Translumbar and Transsacral Motor Evoked Potentials in Patients with Rectal Hyposensitivity

Abstract

Purpose: Rectal hyposensitivity (RH) may contribute to the pathogenesis of fecal incontinence and constipation, but the underlying neurobiologic mechanisms, and more specifically the integrity of peripheral lumbospinal nerves in RH is unknown. Aim: To investigate anal and rectal motor evoked potentials after translumbar and transsacral magnetic stimulation and sensory thresholds after electrical stimulation in subjects with RH and healthy controls. Methods: Subjects with RH defined as abnormally prolonged sensory thresholds for first sensation, desire or urgency to defecate (during rectal balloon distention) were tested by magnetic stimulations, performed with a Cadwell Focalpoint Coil™ (9-cm) placed over the right and left of midline at L3-L4 and S1-S3 levels using 80–100% intensity. A novel anorectal probe containing 2 pairs of bipolar steel ring electrodes (Koningsberg, CA) was used to record motor evoked potentials (MEPs); namely lumbo-anal (TL-aMEP), lumbo-rectal (TL-rMEP), the sacro-anal (TS-aMEP), sacro-rectal (TS-rMEP). Pudendal nerve latency test and rectal sensory thresholds after electrical stimulation were also performed. Student t-test and Spearman's correlation analyses were performed. Results: Thirty-eight subjects with RH (M/F 8/30, mean age 40.2 ± 13.7 yrs) were enrolled. Presenting symptoms were constipation (N = 29) or fecal incontinence (N = 9). Sixteen patients had history of spinal cord injury. Fourteen healthy subjects (M/F 5/9, mean age 42.9 ± 10.0 yrs) served as controls. The bilateral lumbo-anal (TL-aMEP), lumbo-rectal (TL-rMEP), sacro-anal (TS-aMEP), and sacro-rectal (TS-rMEP) motor evoked potentials in subjects with RH were significantly prolonged compared to controls (Table (mean ± 95%CI). After electrical stimulation, the rectal sensory thresholds for first sensation in mA (31 ± 4.37, 21 ± 2.79 (P= 0.026)) and for pain (72 ± 6.22, 46 ± 4.96, P= 0.013) were significantly prolonged compared to healthy controls. PNTML was prolonged bilaterally (P < 0.01) in RH subjects compared to controls. However, correlation between PNTML and MEPs were inconsistent (r ranged from −0.1–0.7).TableConclusion: Subjects with RH demonstrate prolonged motor evoked potentials of the peripheral lumbo-sacral spinal nerves bilaterally indicating that abnormal sensory perception is associated with significant neuropathy of these nerves. Thus, these patients may have both afferent and efferent neurovisceral dysfunction. The inconsistent correlation between PNTML and MEP's suggest that different neurologic pathways may be involved in the pathogenesis of RH. Translumbar and transsacral MEPs provide an objective tool for quantifying the presence of and severity of RH in patients with anal and rectal neuropathy.