Adverse events reported with VNS therapy are generally transient and mild, and are often related to the duration and intensity of stimulation . Serious adverse events have not been reported with standard therapy, and no patients have died or had a higher mortality risk as a result of VNS therapy.4 The most common adverse events reported during the clinical trials were mild hoarseness or voice alterations, coughing, and paresthesia (primarily at the implant site and decreasing over time) and were not considered clinically significant . Other side effects reported less frequently during these studies include dyspnea, pain, headache, pharyngitis, dyspepsia, nausea, vomiting, fever, infection, depression, and accidental injury. Not all of these side effects were related to VNS therapy Outside the clinical trials, occasional reports of additional adverse events such as shortness of breath and vocal cord paresis have been reported, but did not result in discontinuation of therapy. Moreover, many of the side effects tend to diminish or disappear altogether as patients adjust to the stimulation therapy. If side effects persist or are bothersome to the patient, reductions in stimulation intensity or frequency often alleviate them, most of which occur only during active stimulation .16
Pediatric patients seem to have a higher tolerance for VNS therapy. Rare occurrences of increased salivation, increased hyperactivity, and swallowing difficulties have been reported in children Overall, the side effects reported for pediatric patients are often mild and transient
Safety features are built into the VNS therapy system to protect patients from stimulation-related nerve injury The primary safety feature is the "off" switch effect of the magnet If a patient begins to experience continuous stimulation or uncomfortable side effects as a result of VNS therapy, the magnet can be held or taped over the generator to stop stimulation until the patient can visit the physician A watchdog timer also is programmed into the device to monitor the number of pulses a patient receives If a certain number of pulses is delivered without an "off" time, the device will turn itself off to prevent excess stimulation from potentially causing nerve injury
Procedures such as diathermy and full-body magnetic resonance imaging (MRI) scans, which have the potential to heat the device leads around the vagus nerve and thereby result in either temporary or permanent tissue/nerve damage, are contraindi-cated among patients receiving VNS therapy Patients requiring an MRI should have the procedure performed with a head coil, which has been done successfully in VNS therapy patients . As recommended by the FDA, any instructions for MRI imaging that may be in the labeling for the implant should be followed exactly, and information on the types and/or strengths of MRI equipment that may have been previously tested for interaction with the implanted device should be noted. 25 Leads or portions of leads are sometimes left in the body among patients who have had the pulse generator explanted . Therefore, it is important to get information regarding previously implanted devices as the remaining leads could possibly become heated and damage the surrounding tissue
Because the mechanisms of action are not well defined, the selection of patients for VNS therapy does not follow a clear set of guidelines . In addition, the clinical trials for VNS therapy could not distinguish any correlation between patient response and seizure type or etiology, age, sex, frequency of seizures, or frequency of interic-tal spikes on EEG to generate any obvious candidate selection criteria . 27 Therefore, similar to AED therapy, there are currently no markers to predict the success of VNS therapy on a case-by-case basis . Figure 4 . 6 shows a suggested treatment-sequence flowchart that could be helpful in determining which palliative surgical procedures to choose when patients are experiencing refractory seizures
Patients of any age should be considered for VNS therapy if they experience seizures refractory to other therapies, including AEDs, the ketogenic diet, and epilepsy surgery Preliminary data suggest that patients treated with VNS therapy earlier in the course of their epilepsy (i .e . , when seizures fail to respond to treatment with two or three AEDs within 2 years of epilepsy onset) may have a higher response rate to treatment. 24
Precautions should be taken with patients predisposed to cardiac dysfunction and obstructive sleep apnea (OSA) as stimulation may increase apneic events, and chronic obstructive pulmonary disease may increase the risk of dyspnea Lowering the
Treatment Sequence for VNS Therapy
Treatment Sequence for VNS Therapy
stimulus frequency or increasing the "off" time may prevent exacerbation . It is not known whether the effects of VNS on sleep-related breathing diminish over time
Patients who have undergone a bilateral or left cervical vagotomy are not considered candidates for VNS therapy Evaluation by a cardiologist is recommended for patients with a personal or family history of cardiac dysfunction If clinically indicated, Holter monitoring and electrocardiograms also should be done before implant.
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