Twin to twin transfusion syndrome, otherwise known as TTTS, is a severe complication of monozygotic twinning. It is relatively common. It is said to occur in 10 to 15 percent of all mono chorionic twins. TTTS is often a lethal condition. It accounts for 17 percent of perinatal deaths in twins.
The primary defect in cases of TTTS is thought to be abnormal placentation. It is characterized by a paucity of superficial anastomoses together with the presence of a solitary unidirectional deep arteriovenous anastomosis flowing from the donor twin to the recipient twin. In TTTS, one twin may act as the blood donor most of the time, while the other acts as the recipient. This causes an imbalance in the blood distribution, causing the donor twin to be anemic and smaller. The other twin will be abnormally bigger. The said imbalancealong with other vascular anastomotic configurations leads to a circulatory disequilibrium.
Prenatal management strategies in women whose pregnancies are diagnosed to have TTTS are aimed at amelioration of polyhydramnios and at correction of the underlying vascular anomalies in the shared placenta. TTTS case eventually leads to polyhydramnios and eventually to cardiac dysfunction in the recipient twin, and oligohydramnios or anhydramnios due to hypovolemic oliguria in the net donor twin.
Two treatments, the serial amnioreduction and fetoscopic laser ablation of anastomotic vessels are currently undergoing active investigation. Amnioreduction is a variant of amniocentesis. In such procedure, amniotic fluid is removed in order to restore normal fluid volume. Fetoscopic laser therapy with the aid of zoom microscope on the other hand is designed to correct the underlying abnormality by separating the two fetal circulations. Refinements of laser therapy have focused on the selective ablation of those few arteriovenous anastomoses causing disease.
Fetoscopic laser ablation of the placental vascular equator is a means of treating severe twin-to-twin transfusion syndrome. It is a relatively simple fetoscopic technique that can be performed under local anesthesia. The said procedure takes a very short operating time. It uses a preoperative ultrasound identification of the placental common villous district and vascular equator of the placenta by determining the umbilical cord placental insertion sites. Four vessels were ablated during each procedure and operating time ranged from 5 to 25 minutes.
Women with pregnancies complicated by severe TTTS before their 26 weeks of pregnancy were involved in the study. The process of fetoscopic laser ablation of placental anastomoses with the aid of zoom microscope was performed in these women. The article tells that the sonoendoscopic approach was used to identify the placental vascular equator and to photocoagulate crossing vessels.

There are several outcome criteria to be observed in the study. It included observing the overall survival, fetal and perinatal mortalities, and gestational age at delivery, birth weight, operating time and recurrence of TTTS.  The results of the investigation showed that total of 77 women with TTTS underwent the procedure. The mean gestational age at treatment was 20 weeks. This ranges from range 16 to 26 weeks. During the procedure, on an average, four vessels were ablated during every procedure. This ranged from 5 to 25 minutes. The results showed that none of the women required a repeat fetoscopic laser treatment for recurrence of the TTTS. There was at least one survivor in 74 percent of pregnancies. The overall survival rate was 57 percent.

The authors of the study concluded that fetoscopic laser ablation is a safe and effective form of treatment in the management of severe TTTS. The technique of identifying the common villous district of the placenta by ultrasound and photocoagulating any vessels crossing the vascular equator has shown to be an acceptable alternative to both the nonselective and highly selective methods described so far. The said approach is associated with a short operating time, low likelihood of TTTS recurrence or fetal anaemia and with survival results that are equivalent to previously reported techniques. Here is a link to the article