Diagnostic ultrasound ( PDFDrive )

CHAPTER 8 The Gastrointestinal Tract 293
OTHER ABNORMALITIES
Occlusion of the GI tract lumen producing obstruction may be
either mechanical, where an actual physical impediment to the
progression of the luminal content exists, or functional, where
paralysis of the intestinal musculature impedes progression
(paralytic ileus).
Mechanical Bowel Obstruction
Mechanical bowel obstruction (MBO) is characterized by (1)
dilation of the GI tract proximal to the site of luminal occlusion,
(2) accumulation of large quantities of luid and gas, and (3)
hyperperistalsis as the gut attempts to pass the luminal content
beyond the obstruction. If the process is prolonged, exhaustion
and overdistention of the bowel loops may occur, with a secondary
decrease in peristaltic activity. here are three broad categories
of mechanical obstruction: obturation obstruction, related to
blockage of the lumen by material in the lumen; intrinsic
abnormalities of the gut wall, associated with luminal narrowing;
and extrinsic bowel lesions, including adhesions. Strangulation
obstruction develops when the circulation of the obstructed
intestinal loop becomes impaired.
Sonography in patients with suspected MBO is frequently
unhelpful as adhesions, the most common cause of intestinal
obstruction, are not visible on the sonogram. Also, the presence
of abundant gas in the intestinal tract, characteristic of most
patients with obstruction, frequently produces sonograms of
nondiagnostic quality. However, in the minority of patients with
MBO who do not have signiicant gaseous distention, sonography
may be helpful. In a prospective study of 48 patients, Meiser
and Meissner 77 found that ultrasound was positive in 25% of
patients with a “normal” plain ilm. Ultrasound alone allowed
complete diagnosis of the cause of obstruction in 6 patients in
a retrospective study of sonography on 26 patients with known
colonic obstruction; it also correctly predicted the location of
colonic obstruction in 22 cases (85%) and the cause of the obstruction
in 21 cases (81%). 11 Of 13 patients ultimately conirmed
to have adenocarcinoma, 5 had a mass on sonography, 5 had
segmental thickening, and 11 others showed a target sign of
intussusception.
Sonographic study of potential MBO should include assessment
of the following:
• GI tract caliber from the stomach to the rectum, noting
any point at which the caliber alters (Fig. 8.42).
• Content of any dilated loops, with special attention to
their luid and gaseous nature (Fig. 8.43; see also Videos
8.9, 8.10, and 8.16).
• Peristaltic activity within the dilated loops, which is
typically greatly exaggerated and abnormal, frequently
producing a to-and-fro motion of the luminal content.
With strangulation, peristalsis may decrease or cease.
• Site of obstruction for luminal (large gallstones, bezoars, 78
foreign bodies, intussusception, occasional polypoid
tumors), intrinsic (segmental gut wall thickening and
stricture formation from Crohn disease, annular carcinomas),
and extrinsic (abscesses, endometriomas) abnormality
as a cause of the obstruction (Video 8.16).
• Location of gut loops, noting any abnormal position.
Obstruction associated with external hernias is ideal for
sonographic detection in that dilated loops of gut may be
traced to a portion of the gut with normal caliber but
abnormal location (Fig. 8.44). Spigelian and inguinal hernias
are the types most frequently seen on sonograms.
Unique sonographic features are seen in the following:
Closed-loop obstruction occurs if the bowel lumen is occluded
at two points along its length, a serious condition that facilitates
strangulation and necrosis. As the obstructed loop is closed of
from the more proximal portion of the GI tract, little or no gas
is present within the obstructed segments, which may become
dilated and luid illed. Consequently, the abdominal radiograph
may be unremarkable (Fig. 8.45A), and sonography may be most
helpful by showing the dilated involved segments (Fig. 8.45B)
and oten the normal-caliber bowel distal to the point of obstruction.
he features of closed-loop obstruction are well described
on ultrasound and include dilated small bowel, a C- or U-shaped
bowel loop (Fig. 8.45C), a whirl sign, and two adjacent collapsed
loops. 79,80 his last important observation is diicult to observe
on ultrasound, in contrast to CT scan. However, we have correctly
suspected closed-loop obstruction in many patients on the basis
of virtually normal plain ilms, small bowel dilation, and a U- or
C-shaped bowel loop, especially if there is gut wall thickening
or pneumatosis intestinalis suggesting gut infarction.
Aferent loop obstruction is an uncommon complication of
subtotal gastrectomy, with Billroth II gastrojejunostomy, that
may occur by twisting at the anastomosis, internal hernias, or
anastomotic stricture. Again, a gasless, dilated loop may be readily
recognized on sonography in a location consistent with the
enteroenteric anastomosis coursing from the right upper quadrant
across the midline. Its detection, location, and shape should
allow for correct sonographic diagnosis of aferent loop
obstruction. 81
Intussusception, invagination of a bowel segment (the intussusceptum)
into the next distal segment (the intussuscipiens), is
seen on sonography of the abdomen most oten as a transient
and infrequent occurrence. However, it is a relatively infrequent
cause of MBO in the adult, usually associated with a tumor
as a lead point. In our experience, this is oten a lipoma that
appears as a highly echogenic, intraluminal mass related to its
fat content. A sonographic appearance of multiple concentric
rings, related to the invaginating layers of the telescoped bowel
and seen in cross section, is virtually pathognomonic 82 (Fig.
8.46A). Occasionally, only a target appearance may be seen. 83,84 he
longitudinal appearance suggesting a “hay fork” 84 is not as reliably
detected. In both projections, the mesenteric fat invaginating with
the intussusceptum will show as an eccentric area of increased
echogenicity. A lipoma, as a lead point, similarly shows as a
focus of increased echogenicity (Fig. 8.46B and C).
Midgut malrotation predisposes to MBO and infarction. It
is infrequently encountered in adults. A sonographic abnormality
related to the superior mesenteric vessels suggests malrotation. 85
On transverse sonograms, the superior mesenteric vein is seen
on the let ventral aspect of the superior mesenteric artery, a
reversal of the normal relationship.