Introduction
Mirror syndrome was first described in 1892 by Ballantyne. It presents a clinical picture of maternal oedema associated with foetal hydrops and placentomegaly, which must both be present. It is a rare entity, and in many cases, it is underdiagnosed in routine medical practice. Its actual incidence is unknown, and descriptions of this pathology are scarce in the literature. Proof of this is a recent literature review with a sample of only 113 cases reported between 1956 and 2016.1 Maternal oedema, which is the most characteristic sign and symptom, is clinically diagnosed by the presence of fovea after applying digital pressure for five seconds.2 Hydrops fetalis is the pathological accumulation of excess fluid in two or more foetal compartments.3 Placentomegaly is defined as a placental thickness greater than 4 cm in the second trimester or greater than 6 cm in the third trimester, according to the published literature.4
Regarding the main causes of mirror syndrome, the most frequent is foetal hydrops, which can have an immunological or nonimmunological aetiology and can also be due to viral infections or foetal malformations. Other symptoms that are commonly present in pregnant women with this syndrome are arterial hypertension, mild anaemia due to haemodilution, proteins in the urine and elevated liver enzymes. In a high proportion of cases, perinatal death can occur as an adverse event of pregnancy.5
Erythrocyte alloimmunization during pregnancy is caused by the presence of maternal antibodies against antigens present in foetal red blood cells. The destruction of foetal erythrocytes, together with liver damage and endothelial injury resulting from hypoxia, are sufficient pathophysiological mechanisms for the development of hydrops in the foetus. This immune mechanism responsible for hydrops occurs in approximately 15% of cases detected prenatally by ultrasound examination.6,7
Nonimmune hydrops can have multiple causes, including viral infections, genetic syndromes, foetal heart rhythm disorders and malformations. At present, it is much more common than cases with immune causes due to the establishment of programmes for the prevention of isoimmunization during pregnancy.3,8 Specifically, the incidence of immune hydrops has decreased since the 1970s due to the use of anti-D immunoglobulin and the establishment of screening programmes at the population level.9 Most clinically significant maternal erythrocyte alloantibodies are adequately detected with routine screening during pregnancy with an indirect Coombs test; however, there are cases of maternal-foetal isoimmunization due to erythrocyte antibodies against very low frequency and atypical erythrocyte antigens. These may not be detected with routine tests and can led to severe neonatal haemolytic disease.10