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Intraoperative neurophysiological monitoring of the sacral nervous system
6.1
Introduction
The functions involving the genitourinary and the anorectal systems are uniquely controlled by the complex interaction of the autonomic and the somatic nervous systems. Insofar as it is the sacral parasympathetic and somatic systems that comprise the most important peripheral nervous structures controlling these functions, they may also be referred to as sacral functions . The functions themselves (micturition, defecation, erection, etc.) are now better understood because of the application of methods measuring the different functional parameters (urodynamics, faecodynamics, measurements of the sexual response), which provide for better diagnosis of dysfunction. The awareness that such dysfunction is also a consequence of damage to neural structures, also iatrogenic, has also greatly increased. On the other hand, it has become possible to better define the various lesions to the nervous system by electrophysiological methods. These methods by and large document, however only the somatic sacral nervous system and its central pathways . Nevertheless, such information is clinically relevant since (1) the somatic nervous system plays a part in all sacral functions and (2) the somatic and the parasympathetic sacral systems are closely related and information on the somatic system may therefore be a relevant indicator of the overall neurogenic lesion in most clinical situations apart from intraabdominal and pelvic surgery, where the sympathetic and the parasympathetic nerve fibers accompanied by visceral afferents are not close to any somatic nerves .
The common denominator of lower urinary tract (LUT), anorectal, and sexual functions is that a large part of their efferent and afferent pathways travel at least partly in close vicinity. They “share” common spinal cord regulatory segments (the upper lumbar segments—sympathetic efferents; the middle and lower sacral segments—parasympathetic and somatic efferents). Even the long pathways connecting the relevant spinal cord segments with higher levels of the central nervous system are situated close together. Thus it is not uncommon that in lesions to the nervous system, and particularly clearly in lesions affecting the spinal cord, cauda equina, the sacral nerve roots and plexus, and the pudendal nerve, the sacral functions are also affected. Dysfunctions may arise not only from disease or trauma, but also from inadvertent lesions to the above structures during invasive procedures, particularly several surgeries involving the pelvic organs and the spinal canal. Functional consequences of lesions to sacral functions’ neurocontrol include disturbing sensory phenomena such as pain, dysesthesia, urgency and frequency, loss of genital sensation, bladder or rectal fullness, and subsequent retention, obstipation, soiling, incontinence, and erectile dysfunction. Neurogenic damage may lead to lost coordination between detrusor and sphincter function, leading to high bladder pressures and upper urinary tract dysfunction. Incontinence may, particularly in motor disabled persons, lead to problems with hygiene, skin problems, infections, and decubiti. While neurogenic sexual dysfunction may not be life threatening, it can be extremely psychologically disruptive and lead to severe emotional and interpersonal problems. All these may be particularly tragic when they are a consequence of an inadvertent intraoperative lesion.
A whole array of clinical neurophysiological diagnostic methods has been modified for use in the anogenital area, including electromyographic (EMG) methods, reflex, conduction, and evoked potential studies . These methods are routinely employed in uroneurological/neurourological laboratories for diagnostics and follow-up in patients with (suspected) neurogenic sacral dysfunction. The authors have pioneered with trials to establish some of these neurophysiological methods in the operating room to help the surgeon identify particular sacral nervous structures and/or monitor the function of the somatic sacral neuromuscular system during surgery .
6.2
Functional anatomy
Functional anatomy of the genitourinary and anogenital systems is highly complex but needs not be considered in detail as only the gross anatomy of the relevant somatic nervous structures can be approached by clinical neurophysiological methods that are applicable in the operating room environment. Most of the information relevant for intraoperative neurophysiology are summarized as follows.
Afferent fibers from the mucosa and skin from the genitoperineal region travel mostly with the pudendal nerves. The distally most accessible group of sensory fibers are the dorsal nerves of the penis (or clitoris). The sensory fibers from the genital, perineal, and anal region enter through the dorsal spinal roots S2–S5 into the spinal cord and synapse (through interneurons) with sphincteric motor neurons. The afferent information also ascends (the primary sensory neurons synapsing to higher order sensory neurons at various levels) via the spinothalamic and dorsal column tracts, the lemniscal system, and thalamo-cortical tracts finally to the somatosensory cortex (at its interhemispheric location) .
The sphincteric lower motor neurons in the midventral spinal gray matter of the second to fourth sacral spinal cord segments (the “Onuf’s nucleus”) are under voluntary control from the motor cortex. Somatic motor nerve fibers leave through the ventral roots and the sacral plexus, combining the pudendal nerves; direct branches innervate the levator ani and the anal sphincter . The external urethral sphincter may be innervated by sacral somatic fibers traveling via splanchnic nerves or pudendal nerve or, possibly, both.
Further details on functional anatomy and physiology are given for the three different organ systems in the following sections.
6.2.1
Neural control of the lower urinary tract
The LUT is innervated by three sets of peripheral nerves. The pelvic parasympathetic nerves arise at the sacral level of the spinal cord (they excite the bladder and relax the urethra). The sympathetic nerves arise from the upper lumbar segments and inhibit the bladder body, modulate transmission in bladder parasympathetic ganglia, and excite the bladder base and urethra. The somatic efferents and afferents from the S2–S4 sacral roots innervate pelvic floor muscles (levator ani) both through direct branches and by the pudendal nerve, which also innervates the perineal muscles including the anal and urethral sphincter.
All these nerves contain both efferent and afferent nerve fibers that are controlled by centers in the brain and particularly important centers in the brainstem. Long tracts in the spinal cord subserve the spinobulbospinal reflex pathway relevant for coordinated detrusor-sphincter function and normal micturition. The periaqueductal gray and the dorsal pontine tegmentum are established as essential control centers for micturition. While different types of sensation of the LUT travel both in the anterolateral and the dorsal part of the spinal cord, the descending (motor) pathways lie within the lateral aspects of spinal cord.
6.2.2
Anorectum
Touch, pin-prick, heat, and cold stimuli can be perceived in the anal canal to a level of up to 15 mm above the anal valves. The epithelium in the area from about 10 to 15 mm above the valves has rich sensory nerve supply made up of both free and organized nerve endings. The sensory endings in the hairy perianal skin are similar to those in hairy skin elsewhere. The afferent nerve pathway for anal canal sensation is by the inferior hemorrhoidal branches of the pudendal nerve. Sensory pathways from the rectum and the bladder travel in the pelvic visceral nerves to the sacral cord, but some afferent information is probably also related to hypogastric nerves entering the spinal cord at the thoracolumbar level.
Functionally, the most important part of the smooth musculature of the anorectum is the internal anal sphincter which is responsible for about 85% of the resting pressure in the lumen of the canal. The smooth musculature of rectal walls (and of the detrusor!) receives extrinsic motor innervation from the sacral parasympathetic outflow arising in the intermediolateral cell columns of sacral cord segments S2–S4. These first-order neurons send axons that emerge with the ventral spinal nerve roots to synapse with second-order neurons lying within the pelvic plexus or the visceral walls. The sympathetic nerve supply arises from the thoracolumbar chain and travels in the hypogastric nerve to innervate visceral smooth muscle directly, and via a modulatory influence on parasympathetic function at the level of the pelvic plexus. The internal anal sphincter is probably controlled both by sympathetic (hypogastric) and sacral parasympathetic pathways, but the inhibition brought about by rectal distention (the important recto-anal inhibitory reflex) is predominantly an intramural one.
The external anal sphincter is innervated by the pudendal nerve and occasionally also by a perineal branch of S4. The neurons of the sphincter motor nucleus (Onuf’s nucleus) are under voluntary control via corticospinal pathways.
Normal defecation is triggered by filling of the rectum from the sigmoid colon and the signals from stretch receptors in the rectal wall and pelvic floor muscles are interpreted at the conscious level as a desire to defecate. The extension of the rectum causes reflex relaxation of the smooth internal sphincter muscle. Voluntary relaxation of the striated sphincter muscle permits defecation assisted by colonic pressure waves and abdominal straining. If defecation is to be deferred, brief conscious contraction of the voluntary sphincter allows time for recovery of internal sphincter tone, and relaxation of the rectum to accommodate filling. Conscious appreciation of the desire to defecate and intentional control over defecation is conferred by suprasacral neural influences.
The precise way in which the autonomic, pyramidal, extrapyramidal, and sensory pathways integrate to achieve a reliable and predictable anorectal function is not yet fully understood.
6.2.3
Sexual organs
Of the sexual functions affected by neurogenic lesions, research has centered on the male functions, and particularly on erection. Erection can be initiated in the brain and/or follow genital stimulation; in sexual activity, a combination of both is probably involved.
Neurogenic erectile dysfunction due to peripheral lesions can be secondary to the disruption of sensory nerves contributing to the afferent arm of reflex erection or to the disruption of autonomic nerves that mediate arterial dilatation and trabecular smooth muscle relaxation. Erectile dysfunction can occur due to disruption of the relevant pathways in centers within the spinal cord (both suprasacral and sacral), cauda equina, the sacral plexus, the pelvic plexus, the cavernosal nerves, and the pudendal nerves. Particular pelvic surgeries such as radical prostatectomy or cystoprostatectomy lead to a high percentage of mostly neurogenic erectile dysfunction—the lesion occurs in the pelvic plexus or the cavernosal nerves located in the posterolateral aspect of the prostate.
Ejaculation can be abolished by a lesion to the sympathetic innervation of the bladder neck (leading to a retrograde ejaculation), by disruption of the sensory, and (particularly) motor nerves innervating the perineal muscles, whose contraction leads to expulsion of the semen, and by central lesions.
A disturbed sexual response in females is due to (1) afferent lesions leading to loss of sensitivity of the perineal area and (2) efferent lesions leading to a loss of lubrication, loss of clitoral erection, and pelvic floor muscle denervation.
6.3
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