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Obstetric Ultrasound and Abnormalities Detected on Ultrasound

This is a study that was published in prenatal diagnosis just in 2006 and you can see it’s three ultrasound units over a five year period, over 20,000 fetuses screened, and they picked up about a third of the anomalies and not surprisingly, CNS were easier to pick up. Gastrointestinal abnormalities may be lower than you might expect might things like intestinal atresias, tracheoesophageal fistulas are not always so easy to pick up, GU even more surprising and cardiac a low of 5.9%. Having to review our own statistics, we do a little better in cardiac, we actually pick up about 70%.

Let’s start in the beginning and talk a little bit, especially if we are talking about the first trimester scan or the nuchal translucency, it’s really a hot topic now and as you know, it’s performed between 9 and 14 weeks and either absolute cutoffs are used for the thickening 3 mm or what Nicolides group in the UK with the most experienced talks about is greater than the 95^th percentile for that crown rump length measurement, and actually, in high risk patient’s it worked very well, it picks up 80% of the cases.

The interesting thing that is coming up is that even if you wind up having chromosomes tested and they are normal, these fetuses do with an increased nuchal translucency appear to be at increased risk for congenital cardiac defects, coaptation of the aorta, septal defects, and kind of a little laundry list of other things, so in general, these patient’s need to have detailed scans, echos, good comprehensive scan to make sure there are no cardiac anomalies later on.

Let’s talk more when we get into the second trimester, mid trimester about the fetal structural survey, and with rare exceptions except for emergencies, placental locations, and things like that, when you are at the right gestational age, you are really obligated to do a detailed field structural survey if you are picking up the transducer. These guidelines, what you should actually be looking for comes out of AIUM really the credentialing obstetric ultrasound body now, which ACOG is in line with, and again, anywhere, most people say starting at 18 weeks or so, some people are brave and say 16, but the things you need to be looking at are the cerebral ventricles, cerebellum, head shape, spine, thorax, four chamber view of the heart, abdomen, stomach, kidneys, bladder, umbilical cord insertion and the extremities and in fact, if you look at what’s picked up on a routine ultrasound examination, just by these really level one or normal anatomy over here, these are all the abnormalities that you can pick up.

The other thing I think is interesting and I talked to the attendings where I work, I have noticed this trend in patient’s that are over 35 years old, they get level II scans ordered all the time even though they have had the genetic testing, and there is no reason to think in the absence of congenital malformations associated with karyotypic abnormalities that a woman over 35 is going to have increased risk of congenital malformations if you have already assessed the fetus genetically. This is actually a picture of a diagnosis that should never be missed on ultrasound, assuming that you can see it, it’s a first trimester diagnosis, of anencephaly,

Moving down, the other thing we look at, is really something to evaluate, the ventricular system or rule out hydrocephalus, and our landmarks are the falks, your midline echo over there, and then the wall of the lateral ventricle, and generally speaking, this ratio to the hemisphere should not be more than 25% after 24 weeks. Clearly that is a gestational dependent marker, so before that, you are not really sure whether it’s normal or hydrocephalus.

Moving toward the measurement that we use now, the atrium of the lateral ventricle to evaluate the ventricular system for ventriculomegaly or hydrocephalus. This is a gestational age independent marker, as soon as you can get that view, which should never be more than 10 mm, again regardless of gestational age, so it’s very helpful before 24 weeks. This is just a demonstration of mild ventriculomegaly and what I mean by that, is if I said normal is 10, mild is anywhere from 10 to 12 or some people even group them in from 10 to 15, and when you see ventriculomegaly.

Talking about the diagnosis of ACC, absence of a corpus callosum, under normal circumstances, this is a coronal view of the head, you can see this is the third ventricle and really the corpus callosum, really forms the roof of the third ventricle, keeping it in that position and you can see here the lateral ventricles. When you have absence of the corpus callosum, essentially you have unroofing of the third ventricle, you see that it’s superiorly displaced and the lateral ventricles appear more displaced or wider.

Coming down the system, the next thing you usually see, I showed you the choroid just kind of filling the atrium of the lateral ventricle, this is a demonstration of choroid plexus cysts, they are bilateral, this is still controversial, we are still trying to figure out what to do. Initial reports that came out in fact, our experience, when one of our fellows at that time reported on it was that it didn’t matter if it was unilateral, bilateral, if it regressed, etc., but it was still a marker not only for trisomy 18 but 21, even it was isolated and the level II was otherwise normal. Other studies have come out with different results, and in fact, if you take a look, it seems to be very center dependent, how good you are at picking up trisomy 18 and how comfortable you are with your detailed scan, and in fact, probably the largest meta analysis by Gross that was done put them all together isolated and appeared that the risk was about 1 in 374, and so thought or recommended that if it was isolated, it really wasn’t an indication.

Staying in the head and coming down a little bit further into the posterior fossa, one of the views that we get is really our view for cerebellum and the nuchal occipital thickness and our landmarks are really the cavum septi pellucidi in front of the thalami, those three lines, then you have your peduncles over here, you have your nice, round cerebellar hemispheres, you measure it here. Interestingly, the knot, if you look at any one single predictor of trisomy 21, that’s probably the best one we have. It’s increased in 80% of newborns with trisomy 21, the cutoff that we use is greater than or equal to 6 mm during that gestational age. Depending on the study that you quote, the positive predictive value is anywhere between 1 and 4 and 1 in 13 for trisomy 21.