Friday, March 29, 2013

Yolc in Ultrasound

3D ultrasounds imaging my give additional data to functional Doppler studies of research in developmental anatomy and embryology. This method allows a detailed morphologic and volumetric analysis of extraembryonal static structure. Conventional methods for measuring volume of fluid-filled spaces include modeling of shapes (e.g. using and ellipsoidal approximation).
Using 3D planar mode, the position of the yolk sac wall is accurately spatially assessed. Measurement of the volume, rather than estimation from the simple geometric model, increases the accuracy of the measurement. Growth and appearance of the yolk nsac have been correlated with the outcome of the pregnancy. “Kupersik and Kurjak” measured gestational sac volume and yolk sac volume and vascularity in pregnancy between 5 and 12 weeks of gestation. Regression analysis revealed exponential growth of the gestation volume throughout the first trimester of pregnancy.
With the formation of the extraembryonic celomic cavity at the end of the 4th weeks, the primary yolk sac is replaced with newly formed secondary yolk sac. During organogenesis and before the placental circulation is established, the yolk sac is the primary sources of exchange between the mother and the embryo. It has nutritive, metabolic, endocrine, immunological, excretory and hematopoietic functions. At the beginning of the 5th weeks it become is visible as the first structure inside the chorionic cavity. At this time, a circular, well-defined, eco free area measures 3 to 4 mm in diameter, while the gestational sac measure about 8 to 10 mm. the yolk sac grows sliwly until it reaches a maximum diameter of approximately 5 to 6 mm at 10 weeks. Its stalk can be followed from its origin all the way to the embryonic abdomen. As the gestational sac grows and the amniotic cavity expands, the yolk sac as an extraembryocic structure is gradually separated from the embryo. Different theories exist about the destiny of the yolk sac. Until recently, it was assumed the it gets caught and compressed between amnion and chorion and than ultimately disappears by the end of the 11th weeks.
Recent studies emphasized that instead of getting compressed; the yolk sac degenerates first and than consequently disappears. The ultrasound appearance of the yolk sac has already been proposed as a prognostic parameter for the outcome of pregnancy. Kurjak and coworkers established some criteria for distinguishing between normal and abnormal yolk sac appearance. In their experience, yolk sac should always be visible before viable embryo, it measures 4.0 to 5.0 mm in diameter until 7 to 8 weeks of gestation and reaches 6.0 to 6.5 mm by the end of the 9th week. It is evident that sonographic detection of abnormal yolk sac morphology may predict abnormal fetal outcome. Absence of yolk sac, its abnormal size, echogenicity, shap and number are predictive indicators of early pregnancy failure. All these parameter should be defined and assessed prior to 10 gestational weeks. Abnormal yolk sac size may be the first sonographic indicator associated failure. Primarily, the presence of an embryo without the visible yolk sac before 10th gestational week is mostly an abnormal finding.
According to some authors, the inner diameter of the yolk sac is always less than 5.6 mm in a normal pregnancy before the 10th week of the gestational age. Lyons established that for a mean gestational sac diameter of less than 10 mm, the yolk sac diameter should be less than 4 mm. in 15 patient who had abnormally large sacs, six had no embryo, five aborted spontaneously and only one conceptus survived. Out of nine other with embryo and large yolk sac, eight patients aborted and in one patient trisomy 21 was detected at the 24th gestational week. The yolk sac can be too small, and this is accepted as a marker of poor pregnancy outcome. Green and Hobbins reported a group of patient distributed between 8 and 12 weeks gestational age, with a yolk sac diameter less than 2 mm associated with an adverse outcome.
It is unknown whether abnormalities of the yolk sac are related primarily to the yolk sac or secondary to the embryonic maldevelopment. According to the present data, it seems that the yolk sac plays an important role in materno fetal transportation in early pregnancy. Change in size and shape could indicate or reflect the significant dysfunction of this system, and therefore, could influence early embryonic development.
Currently, major benefits of the sonographic evaluation of the yolk sac are:
1. Differentiation of potentially viable and nonviable gestation
2. Confirmation of the presence of an intrauterine pregnancy against decidual cast, and
3. Indication of a possible fetal abnormally

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