Case of the Month #32: Spinal Cord Stimulation (SCS) for Persistent Pain Following Spinal Surgery; presented by Owain Davies

The patient is deemed suitable for SCS and undergoes a trial implantation. Following a successful trial, the patient then proceeds to have a full implant and had a significant improvement in their leg and back pain, enabling them to return to work. 

There are 2 types of spinal cord stimulator, electrodes that are implanted percutaneously, and paddle electrodes, which are inserted via an open unilateral laminotomy (creating a window on the inferior edge of the lamina to permit passage of the electrode). 

Percutaneous implantation is typically done under local anaesthetic, which allows on-table testing to ensure the area stimulated overlaps with the area in which the patient experiences pain. Paddle electrode implantation requires a general anaesthetic but are less prone to migration than percutaneous leads. Occasionally percutaneous implantations are done under general anaesthesia and follow anatomical landmarks and spinal levels rather than on table testing.

Typically, a trial  is done before a full system implantation. During a trial, the electrodes are inserted extradurally into the spinal canal, then tunnelled out through the skin and connected to an external battery. The patient is then discharged and has a trial of stimulation via the external battery for about a week. During this time, the patient is asked to continue with their normal activities and keep a diary of their pain. They may also be given a range of stimulation settings to use. If there is a 50% or greater improvement in the patient’s pain, the electrodes can be connected to an implantable pulse generator (IPG), which is placed subcutaneously below the belt line during a second stage operation. If the patient has no benefit from the trial of stimulation, the electrodes can be removed. 

A UK based RCT (TRIAL-STIM 2018) looked at patients randomised to either trial implant or direct to full implant to establish if a trial implant was required if patients were carefully selected. The 3-year outcomes have recently been published (TRIAL-STIM 2023) which showed no differences between the direct to full implant and trial groups for pain improvement and all secondary outcomes including those who respond to treatment. This suggests that trials may be less commonly used in the future. 

Patients are followed up by the neuromodulation team, with the input from industry representatives to fine tune stimulation settings. There are several medical device companies who market SCS systems, with minor differences in the features offered.

• How does SCS work?

SCS  was developed following the establishment of the ‘gate control theory’ of pain. Electrodes are implanted into the dorsal epidural space over the spinal cord and apply electrical stimulation which is transmitted to the dorsal spinal cord. This is thought to mask pain transmission due to stimulation of Aβ fibres which stops nociceptive C fibres ‘opening the gate’ to projection neurones. Release of GABA from inhibitory interneurons due to Aβ fibre activation, reduces the activation of hyper-excitable wide dynamic range neurones halting the central perception of pain. However, this is an over-simplification of the mode of action, which is not fully understood, and likely also involves altering endogenous inhibitory pathways and central processing of pain. Local changes in neurotransmitter concentrations, such as a reduction in glutamate and substance P release from C-fibres are likely to contribute to the mechanism of action of SCS.  SCS has been shown to alter the activation of supra-spinal areas involved in the processing of pain that inhibit pain transmission via descending serotonergic and noradrenergic projections. 

• What are the different stimulation waveforms?

There are two main waveform types. First, low frequency stimulation (40-100Hz), which creates a paraesthesia sensation over the area of stimulation which is mapped to the distribution of pain. High frequency stimulation (10kHz) and burst stimulation are newer waveforms that don’t create paraesthesia while still delivering pain relief. The stimulation can be tailored via altering the amplitude, pulse width, frequency and which contacts stimulation is applied through. 

• Complications of SCS

SCS implantation has an overall complication rate of approximately 20%. The most common complications of SCS implantation are equipment related, including lead migration, fracture, stimulation side effects and IPG malfunction. Technical complications such as dural puncture, epidural haematoma and neurological deficit are rare (<1%). Infection has been reported in up to 10% of cases, which usually requires removal of the whole system until the infection has been fully treated.