Neurogenic Bowel Dysfunction

The colon is a muscular tube 1.5 m long, and the rectum is 12-15 cm long. The rectum opens to the outside through the anal canal, which is 2-5 cm long. The anus contains the internal anal sphincter, which is composed of smooth muscle and is not under voluntary control, and the external sphincter, which is composed of skeletal voluntary muscle.[6]

The intrinsic nervous system, also known as the enteric nervous system, is composed of the submucosal (ie, Meissner) and myenteric (ie, Auerbach) plexuses (see the image below), which largely regulate segment-to-segment movement of the GI tract.

Illustration of neural control of gut wall by sympathetic, parasympathetic and enteric nervous system. Courtesy of Wikimedia Commons.

The extrinsic nervous supply comprises the parasympathetic, sympathetic, and somatic nerves. The vagus nerve is parasympathetic and innervates the upper segments of the GI tract up to the splenic flexure. The pelvic splanchnic nerves carry parasympathetic fibers from the S2-4 spinal cord levels to the descending colon and rectum.

Sympathetic innervation comes from the superior and inferior mesenteric nerves (T9-12) and the hypogastric nerve (T12-L2). The hypogastric nerve sends out sympathetic innervation from the L1, L2, and L3 spinal segments to the lower colon, rectum, and sphincters. The somatic pudendal nerve (S2-4) innervates the pelvic floor and the external anal sphincter.[7]

Normal bowel function

Fecal contents are propelled in the large intestine by periodic mass movements, and defecation is initiated by involuntary peristaltic advancement of stool into the rectum. An awareness of the need to defecate occurs in the superior frontal gyrus and anterior cingulate gyrus of the cerebral cortex, as a result of a critical level of rectal filling. The rectum stores stool until it is full; fullness stimulates pressure receptors on the pelvic floor that trigger the rectoanal inhibitory reflex, allowing internal anal sphincter relaxation.

The external sphincter is normally contracted until it is voluntarily relaxed; this relaxation reduces pressure and thus permits defecation. The Valsalva maneuver, a voluntary contraction of the diaphragm and abdominal muscles, raises intra-abdominal pressure and triggers peristalsis in the colon and rectum, causing relaxation of the internal sphincter. When rectal pressure exceeds sphincter pressure, defecation occurs.[8, 9]

Bowel dysfunction

Injury to and disorders of the CNS affect bowel function in various ways, depending on the location and severity of the damage.

Spinal cord injury, multiple sclerosis, and myelomeningocele

The pathophysiology of NBD is much the same for SCI, MS, and MMC, even though the nature of the insult differs. Traumatic SCIs are usually well defined, whereas MS lesions may be found at multiple sites, and most patients with MMC have low spinal cord lesions, often at the conus medullaris or the cauda equina.[10]

Spinal cord lesions are classified as either located above the conus medullaris or located at the conus medullaris/cauda equina. A spinal cord lesion above the conus medullaris is an upper-motor-neuron (UMN) lesion. It causes loss of voluntary control, maintained reflex activity in the anorectum, increased colonic transit time, and constipation. Anal tone is increased or maintained.

A lesion at the level of the conus medullaris, the cauda equina, or the inferior splanchnic nerve is considered a lower-motor-neuron (LMN) lesion. It causes loss of voluntary control, loss of reflex activity in the anorectum, prolonged transit time, constipation and rectal impaction, and reduced resting tone in the anal sphincter.[11]

Spina bifida is also associated with significant bowel issues.

Parkinson disease

The pathophysiology of bowel dysfunction in PD is characterized by dystonia of striated muscles of the pelvic floor and the external anal sphincter. Colonic transit time is prolonged as a consequence of loss of dopamine within the CNS and the enteric nervous system.[10]

Brain lesions

Patients with brain lesions and survivors of stroke have bowel dysfunction caused by loss of inhibition of the sacral reflex.[12]

Cerebral palsy can be assoicated with bowel dysfunction as well.

Diabetes mellitus

Patients with diabetes may have fecal incontinence as a consequence of irreversible damage of the autonomic nervous system and impaired rectal sensation. Both motor and sensory dysfunction may occur.[12]

The frequency of fecal incontinence and constipation ranges from approximately 1% to as much as 25% of the general adult population, depending on how the terms are defined.[13, 14, 15, 16] However, bowel dysfunction occurs in most people with neurologic conditions.

Approximately 12,000 new cases of SCI occur in the United States each year, most of them caused by trauma. Bowel dysfunction affects almost all patients with a chronic SCI, with as many as 95% reporting constipation and as many as 75% experiencing fecal incontinence.[17, 18]

MS is diagnosed in young adults and more often in women. Its prevalence is approximately 1 per 1000 individuals, and as many as 70% experience constipation, incontinence, or both.[19, 20] PD affects 1 million people in the US each year, with constipation occurring in 37%.[21] About 25% of stroke survivors experience constipation, and 15% have fecal incontinence.[22, 23]

The age of incidence is variable. No known sexual or racial predilection has been reported for this condition.

The prognosis depends on the severity, location, and presenting comorbid factors in patients with SCI. Patients with complete SCI have a less favorable prognosis. Because of the chronic nature of NBD, it is a significant contributor to reduced quality of life. Patients with SCI have reported that bowel dysfunction is more problematic than bladder dysfunction, sexual dysfunction, pain, fatigue, or perception of body image.[18] Additionally, hospitalizations due to impaction, megacolon, constipation and volvulus are more than twice as frequent in these patients.[24]

A study by Ozisler et al found that an effective bowel program reduced the severity of NBD and lowered the incidence of associated GI problems in SCI patients.[25]

Patients should be educated regarding the long-term management of bowel dysfunction, particularly with respect to the rationale, goals, and techniques of management. They should be instructed in the safe use of assistive devices for bowel emptying and taught efficient techniques for bowel emptying, digital stimulation, and the use of rectal suppositories.

The importance of timing, regularity, and positioning in bowel evacuation should be emphasized. Recommendations for helping prevent bowel-related complications (eg, constipation, hemorrhoids, and impaction) should be provided.

The initial step in the management of neurogenic bowel dysfunction (NBD) is to establish a history of premorbid and current bowel function and patterns. This includes the following:

• Bowel frequency
• Presence of strain
• Presence of hard or lumpy stool
• Presence of incomplete evacuation sensation
• Sensation of anorectal obstruction
• Need for additional manual maneuvers to facilitate defecation

It is important to ascertain whether any gastrointestinal (GI) problems or any other medical conditions (eg, diabetes, irritable bowel syndrome [IBS], lactose intolerance, inflammatory bowel disease [IBD], or rectal bleeding) existed previously. The patient’s functional status should be evaluated. An effort should be made to determine whether the bowel symptoms are affecting the patient’s ability to perform activities of daily living and carry out social and work responsibilities.

Assessment should include the following:

• Ability to learn a bowel program
• Sitting tolerance
• Sitting balance
• Upper-limb strength and proprioception
• Upper-limb function
• Spasticity
• Transfer skills
• Actual and potential risks to skin
• Home accessibility
• Equipment needs

This assessment should involve both the patient and the caregiver.

Symptoms can include the following:

• Abdominal pain
• Abdominal distention
• Early satiety
• Loss of voluntary control over defecation (ie, fecal incontinence)
• Difficulty with evacuation
• Associated neurologic bladder symptoms
• Associated symptoms of autonomic dysreflexia in patients with spinal cord lesions at T6 and above

The physical examination should include the following:

• Complete abdominal and rectal examination
• Assessment of anal sphincter tone
• Elicitation of anocutaneous and bulbocavernosus reflexes to determine if the patient has upper-motor-neuron (UMN) or lower-motor-neuron (LMN) bowel dysfunction
• Neurologic examination to assess the extent of nerve damage

The abdomen should be inspected for distention, increased abdominal muscle tone indicative of spasticity, and bowel sounds.

The tone of the external anal sphincter is assessed by means of digital examination. The external anal sphincter is normally puckered, with LMN impairment being manifested by flattening or scalloping. Tone is reduced or absent in acute complete spinal cord lesions and LMN impairment. Rectal sensation usually is absent in lesions above L3.

The anocutaneous reflex is assessed by means of stimulation with pinprick in the perianal region, which leads to visible reflexive anal contraction. The anocutaneous reflex is normally present if the S2-4 reflex arc is intact. This reflex does not correlate with internal sphincter function.

The bulbocavernosus reflex is assessed by squeezing the glans penis or clitoris (or applying traction on an indwelling catheter), which results in palpable rectal contraction. This reflex is normally present in most patients. The reflex is brisk with UMN lesions and is absent in LMN lesions or spinal shock.

Sensory examination tests the integrity of sacral dermatomes to light touch and pinprick.

Autonomic dysreflexia is an abnormal sympathetic nervous system response to a noxious stimulus below the level of injury in individuals with a spinal cord injury (SCI) above T6. An acute episode results in rapidly rising blood pressure with an accompanying risk of brain hemorrhage and death. Symptoms include flushing, sweating and blotchiness above the lesion, chills, nasal congestion, and headache.

Fecal impaction (see the image below) occurs in almost 80% of patients with SCI and can lead to bowel distention, which, if left untreated, may result in perforation and even death.

Abdominal X-ray showing fecal impaction extending from pelvis upward to left subphrenic space and from left toward right flank, measuring over 40 cm in length and 33 cm in width. Image courtesy of Wikimedia Commons

Gastroesophageal reflux results from chronic overdistention of the bowel. Diverticulosis (see the video below) also results from chronic overdistention, as well as from increased intraluminal pressures brought on by fecal impaction.

Rectal prolapse results from repeated passage of large hard stools in patients with a weakened anorectal mechanism, especially in cases involving LMN lesions. Hemorrhoids result from repeated passage of large hard stools as a result of constipation and can lead to chronically high pressures in the anorectal marginal veins. The prevalence of hemorrhoids in this setting may be as high as 76%. Anal fissures may be caused by increased anal tone or by the passage of hard stool; they may manifest as increased spasm of the sphincter and autonomic dysreflexia.

Megacolon is relatively common in patients with NBD, and it may be associated with sigmoid volvulus, fecal impaction, autonomic dysreflexia, dyspnea from diaphragmatic splinting, weight loss, and chronic malnutrition.

Reduced quality of life occurs as a consequence of fear of incontinence and may lead to reduced social activity and isolation, which are associated with depression and anxiety.

Patients older than 50 years should undergo annual stool testing for occult blood. In patients with diarrhea of unknown etiology, stool examination for fecal leukocytes, Clostridioides (formerly Clostridium) difficile toxin, and ova and parasites should be performed.

An abdominal radiograph is useful for evaluating megacolon and distribution of feces.

Incontinence and evacuation can be investigated by tests that assess sphincter structure and function, such as anorectal manometry and endoanal ultrasonography (EAUS). Anorectal and pelvic floor function can be assessed by means of defecating proctography and nerve conduction studies. Luminal integrity and colonic function can be evaluated by means of endoscopy and transit studies.[8]

Anorectal manometry

Anal manometry is performed by placing a water-based catheter balloon system into the distal rectum and withdrawing it through the anal canal in a stepwise manner or using a solid-state device containing microtransducers to measure anal canal pressure at various points along the catheter. This study can be employed to determine the resting and voluntary squeeze pressures of the anal canal, the length of the canal, the Valsalva reflex, and the presence of the rectoanal inhibitory reflex.[13]

Endoanal ultrasonography

EAUS evaluates sphincter integrity by using an internal rotating microtransducer. Normally, the external sphincter is a hyperechoic circumferential structure, and the internal sphincter is hypoechoic. Defects and scarring appear as incomplete rings.[8]

Defecating proctography

Video fluoroscopy provides structural and functional information during defecation. An oral contrast agent is given to delineate the small bowel, and barium is placed into the rectum. X-rays are taken while the patient attempts to defecate.

Electromyography

Electromyography (EMG) uses needle electrodes placed into the puborectalis or external anal sphincter to assess the state of the muscle and innervating nerve as a function of electrical activity during the resting and contractile phases.

Sensory testing

Rectal sensory function is evaluated by means of rectal balloon insufflation. The balloon is progressively distended until particular sensations are perceived by the patient. The volumes at which these sensations are perceived are recorded. The following three sensory thresholds are usually defined[26] :

• Constant sensation of fullness
• Urge to defecate
• Maximum tolerated volume

Endoscopy

Endoscopic studies, such as rectosigmoidoscopy, anoscopy, and colonoscopy, can be used to visualize anatomic abnormalities or lesions; however, they cannot assess the function of the gastrointestinal (GI) tract.

Treatment of neurogenic bowel dysfunction (NBD) is initially conservative.[2] Patients with suspected bowel rupture or perforation should be transferred to surgical care, as should any patients with rectal prolapse; these conditions are associated with a high morbidity and are best managed surgically.

Consultation with a gastroenterologist, a surgeon, or both should be considered in recalcitrant cases or in cases where complications are suggested or have been observed.

It is important to remain alert for possible complications. Failure to identify bowel impaction could lead to bowel perforation. Failure to identify fecal impaction because of the presence of diarrhea may lead to missing the diagnosis of intestinal obstruction.

A bowel management program, personalized for the patient, should be established that involves adjustment of dietary fiber and fluid intake, modulation of stool consistency, promotion of stool transit through the bowel, and effective reflex or mechanical evacuation of stool from the rectum.[25] Scheduled bowel emptying should be recommended to avoid incontinence and reduce impaction.

Various pharmacologic treatments have been used, though there remains a need for further study in the literature.[27] Pharmacologic options include the following:

• Colonic stimulants (eg, bisacodyl) - Polyethylene glycol–based bisacodyl suppositories may take effect more quickly than vegetable oil–based bisacodyl suppositories in patients with NBD after spinal cord injury (SCI) ^[28]
• Hyperosmolar agents (eg, sodium bisphosphonate)
• Bulking agents (eg, psyllium)
• Stool softeners (eg, docusate sodium)
• Osmotic agents (eg, polyethylene glycol)

Between scheduled bowel-care efforts, most patients should make use of stool softeners, ideally with fiber to increase the bulk of the stools and thereby enhance defecatory response.[29] Patients with reflex bowel function should aim to have soft formed stools. Patients with flaccid bowel function should aim for firmer stools to reduce the likelihood of fecal incontinence.

Other means of triggering the defecation reflex include the following:

• Abdominal massage - This is performed before or during defecation by applying gentle pressure along the colon in a clockwise manner
• Digital stimulation - This is performed by inserting a finger into the anal canal and applying pressure along the circumference of the canal until relaxation of the external canal is felt ^[30]
• Placement of rectal stimulant suppositories
• Enemas (see the image below)
• Various combinations of these techniques

Administration of an enema.

Biofeedback and behavioral training can be beneficial for improving sensory and motor awareness in patients with incomplete neurogenic bowel lesions, especially children.[31, 32]

Another option is transanal irrigation (TAI), which assists in the evacuation of stool by introducing water into the colon and rectum through a catheter in the anus.[33, 34] This may be done daily or every other day. Small prospective trials showed significant improvements in pediatric patients with NBD in whom conservative measures have failed.[35] Other small prospective studies suggested that TAI may be more effective than conservative measures in select groups of patients.[36]

An international expert consensus recommended evaluating all of the previously described treatment methods before surgery is considered.[37]

The Malone antegrade continence enema (MACE) is an approach that makes use of a surgically created entry into the large intestine for irrigation. The appendix serves as a conduit between the skin and the cecum, forming an appendicocecostomy. Enema fluid can be introduced through the stoma via a catheter. This procedure can be employed in patients with chronic refractory NBD, who typically do not have enough rectal tone to allow the use of rectal enemas. The appendicocecostomy is used most often in children with spina bifida.[38]

The sacral anterior root stimulator was developed for SCI patients with neurogenic bladder, but it is also effective for patients with constipation. The implant is placed during a laminectomy of L2-4. The stimulator is triggered by an external device that causes peristalsis of distal colon and rectum.

Sacral nerve stimulation with an electrode placed through the sacral foramen between S2 and S4 is effective for incomplete cauda equina[39] and spinal lesions.[40, 41] Before implantation, a 3-week percutaneous nerve evaluation test is done with a temporary electrode and external battery. Once improvement in fecal incontinence is confirmed, the permanent pulse generator and electrode are placed. The implant provides continuous low-level electrical impulses to the sacral plexus, influencing the anal canal, the colon, and the pelvic floor.[24]

A colostomy or ileostomy is considered in highly refractory cases or when stool incontinence complicates other problems, such as pressure injury management.

Bowel perforation is a surgical emergency resulting from fecal impaction. If clinical and radiologic findings suggest bowel rupture, the patient must be immediately referred for surgical intervention.

Physical therapy can be helpful. In general, any improvement in mobility and activity levels that can be achieved in an affected individual increases the potential for lessening constipation and fecal impaction.[42]

Occupational therapists work hand in hand with nurses to improve toileting and transfer techniques, with the goal of improving independence and thereby potentially lessening fecal incontinence.[30]

Recommendations for monitoring and follow-up include the following:

• Annual follow-up visits for patients with SCI or spina bifida; even more frequent follow-up may be necessary for some patients
• Thorough physical examination, including rectal examination, as part of regular follow-up
• In patients older than 50 years, stool testing for occult blood to rule out colorectal cancer,or colonoscopy
• Evaluation of device use in patients with SCI
• Monitoring to confirm use of appropriate medications
• Advice to ensure that patients’ daily diet contains 15-30 mg of fiber
• Additional diagnostic workup (eg, radiologic evaluation) when necessary (eg, in cases of fecal impaction or potential perforation)

Drugs used for management of neurogenic bowel dysfunction (NBD) include laxatives and stool softeners aimed at stimulating peristalsis with subsequent loosening and expulsion of feces.

Class Summary

Colonic stimulants are used to promote peristalsis.

Bisacodyl (Dulcolax, Bisac Evac, Bisco-Lax)

Bisacodyl is a colonic laxative stimulant that acts by directly stimulating colonic mucosa to produce peristalsis. It is used for relief of constipation and irregularity. This agent is available in 10-mg tablets or suppositories.

Senna (Senokot)

Senna is a natural vegetable derivative that causes neuroperistaltic stimulation. It comes in tablet or syrup form and is available in combination with docusate sodium (Senokot-S).

Class Summary

Hyperosmolar agents are used for short-term treatment of constipation.

Sodium phosphate (Fleet enema)

Sodium phosphate is a purgative and laxative used in constipation and as a component of a bowel-cleansing regimen in preoperative patients.

Class Summary

Bulking agents absorb water in the intestine to form a viscous liquid that promotes peristalsis and reduces transit time.

Psyllium (Metamucil, Citrucel)

Psyllium contains natural fiber that acts to increase the content of feces and, at the same time, promotes bacterial growth. Its main uses are in chronic constipation, irritable bowel syndrome, and bowel management in cases of patients with hemorrhoids.

Class Summary

Stool-softening agents help keep stools soft to facilitate easy, natural passage.

Docusate sodium (Colace, Surfak)

Docusate sodium is a surface-active agent used in painful anorectal conditions and cardiac conditions where maximum ease of stool passage is desired. Therevac minienema is a combination of glycerin and docusate sodium in a polyethylene glycol (PEG) base; it has been shown to produce more rapid results than a bisacodyl suppository. Docusate is available in 100-mg capsules, 20 mg/5 mL syrup, and 200 mg/5 mL microenema.

Class Summary

Osmotic agents promote bowel movement through osmotic action that holds water in the small intestine and colon.

Polyethylene glycol (PEG) solution (Miralax)

PEG solution is used for treatment of occasional constipation. In theory, there is less risk of dehydration or electrolyte imbalance with isotonic PEG than with hypertonic sugar solutions. The laxative effect is generated because PEG is not absorbed and continues to hold water by osmotic action through the small bowel and the colon, resulting in mechanical cleansing. PEG solution is supplied with a measuring cap marked to contain 17 g of laxative powder when filled to the indicated line. It may take 48-96 hours to produce bowel movement.

Class Summary

Use of a peripherally selective opioid antagonist may be considered for treating constipation in patients who have advanced illness necessitating long-term opioid analgesia and who are unresponsive to laxatives.

Methylnaltrexone (Relistor)

Methylnaltrexone is a peripherally acting mu-opioid receptor antagonist that selectively displaces opioids from mu-opioid receptors outside the central nervous system (CNS), including those located in the gastrointestinal (GI) tract, thereby decreasing constipating effects. It is indicated for opioid-induced constipation in patients with advanced illness who are receiving palliative care when their response to laxatives has not been sufficient. Methylnaltrexone is available as a 12 mg/0.6 mL injectable solution for subcutaneous use.