Twin-to-Twin Transfusion Syndrome (TTTS) can occur in monochorionic twin pregnancies when abnormal blood vessel connections form in their single shared placenta and allow blood to flow unevenly between the identical fetuses. Perinatologist and twin expert, Dr. Larry Rand, explains the clinical characteristics of TTTS, how it is diagnosed, the various stages of the condition and why it is so important to differentiate between those stages.
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Today we're gonna talk about monochorionic twin pregnancies and their complications. We said earlier that all monochorionic twins, because they share this one placenta, are connected under the surface by some of their blood vessels, which allows the blood to transfuse back and forth. Because they're identical and they have the same blood type, this is not a problem, and as long as they send the blood back and forth evenly, there's no disease. That's what happens 90% of the time with monochryonic twins. But 10% of the time, one twin sends blood to its co-twin and doesn't get enough back. And when this happens, we have twin to twin transfusion syndrome, or TTTS for short. To fully understand this, we need to look a little bit more closely at the anatomy of these blood vessel connections. First, when we think of blood vessels, there are two kinds in our body arteries and veins. What makes them different is that an artery is surrounded by a layer of muscle, and that allows it to pulse. The purpose of a blood vessel pulsing is so that it can send blood in a direction. Veins on the other hand, don't have any muscular layer and they can't pulse. That's why when an artery connects to a vein, the blood will go from the artery to the vein in a unidirectional channel or a one-way street. We call these one-way streets arteriovenous connections or AVs for short. There's another kind of connection, which is an AA connection that if it exists, is really great and quite protective. Every now and then, an artery from one fetus will meet with an artery from the other fetus. Because each of those arteries has a pulse and is large, these kinds of connections actually allow bidirectional flow. In other words, unlike a one-way street, they act more like an avenue, relieving the traffic congestion. And if a pair of monochorionic twins is lucky enough to have an AA, it will serve a major protective function against forming true twin to twin transfusion syndrome. Having one doesn't make you immune from developing twin to twin transfusion, but it significantly helps. Here's what happens, especially when there's no AA. Let's say there are 3 AV connections going from twin B to twin A. But there's only two AV connections coming back. Over time, twin B is going to send more blood to twin A than twin B can get back. If there were an artery to artery or AA connection here, it would help make up for this imbalance in vascular connections. But without one, twin B will keep donating blood to twin A and will become anemic and have a low count of red blood cells. All of us, including fetuses, need these red blood cells in order to carry oxygen to our tissues. So you can imagine, without enough red blood cells, over time, there can be some significant damage, especially to tissue that's very sensitive to oxygen, like the brain or the heart. In this example, twin A is overloaded with blood that it can't send back to twin B. It acts as the recipient. The recipient twin's heart has to beat all this extra blood volume around its body, which puts quite a strain on it. The way we see this though, is as a difference in the amount of amniotic fluid that's around each baby. So the way that we can tell that twin to twin transfusion syndrome is happening is that the donor, who's anemic and is holding onto everything, isn't peeing enough and has very low amniotic fluid volume around it. The recipient, who's overloaded, is peeing like crazy and has way too much fluid around it. Ultrasound is used to be able to find how much fluid is around each baby. Using ultrasound, we look at each baby's sack individually and measure the deepest pockets from front to back. In order to be diagnosed with true twin to twin transfusion syndrome, you have to meet very specific criteria. One of the twins has to have 2 centimeters or less of amniotic fluid, and the other twin has to have 8 centimeters or more of amniotic fluid. In other words, at the same time, oligohydramniose in one and polyhydramniose in the other. The numbers 2 centimeters and 8 centimeters are actually really important because there can be variations in what a normal amount of amniotic fluid or baby urine is at any given point in the day. So the numbers really need to be at the extremes of being low or being high at the same time in order to be reflective of twin to twin transfusion syndrome as the underlying cause. Once you meet the criteria of having less than 2 and greater than 8 at the same time, We have to determine what stage of TTTS you have. Stage one simply means that you meet the criteria of less than 2 centimeters and greater than 8 centimeters, and there are no other remarkable findings. If the donor twin is so anemic that it's holding onto everything and actually slowing down its urine production so much that its bladder is no longer visible, That's stage 2. To have stage 3 TTTS we see evidence of too much strain on the recipient's heart. Stage 4 TTTS means that the recipient twin's heart is under so much strain that it's actually developed heart failure. This is called hydrops. The word hydrops is actually Latin for fluid retention, something that happens when the fetal heart starts to fail. Stage 5 twin to twin unfortunately means that one of the fetuses has died. If you do have twin to twin transfusion syndrome, in our experience, the most important differentiating factor is whether you are stage 1 disease or if you've progressed to stage 2 or worse. That's because if you have stage 1 TTTS there's a 50% chance that the situation is going to improve on its own. It turns out that fetuses who have an AA connection seem able to balance the blood flow somewhat better and are able to correct the situation or at least keep it at bay without progressing to stage two or beyond. The other 50% roughly, who likely don't have an AA to protect them, will progress and have stage two or worse disease. Once you have stage two disease, there's an 80 to 90% chance that you will lose one or both fetuses. Stop and think about that for a moment. The difference between stage one and stage two is phenomenal. To go from a situation where you have a 50% chance of getting better on your own, to one where there's an 80-90% chance of actually losing both twins, is really night and day. And that's why we make such a big deal about differentiating between stage one and stage two.
