Constipation is a common problem for many patients.21-24 Therapy should be individually tailored; what is very good therapy for one type of constipation may be poison for another.25 It is difficult for young people, who may never have experienced constipation, to relate to the distress caused by this disorder. For those afflicted, the whole tone of the day may be set by the presence or absence of a good bowel movement (BM).
Understanding how a normal BM is created helps us understand how the process can go awry. Our goal in treating constipation is not to "cure" something, it is to help the patient return to the best possible balance that will allow a normal BM to be passed.
Four major components affect the production of a normal BM: solid waste, water, movement, and lubrication. Although we can differentiate these separate components, in fact, they are very much interrelated. For example, decreased movement of intestinal contents allows more water to be absorbed.
Our bodies cannot absorb everything we eat. We also "create" waste, dead cells and bacteria, within our intestines, which needs to be excreted. Too much or too little solid waste can be a problem. Many people eat little fiber, a major component of solid waste. With little fiber stool volume decreases, and there is less material for the intestinal tract to push along. The intestinal tract becomes sluggish, and with slow passage more water is absorbed. Constipation results, and patients experience small, dry stools. Therapy for such patients is to replace the fiber they do not get in their diets. Psyllium fiber can be useful in this regard. The intestine responds to increased stool volume, within reason, with greater contraction. Thus, bowel contents are moved along more efficiently, and less water is absorbed, resulting in healthy BM.
Many older people have taken fiber replacements for years on faith thanks to effective advertising. Although this is good for some, as described above, for others extra fiber can be poison. Extra fiber is medicinal if the problem is a fiber deficiency. However, if the problem is poor intestinal movement and/or inadequate water intake, extra fiber can worsen the situation. Many elderly and dying patients take in little water orally. When psyllium and other fiber products are taken with inadequate water, they turn into something like half-dried oatmeal or partially dried cement. This is difficult for the intestine to move. Sometimes large volumes of stool build up and inhibit intestinal movement. The colon dilates and, in effect, goes "on strike." Large, soft impactions can result. Such impactions also frequently occur if the dominant problem is poor bowel motility (see below). Fiber helps with constipation only if the gut is able to respond by pushing it along. Otherwise, fiber may worsen an already bad situation.
We cannot excrete solid waste without some water. A delicate balance is required; excessive stool water results in diarrhea, and inadequate water results in dry, difficult-to-pass stools. Stool water content depends on how much water we drink, our general hydration status, how much water is secreted and absorbed by the intestine, how fast stool moves through the intestinal tract, and, ultimately, how much water is retained in the stool. Manipulating any of these factors can affect stool water. The easiest factor to manipulate is how much water is retained in the stool.
Water is absorbed and secreted into the intestine much more dynamically than most people realize. If certain nonabsorbable or poorly absorbed molecules are present in the intestine, water absorption becomes limited. This is because absorption of water without these molecules would leave a hypertonic solution in the intestine, which is not physiologically possible. (Note: the fiber and solid waste, while making the major contribution to stool volume, chemically has little effect on how concentrated the liquid component of stool is.) Intestinal water can be absorbed only to the extent it leaves a solution behind that is not more concentrated than body water. We can use this property to hold on to water if we need to do so.
Sorbitol and lactulose are concentrated, unabsorbable sugar solutions. Therefore, they are excreted in the stool. These solutions are concentrated, so they must be diluted in the body in order to keep intestinal fluid concentration the same as that of the body. This is accomplished either through "holding on" to water already in the intestine or by moving water from the body into the intestine. The net result is the same: more water in the stool. Lactulose differs from sorbitol in that lactulose is broken into smaller molecules in the large intestine. These smaller, more acidic components make it harder to absorb nitrogenous compounds that may worsen hepatic encephalopathy. Thus, only lactulose should be used if the goal is the treatment of hepatic encephalopathy. In terms of treating constipation, lactulose and sorbitol work in very similar ways. Dosing of these sugars is easily individualized. Although usually given in multiples of 30 ccs, great flexibility in dosing is possible. Many patients, especially the dying, have difficulty tolerating them because of their sickly-sweet taste. Mixing them with juices, especially apple juice, may make them more palatable.
Magnesium salts such as milk of magnesia and magnesium citrate work in a similar fashion to the above agents. They are concentrated solutions that, in effect, "hold onto" water. The main difference is that they are somewhat absorbable. This is usually a problem only for patients who have significant renal failure. As magnesium is excreted by the kidney, renal failure can result in the development of toxic levels, and thus magnesium salts are contraindicated in renal failure. Milk of magnesia is usually given at night in anticipation of an effect by morning. Magnesium citrate comes as a "sparkling laxative." One-half to one bottle is usually given.
Isotonic solutions such as GoLYTELY also contain difficult-to-absorb salts. In contrast to the above agents, they are given in a more dilute form. Thus, significant volumes may have to be drunk or enterally administered in order to produce an effect. The advantage is that they do not draw water from the body. The relatively large volumes required (compared to the 30 ccs commonly given of the agents noted above) may be difficult for some patients, especially the dying, to tolerate.
Phosphate salt enemas similarly contain concentrated, poorly absorbed salts. Such enemas are useful in actively constipated patients who have hard stool in the rectum. Care should be used in patients with congestive heart failure, renal failure, and others who are salt intolerant, especially if used in high enemas, as salt absorption is increased. Excessive use (daily, for example) may cause damage to the rectum.
Refractory impactions proximal to the rectal vault may require treatment with a high enema. There is probably more art than science to the use of such enemas. A variety of fluids may be infused - mineral oil, lubricants, or water. Soap-suds enemas should not be used, as they can irritate colonic mucosa. There is no need to use sterile water or saline. Large water or saline infusions can be problematic in patients who have tenuous fluid balances, such as patients with congestive heart failure.
There is no M in a BM if there is no movement of the bowel. Intestinal movement is complex. The normal gut responds to a variety of stimuli with contractions intended to move contents along. The intestinal volume of water and solid waste leads to gut dilatation and reactive contraction. However, the ability of the intestine to contract lessens if the gut is overly dilated. Imagine trying to squeeze a very large accordion. If your arms are maximally spread apart, the power of your squeeze is weakened. (Similarly, if your hands are very close together, your squeeze is weakened.) Maximum power and ability to squeeze occur at intermediate volumes, and so it is with the intestine. Very small volumes result in little squeeze (and subsequent constipation). Very large volumes, as occurs with impaction, also weaken the ability of the bowel to move.
The gut also responds to signals transmitted by nerves and to direct stimulation of the intestinal lining. As an example of nervous stimulation of the gut, the gastrocolic reflex results in colonic movement due to dilatation of the stomach. Large boluses of food, therefore, stimulate bowel movements. In contrast, small volumes, such as with continuous tube feeding, do not.
Various substances can stimulate the gut and thus cause contractions. Most cathartic medications, such as bisacodyl (Dulcolax), work in this way by directly stimulating the myenteric plexus (nerves in the wall of the intestine). Strong cathartics given either rectally or orally can be helpful if a strong, definitive contraction is desired. These drugs may be useful in trying to clear a constipated stool. Such drugs are best used intermittently, as excessive use may result in a refractory bowel (the bowel goes "on strike"). Other agents, especially senna, may provide a gentler stimulus around the clock, thus counteracting round-the-clock bowel-slowing drugs, such as opioids. Senna is usually started as one to two tablets (187 mg each) qhs and can be advanced to four tablets BID.
A variety of drugs result in intestinal slowing, such as anticholinergic drugs and opioids. Patients taking such drugs should have their medications reviewed to determine if a reduction in dose or elimination of the drug is possible. My experience has been that a fair dose-constipation response exists for anticholinergic drugs. That is, lowering the dose lessens the constipating effect. This seems less true of opioids. Patients on very high doses of opioids may have only slightly more constipation than those on lower doses. Thus, rarely is constipation a reason to reduce the opioid dose if opioids are otherwise indicated. There is evidence that patients on fentanyl experience less constipation than do those on comparable oral opioids.26-28
Various agents commonly called stool softeners enhance bowel movements by decreasing friction between the stool and the intestinal wall and rectum by lubricating the stool. Stool is thus easier to pass. These agents minimally soften already hardened stool, but when given orally they may help to prevent hardened stools. Patients who have hemorrhoids, rectal fissures, and other anorectal pathology may become constipated because of pain on the passage of stool. Maintaining a softer, looser stool may help, as does avoidance of foods that may give rise to irritating residua, such as peanuts. Lubricants, as described below, may also be of assistance.
Dioctyl sodium sulfosuccinate (DSS; Colace) decreases surface tension on stool, much like soap. By itself it is usually inadequate in treating opioid-related constipation. DSS is usually given once or twice a day in pill form. Although a liquid form of DSS is available, it should be given only via NG or PEG tubes, not orally, as it tastes like concentrated soap. Although perhaps the most commonly ordered laxative by physicians, the evidence base for the use of DSS is weak.29
Mineral oil may be given in enema form and may help to provide lubrication for a previously hard, difficult-to-pass stool. Mineral oil should not be given orally, as it may interfere with fat-soluble vitamin absorption and because aspiration may cause pneumonitis.
Glycerin in either stick form or as an enema may similarly lubricate stool and ease passage. It also may increase rectal water by causing rectal water secretion and may stimulate a bowel movement through mechanical rectal stimulation.
While the principles described above should be helpful in reestablishing as normal an intestinal balance as is possible, there is no substitute for frequent evaluation and reevaluation. Patients, family members, and nurses who are most intimately involved in bowel care should generally be given wide latitude in adjusting constipation medications. While some physician oversight may be necessary, excessive control by physicians often results in poorly managed care. If educated, most patients, families, and nurses should be able to adjust medications appropriately on their own.
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Palliative Care Perspectives
James L. Hallenbeck, M.D.
Copyright © 2003 by Oxford University Press, Inc.
The online version of this book is used with permission of the publisher and author on web sites affiliated with the Inter-Institutional Collaborating Network on End-of-life Care (IICN), sponsored by Growth House, Inc.