Long-term safety and efficacy of intracortical microstimulation in humans

Intracortical microstimulation (ICMS) of somatosensory cortex can restore a sense of touch to people with spinal cord injury. In this early feasibility clinical trial (NCT01894802), we evaluated the safety, efficacy, and longevity of ICMS because there is a paucity of such long-term studies in humans. This information is crucial to the development of clinical neuromodulation devices, particularly for restoring touch, hearing, and vision. ICMS was delivered to five participants with spinal cord injury who were each implanted with two Blackrock NeuroPort microelectrode arrays in the hand representation of Brodmann's area 1. Across implant durations spanning 2 to 10 years, we measured single-electrode detection thresholds, projected fields, quality reports, and electrode health characteristics. ICMS-related adverse events were documented throughout. More than 168 million ICMS pulses were delivered across a combined implant duration of 27 years without any serious adverse events or direct negative effects on electrode health. ICMS consistently evoked sensations localized to the hand. Rarely, sensations persisted for brief periods after stimulation offset (3 to 25 events across participants). ICMS detection thresholds increased slowly over time (about 3.5 microamperes per year), but 64 ± 13% of the electrodes still reliably evoked tactile sensations (about 21% decrease in functional electrodes), including 60% of the electrodes after 10 years in one participant. The quality and projected field coverage of ICMS-evoked sensations were both consistent. Delivering ICMS to the somatosensory cortex was safe and effective, consistently evoking informative somatosensory percepts as long as 10 years, demonstrating the clinical promise of ICMS for sensory restoration.