Evoked Compound Action Potentials Reveal Spinal Cord Dorsal Column Neuroanatomy

Area of Science:

• Neuroscience
• Biomedical Engineering

Background:

• The electrically evoked compound action potential (ECAP) measures neural response to electrical stimuli.
• Assessing ECAPs during spinal cord stimulation (SCS) clarifies the link between stimulation, electrophysiological response, and neuromodulation.
• This understanding is crucial for designing and programming SCS devices.

Purpose of the Study:

• To investigate the relationship between ECAP characteristics and fiber diameter/conduction velocity.
• To explore how ECAP amplitude and dispersion change with propagation distance.
• To identify how these changes inform SCS lead placement.

Main Methods:

• ECAPs were recorded from epidural SCS leads in sheep after a stimulating pulse.
• ECAP amplitude, dispersion, and conduction velocity were measured after filtering and signal processing.
• A single-neuron computational model simulated large-diameter sensory axons to explain observations.

Main Results:

• ECAPs exhibit a triphasic structure (P1, N1, P2 peaks) in both animal and human subjects.
• Conduction velocity in sheep (109 m/s) suggests involvement of fibers up to 20 μm in diameter.
• ECAP amplitude decreased and dispersion increased with propagation distance, with abrupt shifts at specific spinal cord levels.

Conclusions:

• Increased ECAP dispersion with distance is attributed to contributions from slow-conducting, small-diameter fibers.
• Discontinuities in dispersion changes indicate the termination points of these smaller fibers.
• Understanding these ECAP propagation dynamics can optimize SCS lead placement for improved clinical efficacy and reduced side effects.