Small and Large Intestines pdf - D. Maha

GASTROINTESTINAL TRACT

ASS. PROF.DR. MAHA SHAKIR HASSAN

Lec 4

Small and Large Intestines

DEVELOPMENTAL ANOMALIES

• Atresia or stenosis, the former being complete failure of development of
the intestinal lumen and the latter representing only narrowing.
Duplication usually takes the form of well-formed saccular to tubular
cystic structures, which may or may not communicate with the lumen of
the small intestine.
• Meckel diverticuluin is the most common and innocuous of the
anomalies. It results from failure of involution of the omphalomesenteric
duct, leaving a persistent blind-ended tubular protrusion up to 5 to 6 cm
long. The diameter is variable. Usually in the ilium, and composed of all
layers of small intestine. They are generally asymptomatic

Meckel diverticulum. The blind pouch is located on the antimesenteric side of the small bowel.

Hirschsprung Disease: Congenital Megacolon

Distention of the colon to greater than 6 or 7 cm in diameter (megacolon)
occurs as a congenital and as an acquired disorder. Hirschsprung disease
(congenital megacolon) results during development, the caudal migration
of neural crest-derived cells along the alimentary tract arrests at some
point before reaching the anus. Hence, an aganglionic segment is left that
lacks both the Meissner submucosal and Auerbach myenteric plexuses.
This causes functional obstruction and progressive distention of the colon
proximal to the affected segment. In most instances, only the rectum and
sigmoid are aganglionic, but in about a fifth of cases a longer segment,

and rarely the entire colon, is affected. Approximately 50% of cases
result from mutations in RET gene and RET ligands, as this signaling
pathway is required for development of the myenteric nerve plexus

MORPHOLOGY

The critical lesion is the lack of ganglion cells, and of ganglia, in the
muscle  wall  and  submucosa  of  the  affected  segment.  The  affected
segment  is  not  distended.  proximal  properly  innervated  segment  that
undergoes  dilation.  Thus,  when  only  the  distal  colon  is  affected,  the
remainder  of  the  colon  becomes  massively  distended.  The  wall  may  be
thinned  by  distention  or  in  some  cases  is  thickened  by  compensatory
muscle hypertrophy.

Clinical Features.

In most cases a delay occurs in the initial

passage of meconium, which is followed  by vomiting in 48 to 72 hours.
The  principal  threat  to  life  is  superimposed  enterocolitis  with  fluid  and
electrolyte  disturbances.  More  rarely,  the  distended  colon  perforates,
usually  in  the  thin-walled  cecum.  The  diagnosis  is  established  by
documenting  the  absence  of  ganglion  cells  in  the  nondistended  bowel
segment.
Acquired  megacolon  may  result  from  (1)  Chagas  disease,  in  which  the
trypanosomes directly invade the bowel wall to destroy the plexuses, (2)
organic obstruction of the bowel by a neoplasm or inflammatory stricture,
(3)  toxic  megacolon  complicating  ulcerative  colitis  (discussed  later),  or
(4) a functional psychosomatic disorder.

VASCULAR DISORDERS

Angiodysplasia:

Tortuous dilatation of submucosal and mucosal blood vessels, most often
in the cecum or right colon. Usually only after the 6th decade.

The

pathogenesis of angiodysplasia remains undefined but has been attributed
to mechanical and congenital factors.

Degenerative vascular changes

related to aging may also have some role.

Hemorrhoids:

It is variceal dilitation of the anal and perianal submucosal venous plexus.

Pathogenesis: Persistently elevated venous pressure within hemorrhoidal
plexus.

Predisposing conditions:

1.  Chronic constipation.
2.  Pregnancy.
3.  Rarely portal hypertention.

Malabsorption Syndromes and diarrhea

Malabsorption,  which  presents  most  commonly  as  chronic  diarrhea,  is
characterized  by  defective  absorption  of  fats,  fat-  and  water-soluble
vitamins,  proteins,  carbohydrates,  electrolytes  and  minerals,  and  water.
Chronic  malabsorption  can  be  accompanied  by  weight  loss,  anorexia,
abdominal  distention.  A  hallmark  of  malabsorption  is  steatorrhea,
characterized by  excessive  fecal  fat  and  bulky, frothy, greasy,  yellow  or
clay-colored stools.

Most common types:

Gluten-sensitive enteropathy

, also known as celiac disease,

is the prototype of a noninfectious cause of malabsorption resulting from
a reduction in small intestinal absorptive surface area. The basic disorder
in  celiac  disease  is  sensitivity  to  gluten,  the  component  of  wheat  and
related  grains  (oat,  barley,  and  rye)  that  contains  the  water-insoluble
protein  gliadin.  There  is  autoimmune  mechanism  responsible  of
development of such disease. The small intestinal mucosa, when exposed
to  gluten,  accumulates  large  numbers  of  B  cells  and  plasma  cells
sensitized to gliadin; accumulation of lymphocytes in gastric and colonic
mucosa  also  may  occur.  In  addition  to  filling  the  lamina  propria,
lymphocytes  also  cross  into  the  epithelial  space,  with  accompanying
damage  to  surface  enterocytes.  Total  flattening  of  mucosal  villi  (and
hence  loss  of  surface  area)  is  the  outcome,  affecting  the  proximal  more
than the distal small intestine.

The  age  of  presentation  with  symptomatic  diarrhea  and  malnutrition
varies from infancy to mid adulthood; removal of gluten from the diet is
met with dramatic improvement. There is, however, a low long-term risk
of  malignant  disease  on  the  order  of  a  two-fold  increase  over  the  usual
rate.  Intestinal  lymphomas,  especially  T-cell  lymphomas,  are
disproportionately

represented;

other

malignancies

include

gastrointestinal and breast carcinomas.

2-Tropical sprue

Tropical  sprue  resembles  celiac  disease  in  symptomatology  but  occurs
almost exclusively in persons living in or visiting the tropics. No specific
causal  agent  has  been  identified,  but  the  appearance  of  malabsorption
within days or a few weeks of an acute diarrheal enteric infection strongly
implicates an infectious process,  also there is prompt response to broad-
spectrum  antibiotic  therapy.  Small  intestinal  changes  vary  from  near
normal  to  a  severe  diffuse  enteritis  with  villous  flattening  in  contrast  to
celiac  disease,  injury  is  seen  at  all  levels  of  the  small  intestine.

IDIOPATHIC INFLAMMATOR BOWEL DISEASES

Crohn Disease

This disease may affect any level of the alimentary tract, from mouth to
anus. Active cases of CD are often accompanied by extraintestinal
manifistations of immune origin, such as iritis and uveitis, sacroiliitis,
migratory polyarthritis, erythema nodosum, hepatic pericholangitis and
sclerosing cholangitis (bile duct inflammatory disorders), and obstructive
uropathy. Systemic amyloidosis is a rare late consequence. Thus, CD
must be viewed as a systemic inflammatory disease with predominant
gastrointestinal involvement.

Epidemiology.

Worldwide in distribution, CD is much more

prevalent in the United States, Great Britain and is rare in Asia and
Africa. It occurs at any age, from young childhood to advanced age, but
the peak incidence is between the second and third decades of life.
Females are affected slightly more often than males. Whites appear to
develop the disease two to five times more often than do nonwhites.

MORPHOLOGY

In  CD,  there  is  gross  involvement  of  the  small  intestine  alone  in  about
30% of Cases, of small intestine and colon in 40%, and of the colon alone
in  about  30%.  When  fully  developed,  CD  is  characterized  by  (1)
sharply delimited and typically transmural involvement of the bowel
by  an  inflammatory  process  with  mucosal  damage,  (2)  the  presence
of noncaseating granulomas in about 35% of cases, and (3) fissuring
with formation of fistulae.

In  diseased  segments  hypertrophy  of  the  muscularis  propria. As  a  result
the  lumen  is  almost  always  narrowed;  in  the  small  intestine  this  is
evidenced radiagraphically as the  “string sign,” a thin stream  of barium
passing through the diseased segment. Strictures may occur in the colon
but are usually less severe.

A  classic  feature  of  CD  is  the  sharp  demarcation  of  diseased  bowel
segments  from  adjacent  uninvolved  bowel.  When  multiple  bowel
segments  are  involved,  the  intervening  bowel  is  essentially  normal
(“skip” lesions).

In the intestinal mucosa, early disease exhibits focal mucosal ulcers
resembling canker sores (aphthous ulcers).

With progressive disease, ulcers coalesce into long, serpentine linear
ulcers, which tend to be oriented along the axis of the bowel, because the
intervening mucosa tends to be relatively spared, it acquires a coarsely
textured, cobblestone appearance.

Narrow fissures develop between the folds of the mucosa, often
penetrating deeply through the bowel wall all the way to serosa.

Further extension of fissures leads to fistula or sinus tract formation,
either to an adherent viscus, to the outside skin, or into a blind cavity to
form a localized abscess.

Distribution of lesions in inflammatory bowel disease. The distinction between Crohn disease and

ulcerative colitis is primarily based on morphology

By microscopic examination, the mucosa exhibits several
characteristic features :

(1) inflammation, with neutrophilic infiltration into the epithelial
layer and accumulation within crypts to form crypt abscesses;

(2) ulceration, which is the usual outcome of active disease: and

(3) chronic mucosal damage in the form of architectural
distortion, atrophy, and metaplasia.

(4)  Granulomas  may  be  present  anywhere  in  the  alimentary
tract,  even  in  patients  with  CD  limited  to  one  bowel  segment.
However,  the  absence  of  granulomas  does  not  preclude  a
diagnosis of CD.

(5) In diseased segments. the muscularis mucosae and
muscularis propria are usually markedly thickened, and fibrosis
affects all tissue layers.

(6)  Lymphoid  aggregates  scattered  through  the  various  tissue
layers  and  in  the  extramural  fat  also  are  characteristic.
(7) Particularly important in patients with long-standing chronic
disease  are  dysplastic  changes  appearing  in  the  mucosal
epithelial  cells.  It  is  related  to  a  five-told  to  six-fold  increased
risk of carcinoma, particularly of the colon.

Crohn disease of the colon showing a deep fissure extending into the muscle wall, a second,
shallow ulcer (and relative preservation of the intervening mucosa). Abundant lymphocyte
aggregates are present, evident as dense blue patches of cells at the interface between mucosa and
submucosa

Clinical  Features.  The  presentation  of  CD  is  highly  variable
and  ultimately  unpredictable.  The  dominant  manifestations  are
recurrent  episodes  of  diarrhea,  crampy  abdominal  pain,  and
fever lasting days to weeks.

Ulcerative Colitis

Ulcerative colitis (UC) is an ulceroinflammatory disease affecting the
colon but limited to the mucosa and submucosa except in the most severe
cases. UC begins in the rectum and extends proximally in a continuous
fashion, sometimes involving the entire colon. Like CD, UC is a systemic
disorder associated in some patients with migratory polyarthritis,
sacroiliitis, ankylosing spondylitis, uveitis, erythema nodosum, and
hepatic involvement (pericholangitis and primary sclerosing cholangitis).
There are several important differences between UC and CD:
• In UC, well-formed granulomas are absent.
• UC does not exhibit skip lesions.
• The mucosal ulcers in UC rarely extend below the submucosa, and there
is surprisingly little fibrosis.
• Mural thickening does not occur in UC, and the serosal surface is
usually completely normal.
• Patients with UC are at greater risk for carcinoma.

Epidemiology

UC is somewhat more common than CD in the United States and Western
countries, but it is infrequent in Asia, Africa, and South America.

Features That Differ between Crohn Disease and Ulcerative Colitis

Feature

Crohn Disease Ulcerative Colitis

MACROSCOPIC

Bowel region

Ileum ± colon

Colon only

Distribution

Skip lesions

Diffuse

Stricture

Yes

Rare

Feature

Crohn Disease Ulcerative Colitis

Wall appearance

Thick

Thin

MICROSCOPIC

Inflammation

Transmural

Limited to mucosa

Pseudopolyps

Moderate

Marked

Ulcers

Deep, knife-like Superficial, broad-based

Lymphoid reaction Marked

Moderate

Fibrosis

Marked

Mild to none

Serositis

Marked

Mild to none

Granulomas

Yes (

∼35%)

Fistulae/sinuses

Yes

Morphology.

Grossly:

 ulcerative colitis always involves the rectum and extends

proximally in a continuous fashion to involve part or all of the
colon.

 Skip lesions are not seen .
 Disease of the entire colon is termed pancolitis ,while left-sided

disease extends no farther than the transverse colon. Limited distal
disease may be referred to descriptively as ulcerative proctitis or
ulcerative proctosigmoiditis.

 The small intestine is normal, although mild mucosal

inflammation of the distal ileum, backwash ileitis, may be present
in severe cases of pancolitis.

 Grossly, involved colonic mucosa may be slightly red and granular

or have extensive, broad-based ulcers

 Isolated islands of regenerating mucosa often bulge into the lumen

to create pseudopolyps

 . Unlike Crohn disease, mural thickening is not present, the

serosal surface is normal, and strictures do not occur.

 However, inflammation and inflammatory mediators can damage

the muscularis propria and disturb neuromuscular function leading

to colonic dilation and toxic megacolon, which carries a
significant risk of perforation.

Histologic features:

 inflammatory infiltrates, crypt abscesses .
 the inflammatory process is diffuse and generally limited to the

mucosa and superficial submucosa .

 In severe cases, extensive mucosal destruction may be

accompanied by ulcers that extend more deeply into the
submucosa, but the muscularis propria is rarely involved.

 Submucosal fibrosis, mucosal atrophy, and distorted mucosal

architecture remain as residua of healed disease but histology may
also revert to near normal after prolonged remission.

 Granulomas are not present in ulcerative colitis.

The most serious complication of UC is the development of
colon carcinoma. Two factors govern the risk: duration of the
disease and its anatomic extent. It is believed that with 10 years
of disease limited to the left colon the risk is minimal, and at 20
years the risk is on the order of 2%, With pancolitis, the risk of
carcinoma is 10% at 20 years and 15% to 25% by 30 years.

A, Crypt abscess. B, Pseudopyloric metaplasia (bottom).

C, Disease is limited to the mucosa

Clinical Features.

UC  is  a  chronic  relapsing  and  remitting  disorder  marked  by
attacks  of  bloody  mucoid  diarrhea  that  may  persist  for  days,
weeks,  or  months  and  then  subside,  only  to  recur  after  an
asymptomatic  interval  of  months  to  years  or  even  decades.
Extraintestinal

manifestations,

particularly

migratory

polyarthritis, are more common with UC than with CD.
Uncommon but life-threatening complications include:

severe diarrhea and electrolyte derangements, massive
hemorrhage, severe colonic dilation (toxic megacolon) with
potential rupture, and perforation with peritonitis. Inflammatory
strictures of the colorectum.

Diagnosis can  usually be  made by endoscopic examination and
biopsy.  The  most  feared  long-term  complication  of  UC  is
cancer.