The superior vena cava (SVC) is a venous trunk that forms from the junction of the right and left brachiocephalic veins on the right side of the chest, facilitating venous return from the upper body to the right atrium (RA). Various developmental anomalies can arise due to deviations from the normal embryological process^[1].

CT offers high spatial resolution with a short acquisition time, allowing for multi-planar reconstructions and volume-rendered imaging to better visualize anomalies. Optimal enhancement of the SVC is achieved with CT venography, performed 60–75 seconds after intravenous injection of iodinated contrast through a peripheral vein^{[2, 3]}.

Anomalies of superior vena cava comprise of^{[2, 3, 4]}:

1. persistent left SVC (PLSVC) with co-existing right-sided SVC – also known as double SVC - with/or without a bridging vein (left brahiocephalic vein);
2. isolated PLVSC;
3. superior vena cava aneurysm - a rare, asymptomatic finding often detected incidentally on imaging;
4. right upper lobe partial anomalous pulmonary venous return (PAPVR) - a rare congenital anomaly where one or more pulmonary veins drain into a systemic vein or the right atrium instead of the left atrium.

Multiple classifications of PLSVC are described in the literature, with Schummer’s classification being the most commonly used for the supracardial venous system^[5]. This classification includes:

• Type I: normal venous anatomy;
• Type II: isolated PLSVC;
• Type IIIa: double SVC connected by a bridging vein;
• Type IIIb: double SVC without connection.
  

  Fig 1: Schummer classification of PLSVC[5]: Type I: normal venous anatomy; Type II: isolated PLSVC; Type IIIa: double SVC connected by a bridging vein; Type IIIb: double SVC without connection. RSVC: right superior vena cava; PLSVC: persistent left superior vena cava; LBCV: left brachiocephalic vein.

The PLVSC can have multiple drainage sites, namely^[3,6,7]:

• most common: the right atrium through the coronary sinus (CS), which appears dilated;
• left atrium (LA) through:
  • the left atrial appendage;
  • defective separation of CS from LA called Raghib syndrome;
  • the adjoining of the posterior wall of the SVC to the right upper lobe pulmonary vein (also called unroofed);
• inferior vena cava;
• hepatic veins.

The PLVSC occurs in less than 0.5% of the general population but is found in up to 10% in patients  with congenital heart disease ^[8].

Numerous cardiac anomalies (CAs) associated with PLSVC have been identified and categorized into various groups. These include shunt lesions, conotruncal malformations, left-sided obstructive defects, right-sided anomalies, and single ventricular abnormalities. Additionally, PLSVC is linked to aortic arch anomalies, such as a right-sided aortic arch and coarctation of the aorta. According to the literature, PLSVC frequently coexists with heterotaxy, occurring in 50–70% of cases. Heterotaxy encompasses situs inversus and situs ambiguus (right/left isomerism) ^{[9, 10]}.

The clinical significance of PLSVC depends on various factors^{[2,4,11,12,13]}, including:

1. dilatation of the CS:
   • atrioventricular node and His bundle compression, potentially leading to cardiac arrhythmias, such as atrial or ventricular fibrillation;
   • left atrial compression, resulting in decreased cardiac output;
   • complications during mitral valve surgery due to the close anatomical relationship between the CS and the mitral valve.
2. constriction of the CS or atresia of the CS ostium:
   • difficulties in catheterization, with risks of life-threatening arrhythmias, cardiogenic shock, or cardiac tamponade;
   • acute obstruction of cardiac venous drainage in cases where the PLSVC is ligated.
3. importance of preoperative SVC anatomy assessment for invasive procedures and cardiac surgery:
   • central venous catheter (CVC) placement: insertion without fluoroscopic guidance may cause angina, arrhythmias, hypotension, cardiac arrest, or even cardiac perforation;
   • cardiac resynchronization therapy (CRT) lead or pacemaker; implantation: the presence of PLSVC may complicate lead positioning.
   • venous rerouting procedures and major cardiac surgeries, including cavo-pulmonary anastomosis (Glenn, Fontan), heart transplantation, and cardiopulmonary bypass;
   • retrograde cardioplegia: PLSVC can lead to inadequate myocardial perfusion, affecting surgical outcomes.
4. left atrial drainage of PLSVC:
   • may be asymptomatic;
   • can result in severe cyanosis, syncope, exercise intolerance, thromboembolic events, and even brain abscesses.