Functional Neurosurgery involves procedures that are done to modulate the activity and function within a specific part of the brain. In general, the surgeries are performed to prevent abnormal activity and behavior without damaging the normal activity of the brain. This is most commonly done to prevent seizures in patients who cannot be managed well with medications alone or to help patients with various movement disorders such as Parkinson's Disease or Essential Tremor. Seizures originate from abnormal neurons on the surface of the brain within the cerebral cortex where as most movement disorders typically originate within the deep clusters of neurons within the brain known as the deep nuclei.
A seizure focus is the cluster of neurons within the cerebral cortex that is the starting point for a seizure. Neurons normally communicate with each through a series of well-organized electrical impulses. A seizure occurs when a cluster of neurons initiate a disorganized series of electrical impulses that then spread across the surface of the brain. The seizure focus can be at a site where there is an underlying structural abnormality such as a tumor or congenital lesion (an abnormality that occurred during development of the brain). Sometimes the underlying cause is not known and sometimes there may be more than one seizure focus.
The vagus nerve is the tenth cranial nerve. It exits from the brain stem and transmits information to various parts of the body to control or modulate important automatic functions. It receives input from widespread areas of the brain and it is thought to send information back to the brain in a similar widespread pattern. Stimulation of the vagus nerve in the neck has been proven to decrease seizure activity in some patients with epilepsy.
The basal ganglia includes several clusters of neurons deep within the brain with multiple interconnections and shared functions. The specific parts of the basal ganglia include the caudate nucleus, putamen, globus pallidus, and subthalamic nucleus. Clinically speaking, these clusters of neurons are most relevant in regard to their involvement in complex motor pathways. The globus pallidus and subthalamic nucleus are specifically targeted for treatment of several motor disorders including Parkinson's disease and Essential Tremor.
The thalamus is considered the major crossroads within the brain. It directs and integrates information to and from the cerebral cortex with the brain stem and other structures. Essentially all information passes through the thalamus on its way to or from the cerebral cortex. A large portion of the thalamus is dedicated to motor pathways. Information from the deep nuclei pass through the thalamus on its way to the cerebral cortex. The thalamus is therefore a target for treatment of several motor disorders including Parkinson's disease and Essential Tremor.
Seizures and Epilepsy
A seizure is an abnormal activation of neurons within the brain. This may result in alterations of sensation, motor function, behavior, or consciousness. The abnormal activity may be limited to a cluster of neurons and result in a partial seizure or it may spread to involve the surface of the entire brain resulting in a generalized seizure.
A partial seizure can be simple or complex. A simple partial seizure involves the sudden activation of a small cluster of neurons in one side of the brain and results in symptoms related to the function of those neurons. There is no alteration in consciousness with a simple partial seizure. A simple partial seizure most commonly presents with altered sensation or rhythmic jerking of one part of the body.
A complex partial seizure involves the sudden activation of a cluster of neurons in one side of the brain and results in symptoms with an altered level of consciousness. The patient may have rhythmic movements on one side of the body, deviation of the eyes to one side, or staring. They typically will be able to respond to questions during a complex partial seizure.
A generalized seizure involves the sudden activation of neurons throughout the surface on both sides of the brain resulting in loss of consciousness. Generally, there will be rhythmic movements of the muscles including increased tension and rhythmic jerking on both sides of the body.
Epilepsy is a syndrome of recurrent, unprovoked seizures. Another way of saying this is that someone has epilepsy if they have at least two seizures without a known systemic abnormality. Systemic causes of seizures include diabetes, alcohol withdrawal, illicit drug use, and severe metabolic disturbances such as hyponatremia (low sodium levels). Epilepsy is not a single disease - there are many underlying problems that can result in epilepsy. Epilepsy most often presents in childhood or young adulthood. Common causes include an underlying developmental abnormality in the brain, head trauma, or brain tumors. Sometimes, an underlying reason for the seizures is not identified.
Parkinson's disease is a neurodegenerative movement disorder that results in progressive problems with motor capabilities and mental processing. The common symptoms of Parkinson's disease include a tremor while at rest that may improve with activity, rigid muscular tone (cogwheel rigidity), and slowness of movement (bradykinesia). Patients with Parkinson's disease often will develop a stooped posture and shuffling gait. As the disease progresses, patients will often have confusion and other cognitive problems that may be due to medications or the underlying disease process itself. Parkinson's disease is due to the buildup of abnormal proteins (plaques and tangles) within and around the neurons of the brain that interfere with normal function. These abnormalities primarily occur in the basal ganglia which is responsible for coordinating complex motor behavior. Treatment typically involves multiple medications including sinemet which replaces one of the vital neurotransmitters (dopamine) that is depleted by the loss of neurons in the basal ganglia motor pathways. Patients who fail to improve or progress on medications may benefit from a small surgical procedure called deep brain stimulation. This procedure involves placing a small wire within the basal ganglia to stimulate the neurons in the motor pathway and restore normal function.
Essential tremor is a movement disorder characterized by progressive tremor that typically involves one or both arms. The tremor is worsened with activity and improves while at rest. Patients may note that the tremor goes away with sleep and may improved with alcohol consumption. Up to half of all patients may have a family history consistent with essential tremor. The tremor usually presents in young adulthood but can occur at any age. Symptoms usually begin in one arm and may progress to involve both arms over time. Patients with a significant tremor often improve with medications alone. Medications may include propranolol, primidone, or topamax. Those patients with significant tremors that do not improve with medications alone may benefit from a small surgical procedure called deep brain stimulation. This procedure involves placing a small wire within the thalamus or basal ganglia to stimulate the neurons in the motor pathway and restore normal function.
Trigeminal Neuralgia is a facial pain syndrome. Classic trigeminal neuralgia pain is characterized by sudden, sharp, severe bouts of pain on one side of the face only. The pain is due to irritation and abnormal stimulation of the trigeminal nerve which carries most of the sensory information from the face to the brain. The trigeminal nerve has three division that are more or less the upper, middle, or lower portions of the face. The pain is most typical in the middle or lower portion but can include any or all divisions of the nerve. The pain is most commonly sharp and shooting and lasts for a few seconds at a time. The pain can be induced by certain factors such as touching the skin, chewing, or wind blowing on the face. Atypical trigeminal neuralgia is defined as pain due to irritation of the trigeminal nerve with a neurological deficit (decreased sensation), constant pain, or dull/aching pain. Trigeminal neuralgia is caused by pressure on and/or dysfunction of the nerve where it exits from the brain stem. This is most commonly due to a nearby blood vessel (microvascular compression). Symptomatic trigeminal neuralgia is defined as the rare instance in which it is due to another underlying abnormality such as multiple sclerosis or a tumor affecting the nerve. There is a broad spectrum of similar diseases that can cause similar symptoms and trigeminal neuralgia is often confused with dental problems.
There are several treatment options for trigeminal neuralgia. Most patients will respond to tegretol and a good response to this medication is almost diagnostic. There are several other medications that may be of benefit as well. Patients with persistent symptoms or side effects with medication alone may benefit from one of several procedures. Microvascular decompression is considered the best treatment for those patients who are good surgical candidates. Other options include minimally invasive needle procedures to destroy the nerve such as radiofrequency ablation or glycerol injection. Radiosurgery may be performed to target a high dose of radiation on the nerve where it exits from the brain stem.
Hemifacial spasm involves twitching of one side of the face due to irritation of the facial nerve where it exits from the brain stem. The twitching usually begins with the eye muscles and then may progress to involve the cheek and mouth. There is not any pain associated with the twitching but there may be hearing loss. Hemifacial spasm is often differentiated from a tic due to the fact that the twitching will persist even while the patient is asleep. The most common cause of hemifacial spasm is pressure on the nerve due to a nearby blood vessel (microvascular compression). Rarely, it may be due to another underlying abnormality such as multiple sclerosis or a tumor.
There are several treatment options for hemifacial spasm. First line treatment is always medication although it is often ineffective. Tegretol and dilantin are the two most common medications prescribed for hemifacial spasm. Botulinum toxin (Botox) may be used to temporarily paralyze the facial muscles involved but will cause weakness and cosmetic changes. The procedure of choice is microvascular decompression which is a surgical procedure to remove pressure on the facial nerve. Complete or partial improvement with this procedure occurs in up to 90% of patients.
Surgery for epilepsy is reserved only for patients with uncontrolled seizures despite best medical management. Not everyone with seizures is a candidate for surgery. However, for those patients that are deemed surgical candidates there is reason to be hopeful. Patients who have failed full doses of at least two antiseizure medications have approximately a 5% chance of becoming seizure free with medication alone whereas surgery may offer a 50-80% chance of seizure freedom.
A series of evaluations are performed before surgery as part of the decision to determine whether someone is a candidate. Preoperative imaging studies are performed to determine whether there are any obvious structural abnormalities that can account for seizures (tumors, developmental changes, trauma to name a few). Functional studies are performed to evaluate for more subtle changes and localization (SPECT, PET, functional MRI). 24 hour or prolonged surface electroencephalography (EEG) is performed to try and localize the origin of seizures within the brain. Neuropsychiatric testing is often performed to determine subtle abnormalities that may lateralize to one side of the brain or the other. The final stage for evaluating is invasive EEG monitoring with placement of electrodes directly on the surface of the brain. At this point, the patient has been determined to be a good candidate for surgery and is admitted to the hospital for a series of surgeries to complete the localization and ultimate resection of the portion of brain where the seizures are thought to originate from.
Subdural Strip Electrodes
Epilepsy surgery typically has 2 or 3 distinct operative procedures incorporated into a single hospital admission. The first procedure involves invasive neuromonitoring with brain surface electrodes in order to better identify the location of seizure onset. If preoperative testing does not strongly suggest which side of the brain that seizures are originating from then multiple strip electrodes are often placed on the surface of both sides of the brain. Alternatively, if testing strongly suggests a single location then strip electrodes can be placed on top of that particular part of the brain to confirm localization prior to surgical resection. For strip electrodes a small incision is made in the scalp and flat strips with four to eight electrodes are passed on top of the surface of the brain. Up to four strips can be placed from a single incision. Xrays and computer modeling are then used to determine the exact location of the leads relative to the brain and surface landmarks. The patient is then admitted to the intensive care unit and the electrode wires are connected to an EEG for recording brain activity with close video monitoring.
The incision is marked for placement of right temporal subdural strip electrodes; the incision location (solid line) is planned such that it can be incorporated into a larger incision (dotted line) for subsequent resection of the seizure focus. The incision is completely behind the hairline and a hair-sparing procedure may be performed. A color-coded computer generated model (BrainLab) is shown demonstrating the location of the leads after placement.
Intraoperative xrays were taken to confirm location of the strip electrodes along the predetermined path on the surface of the brain. One electrode was passed upwards to the parietal lobe; one was passed anterior along the lateral surface of the temporal lobe; two electrodes were placed along the inferior surface of the temporal lobe.
Computer generated model (BrainLab) that demonstrates the placement of the electrodes relative to the skull base and brain surface. Good coverage along the surface of the temporal lobe was obtained; seizures were determined to be originating from the frontmost electrodes in the inferior placed leads (orange and yellow).
Craniotomy for Subdural Grid Placement
Once preoperative testing has confirmed which side of the brain is the source of the seizures, invasive neuromonitoring can be limited to that one side. If the patient had bilateral subdural strip electrode monitoring, then the incisions for strip placement on the side of the seizures are incorporated into the larger incision. The electrodes on the other side of the brain are removed and the incisions are closed. Typically, the craniotomy will be performed such that the invasive monitoring can be done through the same exposure as the subsequent resection (in a separate surgery). The brain surface is exposed and EEG electrodes are placed on top of the brain. A large grid, usually with six by six rows of electrodes, is placed on the exposed surface. Strip electrodes with a single row of four to eight electrodes are then passed along the surface of the brain to parts that are not directly visualized. The electrodes are positioned such that they cover all the areas of the brain that may be the source for the seizures. The patient is then admitted to the intensive care unit and the electrode wires are connected to an EEG for recording brain activity with close video monitoring.
Craniotomy for Resection of Seizure Focus
The final operation involves removing the portion of the brain that is the source of the seizures. If the patient had bilateral subdural strip electrode monitoring, then the incisions for strip placement on the side of the seizures are incorporated into the larger incision. The electrodes on the other side of the brain are removed and the incisions are closed. If the patient had a craniotomy for subdural grid electrode placement, then the same incision is reopened for the resection. Based on the preoperative testing and invasive neuromonitoring, the portion of the brain that initiates seizures is completely resected. If the seizures originate from areas of the brain with vital functions (language or movement) then cortical mapping may be performed with or without anawake craniotomy. An awake craniotomy entails waking the patient up from anesthesia for a brief period of time during the surgery. The patient is kept comfortable throughout the duration of this procedure and asked to perform specific tasks to confirm that the surgery is performed safely. Acraniotomy is generally a well-tolerated procedure with important risks to safeguard against.
Multiple Subpial Transection
Multiple Subpial Transection is a procedure in which the seizure focus is disconnected from normal brain structures. This procedure is reserved for patients who have a seizure focus in eloquent cortex with important neurological function. Such areas include language, motor, and sensory areas of the brain. Seizure foci in these areas of the brain cannot be resected due to the risk of significant neurological injury. Instead of resecting the area of the brain that is causing seizures, it is simply disconnected. The brain is structured such that normal signals are sent back and forth from superficial cortex to the deeper structures whereas seizures are spread across the surface of the brain. Multiple Subpial Transection involves making several superficial cuts through the cortex that do not disrupt the normal signals as they pass to the deeper structures but do disrupt the spread of seizures across the superficial cortex.
Patients will typically have noninvasive and invasive neuromonitoring beforehand. If a patient is thought to have localizable epilepsy then they will proceed to invasive monitoring either with burr holes for subdural strip electrodes and/or a craniotomy for subdural grid electrodes. If the seizure focus is identified in an important area of the brain then multiple subpial transection may be pursued instead of a resection. Sometimes this procedure will be performed in combination with a resection if only part of the seizure focus is in eloquent cortex. Parallel incisions 5mm deep are made in the cortex and spaced 5mm apart spanning the seizure focus. This will almost always result in transient neurological symptoms related to the underlying function of the cortex. These symptoms typically resolve within a week and rarely result in permanent injury. On average, this procedure will result in seizure freedom approximately 50% of the time and a significant reduction in seizure frequency about 25% of the time. Permanent neurological symptoms remain less than 5% of the time.
Vagal Nerve Stimulator
A vagal nerve stimulator may be implanted by a small surgical procedure in order to decrease the frequency and severity of seizures. The vagus nerve is the tenth cranial nerve and has diffuse connections with many areas of the brain. The vagal nerve stimulator provides a small electrical current to the nerve and suppresses the onset and spread of seizures through the brain. It is thought that the intermittent impulse transmitted through the vagus nerve induces desynchronization of electrical activity in the brain and thus hinders seizure formation. This procedure is done exclusively on the left vagus nerve because the right vagus nerve typically has more input to the heart and thus more potential for side effects.
Patients are usually admitted overnight for placement of the vagal nerve stimulator. One incision is made in the neck to dissect around the left vagus nerve and wrap the small electrode wires around it. The wire is then tunneled under the skin to a second incision made over the chest. The wire is connected to a programmable battery that generates the electrical signal. The battery is placed within a pocket under the skin and will need to be replaced every 5 years on average. The battery is left off for two weeks after implantation to allow for healing. After two weeks, the patient returns to the office to have the vagal nerve stimulator turned on. Typically, stimulation is set to occur for 10 seconds every 5 minutes. Specific settings can be altered to minimize any side effects. The most common side effect is a hoarse voice that occurs during stimulation only.
The goal of the procedure is to reduce seizure frequency. Overall, the vagal nerve stimulator results in an average 45% reduction in seizure frequency. Up to 20% of patients will experience a greater than 75% reduction in seizure frequency. A vagal nerve stimulator does not result in complete freedom from seizures or replace the need for antiseizure medications. However, for patients with severe refractory epilepsy, a significant reduction in seizure frequency can result in a significant improvement in quality of life.
Deep Brain Stimulation
Deep Brain Stimulation is an elective surgical procedure for patients with certain movement disorders that have failed to improve with medications alone. This is most commonly performed for tremors due to Parkinson's disease or Essential Tremor. Using a computerized guidance system, a small wire is placed in the deep structures of the brain (basal ganglia and thalamus) that influence motor activity. The wire is connected to a programmable battery that supplies an electrical impulse. This impulse is transmitted through the wire to the deep structures of the brain and inhibits the tremor.
Deep Brain Stimulation is performed with gentle sedation so the patient can be tested during placement of the wire. After sedation and local anesthetic, a small incision is made in the scalp and a quarter size hole is made in the bone. Using computer guidance and physiological monitoring, a small wire is passed through the brain to the target structure. The wire is then connected to an electrical pulse generator and the patient is monitored for improvement in the tremor. Once a beneficial effect is found, the wire is disconnected and tunneled under the skin for permanent connection to an implanted programmable battery.
Trigeminal neuralgia and Hemifacial spasm are thought to occur due to an artery or vein pushing on the cranial nerve where it exits from the brain stem. This pressure irritates the sensitive nerve and causes facial pain (if pressure is on the trigeminal nerve) or spasm of the facial muscles (if pressure is on the facial nerve). The best way to treat these conditions is to take the pressure off of the nerve. Microvascular decompression involves performing a small craniotomy in order to expose the nerve and relieve the pressure. An incision is made behind the ear and a small craniotomy is performed. Gentle dissection is performed in order to expose the nerve. Any artery or vein that is pressing on the nerve is dissected free and moved away. Sometimes, a soft Teflon spacer is placed between the nerve and the blood vessel. The nerves are sometimes monitored during the procedure to minimize the risk of injury and neurological deficits following the procedure.
Microvascular decompression typically takes 2-3 hours to perform and the patient can expect to be in the hospital for 2-3 days after surgery. Over 90% of patients have complete relief immediately after the procedure. Recurrent pain can occur but long-term benefit is maintained in over 70% of patients. This is the most successful treatment for trigeminal neuralgia and hemifacial spasm but also the most invasive. Risks are low but include facial weakness or numbness, hearing loss, infection, and stroke.
Radiofrequency Ablation of the Trigeminal Nerve
Radiofrequency ablation is a minimally invasive procedure in which a portion of the trigeminal nerve is burned. This is often done with gentle anesthesia and local anesthetic to monitor for effectiveness during the procedure. A small incision is made in the cheek and a needle is guided with xrays to the trigeminal nerve where it exits from the skull base. Once the tip of the needle as next to the nerve, the needle is connected to a wire and a gentle radiofrequency impulse is generated at the tip of the needle to make a lesion in the nerve. This will invariably cause numbness in the face but also stops the pain.
Radiofrequency ablation of the trigeminal nerve is done as an outpatient procedure. There is an initial 90-95% success rate with this procedure. However, recurrences are common. Up to 80% of patients will have recurrent symptoms in 10 years. Because of the relatively high recurrence rate, this procedure is often reserved for patients who are not good surgical candidates or for those who refuse or have failed microvascular decompression.
VIRGINIA BRAIN & SPINE CENTER