CNS Anomalies and Later Pregnancy - SD

Introduction

Hello, I'm Dr. Barbara Hertzberg from Duke University Medical Center.

I'm a professor of radiology and an associate professor of obstetrics and gynecology.

Our talk today is going to be on central nervous system abnormalities in the fetus and on the cervix and on the placenta.

And the format that we'll be using will be case studies.

We'll have several unknown cases that I'll show you and then we'll review the diagnoses for the cases and other relevant information about them.

This talk will be about central nervous system abnormalities and issues of later pregnancy related to the placenta and the cervix and the format will be a case review.

So we'll start by showing some unknown cases.

Case one is a 25-year-old woman who was referred for size and dates.

Case two is a 35-year-old woman with advanced maternal age.

Case three is a 21-year-old patient who was referred for size and dates and case four is a 17-year-old pregnant woman who was also referred for size and dates.

Case 1: Anencephaly

Now we'll go back over the cases and the diagnoses Case one, our 25-year-old woman referred for size and dates.

The coronal image shows lack of calvarium and brain tissue above the level of the orbits.

And likewise, a para sagittal image that I didn't show you before confirms this.

There is lack of normal brain tissue and lack of calvarium above the level of the fetal orbits.

The diagnosis therefore is anencephaly.

Anencephaly is a lethal abnormality.

It's an important abnormality even for future pregnancies as it is associated with a 10 times increased chance of a neural tube defect in a subsequent pregnancy.

While anencephaly refers to absence of the calvarium and of normal brain tissue above the level of the orbits.

Identification of disorganized dysmorphic tissue above the level of the orbits does not exclude an anencephaly, but rather is relatively common particularly when the diagnosis is made early in pregnancy because this tissue corresponds to what has been termed angiostroma.

So this fetus who has this disorganized, abnormal appearing tissue located above the level of the orbits, also has anencephaly even though there is tissue at that level, there is a differential diagnosis to be considered when the thought of anencephaly arises.

And one of the important diagnoses in the differential is amniotic band syndrome To distinguish amniotic band syndrome from anencephaly identification of incomplete loss of the cranium, an asymmetric loss of the cranium can be very useful and anencephaly, there should be no calvarium above the level of the orbits.

On the other hand, in this fetus with amniotic band syndrome on the para sagittal view, we see a small piece of calvarium above the level of the orbits.

And this is confirmed on the coronal view, which also shows a small piece of calvarium above the level of the orbits.

Further confirming the diagnosis of amniotic band syndrome can be defined lesions in a non embryologic distribution in the fetus.

So for example, we see a large amount of herniated abdominal tissue deep to the abdomen in this fetus.

Also helpful of course is identification of bands in the amniotic fluid and here we see a large band in the amniotic fluid, adjacent to some of the eviscerated abdominal tissue.

Another entity that needs to be considered in the differential diagnosis of anencephaly is severe microcephaly, severe microcephaly because if the head is extremely small it can be difficult to find the residual calvarium and brain tissue.

So in a coronal view of this fetus, we see the orbits here but no evidence of calvarium or brain tissue above the level of the orbits.

However, in para sagittal view you can see the calvarium in this very small head.

And when we look in axial view at the remaining brain tissue, you can see that in fact there is thalamus over here as well as posterior phospho tissue here, but the thalamus is abnormal, it's larger than expected taking up most of the brain and the thalami are fused and so this fetus had an appearance that simulated anencephaly due to severe microcephaly, secondary to holoprosencephaly as we can see from the fused thalami.

Finally, in some cases of large cephaloceles the appearance in some scan planes can also simulate anencephaly.

So in this fetus we see no normal brain tissue or calvarium above the level of the orbits in this coronal view.

But when we adjust the scan plane you can see quite a bit of eviscerated brain tissue as well as calvarium and a calvarial defect due to an encephalocele which simply was not seen on the initial image due to the scan plane.

Case 2: Chiari II Malformation

Let's move on to case number two.

This was our 35-year-old woman with advanced maternal age.

As we look at the posterior fossa in this case, we see no evidence of a cisterna magna and where one would expect to see the cerebellar hemispheres.

We see a curved structure.

The structure is curved and it has a configuration of a banana and that is instead of seeing normal cerebellar hemispheres and a normal cisterna magna, let's compare this appearance to the appearance of the posterior fossa in a normal fetus.

Here we see a normal posterior fossa with an approximately dumbbell shape to the cerebella hemispheres and the vermis in between very different from the banana shape of the cerebellum in the case fetus.

In addition, we can see that the cisterna magna is intact and there's a normal amount of fluid in the cisterna magna in the normal fetus.

So this fetus has what has been termed a banana sign and the banana sign is a sign that is seen in fetuses with Chiari two malformations, which is the diagnosis here.

Another sign seen in fetuses with Chiari two malformations is the lemon sign shown here.

The lemon sign refers to flattening or concavity of the frontal bones as we see.

In addition we can often see hydrocephalus in fetuses with Chiari two malformations and that is present here where we see the enlarged ventricle and the relatively diminutive in size choroid plexus.

Let's look at the ultrasound signs of Chiari two malformation in a little bit more detail To see a banana sign to see the abnormal shape of the cerebellar hemispheres and the obliteration of the cisterna magna is never a normal finding and the banana sign can be seen throughout the second and third trimesters.

It is a fixed sign, a fixed finding.

On the other hand, the lemon sign is not necessarily always abnormal.

In fact, a subtle form of lemon sign can be seen in some normal fetuses.

Additionally, the lemon sign is typically seen in the second trimester, but by late in the second trimester and the third trimester, the lemon sign may be gone.

So it is less reliable in terms of diagnosing Chiari two malformation than the banana sign.

This next image demonstrates another potential pitfall in diagnosing the lemon sign.

On the surface it looks like this fetus has a lemon sign.

Certainly it looks like there is some front flattening of the frontal bones, however the posterior fossa is normal in appearance with normal cerebellar hemispheres and a normal cisterna magna.

Additionally, no spine defect was found and almost all fetuses with Chiari two malformations have spine defects.

Although the spine and the posterior fossa appeared normal on this fetus, just about everything else looked abnormal.

For example, the stomach is up in the chest, the heart is deviated into the right side of the chest due to a diaphragmatic hernia.

Additionally, the hand is abnormal with clenching and overlapping of the digits.

So this fetus did not have a lemon sign but had a sign that can closely simulate the lemon sign which has been termed the strawberry sign.

And the strawberry sign is seen in fetuses with trisomy 18.

It consists of pointing of the frontal bones due to hypoplasia of the frontal lobes in conjunction with flattening of the occiput due to hypoplasia of the hind brain.

Many fetuses with Chiari two malformations also have dilated ventricles or ventriculomegaly.

And this fetus with a Chiari two malformation has this.

You can see that the choroid plexus is markedly diminutive in size compared with the size of the ventricle and the ventricle is grossly enlarged.

It measures 15.3 millimeters in diameter when the upper limit of normal is 10 millimeters.

Ventriculomegaly is seen in many but not all fetuses with Chiari two malformations.

It is more common when the posterior fossa abnormality is severe and it's more common later in pregnancy.

If a fetus has a Chiari two malformation, they should be offered karyotyping.

There is a two to 4% chance of an associated aneuploidy.

If the Chiari two malformation and spine defect are isolated ultrasound findings, the likelihood of an aneuploidy increases to about 10 to 17%.

If one takes all comers with Chiari two malformation, including those who have associated other defects fetuses with Chiari, two malformations almost always have a spine defect.

But despite that, the intracranial abnormalities may actually be more apparent than the spine defect and in some cases, if not for the intracranial abnormalities, the spine defect may have been missed.

The spine defect typically consists of a full thickness defect through the skin to the level of the spine as well as a dysraphism of the posterior elements of the spine.

Here you can see that the posterior elements of the spine are pointing apart, they're diverging as one goes more superficially.

Compare this to a normal fetus and notice that in the normal fetus the posterior elements of the spine should converge as one goes posteriorly.

In both fetuses we can see the vertebral body here and here it's the posterior elements that look grossly different in many fetuses.

One can also actually see the meningocele associated with the spine defect as is the case here.

However, in some fetuses, if the spine is up against the uterine wall without much amniotic fluid between it and the uterine wall, it can be hard to actually visualize the meningocele and the spinal dysraphism may be the finding that tips you off to the level of the defect.

Another common finding in fetuses with Chiari two malformations is club feet shown here where we have bilateral club feet, club feet occur in this setting due to unopposed action of muscle groups secondary to a peripheral nerve defect.

Case 3: Encephalocele

This brings us to case three, the 21-year-old woman who was referred for size and dates.

We see a cystic structure located behind the fetal calvarium.

The structure contains a small amount of solid tissue and it is projecting through to the outside of the head a defect in the calvarium as shown here.

Therefore, the diagnosis is an encephalocele.

Okay, now the majority of encephaloceles are occipital but there are exceptions to this majority and this is an example in this fetus.

On the coronal view we see a frontal encephalocele located just superior to the fetal nose and lips.

Likewise, in the sagittal view we see the frontal encephalocele located immediately superior to the fetal lips.

Here the differential diagnosis for an encephalocele for a mass along the back of the fetal head and sometimes neck includes a cystic hygroma, a cervical teratoma, a cervical meningocele, and a skin lesion or a normal structure such as the fetal ear or hair.

As a result, identification of the calvarial defect is a critical step in distinguishing encephaloceles from other entities.

In the differential diagnosis, cystic hygroma is shown here we see a large cystic collection along the back of the fetal head which extends down to the back of the fetal neck.

And when we look at the cystic collection more closely, rather than seeing brain tissue in it, we see multiple septations a finding typical for a cystic hygroma and no calvarial defect was seen.

This is an example of a cervical teratoma in this case a very large complex mixed cystic and solid mass with areas of calcification within it located along the back of the fetal head and neck with no calvarial defect.

Again, this is a cervical teratoma.

On the other hand, in this example of a cervical meningocele, we see a mass along the back of the fetal neck and it is associated with a dysraphism of the posterior elements of the cervical spine.

And finally, a skin lesion or a normal structure such as the fetal ear or hair or a scalp cyst can also closely simulate an encephalocele.

This is an example of a fetal ear.

You can see the normal ridges of the ear because of the way this image was taken, but you can imagine how it could look like a small encephalocele if we hadn't taken the image to show these ridges to best advantage.

Case 4: Dandy-Walker Abnormality

Now we'll move on to case four.

Our 17-year-old patient who was referred for size and dates.

The main findings here are prominence of the fourth ventricle and a increased fluid collection in the posterior fossa connecting to the fourth ventricle through a defect in the expected position of the cerebellar vermis.

So the diagnosis here, seeing also that the cerebellar hemispheres are splayed apart is a Dandy Walker abnormality.

The Dandy Walker abnormality refers to a posterior fossa fluid space that communicates with the fourth ventricle through a vermian defect.

The prognosis for fetuses with Dandy Walker abnormalities is very guarded.

They often are associated with additional central nervous system abnormalities, non-central nervous system abnormalities, as well as a wide variety of congenital syndromes.

There's an increased incidence of aneuploidies associated with Dandy Walker syndrome.

So mothers of fetuses who have these findings are typically offered karyotyping.

The differential diagnosis for an extra or abnormal fluid collection in the posterior fossa includes a posterior fossa arachnoid cyst, mega cisterna magna, and a vascular abnormality.

And in particular, most commonly a vein of Galen aneurysm.

A posterior fossa arachnoid cyst refers to an extra parenchymal intracranial collection that forms from layers of arachnoid tissue.

These posterior fossa arachnoid cysts tend to be more rounded and asymmetric than Dandy Walker abnormalities and are not associated with vermian defects.

Here are two examples.

This one a more rounded collection and this one somewhat asymmetrically located in the posterior fossa.

Mega cisterna magna refers to a big cisterna magna defined as AP diameter of the cisterna magna greater than 10 millimeters.

And this particular cisterna magna measured 12 millimeters.

Distinguishing mega cisterna magna from a Dandy Walker abnormality is presence of an intact fourth ventricle, an intact vermis and absence of splaying of the cerebellar hemispheres.

For some time it had been thought that mega cisterna magna was associated with an increased risk of aneuploidy and in particular trisomy 18.

However, more recently it has been shown that if mega cisterna magna is an isolated finding after doing a detailed fetal scan, then it is not associated with an increased risk of aneuploidy.

Finally, a vein of Galen aneurysm is a dilatation of the vein of Galen which forms due to increased blood flow through the vein of Galen secondary to a communication between the carotid and or the vertebral basilar system and venous plexus in the region of the vein of Galen.

And one sees a cystic structure in the posterior fossa which elongates posterior inferiorly as courses to the straight sinus.

If the scan plane is adjusted.

As we see here, it is not a difficult diagnosis to make if one remembers to always use color doppler anytime there's a cystic structure which does not belong.

And so we see that on color doppler the vein of Galen aneurysm easily filled with blood flow At spectral Doppler evaluation there will be a high frequency turbulence signal within the vein of Galen aneurysm.

As we see here though not needed to make the diagnosis, there are frequently additional ultrasound findings associated with vein of Galen aneurysm.

These include hydrocephalus due to obstruction of the ventricular outflow secondary to the vein of Galen aneurysm and hydrops secondary to high output failure.

So in this particular fetus we also see marked skin edema due to hydrops as well as ascites and a pericardial effusion.

Placental and Cervical Abnormalities

With that we'll move on to abnormalities of the placenta as well as of the cervix and show four additional unknown cases.

Case five is a patient with vaginal bleeding.

Case six is a different patient with vaginal bleeding.

The patient in case seven was referred to assess for size and dates and finally the patient in case eight was pregnant.

Patient referred for pelvic discomfort.

Case 5: Placenta Previa

Moving back to the patient in case five referred for vaginal bleeding, we see the cervix here and the endocervical canal here on this transabdominal image.

Immediately above the cervix we see the placenta.

The placenta is completely covering the cervix in this image and therefore the diagnosis is complete placenta previa.

The following is a typical classification scheme for describing the severity of placenta previa.

There can be some minor variations between different institutions, but for the most part this classification works pretty well.

Complete placenta previa refers to placental tissue that completely covers the internal cervical os partial placenta previa placental tissue partially covers the os.

On the other hand, a marginal placenta previa refers to placental tissue that covers part of the cervix but does not involve the os.

And a low lying placenta refers to placental tissue that is within two centimeters of the internal os.

So in this example at endovaginal ultrasound, we see the cervix here, the endocervical canal here and placental tissue here.

And you can see that this placental tissue covers part of the cervix but it does not involve the internal cervical os and therefore the diagnosis would be marginal placenta previa.

Now there has been some question in the past as to whether endovaginal ultrasound is safe in the setting of placenta previa.

This is a patient who has a complete placenta previa.

We see the cervix here and a cervical canal here and you can see placental tissue covering the internal cervical os completely.

This is an endovaginal ultrasound.

So obviously it was gotten here because it was safe and in fact it is generally considered that if you need to do an endovaginal ultrasound to make the diagnosis or to determine how severe a placenta previa is, it is a safe thing to do.

One just needs to do with caution.

Case 6: Pitfalls in Placenta Previa Diagnosis

Case six was another patient with vaginal bleeding.

In this patient we see at transabdominal ultrasound, the cervix here and the endocervical canal.

Here placental tissue seems to be covering the upper part of the cervix.

However, there is one problem with making the diagnosis of complete placenta previa Here that problem is this is the bladder and the bladder is full.

And that is a known pitfall in the diagnosis of placenta previa because a distended bladder can push the portions of the lower uterine segment together and make it look like there is a longer cervix than there really is.

And like placental tissue is covering that cervix when it really isn't.

So after emptying the bladder in this patient, you can now see here is the cervix, the endocervical canal.

The cervix looks much shorter now that there is an empty bladder.

Placental tissue is here.

This is the lower margin of the placenta and it is nowhere near the cervix.

The diagnosis as shown post void is no placenta previa, but rather there was a full bladder which was causing a spurious appearance of placenta previa.

Other pitfalls in the diagnosis of placenta previa include scanning early in pregnancy.

As a result, if we think that there may be a placenta previa, but we've scanned early in pregnancy, we will always get a follow-up ultrasound at approximately 26 to 28 weeks.

A subchorionic hematoma that is located immediately over the cervix can also simulate placenta previa, particularly if you catch it in an acute stage when the hematoma is very echogenic because then it can look like the hematoma is actually placental tissue over the cervix.

And finally, a uterine contraction can distort the lower uterine segment and make it look like there's a placenta previa when there isn't.

So in this example, we see what one might consider to be cervix here and endocervical canal here, but if you look at this more closely, it's really too long to be the cervix and it is getting wider and wider as one goes superiorly.

But the cervix is really a more cylindrical shape structure.

It's also very bulging.

These are all findings that are seen when there's a big contraction in the lower uterus and both sides of the contraction can be circumferential and touch each other, making it look like there's an endocervical canal when there isn't.

The effect of this is that the placenta is pulled down into the lower uterus by the contraction and it looks like there's placental tissue over the cervix.

But when you see this distorted picture, you should think about the likelihood that there is a lower uterine segment contraction and wait.

And in this patient, when we get an image about 20 minutes later, we can see a non distorted appearance of the cervix and the normal appearing endocervical canal.

And low and behold, there's nothing right over the cervix here because instead the placenta is located here, the edge of the placenta is far away from the cervix.

Case 7: Succenturiate Lobe and Vasa Previa

With that we'll shift focus to the patient in case seven who was referred for size and dates and was pretty much asymptomatic at the time.

We can see the cervix here and a cervical canal here and the placenta here, maybe the placenta is a little low, you'd wonder about a low lying placenta or a marginal previa.

But on this initial image, not much else is seen.

However, after getting the baby's head to be elevated out of the pelvis, we can see that there is more to the story.

Head was here initially and now the head is out of the pelvis.

In addition to seeing the anterior placenta, we can see a posterior lobe of the placenta.

And so the diagnosis is succenturiate lobe of the placenta.

There is an accessory lobe of the placenta, but even that turns out not to be the full story in this patient.

When one looks at endovaginal ultrasound, the initial image looks okay, but again the cervix is here, endocervical canal is here.

When the head becomes elevated out of the pelvis, we can see cervix, endocervical canal, and this membrane and power doppler placed over the membrane shows it's a vascular structure and it's actually arterial.

And so the diagnosis here is actually succenturiate lobe and vasa previa because a blood vessel that is bridging the two lobes of the placenta is located immediately over the cervix.

Vasa previa refers to fetal blood vessels that are unprotected by the umbilical cord and course directly over the cervix.

And this occurs in the setting of either a succenturiate lobe of the placenta as in this case or a velamentous cord insertion in which the cord insert onto the membranes adjacent to the placenta.

And the blood vessel go from the cord insertion, unprotected by the cord to supply the placenta.

In vasa previa there can be rapid fetal exsanguination during labor and it's therefore very important to make the diagnosis because most fetuses with vasa previa are delivered relatively early.

Case 8: Hourglass Membranes and Cervical Issues

Our final case is the patient with pelvic discomfort.

On the left we see the fetal abdomen and on the right we see an unusual shape to the lower uterine segment with an hourglass type of configuration where we see membranes bulging through the area of the cervix.

So this is termed hourglass membranes and the membranes are bulged all the way through the cervix and into the vaginal fornix.

Sometimes you can actually see fetal parts within the hourglass membranes as we see here where we see fetal legs.

So hourglass membranes refer to gestational sac that is prolapsed through the cervix into the vagina.

It's the most advanced stage of preterm labor or preterm cervical dilatation.

And when you see the severe abnormality, delivery is generally imminent.

Sometimes a rescue cerclage is tried to try to stop delivery but only rarely works.

A less severe ultrasound finding for a short cervix would be shortening of the cervix or funneling of the internal cervical os without actual bulging through the external os.

The normal cervix measures greater than two and a half to three centimeters in length when the cervix is short.

Typically the patient either has cervical incompetence or preterm labor.

And the cervical appearance though can change spontaneously just while you watch during the course of an ultrasound exam or can change in response to what's called fundal pressure when one puts firm pressure on the uterine fundus externally.

So here is a patient who looks initially normal, endovaginal image of the cervix shows it measures 3.5 centimeters.

However, just a little bit later on we see a slight funneling of the internal cervical os not a very dramatic change, in response to doing nothing but watching at endovaginal ultrasound.

And just a little bit later though, there's a very dramatic change in the appearance of the cervix where we see marked funneling of the endocervical canal and a very short cervix.

So this is an example of a cervix that initially looked normal, but in response to fundal pressure showed a dramatic change with marked funneling of the internal cervical os and marked shortening of the intact portion of the cervix.

So in the patient who is at increased risk for preterm labor or preterm cervical dilatation, it's very important to not just take a quick snapshot look of the cervix, but to watch it over time and to use a maneuver like fundal pressure to be sure that one does not elicit a change in the cervix because although it might seem on the surface reassuring that the cervix did look normal at one point in this study, it is the worst that the cervix looks, the shortest that the cervix looks, that is most predictive of what the patient outcome will be.

Conclusion

So this concludes my lecture today and I thank you.