Novel Concepts on the Functional Neuroanatomy of the Anorectum: Implications for Anorectal Neuropathy and Neuromodulation Therapy

Anorectal neuropathy causes anorectal dysfunction, yet it is poorly recognized. This stems from both a lack of understanding of the extrinsic and intrinsic innervation of the anorectum and tools for evaluation of neuronal function. Our objective was to provide an improved understanding of the neuronal networks of the anorectum and discuss its functional significance. We performed a comprehensive and up-to-date review of the published literature on anorectal neuroanatomy to generate our findings. Anorectal nerve innervation appears to be much more complex than hitherto known with an extensive overlap, intercommunications, and variations. The innervations arise from 5 sources as evidenced by immunohistochemical markers: Sympathetic innervation arises from the superior hypogastric plexus, hypogastric plexus, and splanchnic nerves (T1-L2); the parasympathetic from pelvic splanchnic nerves (S1-S4); the mixed autonomic from inferior hypogastric plexus; the somatic from pudendal nerves; and the intercommunicating nerves. Furthermore, they are fine, closely packed nerves susceptible to damage from obstetric or spinal cord injury, or pelvic surgery that may not manifest with bowel problems immediately but later in life. This illustrated review provides a new understanding of the afferent and efferent pathways between the rectum, spinal cord, and brain, and a framework for clinical implications of anorectal neuropathy, such as anal sphincter or rectal sensory or rectal accommodation dysfunction, causing bowel problems. Insights into the functional neuroanatomy provide an improved mechanistic understanding of anorectal symptoms and could facilitate the development of neurophysiological tests such as translumbosacral anorectal magnetic stimulation and neuromodulation treatments such as sacral neuromodulation and translumbosacral neuromodulation treatment.