Epidural stimulation of the spinal cord in spinal cord injury: current status and future challenges

A spinal cord injury can occur at any point along its length and can have a wide range of severities and varying effects on sensory and motor function, depending largely on these two factors. While considerable function can be recovered if the injury is ‘incomplete’ – that is, there remain some functional connections between the brain and the spinal cord segments below the lesion – there has been little success in improving function after a motor and/or sensory complete lesion. Recently reported functional gains in an individual with complete motor paralysis was possible because most of the cellular structure below the lesion remains intact 1. It is this network of neurons that can actually control very complicated movements when this spinal network (not the brain itself) receives the appropriate sensory input, and when the excitability of this network is enhanced with a very specific pattern of modest levels of stimulation with epidurally placed electrodes. We refer to this intervention as electro-enabling motor control. Based on extensive animal experimentation of this spinal network, the possibility of regaining locomotor function after complete paralysis in human subjects becomes an important issue. To begin to examine this issue, we performed similar experiments in a single subject with complete motor paralysis. The issues are: is the human spinal cord circuitry without any input from the brain as smart as that of the rat or cat, can it be neuromodulated to stand and to step using epidural stimulation as demonstrated successfully in the rat and cat, and can proprioception serve as a source of control of the spinal circuitry? We made a series of observations demonstrating the feasibility of using epidural stimulation to facilitate the recovery of a series of consciously controlled motor functions, as well as other functions, some of which are largely considered to be autonomically controlled. After implanting an electrode array, recovery of several functions occurred following repeated sessions of epidural stimulation and training to stand and to step, and to exert voluntary control of the lower limbs. The subject is a 23-year-old man with paraplegia from a C7–T1 subluxation as a result of a motor vehicle accident who has had a complete loss of clinically detectable voluntary motor function and partial preservation of sensation below the T1 cord segment. After 170 locomotor training sessions over 26 months, a 16-electrode array was surgically placed on the dura (L1–S1 cord segments). The results from this single subject provide a wakeup call for a change in the perception of the potential for recovery of function using activity-dependent interventions 2. They also highlight the potential advantage of quantitative assessment of multiple parameters in fewer individuals in lieu of the commonly accepted ‘primary outcome’ measure, often with limited objectivity, in many subjects.