Spinal cord stimulators and implications for anaesthesia

The National Institute for Health and Care Excellence has recommended spinal cord stimulation as a treatment option for adults with chronic pain of neuropathic origin.¹ There have been significant advancements in the technology and evidence base over the past decade resulting in its increased use. Patients with an implanted spinal cord stimulator (SCS) are therefore likely to present for other unrelated procedures. This article aims to provide the reader with the essential information required to manage a patient with an SCS safely throughout the perioperative period.

SCS technology

An SCS consists of single or multiple leads containing electrodes that are placed in the dorsal epidural space. Two types of electrode leads exist; cylindrical catheters that are inserted percutaneously using a modified Tuohy needle, or paddles that are placed surgically, often requiring a laminotomy. The exact placement of leads in the epidural space varies depending on the clinical condition being treated. The majority of SCSs are inserted for chronic neuropathic back, leg pain, or both. In these cases, the leads are placed centrally over the dorsal spinal cord at the eighth and ninth thoracic vertebral spaces. The lead is anchored with sutures to the supraspinous ligament and tunnelled subcutaneously to the implanted pulse generator (IPG). The IPG provides the power to the SCS leads. It is situated in a subcutaneous pocket positioned on the patient's back below the scapula, above the belt line, or occasionally the anterior abdomen.

An SCS provides electrical stimulation to the dorsal column, lateral funiculus, and dorsal roots. This stimulation depolarises and inhibits afferent ascending nociceptive pathways and also increases supraspinal descending antinociceptive pathways.² The exact mechanisms of action are complex and have been described in further detail in this journal by Moore and McCrory.³ The strength, duration and frequency of the electrical impulses are programmed remotely by a handheld programmer held over the IPG. Patients are often adept at using their device and can change programs independently, and switch their device on and off.

Perioperative and periprocedural care of patients with SCS

Preoperative period

Multidisciplinary involvement is required to formulate a safe management plan for patients with an SCS. The responsible pain team should be contacted early to provide advice and assistance as they have access to essential information regarding the device and its management. The device should be interrogated and turned off before anaesthesia, to prevent inadvertent reprogramming or activation during surgery from electromagnetic interference. Whilst active, the SCS may also interfere with ECG monitoring causing high-frequency artefacts.⁴

Intraoperative period

Prophylactic antibiotics

Antibiotic prophylaxis is not required for the SCS itself and should be administered as per local recommendations for the surgical procedure.⁵

Positioning

The IPG location should be ascertained, as additional padding may be required to assist in appropriate positioning of the patient and pressure area care. The position of the IPG and leads may also have an impact on the surgical site or technique. Within 6 weeks of SCS insertion, extra care should be taken during positioning of the patient to minimise excessive flexion, extension or rotational movements of the spine, which increase the risks of lead migration.

Neuraxial techniques

Neuraxial techniques are best avoided because of the significant risk of direct damage to SCS leads and the possibility of introducing infection. There are a small number of case reports describing the safe use of neuraxial anaesthesia below the level of the SCS leads, but no high-level evidence to support this practice. Thoracic epidurals are considered to be absolutely contraindicated.

If a neuraxial technique is deemed essential, this should be discussed with the pain specialist. In such circumstances, it should be performed with meticulous attention to sterility, at a site distant to the SCS and under fluoroscopic guidance to ensure accurate and safe needle placement. This technique may be unfamiliar to the anaesthetist and assistance from the pain specialist is likely to be required. It should be noted that epidural spread is often impeded by fibrosis surrounding the electrodes in the epidural space. This may result in incomplete or patchy analgesia.

Diathermy

A careful risk–benefit analysis must be made by clinicians before use of diathermy. The British Pain Society states that monopolar diathermy should be avoided where possible, but if its use is unavoidable then the ground plate should be placed as far from the IPG as possible, avoiding current flow across the device.⁵ Ideally, the ground plate should be placed on the contralateral side to the IPG, with the lowest clinically effective currents used.⁴ Bipolar diathermy can be used safely.

Postoperative period

The device should be switched back on and interrogated by the pain team to ensure functionality before discharge from hospital.⁴

SCS have no role in the management of acute nociceptive pain, which should be managed by conventional means.

Special considerations

MRI

MRI is now not considered an absolute contraindication, as newer models of SCS are MRI-conditional. However, MRI may cause heating, software malfunction, image artefacts, device movement, neuronal damage and unintended stimulation.⁵

The decision regarding MRI safety is complex. Local trust guidelines should be reviewed and then the make and model of SCS attained before discussion with the responsible pain specialist, device representative, and a radiologist and radiographer with appropriate MRI and SCS experience.

The SCS should be interrogated and switched off before entering the MRI suite and then interrogated and reactivated once the patient has left the area. Minimising sedation helps to maintain communication with the patient to assess any periprocedural complications.

CT

Complications in patients undergoing CT are rare and relate mainly to reported unintended stimulation and device failure. To reduce these risks, the SCS should be switched off during the procedure and radiation directly through the SCS device should be minimised.⁴ If any changes to clinical effect are noticed after the procedure, these should be reported to the appropriate pain specialist.

Lithotripsy

The risks of lithotripsy are low. The device should be turned off during the procedure and the lithotripsy beam focus should be avoided within 15 cm of the SCS.⁴

Obstetrics

We are unsure of the effects of SCS in pregnancy and deactivation of the device is recommended by manufacturers. However, this decision must be carefully considered as most analgesics are contraindicated during pregnancy. There are no known interactions with the use of cardiotocography or fetal scalp monitoring.⁶

Early referral to the obstetric anaesthetic preassessment clinic is required so that appropriate delivery planning can be made. This will require discussion with the patient, obstetrician and pain specialist.

Permanent pacemakers, implantable cardioverted defibrillator and external defibrillator use

The presence of an SCS is a relative contraindication for permanent pacemakers (PPM) or implantable cardioverted defibrillators (ICD). The decision to implant a PPM or an ICD is multidisciplinary. Most PPM and ICD devices operate in demand mode and monitor intrinsic cardiac activity. There is a possibility that these devices may misinterpret the SCS electrical activity, which could lead to dangerous arrhythmias or inadvertent defibrillation. If unavoidable, pacemakers with bipolar sensing function are less sensitive to extraneous signals than unipolar and are recommended for concomitant use. A baseline ECG that the SCS is known to induce should be performed. The device should be implanted on the contralateral side to the IPG. During insertion, the maximum sensitivity of the cardiac device against the maximum stimulation of the SCS is set to monitor any interactions.⁵

SCS must not prevent the use of external defibrillation, cardioversion or external pacing during cardiopulmonary resuscitation or unstable arrhythmias. External electrical stimulation can damage the device and induce electrical currents. This is best minimised by using the lowest energy levels required and by placing the defibrillator pads as far away from the device as possible.

Declaration of interests

The authors declare that they have no conflicts of interest.

Biographies

Christopher Bull FRCA is a specialty registrar and advanced trainee in pain medicine at Leeds Teaching Hospitals NHS Trust.

Ganesan Baranidharan FRCA FFPMRCA is a consultant in anaesthesia and pain medicine at Leeds Teaching Hospitals NHS Trust. He is an honorary clinical associate professor at the University of Leeds; is honorary secretary of the Neuromodulation Society of UK and Ireland and a member of the Interventional Pain Medicine Special Interest Group of the British Pain Society. He is also a board member of the UK Faculty of Pain Medicine.

Matrix codes: 1A01; 1A02; 1A03; 2A03; 2A08; 2B03; 3E00