Fetal Gastrointestinal System

Proximal Small Bowel Obstruction

Moving a little farther down in this fetus,
we have a very large distended stomach.
As we look for the double bubble, we see
that there's not just one additional bowel loop,
but several additional bowel loops in the upper quadrant.
And this is an example of a proximal
small bowel obstruction.

Small bowel obstructions are typically due to bowel atresia.
There may be more than one site
of atresia throughout the small bowel.
And what we see on sonography are multiple loops
of dilated fluid-filled small bowel.
Traditionally, we talk about greater than
seven millimeters in diameter.
There is some thought that maybe
greater than 10 millimeters would be more
reasonable to use to diagnose obstruction.

Amniotic Fluid Volume and Location of Atresia

If the obstruction is in the proximal small bowel,
there will be polyhydramnios.
But if the site of the atresia is in the distal small bowel,
the amniotic fluid volume
around the fetus will remain normal
because the fetus is still able to swallow
and digest that fluid.
And the proximal
and mid small bowel is still functioning enough
to reabsorb the fluid.

Etiology and Associated Anomalies

Atresias are felt
to be vascular accidents
and the associated anomalies are all related to some other
vascular etiology.

Number of Dilated Loops and Bowel Characteristics

When you have a small bowel obstruction, the number
of dilated loops will depend on the
level of the obstruction.
So proximal obstruction will have a fewer number of loops
and a distal obstruction will have more loops.
It's important to remember
that the jejunum is easily dilated
and that
proximal obstruction can have larger small bowel loops,
whereas the ileum does not stretch quite as well.
And ileal obstructions tend to
result in perforations at smaller diameters.
About 7% of the time the atresia will be multiple.

Examples of Small Bowel Obstruction

Here's another example of a small bowel obstruction.
We could see multiple distended dilated loops
of fluid filled bowel, which are peristalsing rapidly.

Another example of a more distal atresia.
In this instance, we can see a loop
of distended bowel right across the anterior
abdominal wall, just underneath the skin surface here
with associated polyhydramnios, this fetus is at 41 weeks.
Pretty unusual to see polyhydramnios at 41 weeks,
which brings you back to the fact
that you do have this distended loop of bowel.
And this diagnosis was not made
until the patient was the fetus was post dates.

Risks and Complications

When there is a small bowel obstruction, there's a risk
of ischemia, infarction, and in utero perforation.
The reason for this obstruction could be an atresia
or a volvulus or meconium ileus,
but any one of them can have the risks
of these complications.

Hydroureter Identification

A hydroureter should be identified
by its presence posteriorly in the fetal
abdomen near the spine.

Association with Cystic Fibrosis

About a third of patients
with small bowel obstructions will ultimately be diagnosed
with cystic fibrosis,
and this diagnosis should be
especially considered if echogenic bowel was seen in the
second trimester.

Here's another patient with a bowel obstruction seen at 34
weeks, and we see this very large distended loop
of bowel filled with these internal echoes,
not peristalsing, just sitting, and not moving.
And if you look on the sagittal image,
I think you can appreciate that we have multiple loops here
that have twisted all around itself.
And this did turn out to be volvulus
and this bowel had infarcted
and which explains the lack of peristalsis.
This child lost about a third
of his small bowel after delivery.

Another example of a small bowel process.
In this instance though, we see a short segment of bowel.
You could see rapid peristalsis,
more normal appearing peristalsis all through the abdomen.
But this loop is not peristalsing, it's just sitting there
and sort of squeezing a little bit,
but not giving us normal peristalsis.
And this is an example of meconium ileus.

Meconium Ileus

What happens in meconium ileus is the normal meconium
that's present in the small bowel is abnormally thick,
and it becomes impacted because it is so thick
and sticky once that meconium is stuck.
The result for the imagers is the typical small bowel
obstruction where we see dilated bowel typically filled
with echogenic material that may or may not be peristalsing.

When we see this thick echogenic material within the bowel
loop, these patients nearly always have cystic fibrosis,
and indeed 10 to 15% of fetuses with cystic fibrosis present
with meconium ileus in this instance.

Again, another look at that single loop of bowel,
which is distended and not peristalsing.
Well, when we look closely,
we can see there's a little tiny bit of fluid outlining the
outside of the loop in this patient.
Once we see fluid in association with a bowel obstruction, we can now consider
that we've got the situation of meconium peritonitis.

Meconium Peritonitis

Meconium peritonitis is the result
of a perforation in utero.
The meconium leaks out of the small bowel,
and the result is a subsequent sterile chemical peritonitis.
Because of course, there are no bugs in a fetus.
A fetus is sterile. So this is a chemical peritonitis.
The immediate result is free fluid.
Now that perforation can occur proximal
to meconium ileus to an atresia, to a volvulus,
to an intussusception, or it can be idiopathic.
Often these perforations will seal spontaneously.
So if a fetus comes in with ascites,
it should be considered as a possibility.

When you're trying to determine the source of the
ascites, what will happen to the fetus depends on
what caused the perforation.
Most of the time, fetuses who present with ascites
with their small bowel obstruction do not
have cystic fibrosis.

Once the perforation has occurred,
there's a reactive membrane that forms over the hole.
Typically it's incomplete, but sometimes it is complete.
So we can't predict whether there's going
to be more leaking from that hole or not.
But what happens is that reactive membrane
has some inflammatory process
and the fetus will subsequently deposit calcifications
around that inflammation.
They can be linear, they can be in clumps,
or they can wall off the fluid into a cyst like structure,
or the leak can just seal over
and you'll never know what happened to it
and you'll never know where it was.

Calcifications in Meconium Peritonitis

So here's the fetus. With a small bowel obstruction,
we can see these dilated loops of bowel,
but in this instance, we can see these coarse calcifications
clumping along the anterior abdominal wall that gives us
confident understanding
that at some point this bowel obstruction did perforate
and there was a leak, and then it sealed back up again.

Here's an example of the late outcome
of meconium peritonitis.
We see these scattered calcifications in the fetal abdomen.
There's some here on this view.
There's some over by the stomach.
There's some all around the outside edge of the liver.
And this tells us that at some point this fetus did have a
small bowel obstruction with a perforation
and the chemical peritonitis.
However, by the time we saw the fetus, there was no evidence
of a small bowel obstruction
or a gastric outlet obstruction at all.
And this fetus can be predicted
to have an excellent outcome.

Meconium Pseudocyst

In this instance, however, the meconium was walled off
with the calcium in what is called a meconium pseudocyst.
And it's clearly not a cyst.
There's no through transmission.
It's not anechoic, but it is round.
And so it's gotten the name of the pseudocyst.

And here's another example of this meconium pseudocyst.
The chemical peritonitis has created a wall
around the extruded meconium.
In this instance, on the left,
we don't see any dilated bowel left behind,
but on the right we can still see that the
dilated small bowel obstruction is still present.

When there's a meconium pseudocyst.
It's interesting to realize that the presence
of calcium is very uncommon in cystic fibrosis.
There's a few theories for why this is, it's possible
that the cystic fibrosis fetus meconium is so thick
that it doesn't leak out when the perforation occurs.
Another thought is
that the cystic fibrosis fetus lacks the enzymes
that are necessary to react to the peritonitis.

Differentiating Cystic Fibrosis in Meconium Ileus and Peritonitis

If you're thinking it may be a case of cystic fibrosis
with meconium ileus
and meconium peritonitis, it's worth taking a look around
to see if you can see the gallbladder
because the cystic fibrosis patient typically will not
have a gallbladder in evidence on the ultrasound.
So if you look over in the right upper quadrant
and there's a beautiful gallbladder,
it probably is not cystic fibrosis.