C h a p t e r 2 9
Disorders of Gastrointestinal Function
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Constipation can occur as a primary disorder of intestinal
motility, as a side effect of drugs, as a problem associated
with another disease condition, or as a symptom of
obstructing lesions of the gastrointestinal tract. Some
common causes of constipation are failure to respond to
the urge to defecate, inadequate fiber in the diet, inad-
equate fluid intake, weakness of the abdominal muscles,
inactivity and bed rest, pregnancy, and hemorrhoids.
The pathophysiology of constipation can be classified
into three broad categories: normal-transit constipation,
slow-transit constipation, and disorders of defecation.
Normal-transit constipation (or functional constipation)
is characterized by perceived difficulty in defecation and
usually responds to increased fluid and fiber intake.
Slow-transit constipation, which is characterized by
infrequent bowel movements, is often caused by altera-
tions in intestinal innervation.
Hirschsprung disease
is
an extreme form of slow-transit constipation in which
the ganglion cells in the distal bowel are absent because
of a defect that occurred during embryonic development;
the bowel narrows at the area that lacks ganglionic cells.
Although most persons with this disorder present in
infancy or early childhood, some with a relatively short
segment of involved colon do not have symptoms until
later in life. Defecatory disorders are most commonly
due to dysfunction of the pelvic floor or anal sphincter.
Diseases associated with chronic constipation
include neurologic diseases such as spinal cord injury,
Parkinson disease, and multiple sclerosis; endocrine
disorders such as hypothyroidism; and obstructive
lesions in the gastrointestinal tract. Drugs such as nar-
cotics, anticholinergic agents, calcium channel blockers,
diuretics, calcium (antacids and supplements), iron sup-
plements, and aluminum antacids tend to cause consti-
pation. Elderly people with long-standing constipation
may develop dilation of the rectum, colon, or both. This
condition allows large amounts of stool to accumulate
with little or no sensation. Constipation, in the context
of a change in bowel habits, may be a sign of colorectal
cancer.
Diagnosis of constipation usually is based on a his-
tory of infrequent stools, straining with defecation, the
passing of hard and lumpy stools, or the sense of incom-
plete evacuation with defecation. Rectal examination is
used to determine whether fecal impaction, anal stric-
ture, or rectal masses are present. Constipation as a sign
of another disease condition should be ruled out. Tests
that measure colon transit time and defecatory function
are reserved for refractory cases.
The treatment of constipation usually is directed
toward relieving the cause. A conscious effort should be
made to respond to the defecation urge. A time should
be set aside after a meal, when mass movements in the
colon are most likely to occur, for a bowel movement.
Adequate fluid intake and bulk in the diet should be
encouraged. Moderate exercise is essential, and persons
on bed rest benefit from passive and active exercises.
Laxatives and enemas should be used judiciously. They
should not be used on a regular basis to treat simple
constipation because they interfere with the defecation
reflex and actually may damage the rectal mucosa.
Acute Intestinal Obstruction
Intestinal obstruction refers to impaired movement of
intestinal contents in a cephalocaudal direction. The
condition can be acute or chronic and may affect the
small intestine or colon. In contrast to chronic obstruc-
tions, which often involve the colon and may last for
weeks or months, acute obstructions usually present as
severe disorders of the small intestine that are poten-
tially lethal if not recognized early.
50,51
Acute intesti-
nal obstruction can be mechanical or nonmechanical,
resulting from paralytic obstruction of the ileus.
Mechanical obstruction can result from a number of
conditions, intrinsic or extrinsic, that encroach on the
patency of the bowel lumen (Fig. 29-10). Major inciting
causes include an external hernia (i.e., inguinal, femo-
ral, or umbilical) and postoperative adhesions.
52
Less
common causes are strictures, tumors, foreign bodies,
intussusception, and volvulus.
7
Intussusception involves
the telescoping of bowel into the adjacent segment (Figs.
29-10A and 29-11). It is the most common cause of
intestinal obstruction in children younger than 2 years
of age. The most common form is intussusception of
the terminal ileum into the right colon, but other areas
of the bowel may be involved. In most cases, the cause
of the disorder is unknown.
52
The condition can also
occur in adults when an intraluminal mass or tumor
acts as a traction force and pulls the segment along as
it telescopes into the distal segment. Volvulus refers to
a complete twisting of the bowel on an axis formed by
its mesentery (see Fig. 29-10B). It can occur in any por-
tion of the gastrointestinal tract, but most commonly
involves the cecum, followed by the sigmoid colon.
Mechanical bowel obstruction may be a simple obstruc-
tion, in which there is no alteration in blood flow, or a
strangulated obstruction, in which there is impairment
of blood flow and necrosis of bowel tissue.
Paralytic, or adynamic, obstruction of the ileus results
from neurogenic or muscular impairment of peristalsis.
Paralytic ileus is seen most commonly after abdominal
surgery. It also accompanies inflammatory conditions of
the abdomen, intestinal ischemia, pelvic fractures, and
back injuries. It occurs early in the course of peritonitis
and can result from chemical irritation caused by bile,
bacterial toxins, electrolyte imbalances as in hypokale-
mia, and vascular insufficiency.
The major effects of intestinal obstruction are
abdominal distention and loss of fluids and electro-
lytes
51
(Fig. 29-12). Distention is further aggravated by
the accumulation of gases and fluid proximal to the site
of obstruction. Approximately 70% to 80% of these
gases are derived from swallowed air, and because this
air is composed mainly of nitrogen, it is poorly absorbed
from the intestinal lumen. As the process continues, the
distention moves proximally (i.e., toward the mouth),
involving additional segments of bowel. Either form
of intestinal obstruction eventually may lead to stran-
gulation (i.e., interruption of blood flow), gangrenous
changes in the bowel wall, and, ultimately, perforation
of the bowel. The increased pressure in the intestine
tends to compromise mucosal blood flow, leading to
