Challenges and opportunities in restoring function after paralysis.

Neurotechnology has made major advances in development of interfaces to the nervous system that restore function in paralytic disorders. These advances enable both restoration of voluntary function and activation of paralyzed muscles to reanimate movement. The technologies used in each case are different, with external surface stimulation or percutaneous stimulation generally used for restoration of voluntary function, and implanted stimulators generally used for neuroprosthetic restoration. The opportunity to restore function through neuroplasticity has demonstrated significant advances in cases where there are retained neural circuits after the injury, such as spinal cord injury and stroke. In cases where there is a complete loss of voluntary neural control, neural prostheses have demonstrated the capacity to restore movement, control of the bladder and bowel, and respiration and cough. The focus of most clinical studies has been primarily toward activation of paralyzed nerves, but advances in inhibition of neural activity provide additional means of addressing the paralytic complications of pain and spasticity, and these techniques are now reaching the clinic. Future clinical advances necessitate having a better understanding of the underlying mechanisms, and having more precise neural interfaces that will ultimately allow individual nerve fibers or groups of nerve fibers to be controlled with specificity and reliability. While electrical currents have been the primary means of interfacing to the nervous system to date, optical and magnetic techniques under development are beginning to reach the clinic, and provide great opportunity. Ultimately, techniques that combine approaches are likely to be the most effective means for restoring function, for example combining regeneration and neural plasticity to maximize voluntary activity, combined with neural prostheses to augment the voluntary activity to functional levels of performance. It is a substantial challenge to bring any of these techniques through clinical trials, but as each of the individual techniques is sufficiently developed to reach the clinic, these present great opportunities for enabling patients with paralytic disorders to achieve substantial independence and restore their quality of life.