Neurogenic bowel is often overshadowed by other medical complications of spinal cord injury (SCI) but can significantly impact one’s quality of life. This article aims to illustrate the intricacies of anatomy and physiology within the gastrointestinal (GI) tract as it pertains to SCI and how to navigate neurogenic bowel management–from the acute postinjury phase through the lifelong journey of adaptation and care in outpatient settings.
Key points
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Autonomic and enteric nervous system changes lead to bowel dysfunction after SCI.
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Acutely, these changes predispose to GI complications including stress ulcers. Chronically, they lead to neurogenic bowel dysfunction.
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Comprehensive and interdisciplinary evaluations are needed to identify the appropriate neurogenic bowel management plan for a patient; these management plans should be individualized.
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Neurogenic bowel dysfunction management plans should be reevaluated over time given changes in patient medical, functional, and psychosocial status.
Introduction
The consequences of spinal cord injury (SCI) extend well beyond the loss of motor function and sensation. Among the challenges faced by individuals with SCI, the neurogenic bowel is often overshadowed by more pressing concerns of neuropathic pain, spasticity, and sudden mobility decline. The term neurogenic bowel describes the impairment of bowel function due to a neurologic injury, significantly impacting one’s quality of life and thus requiring comprehensive management strategies. This chapter aims to illustrate the intricacies of anatomy and physiology within the gastrointestinal (GI) tract as it pertains to SCI and how to navigate neurogenic bowel management–from the acute postinjury phase through the lifelong journey of adaptation and care in outpatient settings.
History
The management of the neurogenic bowel is rooted in the earliest observations of organ system dysfunction following SCI. Initially, the focus was to prevent life-threatening complications, including bowel perforations and sepsis. As the medical community’s understanding of GI physiology broadened, particularly about the autonomic nervous system, strategies evolved to address bowel management in persons with SCI. These strategies include surgical interventions, specialized techniques, and pharmacotherapy, from the use of nonspecific stimulant laxatives to drugs designed to target specific pathways in the enteric nervous system.
Definitions
Spinal Cord Injury
An injury to the spinal cord can result in a loss or impairment of motor, sensory, and autonomic function below the level of injury. Depending on the level and completeness of the injury, SCI can lead to partial or complete paralysis, sensory loss, and dysfunction of the bowel and bladder.
Neurogenic Bowel
The bowel dysfunction in individuals with nervous system diseases or injuries, particularly SCI. This condition is characterized by decreased bowel function control due to disrupted nerve signals between the bowel and the brain. Specifically, after SCI, autonomic nervous system imbalance leads to unchecked parasympathetic activity and lack of continence.
Background
The GI tract is an intricate system designed to optimize the digestion and absorption of nutrients while maintaining fluid and electrolyte balance. The anatomy of the colon, with its specialized muscles and innervation, is central to the process of fecal continence. The internal and external anal sphincters, along with the pelvic floor muscles, work in concert to retain stool until defecation is appropriate. At the same time, the colon sections have specific roles in processing fecal matter.
The physiology of the colon is particularly relevant to the management of neurogenic bowel in SCI patients. Peristalsis, the wave-like muscular contractions that move food through the digestive tract, is critical to bowel management. Peristalsis mixes fecal contents in the colon and propels them toward the rectum. The coordination of these movements is complex and involves a combination of neurogenic and myogenic mechanisms and chemical signaling from hormones and neurotransmitters.
Understanding peristalsis is critical because SCI can disrupt these coordinated movements, leading to constipation, incontinence, or a combination of both. The level of injury and whether the bowel is classified as areflexic or reflexic will determine the management approach. For example, a higher-level injury may lead to a reflexic bowel, where the control of defecation reflexes is lost. In contrast, a lower-level injury may result in an areflexic bowel characterized by reduced or absent reflex activity.
Discussion
Anatomy and Physiology of the Gastrointestinal Tract
The GI tract is a complex system that is central to the digestion process. Beginning at the mouth and extending to the anus, the GI tract is a continuum of organs, each with a specific function in the breakdown, absorption, and expulsion of food and waste. When delving into the colon’s anatomy, the colon absorbs water and electrolytes from food residue before elimination from the body. The colon is not just a storage tube for waste but a dynamic organ with a rich supply of nerves and muscles that facilitate the movement of contents. At the end of the colon, the continuous inner smooth muscle layer of the rectum thickens to produce the internal anal sphincter (IAS), which is innervated by sympathetic (L1-2) neurons and accounts for about 80% of resting sphincter pressure.
Similarly, the external anal sphincter (EAS) is a circular band of striated muscle that contracts with the pelvic floor, which is under voluntary control and is innervated by the pudendal (S2-4) somatic nerve. The puborectalis muscle loops around the proximal rectum and maintains the anorectal angle. Altogether, the IAS, EAS, and puborectalis act together to maintain fecal continence.
The coordination of colonic motility involves three mechanisms: chemical, neurogenic, and myogenic. The chemical mechanisms include a variety of neurotransmitters and hormones, such as amines, peptides, acetylcholine, and fatty acid derivatives (prostaglandins and sex steroids) that adjust the motility according to the body’s needs. The enteric nervous system mediates neurogenic control, often referred to as the “second brain,” which can operate independently of the central nervous system to manage the intricate process of digestion. Finally, myogenic control refers to the smooth muscle’s inherent ability to contract through the electrical coupling of cells through gap junctions and maintain a rhythm that propels waste along the GI tract.
Innervation of the Gastrointestinal Tract
The autonomic nervous system (ANS) plays a pivotal role in the function of the GI tract. The parasympathetic arm, primarily through the vagus nerve and sacral parasympathetic fibers, enhances digestion, while the sympathetic division inhibits GI activity during stress.
The enteric nervous system (ENS) is a network of neurons that governs most colonic wall movement to mix and locally advance stool through the colon. Auerbach’s plexus (myenteric) and Meissner’s plexus are part of the ENS.
Along with autonomic innervation, somatic innervation plays a crucial role in voluntary control over defecation. The pudendal nerves (S2-S4) control the external anal sphincter, allowing for voluntary contraction and thus contributing to continence.
Colonic Reflexes
Colonic reflexes are vital in maintaining bowel function. The vagal reflexes control the upper GI tract, while the sacral parasympathetic reflexes are essential for the motility of the lower colon and rectum. The pelvic nerve reflexes (spinal cord mediated) are initiated from enteric circuits in response to colonic dilation and reinforce colonic-initiated propulsive activity in defecation. The rectocolic reflex, a pelvic nerve-mediated pathway, produces propulsive colonic peristalsis in response to chemical or mechanical stimulation of the rectum and anal canal; mechanically stimulating this reflex by digital stimulation is imperative in a typical SCI bowel regimen. The gastrocolic reflex, which may or may not be present in patients with SCI, is triggered by feeding, producing propulsive peristalsis of the small intestine and colon that is mediated by neural (cholinergic) and hormonal (gastrin, motilin, CCK) influences. The colo-colonic intramural reflex (enteric nervous system) does not require extrinsic colonic innervation and provides an inherent tendency of the intestine to produce peristalsis toward the anus.
Acute Complications of the Gastrointestinal Tract After Spinal Cord Injury
After a spinal cord injury, the risk of acute GI complications increases. Common symptoms include nausea, vomiting, anorexia, abdominal spasticity, and referred shoulder pain. Stress ulcers can form due to reduced blood flow and heightened gastric acid secretion. An ileus, or the cessation of bowel movements, is a common problem that can lead to gastric distension and is common in cervical and complete SCI. Hypercalcemia of immobility, pancreatitis, and abnormal gallbladder activity can also be seen. , Superior mesenteric artery (SMA) syndrome is a rare but serious condition whereby the third part of the duodenum is compressed between the SMA and the aorta, causing obstruction.
Chronic Gastrointestinal Complications After Spinal Cord Injury
Chronic complications often impact the quality of life of an individual with SCI. Hemorrhoids, abdominal distension, and autonomic dysreflexia due to GI dysfunction are prevalent and can lead to considerable discomfort and health risks. Gallstone risk is also notably higher in individuals with SCI.
Neurogenic Bowel Overview
Neurogenic bowel dysfunction is a common and debilitating condition that affects individuals with SCI. This dysfunction is primarily due to an imbalance in the autonomic nervous system, leading to unchecked parasympathetic activity. This heightened activity can enhance glandular secretions, such as those from the pancreas and stomach, and relax junctional sphincters, including the cardia, pylorus, and ileocecal valve. Additionally, the external anal sphincter, a striated muscle, is released from central control, resulting in a lack of continence ( Fig. 1 ).
The impact on the quality of life for patients with SCI is profound, with 27% to 61% of patients ranking bowel dysfunction as a major life-limiting problem. Unplanned evacuations and extended time spent on bowel care significantly contribute to psychological distress.
Furthermore, colonic transit time for these patients is more than double that of neurally intact individuals, with the main delays reported at the descending colon and anorectum. However, some studies suggest a general slowing throughout the entire length of the colon. These disruptions can lead to a considerably decreased quality of life, emphasizing the need for effective management strategies for those living with SCI-induced neurogenic bowel dysfunction.
Neurogenic bowel dysfunction following SCI can be classified into two types: areflexic and reflexic. The areflexic bowel results from injuries that affect the parasympathetic cell bodies at the conus medullaris, their axons in the cauda equina, or the pelvic nerve, leading to a loss of coordinated bowel contractions and, ultimately, constipation and incontinence. On the other hand, the reflexic bowel results from spinal cord injuries above the conus medullaris, causing hyperreflexia of the colon and external anal sphincter.
Assessment of Gastrointestinal Function After Spinal Cord Injury
In SCI, the assessment and management of GI function often take precedence due to the significant impact on patient morbidity and quality of life. , It is essential to understand that bowel function and impairment evaluation should begin at the onset of SCI and must be re-evaluated at least annually to adapt the bowel program as the patient’s condition changes over time.
A thorough patient history helps establish a premorbid baseline of GI function. This includes an understanding of pre-existing conditions such as laxative dependency, diabetes, irritable bowel syndrome, lactose intolerance, and inflammatory bowel disease that could influence colonic transit time and gut responsiveness to medications and predispose patients to life-threatening conditions like toxic megacolon.
The current bowel program is reviewed for patient satisfaction, noting one’s nutritional status via a daily dietary intake analysis. Physicians should be vigilant for symptoms that impact the patient’s quality of life, such as delayed or painful evacuation, constipation, and unplanned evacuations, which can occur between bowel care sessions. The frequency, duration, and nature of defecation, along with stool characteristics, should be recorded in detail. The patient’s medication regimen likewise requires review to assess its impact on bowel care, given that many drugs can alter gut flora or, at worse, significantly inhibit bowel motility, including anticholinergics, antidepressants, and narcotic pain medications. ,
It is likewise critical to evaluate the patient’s ability to comply with bowel care. A significant proportion of SCI patients require assistance, with those having tetraplegia being three times more likely than those with paraplegia to need help. Patients with injuries at C5 and above typically depend entirely on all aspects of bowel care. In contrast, those with injuries at C6 and C7 may be capable of independent care, but many still opt for assistance. The patient’s ability to learn and instruct others in their bowel care is critical to successful bowel care.
The physical examination is the cornerstone of the annual assessment, mostly when the patient is seen in the outpatient clinic, which includes a complete abdominal assessment to screen for colonic and pelvic floor dysfunction and other complications related to the neurogenic bowel. Palpation, auscultation, and percussion of the abdomen are performed, along with rectal examinations, to gather information about sensation, sphincter innervation, and the presence of masses or hemorrhoids. The assessment of sacral reflexes, anocutaneous and bulbocavernous, provides valuable insights into the type of neurogenic bowel present, differentiating between upper and lower motor neuron bowel patterns. Positive anocutaneous and bulbocavernosus reflexes suggest the presence of an upper motor neuron bowel pattern. Functional examinations also evaluate sitting tolerance, upper extremity strength, spasticity, and transfer skills. The patient’s anthropometric characteristics, such as weight, height, and living environment, are also weighed into tailor one’s bowel program.
Diagnostic tests, including stool testing for occult blood starting at age 50 and abdominal plain film x-rays, are recommended at annual outpatient assessments. These tests help confirm fecal retention and evaluate serious conditions like megacolon.
Bowel Program Overview
Overall, the primary goal of a comprehensive bowel program is to provide predictable and effective elimination, reduce GI complaints, and be adaptable over time, ensuring that it is revised as needed throughout the continuum of care ( Fig. 2 ).
