- outline the defecation reflex;
- describe the factors that contribute to the development of constipation;
- discuss the pharmacological management of constipation;
- outline the side effects associated with the use of laxatives.
Mr Benjamin is a 75-year-old man with no close relatives who has lived alone since he lost his wife three years ago. He has become increasingly frail over the past two years. He cooks infrequently, eats little fruit and almost no vegetables. Mr Benjamin rarely visits his friends or the shops; if he goes for a walk, it is a short one, as he is now frightened of the traffic. He has severely restricted his intake of fluids in the evening and has even cut out his cup of hot milk before bed, as he does not want to visit the toilet during the night. Mr Benjamin has never had any gastrointestinal complaints in the past, but recently he has not opened his bowels for more than two weeks. His doctor has advised him to drink more fluids and has prescribed lactulose.
Describe the normal process of defecation.
Faecal material usually remains in the colon for about 24 hours, but the rectum is normally empty. There are generally slow mixing and propulsive contractions in the colon but mass movements of colonic content also occur, usually after a meal, when strong contractile waves push the content into the rectum and distend it. Distension stimulates sensory receptors in the rectum and initiates the defecation reflex, a reflex involving parasympathetic nerves in the sacral spinal cord, together with conscious awareness of the urge to defecate. At the same time the smooth muscle of the internal anal sphincter is relaxed and the somatic nerves supplying striated muscle in the external anal sphincter are inhibited, allowing the sphincter to relax. Voluntary control of defecation is learnt in early childhood and involves voluntary contraction of the external anal sphincter.
What are the causative factors for the development of constipation?
Constipation is a condition in which faecal material moves too slowly through the large intestine. As a result too much water is reabsorbed; hard, dry faeces which are difficult to move and very abrasive are produced. Infrequent or difficult defecation is a common problem in the elderly as ageing is associated with a decline in both secretory activity and motility in the gut. Constipation could develop because of emotional problems, inactive or sedentary lifestyle, lack of fibre and fluid in the diet, intestinal muscle weakness, a neurogenic disorder or an iatrogenic effect. Iatrogenic conditions are those caused by drugs or other medical treatments.
Outline the factors in the history which may be contributing to this patient’s constipation.
Mr Benjamin eats little fruit or vegetables and is therefore likely to have inadequate fibre (roughage) in his diet; he also drinks very little fluid. This results in a small volume of faecal material moving through the colon. Slow passage of the material through the colon favours fluid reabsorption: excessive drying and compaction of the faecal material may then occur. Mr Benjamin is also inactive, taking only occasional short walks, which also increases the likelihood of constipation.
Which types of drug may cause constipation as a side effect?
Constipation can be a troublesome side effect of opiates used for pain relief, for example morphine and codeine. It is also a side effect of some calcium channel blocking agents, antacids containing aluminium compounds and iron salts used in the treatment of anaemia.
Comment on the pharmacological management of constipation.
Laxatives are used to treat constipation. They change the consistency of the faeces, increasing the frequency of defecation by accelerating the rate of faecal passage through the colon and elimination of stool from the rectum. There are four main types of laxative: bulk-forming preparations, such as sterculia and ispaghula; hyperosmolar or saline solutions, such as magnesium sulfate; faecal softeners/wetting agents, such as docusate (dioctyl sodium sulfosuccinate); and stimulant or irritant laxatives, such as senna and bisacodyl.
Before a laxative is prescribed, it is important to ensure that the patient really is constipated, as the frequency of normal defecation varies considerably between patients, ranging from three times a day to one defecation every three days. Constipation may be secondary to another, possibly serious, condition such as intestinal obstruction, and this should be excluded before treatment begins. In general, a bulk-forming or hyperosmolar laxative is tried before stimulant compounds are used.
To which category of drugs does lactulose belong? Comment on its mechanism of action and the recommended dose.
Lactulose is a hyperosmotic liquid containing a disaccharide of galactose and fructose which is not absorbed from the intestine. The recommended dosage is 15 ml twice a day. It passes unchanged into the colon and produces an osmotic effect, directing fluids into the colon content, which expands the bowel and initiates peristalsis. Production of lactic and acetic acids from lactulose is brought about by bacteria in the large intestine, which in turn further stimulates peristalsis. Lactulose is safe for diabetic patients since the sugar content is not absorbed.
Comment on the adverse effects which are associated with the use of laxatives.
Laxatives are often misused/abused, for example in slimming disorders, to increase gut transit rate and so limit absorption of foods. Side effects which may occur include: flatulence, and abdominal distension or discomfort with bulk-forming and osmotic laxatives. Other adverse effects may include: diarrhoea, nausea, vomiting, weakness, dehydration and electrolyte imbalances, for example hypokalaemia. The most prominent side effect of the powerful stimulant/irritant laxatives is abdominal cramping, which is due to increased peristalsis.
What advice might be useful for this patient?
Mr Benjamin has a restricted, low-roughage diet and his colonic motility will be helped by increasing its fruit and vegetable content. It may be possible to find him a day care place at which a healthy, balanced meal can be provided; he will also benefit from the exercise and social interaction involved. It is important that Mr Benjamin maintains his fluid intake during the day, to avoid dehydration and promote colonic transit.
Key Points
- Constipation is associated with slow transit of faecal material through the large intestine and increased fluid absorption, resulting in hard, dry faeces.
- Constipation is common in the elderly, in people with emotional problems or those with an inactive/sedentary lifestyle, and also with lack of fibre and fluid in the diet, intestinal muscle weakness and neurogenic disorders.
- Constipation can be associated with the use of opiates, such as morphine and codeine, calcium channel blocking agents, antacids containing aluminium compounds and iron salts used in the treatment of anaemia.
- Drug treatment involves the use of laxatives. There are four main types: bulk-forming preparations, hyperosmolar or saline solutions, faecal softeners/wetting agents and stimulant or irritant laxatives.
- Adverse effects of laxative use or misuse include: flatulence, abdominal distension, cramps and discomfort, diarrhoea, weakness, dehydration and electrolyte imbalances.
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