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. 1999 Jul;45(1):58–63. doi: 10.1136/gut.45.1.58

Laterality effects of human pudendal nerve stimulation on corticoanal pathways: evidence for functional asymmetry

S Hamdy 1, P Enck 1, Q Aziz 1, S Uengoergil 1, A Hobson 1, D Thompson 1
PMCID: PMC1727586  PMID: 10369705

Abstract

BACKGROUND—Although motor and sensory pathways to the human external anal sphincter are bilateral, a unilateral pudendal neuropathy may still disrupt anal continence. Anal continence can, however, be preserved despite unilateral pudendal damage, and so to explain those differing observations, we postulated that pudendal innervation might be asymmetric.
AIMS—To explore the individual effects of right and left pudendal nerve stimulation on the corticofugal pathways to the human external anal sphincter and thus assess evidence for functional asymmetric pelvic innervation.
METHODS—In eight healthy subjects, anal sphincter electromyographic responses, evoked to transcranial magnetic stimulation of the motor cortex, were recorded 5-500 msec after digital transrectal electrical conditioning stimuli applied to each pudendal nerve.
RESULTS—Right or left pudendal nerve stimulation evoked anal responses of similar latencies but asymmetric amplitudes in six subjects: dominant responses (>50% contralateral side) from the right pudendal in four subjects and from the left in two. Cortical stimulation also evoked anal responses with amplitude 448 (121) µV and latency 20.9 (1.1) msec. When cortical stimulation was preceded by pudendal nerve stimulation, the cortical responses were facilitated at interstimulus intervals of 5-20 msec. Dominant pudendal nerve stimulation induced greater facilitation of the cortically evoked responses than the non-dominant nerve.
CONCLUSIONS—Cortical pathways to the external anal sphincter are facilitated by pudendal nerve conditioning, in an asymmetric manner. This functional asymmetry may explain the presence and absence of anal incontinence after unilateral pudendal nerve injury.


Keywords: cerebral cortex; continence; electromyography; external anal sphincter; incontinence; magnetic stimulation

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Figure 1  .

Figure 1  

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External anal sphincter EMG responses are shown following right (dominant) and left (non-dominant) pudendal nerve stimulation in one individual. Three traces are superimposed to show reproducibility. Stimulus at 0 ms; arrows indicate the onset of the EMG response. The response amplitude from the dominant pudendal nerve is larger than from the non-dominant nerve, despite similar latencies.

Figure 2  .

Figure 2  

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Cortically evoked external anal sphincter EMG responses from one individual following cortical stimulation alone and following dominant and non-dominant pudendal nerve stimulation at different ISIs. Three traces are superimposed to show reproducibility. Cortical stimulus at 0 msec; arrows indicate the onset of the EMG response. Following dominant pudendal nerve stimulation, the cortically evoked responses are facilitated to a much greater degree than with non-dominant pudendal stimulation.

Figure 3  .

Figure 3  

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Effects of: (A) dominant (closed circle) and non-dominant pudendal nerve (open circle) stimulation (n=6); and (B) symmetric right (closed square) and left (open square) pudendal nerve stimulation (n=2) on the amplitudes and latencies of the cortically evoked anal responses at increasing ISIs. The vertical axes show the percentage of the response to cortical stimulation alone; the horizontal broken line indicates 100%. Dominant pudendal nerve stimulation produces relatively greater facilitation of the cortically evoked responses than non-dominant pudendal nerve stimulation (p<0.001); in contrast, prior symmetrical pudendal nerve stimulation has equal effects from either side.

Selected References

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