In recent years, there has been a great increase in the number of patients with implanted electronic medical devices who require surgery, anesthesia, or procedural sedation. After the clinical introduction of the cardiac pacemaker in the 1970s, there has been an explosion in numbers of other implantable noncardiac neuromodulating devices. These devices exert their biological effects via a diverse group of stimuli, including but not limited to electrical discharges, release of neurotransmitters, elution of neuromodulators or immunomodulators, and release of gene-modulating endogenous chemicals. The ambient electromagnetic environment of the hospital operating room, gastrointestinal (GI) procedure suite, imaging suite, magnetic resonance imaging (MRI)/computed tomography (CT), etc, can interact with these devices, causing device malfunction, device destruction, and patient harm. The demand for implantable neurostimulation devices has grown significantly during the past several decades, reaching $1.9 billion in 2012. This market that includes deep brain stimulators (DBSs), gastric nerve stimulators, sacral nerve stimulators (SNSs), spinal cord stimulators (SCSs), vagal nerve stimulators (VNSs), and cochlear implants is projected to enlarge by more than 13% annually, nearing $3.5 billion by 2017 (lifescienceintelligence.com, US Neurostimulation Devices Market Analysis).
Increasingly, human beings are becoming electrochemical creatures,1 so beware the “Cyborg” that may be your next patient!2 This review article describes and catalogs the devices available today (and in the near future), and points out their vulnerabilities in the environments of:
- (1) Electromagnetic fields (EMF) (eg, cautery, GI endoscopy, diathermy, lithotripsy, and external electrical impulses such as defibrillation, electroconvulsive therapy [ECT], and nerve stimulation); and
- (2) Diagnostic tests and treatments (eg, MRI, CT, ultrasound, mammography, transcutaneous magnetic stimulation [TMS], laser or phacoemulsification eye surgery, and external beam radiotherapy [XRT]).
We will also describe important preoperative, intraoperative, and postoperative anesthetic considerations for patients having these devices. Understanding some basic principles related to these noncardiac implantable electronic medical devices (IEMDs) is required to ensure patient safety.
The devices to be considered are as follows:
- (1) Deep brain stimulator,
- (2) Vagal nerve stimulator,
- (3) Cochlear implant (CI),
- (4) Retinal nerve stimulator (RNS),
- (5) Tantalum markers,
- (6) Hypoglossal nerve stimulator,
- (7) Gastric nerve stimulator,
- (8) Phrenic nerve stimulator (PhNS),
- (9) Sacral nerve/bladder stimulator,
- (10) Spinal cord stimulator,
- (11) Peripheral nerve stimulator (PNS),
- (12) Dorsal root ganglion (DRG) stimulator, and
- (13) Bone stimulator (BS).
In general, the IEMD is made up of 3 components:
- (1) A pulse generator (containing a battery and programmable electronic circuit within an implantable case);
- (2) Electrodes (the specialized contacts between the generator cables and target tissue); and
- (3) Cable(s) (usually insulated wire connecting the pulse generator to the electrode(s).
Depending on the permanence of the neuromodulating device, its components may be percutaneously or surgically implanted. The exact body location of the patient’s specific IEMD can also vary. Thus, the location of the IEMD relative to the site of therapeutic intervention is very important information to ensure patient safety. As the use of these IEMDs increases, the anesthesiologist will be compelled to include them in the perioperative plan.
Deep Brain Stimulator
The DBS, initially approved by the Federal Drug Administration (FDA) in 1997 for essential tremor and Parkinson’s disease (PD) tremor, is the most well known and widely implanted of the noncardiac neuromodulators. Recently, approved DBS indications have widened to include use for dystonia and refractory obsessive-compulsive disorder (OCD).3,4 Clinical trials are underway for the use of the DBS in depression, severe anorexia nervosa, post-traumatic stress disorder, refractory OCD (FDA, 2009), central pain syndrome, phantom limb pain, seizure disorder, tinnitus, and Tourette’s syndrome (FDA, 1999).
The brain targets of these diseases relieved by the DBS are different
(Figure 1 [Original Chart]).
Treating PD patients with drugs costs $1000 to $6000 per year, and annual health care costs range from $2000 to $20,000 per year.5,6 Despite the initial upfront costs of implanting the DBS device ($25,000 device alone, $125,000-$150,000 US hardware plus hospital), the overall cost-effectiveness of DBS over 10 years has been shown to be beneficial to patients with advanced disease (Figure 2).7
Typically, the PD patient has a favorable response to medications for the first 5 to 10 years. After a decade, symptoms may return as resistance to dopamine drugs recurs and the patient now becomes a candidate for DBS.
DBS, a surgical treatment for advanced PD, involves placing a stimulating electrode(s) in a targeted region of the brain globus pallidus intermedius or subthalamic nucleus and connecting it to a pulse generator, which then delivers high-frequency, preprogrammed stimulation. The exact mechanism of how DBS exerts its effects is not completely understood. By changing the firing rate and pattern of individual neurons in the basal ganglia,8 DBS causes stimulation of adenosine, glutamate, blood flow, and neurogenesis.9–12 These stimulatory and inhibitory effects ultimately result in improvement in PD symptoms, lasting a decade or more. New “closed-loop smart DBS devices for PD” may come equipped with neurochemical sensing feedback loops, allowing detection of local dopamine neurotransmitter levels.13
How the DBS interacts with the hospital environment is summarized in the Table. In general, the head and neck neurostimulators (DBS, VNS, CI, and RNS) are not compatible with each other in the same patient. Each has its own battery and emits electromagnetic interference (EMI), which can adversely affect the other devices (see section on “Developing a Perioperative Plan” and “How the Environment Influences the Device” for details).
Vagal Nerve Stimulator
The FDA approved the use of VNS for the management of medically refractory epilepsy in 1997.
Under general anesthesia, the vagal nerve–stimulating electrode is implanted within the carotid sheath directly on the left vagal nerve, whereas the pulse generator is placed just below the left clavicle.
The proposed mechanism of seizure suppression includes increasing vagal nerve afferent signaling to the thalamo-cortex, reducing the number of seizures, seizure duration, and the number of interictal spikes.14 Possible complications of VNS insertion include infection, lead fracture, hoarseness, and dysphagia. New potential indications for VNS now include intractable hiccups, treatment-resistant depression (FDA, 2005),15–18 obesity modification (FDA, 2015 VBLOC by EnteroMedics), and immune-modulation of autoimmune or inflammatory diseases like rheumatoid arthritis19 via efferent vagal inputs to the spleen.1
Chronic vagal nerve stimulation can cause some very serious side effects, including:
- (1) Respiratory side effects: Dyspnea on exertion, altered breathing patterns (gasping), and snoring while sleeping.20 Together, these may worsen existing obstructive sleep apnea (OSA) in a patient with epilepsy and a new VNS.
- (2) Laryngo-pharyngeal dysfunction: Because the vagus nerve supplies both motor and sensory innervation to this area, chronic stimulation may increase the risk of aspiration. Abnormal motion of the vocal cords or partial adduction21 may cause partial airway obstruction during general anesthesia with laryngeal mask airway. Possible trauma may also result from intermittent adduction of the vocal cord against a rigid endotracheal tube.
- (3) Changes in heart rate variability: These could occur because of vagal alterations of the sympatho-vagal balance of the heart: potential bradycardia22 with asystole.23,24
The VNS can be modified using a special watch-shaped magnet. Understanding the applications of this magnet is important for patient safety as it differs from the magnet applied to pacemakers.
Close proximity of the magnet to the patient VNS generator can initiate several responses. If the magnet is placed over the VNS generator for more than 1 second and then quickly removed, the generator will deliver a burst of vagal nerve stimulation based on the preprogrammed settings. This may stop the seizure. The pulse generator will then go back to its original settings. However, if the magnet is placed over the generator for more than 65 seconds, then the preprogrammed output from the generator will be inhibited. After this, removal of the magnet from the generator will resume the original VNS output settings.25
After the operation, the generator function should be formally tested. Depending on the indication for the VNS, the clinician can choose to leave the stimulator “on” or turn it “off” for the case. For example, if seizures are the indication and the patient is having a sedation case for a wide local excision of a skin lesion, then perhaps leaving the VNS on may be more appropriate. If obesity modification or treatment-resistant depression is the indication for the VNS, and the surgeons are planning a thyroidectomy with monopolar electrocautery (close proximity of EMI), then turning the VNS off for the procedure may be appropriate.
See the Table for a summary of the effects of the environment on the VNS.
CIs have been successfully used since 1973 for patients with severe sensorineural hearing loss.
CI functions by an integrated internal and external system, which communicates via a magnet and results in an electrical stimulus of the cranial nerve 8 (auditory nerve) in the cochlea of the inner ear (Figure 3).
Speech sound waves are eventually converted to electrical nerve impulses via the integration of a microphone, speech processor, external transmitting coil, receiver/stimulator, and an electrode array. There are 3 main manufacturers: Advanced Bionics (United States), Cochlear Corporation (Australia), and MED-EL (Austria). The magnet present in the CI could interfere with a brain MRI magnet because of its ferromagnetic properties. Consequences of this could be: (1) twisting/torque of the CI device; (2) burning of tissue secondary to heating; (3) visual artifacts on the MRI scan; and (4) weakening of the internal magnet, leading to CI device malfunction.26 For a summary of the interaction of the CI in the hospital environment, see the Table.
Brain MRI compatibility deserves special discussion. Most CIs are MRI compatible up to 1.5 Tesla. The clinician must consult with the exact manufacturer for the device in question. In some CI devices, the CI magnet is enabled such that it can be surgically extracted before the MRI without removing the entire CI device: a same-day procedure. A temporary fixing disc is then applied to hold the external coil in place. The CIs with a removable magnet are Cochlear Nucleus 5 (CI 500 series), Cochlear Nucleus Freedom, Nucleus 24, and Nucleus 22. An X-ray of the CI can help the clinician discover whether it has a removable magnet. If the radiopaque lettering of the middle character is C, G, H, J, L, P, T, 2, 5, or 7, then the CI has a removable magnet (www.cochlear.com). After the MRI is completed, another sterile magnet is reinserted into the CI. If left in place during the scan, some CI magnets can be weakened 54% to 79% by the 1.5-Tesla brain MRI.27
The CI devices that do not have removable magnets must be explanted whether a good-quality brain MRI is critically needed in the area of the implant or not. On the other hand, spine MRI can be performed safely without visual scan artifacts, and there is no demagnetization if the CI is more than 30 cm from the entrance to the MRI bore.
CI devices that are 1.5-Tesla MRI-compatible have their magnets placed in a robust ceramic case.
Recently, “magnetless” devices (“CLARION” Multi-strategy Cochlear Implant) have been developed for patients who may receive many lifetime MRI tests.
Testing also noted that 1.5-Tesla brain MRIs did not heat tissues more than 0.1°C, well below the acceptable limits of 1°C. Even 3.0-Tesla MRIs only heated tissues up to 0.5°C .28 The “SYNCHRONY” CI by MED-EL allows a 3-Tesla MRI without magnet removal because it has a revolutionary self-aligning magnet.
Thus, as the population ages and more patients receive CIs, understanding the integration of CIs with other IEMDs and MRI scanners will be important to ensure patient safety.29 In general, the head and neck neurostimulators (DBSs, VNSs, CIs, and RNSs) are not compatible with each other in the same patient because each has a battery that emits EMI and can adversely affect the other devices.
Retinal Nerve Stimulator
In March 2013, the FDA approved the Argus II Retinal Prosthesis System (by Second Sight), which has championed the new age of the “bionic” eye. Patients with severe to profound retinitis pigmentosa and little or no light perception in both eyes can begin to see again. They must have a healthy optic nerve, brain visual cortex, and retina with only damaged photosensors (rods and cones). Retinitis pigmentosa affects 1 in 4000 people and leads to rapidly progressive visual loss in patients with otherwise normal vision. The Argus II prosthetic is implanted into only one eye. The apparatus starts with a pair of eye spectacles holding a special miniature eye camera. The camera sees the image and then sends it to a visual processing unit, which communicates back with the glasses (Figure 4).
The information is wirelessly sent to an implanted eye receiver, which then generates an electrical signal that triggers the remaining healthy rods/cones on the retina to emit an impulse to the occipital (visual) cortex of the brain. This signal is then interpreted by the brain as an image. This new bionic sight comes at a significant price of $300,000 (US). The current resolution of the image is fairly low, at 60 pixels, but it is rapidly being upgraded by many hundreds of times, along with potential for color vision in the future.
Recently, this retinal eye prosthetic IEMD has been approved for age-related macular degeneration, the most common cause of visual loss in the developed world.
Contraindications to the retinal implant are other nearby implants that emit EMI in the head such as CIs, DBSs, and VNSs. Devices that emit direct and indirect release of electrical energy can also damage the patient/stimulator and are contraindicated: electrocautery (monopolar surgical bovie), ECT, diathermy, direct lithotripsy, defibrillation, ionizing radiation therapy, eye laser/phacoemulsification/fragmatome, and certain MRI scanners. It is classified as an “MRI-conditional device,” meaning that 1.5- or 3.0-Tesla scans should only be performed according to specific manufacturer instructions (Table). The external (microcamera and visual processing unit) should remain outside the MRI room at all times.
Although CT scans and ultrasound can safely be performed, the implant can leave an artifact in the image, making interpretation inaccurate. Finally, the long-term effects of chronic electrical stimulation to the retina are unknown but may include deterioration of the retina and/or optic nerve.
Not to be confused with the retinal eye prosthetic, the tantalum marker will be discussed, although it does not emit electrical currents and is nonferrous.
The markers are small 2.5-mm diameter buttons made of tantalum, a nonmagnetic, noncorrosive metal. The markers are implanted permanently into the sclera of the eye for external demarcation of the location of a choroidal ocular melanoma. This facilitates treatment with a highly precise (accuracy to 1 mm) type of external beam proton radiotherapy. Many of the tantalum markers are compatible with 1.5- but not 3.0- Tesla brain MRI.30,31
Hypoglossal Nerve Stimulator
OSA, a common disorder in the United States, often presents with moderate-severe airway obstruction during sleep, causing numerous nocturnal oxygen desaturation events. Stimulation of the hypoglossal nerve (genioglossus muscle)32 has been proposed as a treatment for OSA by causing anterior tongue retraction and opening of the obstructed airway.33 Upper airway stimulation by this unilateral hypoglossal nerve device (funded by Inspire Medical Systems) had a very successful recent feasibility trial published in the New England Journal of Medicine (January 9, 2014—STAR Clinical Trial.gov, # NCT01161420). This study showed that patients with a body mass index of 32 or less or apnea hypopnea index of 50 or more events per hour were good candidates34 (Figure 5).
The hypoglossal nerve stimulator enables the nerve stimulation to be coordinated with the patients’ ventilation while they sleep. Because hypoglossal nerve stimulator helps maintain airway patency, the authors recommend continuing their function in the perioperative period. Because these patients have a high risk of airway obstruction and apnea, additional care such as telemetered pulse oximetry and continuous supplemental oxygen should be considered for these patients.
Gastric Nerve Stimulator
Gastroparesis causes chronic protracted nausea, vomiting, and weight loss. In late 1999, the FDA approved the “Enterra” Therapy System as a Humanitarian Use Device for the treatment of gastroparesis.35 Through high-frequency stimulation electrodes placed in the muscularis propria muscle of the stomach, it can be encouraged to empty normally, decreasing nausea, vomiting, and consequent malnutrition.
The University of California San Francisco Intestinal Rehabilitation and Transplant Program began treating patients with this IEMD in 2006, and it has had impressive results, with some patients reporting a reduction of vomiting by 60% to 80%. The device (2.5 × 2 × 2.5 inches) is laparoscopically implanted under general anesthesia in a 1- to 3-hour operation and requires a 1- to 5-day postoperative hospital stay.
A similar device with a slightly different indication recently surfaced in 2009. The Diamond System (MetaCure, Orangeburg) is used to treat “diabesity” (ie, patients with obesity metabolic syndrome arising from type II diabetes mellitus). Implanted laparoscopically in the stomach, the Diamond System stimulates the vagal afferent nerve, generating a sense of satiety. Shown to cause significant weight loss and correction of blood sugar and cholesterol, this could be the new noninvasive alternative to gastric bypass or banding operations.36 Because postoperative ileus is common, it would seem reasonable to turn off the gastric stimulator during the operation and resume it in the postoperative period as the bowel motility recovers. Because these patients likely have a “full stomach” even when not eating, rapid sequence induction and intubation with cricoid pressure along with an orogastric/nasogastric tube to suction should be considered.
Phrenic Nerve Stimulator
PhNS s (pioneered in 1872 by Duchenne) were invented to reanimate the diaphragm and attempt ventilator independence in patients with lack of central respiratory drive.37 Clinical indications for this “electrophrenic respiration” may include congenital central hypoventilation syndrome (Ondine’s Curse), spinal cord injury above C4, brainstem injury, and idiopathic severe sleep apnea. Future candidates could include stroke, encephalitis, meningitis, and ShyDrager syndrome patients.
Before being considered a candidate for this IEMD, the patient must have an intact phrenic nerve, diaphragm, and lung alveolar oxygenation. A PhNS and receiver implantation is performed via a cervical or thoracoscopic surgical approach. Small multicenter studies from 1966 to 1988 showed that 45% to 84% of patients were able to pace for 12 to 24 hours per day, allowing improved ability to swallow, speak, and smell. The Avery Mark IV Breathing Pacemaker System, one of 3 worldwide, is used in the United States and Europe.
Case reports exist of patients with spinal cord injury who have successfully received both phrenic nerve pacers and SCSs together.
Patients with spinal cord injury below C4 anterior horn cells do not benefit from PhNSs, but they can benefit from a modified technique using direct diaphragmatic muscle stimulator pacing. These patients may present with poliomyelitis and amyotrophic lateral sclerosis. Some amyotrophic lateral sclerosis patients can delay ventilator dependency for up to 2 years by using a diaphragmatic pacer initially.38,39
In conclusion, “electrophrenic stimulation” can potentially benefit patients in a 2-fold manner. While improving the independent quality of life of quadriplegic ventilator-dependent patients, it can also reduce the high cost of health care needed for these individuals.
The anesthetic implications of PhNSs include the issue of using mechanical ventilation during surgery. Disabling the stimulator during surgery should be considered to avoid ventilator dysynchrony. The Avery Mark IV transmitter (one type of PhNS) has 2 on/off switches, which can be turned off when the phrenic pacer is not in use. In this device, there are no implanted batteries, and the pacer will not operate if the external transmitter is off or removed from contact with the patient
Reactivation and assessment of the function of the device after surgery is also required. Patients with these devices should be considered high risk for perioperative respiratory failure.
The Sacral Nerve/Bladder Stimulator
The SNS, approved by the FDA in 1997 for urinary retention and incontinence, has recently expanded its indications to include pelvic floor dysfunction (urge fecal incontinence, female pelvic pain, and interstitial cystitis).40,41
After test placement of the lead electrode at the sacral nerve (via the S3 foramen), the InterStim (Medtronic sacral neuromodulation device) stimulates the nerves going to the bladder, bowels, urinary/anal sphincters, and pelvic floor muscles; 50% to 90% of patients with urinary/fecal urgency may experience significant improvement in symptoms.42 Risks include infection (3%–4%), mechanical failure, lead movement, and lead fracture. Every 3 to 5 years, the battery change will require complete replacement of the device.
Some medical restrictions include inability to perform spine MRI (even if device is turned off), diathermy, monopolar electrocautery, or radiation therapy. Ultrasound, lithotripsy, BSs, and cardio version/defibrillation paddles should be kept at least 15 cm away from the SNS device.
Pacer/cardioverters and SNSs can have mutual compatibility achieved under certain conditions. Consult the manufacturers for the details. Of note, SNSs can cause significant artifact on the EKG, which may lead to potential misdiagnosis of dysrhythmias.43
Spinal Cord Stimulator
First introduced in 1967, spinal cord stimulation is one of the oldest forms of IEMDs used to treat pain such as failed back syndrome, complex regional pain syndrome, cancer pain, ischemic pain from peripheral vascular disease and severe cardiac angina, and neuropathic pain.44 The exact mechanism of action of SCSs in relieving chronic pain has been extensively debated and is probably multifactorial involving stimulation of the dorsal spinal cord columns.45
Today, SCSs use sophisticated combinations of electrodes, contacts, and current delivery (high frequencies, 1–10 KHz; classic frequencies, 10–50 Hz).46
Before permanent placement of an SCS, a temporary trial SCS electrode is placed to ensure that this is a successful treatment for the perceived pain. Percutaneous electrodes (4–16 in a group) are guided into the epidural space using fluoroscopy and paresthesia sensation by the patient. These are then connected to an external pulse generator, and several different stimulation pulses are tested to ensure relief of pain. Sometimes, trials fail because patients are not able to tolerate the paresthesia, and pain is not relieved. A new type of high-frequency stimulation of the spinal cord, 1 to 10 KHz, has recently been discovered to not cause uncomfortable paresthesia while concurrently relieving pain; 1-KHz stimulation seems to reduce the action potentials in A α/β fibers and may be more specific treatment in certain types of pain. On the other hand, C-fiber transmission and neural windup seemed to respond better to 50 Hz but not higher frequency currents.
SCSs are placed in the epidural space anatomically based on the location of the pain:
- (1) C4 to T1 for upper limb pain,
- (2) C6-T2 for ischemic angina, and
- (3) T9-L1 for lower limb pain.
Possible complications of SCSs include electrode migration, infection, and hardware malfunction. Depending on the specific manufacturer, most SCSs are contraindicated for body MRI. However, some SCSs allow brain/head and extremity MRI to be performed. Medtronic has recently released a full spine SCS that is compatible with the 1.5-Tesla MRI for full body scan.
Case reports exist confirming compatibility of SCS with PhNSs in patients with spinal cord injury. It is recommended to continue spinal cord stimulation uninterrupted during surgery unless the surgery involves potential disruption of stimulator function with monopolar electrocautery or electrode displacement. Bipolar electrocautery is indicated if the stimulator is left on. If monopolar cautery is used, then follow the recommendations listed in the Table and the text in the section entitled “How the Environment Influences the Device, Surgical Electrocautery.”
Peripheral Nerve Stimulator
A recently clinically introduced, still somewhat experimental, mode of chronic pain management of the extremities is the permanent PNS.47
The PNS is the direct, electrical stimulation of and inhibition of the primary afferent pain signals of specific nerve(s) that travel outside of the spinal cord. PNSs may be especially useful in cases in which the SCS fails to elicit therapeutic paresthesia or patients are not candidates for an SCS because of coagulopathy.
The PNS has been used in: (1) facial pain and chronic intractable headaches, (2) reflex sympathetic dystrophy, and (3) peripheral extremity pain. A PNS used to treat facial pain and headaches implanted in the head and neck may be incompatible with other neurostimulators of the head and neck: DBS, VNS, CI, and RNS (Figure 6).
These PNS devices are typically not compatible with other IEMDs in the same patient, with one exception: Freedom (trademark) 4 and 8 (Stim Wave PNS). Because this PNS device uses an external battery worn around the waist instead of an internal pulse generator (IPG), it is also MRI compatible. Future PNS devices may include options such as wireless operation, external patch strategies, and miniaturized IPG (allows easy implantation in the periphery). Continuing the nerve stimulation in the perioperative period is warranted, but the device should be deactivated whether or not it will be close to electrocautery or at risk of disruption attributable to surgery.
Dorsal Root Ganglion Stimulator
Another recently emerging neuromodulation pain treatment is the DRG stimulator, which operates at the interface of the peripheral and central nervous systems.48 With electrodes placed via the epidural space, the DRG can be stimulated in an approach akin to the SCS. The mechanism for DRG stimulation includes both interruption of afferent pain signals to the brain as well as some partial downstream spinal cord stimulation modulation.
DRG stimulation, like the SCS, is effective in neuropathic pain for: (1) failed back surgery syndrome, (2) complex regional pain syndrome,49 and (3) chronic postsurgical pain. An advantage of DRG stimulation over an SCS is that the leads do not migrate as often, so operative revisions are less frequent.
Bone stimulation with electrical impulses is being used to improve bone fracture healing times in patients with delayed or nonunion of fractures.50 Electric currents create regions of polarity, affecting transmembrane potentials within the bone, resulting in new bone growth, or remodeling in the physis and newly fractured bone.51–53
The modern BSs of today can be classified into 2 groups:
- (1) Electromagnetic: pulsed EMF, direct current, and capacitative coupling; and
- (2) Ultrasound: low-intensity pulsed ultrasound.
Direct current BSs, an invasive treatment, require a surgically placed metallic cathode within or next to the fracture site. The pulsed EMF and capacitative coupling are noninvasive devices placed near the osteotomy/fracture site. Mostly used in tibial, femur, and spine fractures, these devices claim a significantly increased rate of bone healing, as evidenced by radiographic union in acute fractures.54 Alternately, low-intensity pulsed ultrasound works by sending an ultrasound-induced mechanical signal through soft tissue and bone that creates “micromotion” at the fracture/osteotomy site. Initiating this micromotion 20 minutes per day for 2 to 6 months noninvasively stimulates bone healing by promoting endochondral ossification. Originally, bone stimulation indications included acute fracture, delayed union/nonunion (3 months of no radiographic changes of healing), arthrodesis, and osteochondral defects. New indications have expanded to include osteoarthritis, rheumatoid arthritis, osteoporosis, and possibly neuropathic pain.55–57
Thus, the preoperative presence of a BS in your patient presenting for another type of nonorthopedic surgery may become more commonplace, especially as the United States tries to wean its population off the widespread use of narcotics.
Developing a Perioperative Plan for Patients With Implanted Electronic Medical Devices
Because there are no current official guidelines for the preoperative, intraoperative, and postoperative management of surgical patients with IEMDs, each patient must be considered for their specific device(s) and potential interaction with the new surgical/diagnostic environment. The primary reason for receiving the IEMD (eg, PD, epilepsy, and sleep apnea) may in itself increase risk of perioperative injury.
Preoperative consultation in the anesthesia screening clinic should reveal some of the following information:58
- (1) Device type and location in the body of the patient (electrodes, IPG, cable(s);
- (2) Device manufacturer, support line phone number for emergencies, internet web site;
- (3) Names of the device implanting team (doctor, hospital, and specialty);
- (4) Date of last check for device function, battery life, and implantation;
- (5) Complications during anesthesia for insertion of IEMD;
- (6) Quality of symptom control by the IEMD and consequences of turning the IEMD off. Postoperative evaluation of IEMD;
- (7) How to disable, restart, and reprogram the device;
- (8) Status of the current medications for the medical condition being treated and possible adjustment of medications when the device is disabled or altered during the intraoperative period; and
- (9) Phone number and contact information for industry/device representatives and a plan to have them be present during the perioperative period or postoperative period to check device function if indicated.
To ensure patient safety and avoid surgical/diagnostic equipment interaction with the proper function of the IEMD, the anesthesia, surgery, and nursing teams should all be aware that the patient harbors the device. There are 2 general environments that threaten the device function.
- (1) EMF: EMI is the effect of EMF on the function of the IEMD. The EMF can be “conducted” or “radiated.” “Conducted” EMI results when EMF directly contacts the body (surgical monopolar cautery, GI endoscopy with biopsy, diathermy, lithotripsy, defibrillation, ECT, and nerve stimulation). “Radiated” EMI occurs when the patient with the IEMD is placed in an EMF (eg, MRI scanner).
- (2) Diagnostic tests or treatments: MRI, CT scan, ultrasound, mammography, TMS, laser or phacoemulsification eye surgery, XRT, and neuraxial anesthesia. These environments either interfere with IEMD function by causing “radiated” EMI. Or, the IEMD interferes with the quality of scan visualization of the structures by creating a defect in the scan.
EMI can affect the IEMD by:
- (1) Turning the device on/off;
- (2) Resetting the IPG to new frequencies, amplitudes (unacceptable for treatment).
- (3) High levels of current can pass through the electrodes to the target tissue (eg, brain, spinal cord, vagal nerve, cochlear nerve, and retina) and cause inappropriate stimulation, burns, permanent nerve injury, and stroke;
- (4) A high current can permanently damage the battery power or destroy the device.59,60
How the Environment Influences the Device
Electrocautery used in surgery can potentially damage the IEMD by creating an EMI that damages the device, as described in 1 through 4 above. Bipolar cautery is less likely to create EMI with the device than monopolar because bipolar cautery uses radiofrequency current to cut and/or coagulate tissues by passing the current through the 2 tips of the instrument at a very short distance apart, and the current does not traverse the patient towards a distant grounding pad. Thus, bipolar generates much less EMF/EMI that could potentially interfere with the IEMD. If monopolar cautery is used in a patient with a neurostimulator, then consider these guidelines:
- (1) Turn IEMD off, turn voltage to 0;
- (2) Place the grounding pad as far as possible from the IEMD (generator, leads); and
- (3) Avoid the use of full-length grounding pads. Use a nonconductive surface or bed.
GI Endoscopy and GI Electrocautery
GI endoscopy without biopsy is completely safe with IEMDs. However, when tissue biopsy, sphincterotomy, and coagulation for hemorrhage are necessary, then consideration of the EMF/EMI generated by the endoscopists’ radiofrequency monopolar/bipolar probe must be considered. The electrocautery device typically sends its current via a monopolar device that passes through the patient’s body to a distant grounding pad and then back to the generator. Resistance to the flow of this current by the patient’s tissues generates heat, which then allows
electrocoagulation (short bursts of low energy) or
Electrosection is enabled by continuous current-generating high temperatures (for longer periods of time), which then cause cell explosion and evaporation. Some of the newer models have programmable blends of coagulation and cutting current.
Most endoscopy probes are monopolar:
- (1) Endoscopic polypectomy (colon, stomach);
- (2) Endoscopic shincterotomy (biliary, pancreatic);
- (3) “Hot” biopsy, polypectomy (special type in which coagulation and cutting are used to remove the polyp while simultaneously ablating the base of the biopsy); and
- (4) “Argon plasma coagulation” for control of hemorrhage, ablation of mucosal lesions (vascular malformations), or residual polyp tissue.
Bipolar and “multipolar” cautery is used in “bicap” probes that control local hemorrhage arising from ulcers or vascular lesions. The Olympus heat probe (Olympus Optical, Tokyo, Japan) is unique because it uses a nonconductive hemostatic probe, coated by a Teflon tip, heated by an internal electric resister. The induced EMF/EMI is minimal because no current travels through the body tissues to either a local (bipolar) or distant (monopolar grounding pad) target59 (see Table).
Diathermy (short-wave, microwave, therapeutic ultrasound, and high-frequency electromagnetic radiation), not surgical electrocautery as discussed above, is contraindicated in all patients with IEMDs according to the manufacturer manuals.60 Two published case reports describe severe brain damage when diathermy was used in a patient with deep brain stimulation.61,62 Considered one of the most dangerous medical interventions in a patient bearing an IEMD, diathermy causes local heating of soft tissues by electric current and, thus, large EMI. It is used to treat musculoskeletal pain, joint pains, tendinitis, and inflammatory conditions (see Table).
Lithotripsy (extracorporeal shock wave lithotripsy) is generally contraindicated by IEMD manufacturers, especially with IPGs in the abdomen or buttock. Extracorporeal shock-wave lithotripsy generates strong shock waves that dissolve biliary, renal, and more recently, salivary gland stones. These shock ultrasonic waves can disrupt or damage the internal working parts of the IEMD or IPG60 (see Table).
Emergency Defibrillation, Cardioversion, and Sensing Pacemakers
Emergency defibrillation or cardioversion is not contraindicated in patients with IEMDs. However, the operator should note that this electrical energy is a direct current flowing through the body and the IPG/IEMD device. This current may seriously damage the device or inflict bodily injury.59,60
When applying the defibrillator, consider these guidelines to minimize the injury:
- (1) Position the paddles as far away as possible from the implanted device;
- (2) Position the paddles perpendicular to the wires; and
- (3) Use the lowest clinically effective energy to defibrillate/cardiovert.60
The interactions between separate IEMDs in a single patient can be complex and should involve a discussion with the device manufacturer. Cardiac devices with sensing functions may be particularly problematic. Some recommendations include placing the DBS generator on the opposite side of the body to the cardiac pacer and programming it in bipolar mode to minimize the risk of interference63–65 (see Table).
ECT is used for severe depression, refractory catatonia, acute suicidal ideation, psychosis, and bipolar mania. Under general anesthesia, the electrodes are placed in a bitemporal fashion, and an electrical current is passed to induce a seizure lasting at least 20 seconds. In general, manufacturers are not recommending ECT in patients with existing head or neck IEMDs.
Among the concerns are that the ECT current may cause induction heating of DBS electrodes. In addition, the induced seizure may cause movement of the DBS electrodes (see previous section on DBS in this review).60 Despite this, there are several case reports of successful ECT in patients with DBS.
If the IEMD is far away from the head and neck of the patient, then ECT may be safely possible. The current used to induce an ECT seizure is far less than for cardioversion, and there are several case reports noting that heating is minimally induced in these cases.66–69
Interestingly, DBS is now being placed in some patients with psychiatric disorders that may also be treated with ECT (severe, refractory depression, bipolar disorder, catatonia, and related symptoms [OCD, and post-traumatic stress disorder] see Table).
Peripheral nerve stimulation (PNS) is used to guide placement of regional anesthesia blocks (ie, to locate proximity of needles to specific nerves). Several safety concerns should be considered in using PNS guidance in patients with IEMDs. If possible, use ultrasound to guide nerve blocks rather than a PNS.
However, if using a PNS for guidance, then consider:
- (1) Applying the current so it does not cross the IPG pulse generator or lead system; and
- (2) Avoiding high-pulse duration and high frequency.58
Several case reports have been published noting that PNS can but may not necessarily interfere with IEMD functions. One case describes PNS for placement of a supraclavicular block where there was no interference with the patient’s existing DBS.66 Please consult with the IEMD manufacturer for more detailed guidelines.
Of note, a transcutaneous electrical stimulation (TENS) unit used for chronic pain should not be placed over an SCS because it may injure the patient67 (see Table).
Diagnostic Tests or Treatments
Most imaging devices (CT scans, ultrasound, mammography, standard fluoroscopy, plain X-rays, nuclear medicine scans, and XRT) are generally considered safe in a patient with an IEMD. However, specific limitations in the quality of the image produced in a patient with an IEMD having one of these diagnostic tests should be contemplated (see Table).
CT scans are generally safe and do not usually interfere with the functioning of the IEMD. However, one manufacturer warns against the use of the CT scan in an SCS because it can cause a momentary increase in the stimulation level, and the patient may experience a “jolt-like” sensation.60 Turning the voltage output to 0 and turning the device off can mitigate this problem during CT scans (see Table).
Ultrasound, also considered a safe device, is routinely used to help find and place DBS wires to IPGs. Some manufacturers will suggest not placing an ultrasound directly over the IPG because this may cause some mechanical damage60 (see Table).
A mammogram in patients with a chest IPG is safe but may not produce a good breast image because some of the breast will not be seen secondary to the IPG artifact (see Table).
Standard Fluoroscopy and Plain X-Rays
Plain X-rays and standard fluoroscopy are considered safe tests with IPG and do not affect IEMD normal functions. Radiographs generate clear pictures without artifacts and are often the study of choice when investigating IEMD malfunction with suspected wire fracture60 (see Table).
Nuclear Medicine Scans
Nuclear medicine studies are considered safe (see Table).
External Beam Radiotherapy
XRT, used to treat cancer, has an unknown effect on the neurostimulator located in close proximity to the XRT field. Manufacturers recommend shielding the IEMD with an external lead drape. A case report described XRT to the neck of a patient with squamous cell carcinoma who also had a DBS for PD. The IPG was shielded with lead, and the stimulator was not turned off because the PD tremors would affect the quality of the XRT treatment. This patient did well with his treatment, and the DBS neurostimulator was not affected.70 Another case report described a woman with PD and breast cancer who developed cancer on the same side as the IPG for DBS. After her surgery, the IPG was moved to the opposite side of the chest, which allowed her to receive XRT postoperatively as well as follow-up mammograms60 (see Table).
Single-shot spinal anesthesia is not contraindicated in patients with SCSs or SNSs. However, the clinician must avoid needle insertion at the site of the neurostimulator/IEMD. SCSs that are inserted above L1 can accommodate L2-L3 placement of spinal anesthesia. SNSs in the sacral area will also not be affected by an L2-L3 spinal needle. Thus, the clinician must know the exact location of the spinal neurostimulator/IEMD and where the connecting cables run. However, epidural catheters are not recommended because the catheter can travel up or down its insertion site and disrupt the existing neurostimulator. An exception to this concept may be placement of a lumbar epidural in a patient with a preexisting cervical SCS58 (see Table).
Laser or Phacoemulsification of the Eye
With our aging population, more patients with head and neck IEMDs (DBSs, VNSs, CIs, and RNSs) will present for cataract or other types of eye surgery.
The use of laser, phacoemulsification, or fragmatome may damage the Argus II Retinal Implant and is mostly contraindicated. See the manufacturer and physician guide recommendations. There are no established guidelines or safety protocols to guide the clinician concerning cataract surgery on patients with a DBS. In general, manufacturers recommend turning the DBS voltage to 0 and turning the device off. However, a returning tremor may then prohibit cataract surgery under local anesthesia and commit the patient to general anesthesia.
Two published case reports exist in patients with continuously functioning DBSs (left on) during phacoemulsification cataract eye surgery.71,72
Pulsed-mode phacoemulsification rather than continuous mode was used under sub-Tenon’s local eye anesthesia. Pulsed mode decreased the rate of anterior eye segment temperature elevation (3.5°C) and thus, theoretically, likely decreased the rate of heating of the intracranial DBS electrodes. Also of note, phacoemulsification acoustic waves do not interfere with the performance of the electrical parameters of the DBS system. These cases proceeded successfully and uneventfully.
It is unknown whether laser-assisted phacoemulsification cataract surgery affects head and neck IEMD functioning (DBSs, VNSs, CIs, and RNSs). Lasers use focused light to generate heat and cut tissues so they may emit EMI
Clinicians should be aware of the interaction of the noncardiac neurostimulators (IEMDs) with MRI, especially because it is considered the gold standard for evaluating intracranial and spine abnormalities. In contrast to the cardiac pacers/stimulators absolutely contraindicated in MRI, some IEMDs can be considered conditionally compatible with MRI on a case-by-case basis. Despite this, many manufacturers will list these IEMD devices as absolutely contraindicated in MRI scans. The patient should have been provided with the emergency device hotline and the contact information of the implanting physician team. MRI scans can be required to troubleshoot IEMD positioning, migration, and injury. Also, MRI may be a preferred scan in children because it does not emit ionizing radiation as in X-ray, CT, and fluoroscopy. Whether or not an IEMD device is compatible with MRI-generating EMF/EMI depends on several factors:
- (1) Location of MRI scan relative to the IEMD in the body;
- (2) Type of MRI technique used: type of coil (location of transmit and receive coil), specific absorption rate, IEMD lead positioning, strength of magnet (ie, 1.5 or 3.0 Tesla); and
- (3) Ferromagnetic quality of the IPG (implanted pulse generator). Most modern IEMDs are made of nonferrous parts.
Device heating at the wire/metal-tissue interface and its effects on the target biological tissue is the main concern. In a DBS, an increase in temperature of 5°C to 7°C can cause reversible tissue damage, but more than 8°C can cause permanent tissue damage leading to stroke. Also, EMI may turn the pulse generator on and off or reprogram the settings of the DBS.
Medtronic DBS manufacturers recently published revised safety guidelines for MRI scans in 2016. Because most DBS devices implanted in the United States are made by Medtronic, we will discuss their recommendations:
- (1) For devices: Activa PC model #37601, Activa RC model #37612, Activa SC model #37603. These are “full body” MRI eligible and “head” eligible under several conditions: (a) 1.5 Tesla horizontal closed bore magnet, (b) Radiofrequency (RF) transmit/receive body coil or radiofrequency transmit/receive head coil, (c) along with other technical MRI magnet power and rate settings (http://professional.medtronic.com/mri/clinicians).
- (2) For devices: Activa SC model #37602, Kinetra model #7428, and Soletra model #7426. These devices are only eligible for MRI “head” scans. Not eligible for MRI “body” scans.
Before MRI scanning, a patient with a DBS should consider adjusting the DBS settings: (a) turn the IPG off and voltage to 0, (b) consider leaving the stimulator on but put in “bipolar” mode, and/or (c) avoid sedation during the scan. After the scan is completed, turn the off DBS stimulator back on and consider resetting it if it was placed in “bipolar” mode. Note that previous guidelines from Medtronic 2010 to 2016 did not recommend body MRI in DBS patients but only head MRI. New guidelines are more permissive in certain specific DBS neurostimulator models.
For VNSs, MRI guidelines suggest head only transmit/receive coils that do not extend over the IPG. Avoid body coils and spine MRI. Body MRI scans are generally contraindicated in patients with SCSs. But Medtronic has a new SCS (RestoreSensor SureScan MRI), which is spine MRI compatible. The use of MRI head transmit and receive coils in a patient with an SCS is conditionally compatible. Some SCSs allow MRI scanning of parts of the extremities and the head. Check with the specific manufacturer and model number. Observe caution in patients with spinal BSs requiring MRI scans. Most are MRI incompatible according to the manufacturers. The Argus II RNS appears to be conditionally compatible with both the head and body MRI according to their manufacturers. Thus, for each IEMD that has MRI labeling approved by a government agency, specific guidelines, procedures, and conditions should be respected to ensure patient safety (see Table).
Transcutaneous Magnetic Stimulation
TMS is a noninvasive, FDA-cleared medical procedure that uses repetitive magnetic fields or pulses created by a small coil magnet to stimulate nerve cells in specific areas of the brain to treat a variety of maladies.73 Some proposed, still experimental, applications include treatment for migraine pain with aura,74 movement disorders, seizures, depression, anxiety, OCD, and addiction,75 amongst others. By holding the magnet over the scalp, the pulses generated by the repetitive TMS create a brief magnetic field that generates a current in the targeted brain neurons, which may reverse some of the symptoms experienced by the patient. This does not require sedation or anesthesia.
TMS is contraindicated in patients with IEMDs, especially in the head and neck (DBSs, CIs, retinal implants, and VNSs) because the TMS magnet will cause an EMI that may affect the neurostimulator (see Table).
In the postoperative period, the patient and the IEMD should be checked for appropriate functioning. Formal interrogation of IEMD settings, battery, and position/function should be obtained. If it is not functioning normally, then the IEMD should be evaluated and reprogrammed by appropriately qualified individuals.
The prudent and well-informed anesthesiologist and surgeon should know whether their patient has an IEMD(s) preoperatively. The clinician should understand how this specific IEMD functions, as well as how it interacts with other IEMDs and the ambient electromagnetic environment of the hospital, operating room, and procedure room. Finally, recognition of device malfunction through careful postoperative interrogation is critical to ensure patient safety.
Name: Una Srejic, MD.
Contribution: This author helped prepare the manuscript.
Name: Paul Larson, MD.
Contribution: This author helped prepare the manuscript.
Name: Philip Bickler, MD, PhD.
Contribution: This author helped prepare the manuscript.
This manuscript was handled by: Maxime Cannesson, MD, PhD.
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