Neuromodulation & Spinal Cord Stimulation

SCS Mechanism of Action

Spinal cord stimulation works primarily through activation of the dorsal columns of the spinal cord. The foundational concept derives from gate control theory (Melzack and Wall, 1965), which proposes that stimulation of large-diameter myelinated A-beta fibers in the dorsal columns can inhibit pain signal transmission carried by smaller C-fibers and A-delta fibers. In practice, an electrical field delivered to the dorsal epidural space recruits these large fibers, effectively "closing the gate" on nociceptive input ascending to higher cortical centers.

Newer waveform paradigms (high-frequency, burst, and differential target multiplexed stimulation) may also engage supraspinal mechanisms, descending inhibitory pathways, and glial cell modulation, though the precise mechanisms remain areas of active investigation.

Device Components

Leads

Two main lead types exist for SCS:

• Percutaneous (cylindrical) leads: Placed through a Tuohy needle into the epidural space under fluoroscopic guidance. These are the standard for trial procedures and can also serve as permanent leads. They typically have 8 or 16 electrode contacts arranged in a linear array.
• Paddle (surgical) leads: Placed via a small laminotomy or laminectomy performed by a surgeon. Paddle leads have a flat profile that sits against the dorsal dura, offering broader electrode arrays (up to 32 contacts in some configurations), more stable positioning, and potentially lower energy requirements due to unidirectional current delivery.

Implantable Pulse Generator (IPG)

The IPG is the battery and processing unit of the system. Modern IPGs are either rechargeable (lasting 10-25 years with regular charging) or primary cell (non-rechargeable, typically lasting 3-5 years depending on stimulation parameters). The IPG connects to the leads via extensions tunneled subcutaneously. Pocket locations include the upper buttock, flank, or lower abdomen.

Extensions

Extension wires connect the leads to the IPG. They are tunneled subcutaneously from the spinal entry point to the IPG pocket. Extension-lead connection sites can be a source of failure if not properly secured.

The SCS Trial

A trial period is considered the standard of care before proceeding to permanent implant. The purpose is to confirm that stimulation provides meaningful benefit in a real-world setting.

Trial Procedure

1. Lead placement: Under fluoroscopy, one or two percutaneous leads are advanced into the posterior epidural space, typically through a paramedian approach at one to two levels below the target dermatome
2. Awake testing: The patient provides feedback during intraoperative programming to confirm that stimulation coverage overlaps the painful area (for paresthesia-based paradigms)
3. Externalization: Lead extensions are tunneled to exit percutaneously and connected to an external pulse generator
4. Trial duration: Typically 3-7 days, though some centers extend to 10-14 days
5. Patient activity: Patients are encouraged to perform their usual daily activities during the trial to assess real-world benefit

Trial Success Criteria

A trial is considered successful when the patient achieves at least 50% pain reduction AND demonstrates meaningful functional improvement (increased activity, decreased medication use, improved sleep). Both components matter — pain relief alone without functional gain may not justify permanent implantation.

Permanent Implant

If the trial is successful, permanent implantation is typically performed 2-4 weeks later.

Surgical Steps

1. Lead placement: Either new percutaneous leads are placed or the trial leads are replaced. If paddle leads are chosen, a small laminotomy is performed for placement.
2. Anchoring: Leads are secured to the supraspinous ligament or deep fascia using strain-relief anchors to minimize migration
3. IPG pocket creation: A subcutaneous pocket is fashioned in the upper buttock or flank. The pocket should be large enough for the IPG to sit without tension but small enough to prevent flipping (Twiddler syndrome)
4. Tunneling: Extensions are tunneled subcutaneously from the lead exit site to the IPG pocket
5. Connection and closure: All connections are secured, impedances checked, and incisions closed in layers

Recovery typically involves 2-4 weeks of activity restriction (avoiding bending, twisting, and lifting greater than 5-10 pounds) to allow lead scarring and stabilization.

Dorsal Root Ganglion (DRG) Stimulation

DRG stimulation represents a targeted neuromodulation approach that places electrodes directly at the dorsal root ganglion within the neural foramen.

Key Features

• Lead placement: Electrodes are advanced through the epidural space and positioned in the lateral recess or foramen adjacent to the DRG at the target level
• Focal coverage: DRG stimulation provides highly specific dermatomal coverage, making it particularly effective for localized pain distributions (e.g., foot, groin, knee)
• Positional stability: Because the DRG is a fixed anatomic structure, stimulation is less affected by body position changes compared to traditional dorsal column SCS
• Awake placement: The American Society of Pain and Neuroscience (ASPN) recommends awake or minimal sedation placement to optimize electrode positioning through intraoperative feedback

Peripheral Nerve Stimulation (PNS)

PNS has experienced a resurgence with modern percutaneous systems designed for temporary implantation.

Sprint PNS System

• Concept: A 60-day temporary percutaneous stimulation system where fine-wire leads are placed under ultrasound or fluoroscopic guidance at the target peripheral nerve
• Mechanism: Delivers stimulation for a defined treatment window, after which the leads are removed. The therapeutic effect often persists well beyond the stimulation period, suggesting neuroplastic changes in central pain processing
• Target sites: Can be placed at virtually any accessible peripheral nerve, including sites near mobile joints (shoulder, knee, hip)
• Removal: Leads are designed for easy office-based removal after the 60-day treatment window