Obstetrics is full of acronyms. One of those that can inspire fear and fascination even in a perinatologist is TTTS (Twin-twin transfusion syndrome). Such a simple name, such a complicated disease.
Twins come in many different varieties. It is those that share a placenta that have the potential for this unique complication. Because the placenta is shared, their blood volume is shared. As we know, sharing is not always equal. This is not a disease that a mother can cause or prevent, it is a known complication of a shared placenta. We know is can be mild or severe, it can progress slowly or quickly, and some times is can even correct it self. There are some treatment options, but there are risks to each of the choices.
The twin that is giving blood is call the donor. It is frequently smaller, but not always. In mild disease, it will develop low fluid. Next is lower blood pressure leading to decreased blood supply to the kidneys, so less urine production, in more severe cases this baby will have an empty bladder. More severe disease will be associated with blood flow alterations in the baby, possibly leading toe heart failure (hydrops) and death. The twin receiving blood is the recipient. Again, although this twin tends to be the larger twin, this is not always the case. This baby will receive extra blood and therefore extra fluid. They have more fluid, so they pee more and may develop excessive fluid. This baby is also at risk for alterations is blood flow, heart failure and death.
A placenta is made of a multiple functional units called cotyledons. Each unit has an arterial supply from the umbilical cord and a vein that brings the oxygenated blood back to the baby. You can image the placenta represented by a complex of small circles. (ttts1)
Each baby develops its umbilical cord independently. Each functional unit is attached to an umbilical cord. There may not be an equal distribution of cotelydons, one baby may start off with a greater share of the placenta from the beginning, and this may contribute to difference sizes in the twins. Dark red on the left is twin A, light pink on the right is twin B. (ttts2)
Each functional unit also has a vein returning to the umbilical cord. For the most part, the veins and arteries are paired. Veins for A are dark blue and for B they are light blue. (ttts3)
So far, we have only examined the part of the placenta that is unique to each baby. Because the babies share a placenta, there must be connections in the blood vessels. There are three types of connections: arterial-arterial, vein-vein and arterial-vein. Each has a different impact on the outcome of the pregnancy.
A-A connections. These are actually the least common in healthy mono/di twins. There are often implicated in more serious twin conditions (like TRAP sequence). Because they are both high pressure vessels, there is not a lot of flow between the babies. See the connection highlighted with the yellow ring. (ttts4)
V-V connections. These are the good connections. The more V-V connections, the less risk there is for TTTS. These are both low pressure vessels. Blood will flow in the direction of least resistance (or from higher to lower pressure). If the babies are sharing blood unequally, these are the “escape valve” that allow the blood to travel back to the donor baby. Unfortunately, as the disease progresses, the pressure from the excessive amniotic fluid in the recipient twin can actually compress these vessels. When the escape valve is closed, the disease can suddenly progress quickly. (ttts5)
A-V connections. These are the ones that cause problems. They are not direct vascular connections. Rather one baby supplies and artery to a cotyledon and the other baby supplies the vein. Yellow circles show cotyledons supplied by baby A, orange from baby B. (ttts6)
Because the functional units are made up of microscopic blood vessels that allows the transfer of nutrients and oxygen, these connections are essentially one way. The blood goes from one baby via the artery to the other baby via the vein. Plus the receiving baby receives the nutritional benefit, while the other baby does the work to supply the blood. (ttts7)
TTTS occurs when there is a net transfusion in a single direction. If there are only two A-V connections and one goes each direction, this is likely to be well tolerated. It is when there is a disproportional flow in one direction, particularly with fewer V-V connections to allow the blood to equalized, there is a risk for TTTS. This occurs in about 15% of mono/di twins. Not surprisingly, the most effective treatment we have is to disconnect the A-V connections and stop the one way flow.
Unfortunately, it is difficult to assess the placental blood vessels in pregnancy. When surgery is performed to treat TTTS, the surface vessels are mapped. It has been attempted to do this with ultrasound, but is it not a reliable why to screen for TTTS. The best we can do it assess these babies every two weeks to assess the fluid in each sac. If you are pregnancy with twins who share a placenta, please ask for you ultrasounds at a minimum of every two weeks from 16 weeks onward. If there is a difference, the patient needs to be referred to perinatology. If TTTS is diagnosed, they will then need to be referred to a treatment centre, as the surgery for this disease is highly specialized and concentrated into a few centres.
**This is my first attempt to draw my own picture for the blog. The diagrams are based on the ones I draw for my patients when explaining the risks associated with mono/di twin. They are all PDF attachments**