The Cyberonics® NeuroCybernetic Prosthesis (NCP®) System is comprised of an implantable Pulse Generator and Lead, and an external programming system used to change stimulation settings.
The Model 100 NCP Pulse Generator is an implantable, multiprogrammable bipolar
Pulse Generator that delivers electrical signals to
the vagus nerve for the purpose of reducing the frequency of partial onset
seizures. The Pulse Generator is housed in a hermetically
sealed titanium case and is powered by a single lithium thionyl chloride battery.
Electrical signals are transmitted from the NCP Pulse Generator to the vagus
nerve.
Go here for a review of EPILEPSY.
INTRACTABLE
The
EPILEPSY BILL OF RIGHTS states a person should expect to be seizure
free for life and have no side effects from treatment:, thus putting
quality of life first, seizure control second. They are also encouraged
to be educated about the medical, legal, economic, and, political
aspects of their illness. Within this context, anyone who has not had
ALL their seizures 100% controlled (auras, complex partial, absences,
myoclonic, convulsive) within 2 years of treatment with several
antiepileptic drugs should be considered medically intractable. At this
point their prognosis is poor for gaining complete control using
additional drugs and significant neural damage has already resulted.
Further delay will risk neurologic dysfunction (memory, further
seizures) and social, emotional and financial disability.
FEDERAL DRUG ADMINISTRATION
FDA approval for the Vagal Nerve Stimulator (VNS) came July 1997 as
adjunctive treatment of complex partial seizures for patients over 12
years old. As with any approved treatment, "off label" use is permitted
and epileptologists have already implanted children younger than 12
(three y.o.), and treated patient with other than complex partial
seizures(absences, Lennox-Gastuat).
TREATMENT
There is a long list of
AEDs:
carbamazepine, clonazepam, clorazepate,
ethosuximide, felbamate, gabapentin, lamotrigine, methsuximide,
phenobarbital, phenytoin, primidone, tiagabine, topiramate, and valproic
acid. Newly introduced antiepileptic drugs do not always
appreciably altered the success rate for controlling seizures.
Non-medicinal treatments of epilepsy have always co-existed with medicinal treatments. Among these includes epilepsy surgery, which is not synonymous with epilepsy BRAIN surgery. For example, based on what was then current and accepted beliefs about causation, surgical treatments have included cauterization, amputation, cervical sympathectomy and trephination. These were still common American and European practices into the 20th century and were reviewed at the 1902 meeting of the National Association for the Study of Epilepsy. Despite our present views about the value of these practices, none of these techniques were entirely without their successes.
Brain surgery, which is 100 years old, is an alternative treatment for patients with intractable epilepsy.
The most common operation is resection of the anterior portion of either the right or left temporal lobe. Resections of portions of the frontal, parietal or occipital lobes, and section of the anterior two-thirds of the corpus callosum constitute the majority of operations for the rest of the patients. Resective surgery, especially of the temporal lobe, or removal of a tumor, can lead to complete control of seizures (on or off AEDs).
HISTORY
In the third quarter of this century Dr. Cooper championed the use of
cerebellar stimulation for various neurological conditions, including
epilepsy. This was viewed as a way to use the brain's natural inhibitory
mechanisms to raise the seizure threshold and reduce seizures.
Cooper IS, Amin I, Gilman S,
The effect of chronic cerebellar stimulation upon epilepsy in man.
Transactions of the American Neurological Association. 98:192-6, 1973.
Interest peaked during the 1970's and 80's with a hundred reports
appearing in the medical literature. The lack of efficacy, and
inconvenience of intracranial brain surgery limited its applicability
and use.
Recently, the theoretical basis for cerebellar stimulation has been revived as cervical stimulation of the vagus nerve. The predominant neurotransmitter for both is GABA. A key difference is "amplification". Stimulation through electrode contacts in the cerebellum could only recruit a limited number of cerebellar neurons which then projected their output to limited cortical regions. However, stimulation of the whole vagus nerve is capable of producing retrograde firing of the entire vagal nucleus. The inhibitory projection of the nucleus then influences the entire cerebral cortex. This amplification increases the potential effectiveness by several orders of magnitude.
ANATOMY
There is a difference in function between left and right vagus nerve.
The right subserves cardiac rhythm, blood pressure and GI motility to a
much greater extent than the left. Thus only the left vagus is used for
stimulation. However, the recurrent laryngeal nerve does come off the
left vagus so that hoarseness can be a permanent sequelae from surgery.
The most frequent and significant complications of VNS implantation are
1% non-permanent hoarseness and 1% infection of the operative site.
SURGERY
The operation is done on an outpatient basis and takes one-two hours.
The approach to placing the wires around the vagus nerve is similar to a
carotid endarterectomy. The pacemaker looks just like a heart pacemaker,
and is placed below the left clavicle and above the breast. A bulge will
show under the skin but is hidden by clothing.
PACEMAKER
So as not be confused by side effects of the surgery versus side effects
of the actual electrical stimulations, we delay activating the device
until 2 weeks post-op. Seizure medications are continued after the
implantation and should not be changed for the first month so as to
better evaluate the initial effects of stimulation. Afterwards, both
drugs and stimulation variables (current, time on/off) should be
adjusted to maximal effect.
The usual stimulation protocol is for the device to be on for 30 seconds, then off for 5 minutes. This cycle continues 24 hours/day. The lowest current (0.25 mA) may cause some discomfort in a few patients and the time_on can then be reduced to 14 or even 7 seconds. But this is rare. Most patients start at 0.50 - 0.75 mA and increase to 1-3 mA on their return visits. The duty cycle (time_on divided by time_off) should not exceed 40% to prevent injury to the nerve. Longer duty cycles also deplete the battery faster. The usual battery life is 7 years. Replacement is an outpatient procedure taking 45 minutes.
SIDE EFFECTS
Despite the intended inhibitory influence of the treatment, no cognitive
side effects have been noticed. Most patients do feel a tingling in
their throats with either hoarseness or coughing during the actual
stimulation. This is handled by reducing the current. Patients usually
develop tolerance to this effect and request the electrical current be
increased after a month.
PRECAUTIONS
Microwave diathermy (not related to microwave oven use), radio / TV transmitter. Best to have device turned off
prior to elective surgery, use of bovie might affect it. No body MRI,
but can do head with "coil". Aspiration with swallowing if current too
high. Can go through metal detectors at airport. Can swim. Can
rough-house. Steel casing not easily damaged, wires not easily crushed.
A million heart pacemakers in use.
RESULTS
On average, most patients can expect a 50% reduction in seizures. This
is as good as reported in experimental new drug trials. Of course some
patients will not see this benefit persist (again tolerance as with any
seizure medication). However, some patients get better and better
seizure control with time. Some have become seizure free over 1-2 years.
Nevertheless, VNS is not proposed as a curative procedure and patients
should be plainly told they will most likely need to continue seizure
medicines.
Probably the most unique, and important, aspect of VNS is the patient's
ability to turn the stimulator on whenever he or she wants. The patient
is supplied with a magnet to carry around. When the magnet is placed
over the device, the VNS is shutdown, and stays off until the magnet is
removed. This is a fail safe measure in the event the stimulator
malfunctions and stays on continuously. In practice, the magnet is used
as soon as the patient feels a seizure coming on. The VNS can then be
activated on demand. By timing the stimulus to the onset of the seizure,
many patients can terminate or modify the seizure and/or shorten the
postictal period. This allows the patient to regain some control over
their lives!
I CAN NOT EMPHASIZE ENOUGH HOW IMPORTANT THIS IS FOR THE PATIENT.
REIMBURSEMENT
Third Party, Medicare, County Indigent. Cyberonics has set
up 800 hotline (1-888-508-8082). Presently $15,000 to implant, less to
replace battery.
FUTURE
What is the future for VNS. Brain surgery has the ability to cure some patients and help
most patients. However, the evaluation process is expensive and
difficult, partly to insure that surgery is performed ONLY on patient
with real and uncontrolled seizures. After all, there is a 1% risk of
stroke and death from brain surgery. This has limited the use of
epilepsy brain surgery to academic centers. It will never be available
in community hospitals, and thus will never be available to the 100,00
patients who might benefit.
Now consider seizure medicines. There is no restriction on their use. Pediatricians, Internists, Family Practice, OB-GYN, even surgeons prescribe seizure drugs. No one would image that it should be limited to Neurologists or Epileptologists! This insures that every patient with seizures will have access to treatment. Of course, the inevitable missed diagnosis, under dosage or wrong type of medicine will occur. This is the price we pay to insure maximum availability. And this occurs in all areas of medicine and surgery.
Where does VNS fall within this spectrum. It should be used like seizure medicines, with access to every patient through community hospitals and private practices. The risk of death and injury is vastly smaller than for epilepsy brain surgery. The pre- surgical evaluation is also vastly more simple. Any patient thought to have epilepsy and who has failed to completely respond to medication is a candidate. A CT or MRI and an EEG should be done. Keep in mind that 60% of patients with epilepsy have normal EEGs, so a normal EEG in a patient with a good history of seizures is still a candidate.
Neurosurgeons are typically called on to perform this operation, however, any surgeon with experience with this anatomy can do it. Vascular, ENT, Plastics. This approach has already been applied in West Texas.
Maximal benefit to the public regarding seizure control demands widely
available treatment. Imagine if Dilantin could only be prescribed if the
patient had a seizure captured with long-term video/EEG monitoring. The
clinical trials show that VNS is as effective as any of the
antiepileptic drugs in patients who do not respond to initial treatment.
The surgery is "outpatient surgery". And the hospital and physician are
compensated for this $15,000 procedure. Increasing its availability will
help identify other indications for the procedure and clarify who will
benefit best. This has always been the case after a new drug is released
and the first 100,000 patients have tried it.
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