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