CASE REPORT Occlusion of Arterio-Arterial Anastomosis Manifesting as Acute Twin–Twin Transfusion Syndrome T. Y. T. Tan a,b , M. L. Denbow a,b , P. M. Cox c,d , D. Talbert b and N. M. Fisk a,b,* a Centre for Fetal Care, Queen Charlotte’s and Chelsea Hospital, Hammersmith Campus, Du Cane Road, London, W12 0HS, UK; b Institute of Reproductive and Developmental Biology, Imperial College, Hammersmith Campus, London, UK; c Department of Histopathology, Imperial College, Hammersmith Campus, London, UK; d Department of Histopathology, Birmingham Women’s Hospital, Metchley Park Road, Birmingham B15 2TG, UK Paper accepted 11 August 2003 In vivo, ex vivo and modelling studies suggest that arterio-arterial anastomoses (AAAs) protect against haemodynamic imbalance in monochorionic twins and thus the development of TTTS. We report the acute onset of severe TTTS at 34 weeks’ gestation in a patient with an antenatally visualized AAA which was shown at injection studies to have been obliterated, presumably by thrombosis. Computer modelling with the relevant clinical data confirmed that occlusion of the AAA alone was sufficient to reproduce the clinical manifestations. A study of the vascular configuration of AAA in the fixed placenta suggested that its small diameter and turbulent flow may have contributed to its occlusion. This case report shows that the unmasking of unbalanced AVA configurations by occlusion of a protective AAA can manifest as TTTS.  2003 Elsevier Ltd. All rights reserved. Placenta (2004), 25, 238–242 INTRODUCTION It is now well accepted that almost all monochorionic placentae contain vascular anastomoses resulting in inter-twin trans- fusion, and this has been confirmed in vivo by marker studies [1,2]. Superficial anastomoses, both the common arterio- arterial anastomoses (AAA) and the rarer veno-venous anasto- moses (VVA), directly link the twins’ circulations on the chorionic plate to allow bi-directional flow. Deep arteriovenous anastomoses (AVAs) supply shared cotyledons, allowing unidirectional flow from a chorionic artery of one twin to a chorionic vein of its co-twin. Several lines of evidence suggest that bi-directional anasto- moses compensate for unidirectional net transfusional imbal- ance mediated by arterio-venous anastomoses. Firstly, the presence of AAAs is significantly less frequent in placentae of cases with twin–twin transfusion syndrome (TTTS) compared to placentae of those without TTTS [3–5]. Secondly, the most common anastomotic pattern in TTTS is absent AAA with the presence of one or more AVAs [5]. Thirdly, cases of TTTS that occur in association with an AAA are more likely to have a milder phenotype and better survival than those that occur without an AAA [6,7]. Using Doppler, we have confirmed in vivo this predictive value of absent AAA for TTTS and suggested a role for AAA detection in the routine assessment of monochorionic twins [8]. Lastly, our own earlier work on computer modelling of bi-directional anastomotic flow [6] and recent work of others in a more refined model [9] demonstrate that even small diameter AAAs can compensate for a wide range of AVA flow. Here, we report the occurrence of an acute and atypical late onset TTTS in association with occlusion of an AAA and use computer modelling to demonstrate that the pheno- typic appearance of TTTS was due to acute cessation of compensatory AAA flow. CASE REPORT Clinical history Monochorionic diamniotic placentation was diagnosed at 12 weeks’ gestation on ultrasound which showed normal nuchal translucency of 2.0 mm each. Single puncture amniocentesis at 16 weeks for maternal age showed a normal karyotype. The mother was then monitored fortnightly for fetal growth and Doppler studies, and to detect early signs of TTTS. Fetal echocardiography was normal in both twins at 20 weeks. At 26 weeks, an AAA was detected [6,7]. The twins showed appro- priate growth, amniotic fluid volume and normal umbilical Doppler studies on fortnightly assessment, and at 32 weeks * To whom correspondence should be addressed. Tel.: +44-(0)207- 594-2144; Fax: +44-(0)207-594-2156; E-mail: n.fisk@imperial.ac.uk Placenta (2004), 25, 238–242 doi:10.1016/j.placenta.2003.08.018 0143-4004/04/$–see front matter  2003 Elsevier Ltd. All rights reserved.