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Re: OB-GYN-L digest 695From: George Saade (george.saade@utmb.edu)Thu Oct 31 08:47:48 1996
In response to Dr. Brown's question about the mechanism by which septostomy works in oligohydramnios/polyhydramnios sequence: We do not have a definite scientific reason. The same can be said about decompression amniocentesis. One major drawback is that we really do not fully understand the pathogenesis of TOPS. If one looks carefully, all monochorionic placentae have vascular communications. Why then do we see TOPS in only a few cases? Why we do not see it in monochorionic-monoamniotic pregnancies? My speculation is that a difference in pressure may promote whatever mechanism is causing TOPS. By equilibrating pressure, either by septostomy or decompresion amniocentesis, the mechanism is reversed or halted. I have been asked whether we know if the pressures were different between the sacs. We tried measuring pressure and failed because we cannot obtain a reading unless the tip of the needle is free within a pocket of fluid, which was impossible in the oligo sac. Others have measured amniotic fluid pressure in polyhydramnios and found it increased. Furthermore, Doppler indices improved following decompression. We also have an indirect evidence that there is a pressure difference between the two sacs. When the septostomy is performed, we note a relatively immediate increase in fluid in the oligo side (within half an hour). This is faster than can be explained by fetal urination, especially that we did not notice bladder filling in the oligo fetus. The only explanation is that the pressure difference between the two sacs drives the fluid from the poly side to the oligo side. In fact, if we observe the membrane carefully, we can see it move while still bulging toward the oligo side. If the ultrasound is repeated the next day, the membrane is seen floating from one side to the other without tension on it. I hope this makes sense. I will be happy to answer other questions.
-- George Saade, M.D. The University of Texas Medical Branch Galveston, TX Tel: (409)747-0482 Fax: (409)772-2261
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