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The persistence of a left superior vena cava (LSVC)
is due to a development abnormality of the sinus venosus in the early stages of fetal life. In the
4 mm embryo, this structure consists of 3 distinct parts, namely the right horn, the transverse
part, and the left horn; due to the rightward direction of blood flow, the right horn undergoes
preferential growth, and becomes the only communication between the atrium and the venous
sinus. The transverse part and the proximal left horn merge to form the coronary sinus, whereas
the distal left horn and the left cardinal vein obliterate. The remnant of these two structures in
the adult subject is termed as the ligament of Marshall. This peculiar embryogenetic development
clearly depicts the anatomy of LSVC persistence: in case of permanent patency of the left cardinal
vein, its drainage reaches the right atrium through the coronary sinus, which becomes enlarged to
variable degrees.
This abnormality has been reported to occur approximately in 0.5% of the general population, and
may be associated with other congenital cardiac abnormalities besides drainage in an enlarged
coronary sinus. When isolated, it is usually not recognised until a left superior access to the heart
is required; it constitutes then a relevant anatomical finding. In fact, it can complicate proper
positioning of left sided pacemaker (PM) and cardioverter-defibrillator (CD) leads, since the access
to the right atrium occurs posteriorly and at the os of the coronary sinus1-4; moreover 10% of
these subjects do not have a Right Superior Vena Cava (RSVC).
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