Effects of Pudendal and Cortical Paired Associative Stimulation on Plasticity of Sphincter Responses after Incomplete Spinal Cord Injury: A Feasibility Study

Abstract

Objectives: To assess whether paired associative stimulation of the dorsal penile nerve and the motor cortex produces change in anal sphincter muscle responses in incomplete spinal cord injury (iSCI). Methods:This was a prospective experimental study. Eighteen male iSCI subjects with neuropathic bladder history were recruited. Incontinence was assessed using the International Consultation on Incontinence Modular Questionnaire (ICIQ). Electromyographic activity of the external anal sphincter was recorded. Pudendo-anal reflexes (PAR) were elicited by electrical stimulation of the dorsal penile nerve (DPN) and anal sphincter motor evoked potentials (MEP) by transcranial magnetic stimulation (TMS). Paired associative stimulation (PAS) (DPN and TMS, interval 40ms) was applied for 8 min at 0.25Hz using either real or sham TMS of the motor cortex. Pudendal somatosensory evoked potentials (pSSEPs) were recorded.Results: A PAR could be recorded in all subjects and an MEP in 12 subjects. The PAR was facilitated by prior (30ms) conditioning TMS. Group mean amplitudes of the PAR, the conditioned PAR and MEP, and ICIQ scores showed no change after real or sham PAS. However, 13 subjects individually showed significant changes (increases or decreases) in one or more sphincter responses to real or sham PAS. Individual responses were not correlated with the presence or latency of pSSEPs. Conclusions:Paired pudendal nerve and cortical stimulation altered the excitability of cortico-spinal and reflex circuitry controlling the anal sphincter in iSCI individuals. Future work should investigate whether such changes in individuals might lead to altered neural circuitry accompanied by functional restoration of continence.