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Precautions and adverse effects

As with all surgical procedures in epilepsy, patients must be fully informed of the potential
risks and the long-term consequences of VNS insertion. Despite little vagal viscero-effector
activity during therapeutic VNS in humans, caution is advised in patients with heart disease
and severe asthma. One study concluded that ‘long-term vagus stimulation in patients without
concomitant lung disease does not induce any significant changes in FEV1. However, in
patients with obstructive lung disease, intense vagus stimulation can cause a deterioration of
lung function’10. It is recommended that there is minimal handling of the vagus intra-
operatively. Transient bradycardia or sinus arrest may occur during the lead test in 0.1% of
cases but is not necessarily a contraindication to switching on the device after an interval.
Infection of the lead or generator site, likely the most serious adverse effect, may occur in up
to 3% requiring removal of the device in about 1%. Lead breakage may occur. Horner’s
syndrome, unilateral facial weakness and vocal cord paresis have been reported. Stimulation
and, to a lesser extent, implantation may be associated with hoarseness, cough, dyspnoea,
pharyngitis, paraesthesia and pain. Pre-existing dysphagia may be exacerbated, as can
obstructive sleep apnoea, although these features do not seem to emerge de novo following
VNS.

If a patient requires removal of the device for infection or if removal is requested due to lack
of efficacy, it is usual to remove the generator box only and to leave the lead in place. The
lead can be removed but this entails more difficult surgery and carries some risk of a hoarse
voice owing to injury to the vagus nerve.

Common side effects of ataxia, dizziness, fatigue, nausea and somnolence as may be seen
with AEDs were absent from the list of statistically significant side effects of VNS in a
Cochrane review11.

Practical considerations

Strong electric or magnetic fields may damage the generator and should be avoided. A
detailed account regarding risks associated with defibrillation, lithotripsy, therapeutic
ultrasound and therapeutic and surgical diathermy can be found on the website
www.cyberonics.com. The system is not affected by home microwave ovens or mobile
phones. Airport security systems and shop theft detectors may be activated by VNS although
there are no reports of the VNS itself being affected.

There is some concern regarding limitations of new generation MRI in those with VNS
implantation, including those where the battery pack has been removed but the wire remains.
The potential risks of performing MRI on patients with an implanted VNS include heating
effects, especially of the stimulation electrodes, inadvertent resetting of the device or magnet
mode activation, image distortion and artefacts, magnetic field interactions and device
malfunction or damage. If performing an MRI scan, VNS output should be set to zero
beforehand and reset afterwards, meaning that an appropriately trained person must be
available. VNS is approved in MRI scanning using only transmit-and-receive type head coils
at both 1.5 and 3 Tesla field strength. Some modern head coils are of the phased-array type
which should not be used.

In practice, good diagnostic quality brain scanning can be achieved if appropriate precautions
are in place, however body or extremity imaging (receive-only coils) and experimental brain
protocols may not be risk-free, even if the generator has been explanted and only the wire
remains. Work is being carried out to develop full MRI compatibility but restrictions remain