complete guide. The full exchange with Dr. Rattnani on complications, hemispherectomy and paralysis is in the live discussion. Indicative costs for related operations are listed under brain and neurosurgery procedures.