New Neurostimulation Strategy and Corresponding Implantable Device to Enhance Bladder Functions

Abstract

Spinal cord injury (SCI) is one of the most complex and devastating medical conditions. Its worldwide incidence ranges from 11 to 112 per 100,000 Population (Blumer & Quine, 1995; DeVivo, 1997). SCI leads to different degrees of dysfunction of the lower urinary tract due to a large variety of possible lesions. Immediately after SCI, flaccid paralysis sets in, followed by the absence of reflexes and a complete loss of sensory and motor control below the level of lesion, rendering the urinary bladder areflexic and atonic. This period, termed spinal shock, can extend from a few days to several months (Chai & Steers, 1996). Most patients with suprasacral SCI suffer from detrusor over-activity (DO) and detrusor sphincter dyssynergia (DSD) (Blaivas et al., 1981). DSD leads to high intravesical pressure, high residual urine, urinary tract infection, and deterioration of the upper urinary tract. In order to recover the voluntary control of micturition, functional electrical stimulation (FES) has been investigated at different sites of the urinary system: the bladder muscle (detrusor), the pelvic nerves, the spinal cord and the sacral nerve roots. Among these, sacral nerve root stimulation is considered the most efficient technique to induce micturition and has been prevalent in clinical practice over the last two decades (Elabaddy et al., 1994). Using cuffelectrodes, this technique offers the advantages of a safe and stable fixation of electrodes as well as confinement of the spread of stimulation current within the targeted nerves. However, the detrusor and the external urethral sphincter (EUS) muscles share the sacral nerves as common innervations pathways, and stimulation of the entire sacral root induces contraction of both. Thus, the efficiency of micturition by means of sacral neurostimulation depends on the capability to contract the detrusor without triggering EUS contraction. In order to improve this neurostimulation selectivity, several techniques have been proposed, among which are rhizotomy, and EUS blockade using high-frequency stimulation. Dorsal rhizotomy consists of selectively severing afferent sacral nerve roots that are involved in pathological reflex arc in suprasacral SCI patients. Rhizotomy abolishes DO, reduces DSD, and prevents autonomic dysreflexia. As a beneficial result, the uninhibited bladder contractions are reduced, the bladder capacity and compliance are increased, urine flow is improved, and consequently the upper urinary tract is protected from ureteral reflux and hydronephrosis. In case of a complete SCI, dorsal rhizotomy is combined with an