202409221429

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Tags: Neuro

Vagus nerve stimulation

Epilepsy surgery

Background

Vagus nerve stimulation affects a variety of monoamine- (such as serotonin, noradrenaline, dopamine and GABA) producing nuclei in the brainstem (e.g. locus coeruleus, raphé nuclei). Vagus nerve stimulation causes activation in the limbic system (e.g. amygdala, hippocampus) and modulation of functional connectivity in different brain regions (e.g. prefrontal cortex)

Typically, the device generator is implanted on the left anterior chest wall in a subcutaneous pocket below the clavicle. A lead runs from the generator and terminates in three helical coils (a positive and negative electrode and an anchoring coil), which wrap around the vagus nerve in the neck

Vagus nerve stimulation (VNS) therapy involves intermittent electrical stimulation of the left cervical vagus nerve.

Therapy stimulation parameters are individualised to each patient. Typically, patients receive vagus stimulation 10–50% of the time.

In all models, continuous application of the magnet to the generator will deactivate the device, e.g. taping the magnet over the generator.

VNS magnets can be used to deliver an additional period of vagal stimulation to help terminate a seizure if swiped across the generator.

Newer VNS models have an optional ‘autostimulation’ mode which will deliver periods of continuous vagus nerve stimulation when it senses a tachycardia (the device assumes that a tachycardia means that a seizure is commencing; 80% of patients with drug-resistant epilepsy experience tachycardia at the time of seizure).

Battery replacement for VNS devices is a quick procedure which can be undertaken as a day case, and in most instances use of a supraglottic airway is sufficient.

Pre-op preparation

For initial VNS implantation and lead revision surgeries, a group and screen blood test should be processed due to the risk of intra-operative vascular injury.

Salient info:

VNS implantation

usually under GA but RA possible: cervical plexus block + LA infiltration to anterior chest wall

ETT safer than SGA ∵ risk of airway obstruction

2 incisions: neck & chest wall

potential for occurrence of bradyarrhythmias and asystole a/w initial delivery of a vagal stimulus during system testing

Device is activated 2 weeks post-insertion with incremental increases of stimulation current to a therapeutic target range of 1.5–2.25 mA as quickly as tolerated by the patient.

Lead revision surgery: challenging ∵ fibrosis

Battery replacement: day case procedure under LA

Peri-op Mx of existing VNS device

Deactivate VNS before elective surgery (esp OT a/w ↑vagal tone)
check / interrogate & reactivate post-op

It is generally safer to formally deactivate the VNS device before elective surgery to avoid potential complications of vagal stimulation. There should be a plan for reactivation of the device in the postoperative period. If formal deactivation and reactivation is not readily available and the VNS device is to be left activated for surgery, then the patient's personal device magnet must be available in the operating theatre to allow for emergency deactivation should complications arise.

In an emergency, if a VNS-specific magnet is not available, any magnet with a strength over 50 gauss (e.g. a typical pacemaker magnet = 90 gauss) can be secured over the pulse generator to deactivate the device.

If the VNS device is active, vagus nerve stimulation can cause unilateral vocal cord adduction with consequent airway obstruction in the presence of a supraglottic airway device. Consideration should thus be given to securing the airway by tracheal intubation.

Neuraxial / Regional

Vagus nerve stimulation devices should be deactivated before spinal anaesthesia due to potential cardiovascular complications

Intrathecal blocks (spinal anaesthesia) are associated with the inhibition of small unmyelinated sympathetic fibres first, after which larger myelinated sensory and motor fibres are blocked. Hypotension and bradycardia are common side effects, representing the normal physiologic response to the blockade of spinal sympathetic fibres, decreased venous return and parasympathetic dominance. The addition of vagal stimulation from an active VNS device may theoretically exacerbate these changes.

safety of continuing VNS therapy during epidural analgesia or anaesthesia is unclear. If VNS is continued in this situation, monitoring with continuous telemetry should be used and the patient's device magnet should be immediately available to turn off stimulation if required.

Avoid ipsilateral cervical region and supra/infraclavicular blocks

Peripheral nerve stimulators may in theory cause current to enter the VNS device. If necessary, use lowest current possible with path not crossing the generator or lead.

No contraindications to contralateral upper limb, abdominal or lower limb blocks

Electrocautery

The main risk to the patient with these techniques is heating of the VNS device, which can cause temporary or permanent nerve, tissue or vascular damage.

Diathermy can concentrate, or reflect, its energy off any portion of the implanted VNS system, including just a small portion of the lead or electrode (including from previous leads still in place). Injury or damage can occur during diathermy treatment whether the VNS system is turned ‘on’ or ‘off’.

The manufacturer advises against the use of shortwave and microwave diathermy in patients with VNS devices, due to the risk of the device heating and causing tissue damage.

General precautions:

Peri-op risks

Airway obstruction

Up to 1/3 of patients with medically refractory epilepsy have OSAS
severity of airway obstruction is shown to be exacerbated by VNS and may even arise de novo in patients without a previous history of OSA

Symptoms of laryngopharyngeal dysfunction in ~2/3 of VNS patients.
∵ RLN stimulation → Lt VC adduction
transient episodes of dysphonia, SOB and coughing only during stimulation periods

Left VC paresis has also been described due to vagus nerve damage during lead implantation. This is usually temporary but there are rare reports of permanent vocal cord paralysis

Case reports of VNS-induced Lt VC adduction → airway obstruction under GA w/ SGA

Bradyarrhythmia

VNS device is preferentially implanted around the left vagus nerve to minimise the risk of bradyarrhythmias, as the sino-atrial node is predominantly innervated by efferent fibres of the right vagus nerve.
Despite this, bradyarrhythmias, hypotension, syncope and, rarely, asystole have still been reported in association with stimulation of the left vagus nerve

Bradyarrhythmias most commonly occur during the initial device implantation, at the time of lead impedance testing

Obs / ICU patients

MDT
affecting airway (e.g. NIV), circulation (e.g. shock), CVC insertion site (avoid ipsilateral IJ / SC)
theoretical risk of repetitive VC adduction against ETT

MRI

Vagus nerve stimulation devices must be deactivated in the MRI suite

The implanted VNS is an MR conditional device, meaning it is an item that has been demonstrated to pose no known hazards in an MRI environment with specified conditions of use as set out by the manufacturer

The programming wand and patient magnet are MRI-unsafe and should not be brought into the scanning room

If the patient has recently undergone initial VNS device implantation or revision surgery, a deferral period of 2 weeks should be observed before MRI scanning

An important principle to highlight is the unsafe MR exclusion zone. This is the area of the body which cannot be scanned under any circumstances. It is dependent on the VNS and MRI model and varies from the area between vertebral levels C7 to T8–L3, with further information available in the manufacturer's MRI guidelines

References

2023 Guideline by AA & RCoA

Management of Vagus Nerve Stimulation Therapy in the Peri-Operative Period
Management of vagus nerve stimulation therapy in the peri‐operative period - Broderick - 2023 - Anaesthesia - Wiley Online Library