Epidural Stimulation vs. FES: Understanding Electrical Stimulation in Rehabilitation

There are many different types of electrical stimulation, and two commonly used in rehabilitation after neurological injury or disease are functional electrical stimulation (FES) and epidural stimulation. This article will explore these techniques, their applications, benefits, and key differences.

What is Epidural Stimulation? 

Epidural stimulation is a form of spinal cord stimulation that involves implanting electrodes directly onto the spinal cord. Originally used for pain management, it has recently been applied to facilitate functional movements in individuals with upper motor neuron lesions, making it an emerging form of FES.

Another form of spinal cord stimulation is transcutaneous spinal cord stimulation (TSCS), which also targets the spinal cord but non-invasively. We will explore the similarities and differences between TSCS and FES in a future article.

How Epidural Stimulation Works

Electrodes are surgically implanted in the epidural space between the spinal cord and vertebrae. These electrodes connect to a battery-powered stimulator implanted in the trunk or flank. The device sends low levels of electricity directly into the spinal cord to relieve pain or assist with movement. 

Epidural stimulation assists movements by making lower motor neurons more sensitive to activation from undamaged upper motor neurons, making it easier for people to move despite upper motor neuron lesions. Epidural stimulation can also be used to directly cause muscle contractions if the stimulation intensity is increased beyond the motor activation threshold.

Epidural Stimulation for Spinal Cord Injury

Epidural stimulation is a promising technique being researched and developed for treating spinal cord injury (SCI). Here is a concise overview:

• Purpose: The stimulation aims to help activate neural circuits below the level of injury that control movement and other functions.
• Results: Studies have shown improvements in voluntary movement, cardiovascular function, and bladder control in some patients with SCI.
• Current status: While promising, epidural stimulation for applications beyond pain management is still largely experimental and not yet widely available as a standard treatment.
• Limitations: Effects vary between individuals, and it cannot fully restore all functions lost due to spinal cord injury.
• Ongoing research: Researchers are working to optimize the implanted electronics and stimulation patterns and to combine this approach with other therapies for better outcomes.

Chalif et al. recently published a systematic review of the research on epidural stimulation for spinal cord injury in humans in the Journal of Clinical Medicine. The authors wrote a compelling summary of their study that is worth quoting:

“. . . The objective was to analyze how [epidural stimulation] contributes to the recovery of neurological functions in SCI patients. . . . Results: A total of 64 studies encompassing 306 patients were identified. Studies investigated various stimulation devices, parameters, and rehabilitation methods. Results indicated significant improvements in motor function: 44% of patients achieved assisted or independent stepping or standing; 87% showed enhanced muscle activity; 65% experienced faster walking speeds; and 80% improved in overground walking.

Additionally, [epidural stimulation] led to better autonomic function, evidenced by improvements in bladder and sexual functions, airway pressures, and bowel movements. Notable adverse effects included device migration, infections, and post-implant autonomic dysreflexia, although these were infrequent. . . . Conclusion: Epidural spinal cord stimulation is emerging as an effective and generally safe treatment for chronic SCI, particularly when combined with intensive physical rehabilitation. Future research on standardized stimulation parameters and well-defined therapy regimens will optimize benefits for specific patient populations.”

Challenges and Considerations

While promising, epidural stimulation faces some challenges:

• It is an invasive procedure requiring surgery.
• Potential complications include lead migration, breakage, infection, hardware malfunction, and unwanted stimulation.
• The FDA has not yet approved any epidural stimulation systems for restoring motor function (as of July 2024).

What is FES? 

Functional Electrical Stimulation (FES) is a technique that uses electrical pulses to activate nerves and muscles, helping individuals with paralysis or muscle weakness regain functional movement. To understand FES, we first need to understand its necessity.

Our brain controls body movement by sending electrical signals through nerves to muscles. When the brain or nerves are damaged, these signals cannot reach the muscles, resulting in paralysis (complete loss of muscle control), paresis (partial paralysis), or muscle weakness. Additionally, uncontrolled muscle contractions can occur, known as muscle spasms or spasticity.

How FES Works

FES replaces lost electrical signals with artificial stimulation. A stimulator device converts electricity from a power source into electrical pulses, which are then sent through cables and electrodes into the body. These pulses bypass the damaged nerves and activate undamaged nerves, causing muscles to contract. FES can be invasive or non-invasive and is typically used to stimulate peripheral nerves to facilitate or cause muscle contractions.

FES systems like the MyoCycle Home are non-invasive and send electrical pulses through adhesive electrodes stuck on the skin over target muscle groups.

Applications of FES

FES is used to help individuals with various neurological disorders, including:

• Spinal cord injury (SCI)
• Multiple sclerosis (MS)
• Stroke
• Cerebral palsy (CP)
• Muscle weakness after surgery (e.g., knee replacement)

Benefits of FES:

• Muscle re-education
• Relaxation of muscle spasms
• Prevention of muscle atrophy
• Increased local blood circulation
• Maintained or increased range of motion

Some studies suggest FES may also increase bone density, improve voluntary motor function, and enhance cardiovascular health.

Comparing FES and Epidural Stimulation

1. Invasiveness:
   • FES can be non-invasive (applied through the skin) or invasive (implanted)
   • Epidural stimulation is always invasive, requiring surgical implantation
2. Application:
   • FES is typically applied to peripheral nerves or muscles
   • Epidural stimulation targets the spinal cord directly
3. Functionality:
   • FES is used for specific functional tasks (e.g., walking, cycling)
   • Epidural stimulation aims to facilitate overall motor control and pain management
4. Approval status:
   • Many FES devices are FDA-approved for various applications
   • Epidural stimulation for motor function restoration is still largely experimental

Both FES and epidural stimulation offer promising avenues for rehabilitation in individuals with neurological injuries or diseases. While FES is more established and offers non-invasive options, epidural stimulation shows potential for broader motor function restoration. As research continues, these techniques may provide increasingly effective solutions for improving quality of life in affected individuals. Note: For the most up-to-date information on clinical research and FDA approvals, please consult recent medical literature and official regulatory sources.