User:Simmon~enwiki/Transcutaneous lumbar spinal cord stimulation

The major clinical application of spinal cord stimulation is the control of neurological pain and of impaired motor functions. Until recently, spinal cord stimulation in humans was only possible with epidural electrodes implanted close to the back side of the spinal cord. With the development of transcutaneous spinal cord stimulation, a non-invasive technique using skin electrodes placed over the lower back and abdomen became available.

Transcutaneous spinal cord stimulation activates large-diameter afferents of posterior roots of several lumbar and upper sacral spinal cord segments. Single pulses evoke brief contractions of effectively all lower limb muscles bilaterally – so-called posterior root-muscle reflexes (PRM reflexes). Physiologically, PRM reflexes are closely related to the H reflex. The method hence allows for new designs of human neurophysiological studies, both in people with and without neurological conditions.

The consistent stimulation of sensory nerve fibers of multiple posterior roots further allows for the application of transcutaneous lumbar spinal cord stimulation as a neuromodulation technique. The potential of its therapeutic relevance in spasticity control and enhancement of residual motor control after spinal cord injury is currently being investigated, and first results are very promising.

Background
Spinal cord stimulation is currently used to control neurological pain and impaired motor functions. In these clinical applications, epidural spinal cord stimulation systems are being used with the electrode or ‘lead’ being surgically placed in the epidural space (the outermost part of the vertebral canal outside of the meninges covering the spinal cord). The electrode is used to apply electrical stimulation to the posterior aspect of the spinal cord from a close distance of few millimeters. Epidural spinal cord stimulation works through the principle of neuromodulation, i.e. the modulation of neural signal transfer properties and the augmentation of neural circuits’ activity of the spinal cord.

Specifically, Pinter and colleagues (2000) demonstrated that epidural SCS is highly effective to control lower limb spasticity of patients with traumatic spinal cord injury. There was a remarkable antispastic effect across thigh and leg muscles when stimulating the lumbar posterior roots with frequencies of 50 Hz -100 Hz. In patients with motor complete (functionally complete or discomplete) spinal cord injury in supine position, continuous constant epidural SCS of the same site of the spinal cord (with higher intensity and 25 Hz -50 Hz) could generate automatic, rhythmic stepping-like activity in the paralyzed lower limbs (Dimitrijevic et al., 1998; Gerasimenko et al., 2002; Jilge et al., 2004; Minassian et al., 2004; 2007a). Figure 1 shows an example of such rhythmic electromyographic activities recorded from the lower limb muscles of a patient with long-standing (chronic) complete spinal cord injury. Early work has begun to investigate the clinical relevance of epidural spinal cord stimulation to enhance locomotor activity (Herman et al., 2002; Minassian et al., 2005; 2007a; Huang et al., 2006; Harkema et al., 2011) and other functional movements (Harkema et al., 2011).

The method
Transcutaneous lumbar spinal cord stimulation uses commercially available, self-adhesive skin electrodes placed over the lower back and abdomen (Figure 2; Minassian et al., 2007b; Hofstoetter et al., 2008).

The stimulation activates sensory fibers within the lumbar and upper sacral posterior roots. The specific, localized depolarizations of posterior root fibers in spite of the distant stimulation are made feasible by the tissue heterogeneity of the volume conductor in-between the electrodes and by the neuroanatomy of the terminal spinal cord (Ladenbauer et al., 2010, Danner et al., 2011; Szava et al., 2011).

The direct, electrical stimulation of large-diameter afferents is reflected by the elicitation of so-called posterior root-muscle reflexes (PRM reflexes) in many lower limb muscles, when single pulses are applied (Figure 3). Neural structures within the spinal cord are not directly electrically stimulated, but transsynaptically activated by the posterior root-stimulation.

The reliable stimulation of sensory nerve fibers of multiple posterior roots is essential for the application of transcutaneous spinal cord stimulation in human electrophysiological studies (delivering single or few pulses to elicit PRM reflexes) and as a neuromodulation technique (with continuous stimulation).

There is an increasing number of human neurophysiological studies applying the non-invasive method of spinal cord stimulation to elicit short-latency reflexes in the lower limb muscles of individuals with intact nervous system or spinal cord injury (Courtine et al., 2007; Hofstoetter et al., 2008; Kitano & Koceja 2009; Dy et al., 2010).

The potential of transcutaneous lumbar spinal cord stimulation in controlling spinal spasticity and augmenting mobility in spinal cord injured persons (Minassian et al., 2010) is currently being investigated. Figure 4 shows an example how transcutaneous stimulation of the lumbar spinal cord can modify active treadmill-stepping of an incomplete spinal cord injured person.

Transcutaneous lumbar spinal cord stimulation can become a promising new evaluation and intervention approach meeting the principles of restorative neurology, that is for the assessment of mechanisms responsible for neurological deficits and for the improvement of impaired nervous system function through modification of altered neural control.