Borderlands in Obstetrical Ultrasound

Nuchal Fold

This is the fetal translucency or nuchal fold.

This is a sentinel patient, a 21 week fetus for morphology, routine morphology, who was actually referred in from an outside institution because she had a small, relatively thick nuchal fold of six millimeters.

Historically, the thickened nuchal fold was first mentioned in around 1985 when the second trimester screening started relatively vigorously.

And then we had the introduction of the genetic sonogram in the mid 1990s promulgated by Dr. Bonf among other physicians.

And then in the early two thousands, we had the initiation of the first trimester nuchal translucency screening.

And here's an example of a slightly thickened nuchal fold of six millimeters.

So another interactive slide here. What is too thick for nuchal fold? And how late do we measure nuchal fold? There'll be four choices here. We'll come back to this in a few minutes here.

But an interesting study done in the American Journal of OB GYN in 2008, looked at nuchal fold various thresholds and seeing what the various sensitivity and specificity performance characteristics were.

And with nuchal fold more than six millimeters since c was quite low, but the likelihood ratio was quite high around 12.

Whereas if the nuchal fold threshold was chosen at five millimeters, you picked up a few more cases, 15%, you had slightly lower specificity of 97% compared to six millimeters, but the likelihood ratio is five.

So those are kind of important things to remember. We'll talk about that in just a second too.

And one thing I wanna review is the base theorem where you have a pretest probability of something occurring. You do a diagnostic test, in this case ultrasound, and you come up with a post-test probability or percentage.

And this is a really relative simple way to get base theorem. What we can, one can do is calculate the odds of an event occurring in one theoretical possibility I have is of the Red Sox winning the World Series, which is probably coming up in the next couple weeks, we'll say at 70% some fanatic people think may occur, but the odds or probability of an event occurring in this case would be 0.7 over 0.3 or one minus the probability of an event, or two and one third.

And then if you convert that back from probability into odds, probability equals odds over one plus odds or two, and one third over three, and one third or 0.7.

So that's how the two are closely interrelated.

So you take the pretest odds times the likelihood ratio of a diagnostic test, and likelihood rate shows just the sensitivity over one minus the specificity that multiplication will give you the post-test odds and convert it back to probability, and you get the chances of something happening.

So in our patient, a 24-year-old has a low baseline pretest odds or prevalence of risk of aneuploidy, of trisomy. One in 1400 then has the thick nuchal fold, which has a likelihood ratio of 11, as we showed with the ultrasound screening form as an isolated finding.

So we multiply the pretest risk of one in 1400 times likelihood ratio of 11 equals the post-test risk of one in 127, which is much higher risk than the age-based risk of a 35-year-old woman of one in 250, which is our cutoff for offering women amniocentesis.

So she had a higher risk than one in 250, but she declined genetic counseling and her outcome was quite healthy and normal, a normal spontaneous vaginal delivery term infant.

So what is too thick after 21 weeks really is not really any good data. So we can't determine that.

So what is the answer to our question here? It's really the first two, either six millimeters or five millimeters, but the latest you can really measure the nuchal fold is 20.6 weeks after that. There's no good data at Dartmouth Hitchcock for various reasons, we have chosen five millimeters.

Nuchal Translucency

Okay, moving on to the closely related nuchal translucency. We've just backed up the screen process to the first trimester.

And this is a very popular phenomenon.

Generally the thick nuchal translucency is considered to be more than 2.5 to three millimeters.

The way we measure this is obviously known to most of you sonographers or sonologist. It's a very precise meticulous measurement.

Sonographers or doctors have to become credentialed to do this submitting test cases to a central agency.

You measure the inner to inner border of the skin to the soft tissues over the cervical spine, and it is strongly associated with aneuploidy, also with cardiac defects, lymphatic disorders, fetal infections and anemia.

And some people think it's a form of cystic hygroma.

It's also important to remember that some cases of thick nuchal translucency may be normal variance in a few cases.

Here's an example of a very thick nuchal translucency five millimeters here, and it actually extends down over the trunk in pelvis.

This is a cystic hygroma that's actually much thicker than the average nuchal translucency.

Here's a very markedly thickened nuchal translucency that is actually a cystic hygroma with septated cystic collections here, and very thick two to three centimeters.

Nuchal translucency appears to arise from altered composition of the extracellular matrix that is encoded on chromosomes 21, 18 and 13.

So if you have a trisomy, an extra chromosome of one of these chromosomes, you'll therefore get extra cellular protein matrix thickening.

Other causes are cardiac failure or cardiac abnormalities, venous congestion, abnormal delayed development of lymphatic system, poor lymphatic drainage abnormalities, and finally, fetal anemia and infection.

A recent study on the ultrasound findings in Trisomy 21, besides nuchal translucency or absent nasal bone at about 43%, a major heart anomaly or defect in 41%, an aberrant right subclavian artery in 25%, an extra cardiac abnormality in 24%, and mild ventriculomegaly defined as greater than five millimeters and 17%, and a thick nuchal fold greater than or equal to five millimeters and 16%.

Mild Ventriculomegaly

So moving onto our fourth topic of mild ventricular megaly.

This is a mildly dilated ventricle measuring 13 millimeters.

The conventional thinking has been all along that normal ventricles should not be greater than 10 millimeters throughout gestation.

I think people like this number, it's easy to remember, it's a relative round number, but I think there's some data that's been around for actually quite a while that shows some ventricles may be up to 11, 12, even 13 millimeters in normal situations.

Place to measure the ventricular atrium is at the level of the glomus, near the atrium.

But always be aware that some sonographers or doctors may measure the lateral sulcus of the white matter as it dives down to go more caudal at in the brain.

And here's an example. This is not the ventricle up here. This is the lateral sulcus of the white matter.

Here's another picture. L is the lateral sulcus of the white matter. Here's the F for the fox, and another shot here.

And here's where you wanna measure the ventricles back in the atria where the choroid plexus fills it.

Some not good measurements that you recognize probably from being a astute sonographer or sinologist.

One can see that the ventricle here is actually measuring 14 millimeters, but it's a obliquely sagittal view. So you're actually seeing the ventricle from a sagittal plane more so than an axial plane.

Here is a place where the sonographers got a more axial view, but they placed the calipers in the frontal horn, not the place to measure the ventricles.

Another view showing, again, frontal horn placement. Here. Again, not good. And this is a more of a coronal shot here. We're getting some of the cerebellum and not the position to measure the ventricles.

So you wanna have an axial shot just above the thalami at the level of the atria.

This is a good picture for measuring the ventricles, but the tech has placed the calipers on the white matter here, so getting both the white matter and the ventricles getting a ventricle that's purportedly 13 millimeters, but the ventricle is actually quite normal. Seven millimeters here when the caliper is correctly positioned.

Ventricular megaly occurs not uncommonly, it's estimated to be one per a thousand live births, but most people think this is an underestimate due to some stillborns or patients not having a ultrasound in the first or second trimester.

And the ventricles in males are felt to be slightly larger than females normally. And most or some experts allow up to 11 to 12 millimeters.

So we'll talk about mild ventricular megaly now that's defined as 10 to 15 millimeters.

And the bottom line is that isolated finding of mild ventricular megaly may not have serious long-term consequences.

There is a slightly increased risk of CNS and non CNS anomalies if it's an isolated finding, especially in females, much more so than males, or if both lateral ventricles are dilated or if the ventricles are more than 12 millimeters.

A few studies talking about this, this is one from the white journal. Ultrasound and obstetrics and gynecology had an abnormal outcome in a quarter of patients. 4% had perinatal death, 4% had aneuploidy, 9% had ultrasound false negatives for malformations, and 11% had neurologic sequela, female fetuses much more neuro sequela, 23% versus 5%.

When the ventricles measured more than or equal to 12 millimeters. They found that when the ventricles were only dilated on one side, usually a benign process when it was bilateral, they were more often associated with aneuploidy.

And we offer amniocentesis when it's bilateral mild ventriculomegaly.

We also recommend torch screening as one study from Europe found 16% of hydrocephalus cases were related to torch infections.

Rarely is it a marker of more severe CNS lesions.

This is a larger study of 176 patients, and patients were broken down into mild, moderate, or severe ventricular megaly.

Most of the cases were in the isolated group, around 60%, about a quarter in the group B, and about 40% in group C.

And they found if it was an isolated finding, those patients who were alive at two years were 98% of group A, 80% of group B, and only 33% of group C.

And of those who were still living at two years, they found normal neurodevelopmental outcome in 93% of group A patients, 75% of group B, and 63% of group C, which is somewhat surprising finding given the more severity of the hydrocephalus in those cases.

They did find an increased risk for developmental delay in about a third of the cases versus 5% of controls.

And they did also find a serious significant correlation between ventricular size and mental development index.

But again, they found that the ventricles were only borderline dilated up to 12 millimeters. Patients did well.

If they were above this level, they were monitored more closely, and they tended to have a worse prognosis.

And the natural course during pregnancy was that the 60% of the dilated ventricles remained stable, 30% resolved and 10% worsened.

They also found a developmental delay to be greater in more than 12 millimeter dilatation than in less than 12 millimeters, 23% compared to 3%.

And they found that most fetuses with isolated mild ventricular megaly resulted in healthy infants.

But you can see that this, there's somewhat conflicting data on this and I think most reasonable experts in this would recommend a fetal MRI if there is hydrocephalus detected in utero.

But you can see the counseling patients is difficult. There's conflicting data that's pretty much all over the map.

Marked Ventriculomegaly

Marked ventricular megaly, I think is not such a dilemma. Ventricle is more than 15 millimeters can be either isolated, usually due to aqueduct stenosis or communicating hydrocephalus.

Other neural tube defects can cause it, including Chiari malformation, midline defects, or aneuploidy, chromosomal disorders and syndromes.

These tend to have a bad prognosis.

Here's a case of moderate to severe hydrocephalus at 18 weeks, 15 millimeters and 18 millimeters, and at 35 weeks has progressed to 49 millimeters with not much cortical cortex left.