Cor triatriatum is a rare congenital cardiac anomaly in which a fibromuscular membrane divides the atrium in two. In its most common form, cor triatriatum sinister, the left atrium is divided into an upper chamber that receives the pulmonary veins and a lower chamber that is related to the left atrial appendage and the mitral valve orifice. 
The location of the atrial appendage is a key landmark in this congenital malformation. It differentiates cor triatriatum from a physiologically similar condition, supravalvular mitral stenosis. In cor triatriatum the left atrial appendage is invariably associated with the lower chamber, which is below the membrane.
Rarely, a fibromuscular membrane that is a remnant of the right valve of the sinus venosus divides the right atrium; this finding, so-called cor triatriatum dexter, is usually asymptomatic and is mostly reported as an incidental finding.  However, fetal right ventricular hypoplasia has been described associated with this anomaly and has been hypothesized to result from abnormal fetal circulation leading to growth restriction of the fetal right ventricle.  Owing to its clinical relevance, the remainder of this chapter focuses on cor triatriatum sinister.
Cor triatriatum represents 0.1% of all congenital cardiac malformations and may be associated with other cardiac defects in as many as 50% of cases. Examples of associated cardiac defects include atrial septal defect, persistent left superior vena cava with an unroofed coronary sinus, partial anomalous pulmonary venous connection, ventricular septal defect, and more complex cardiac lesions, such as tetralogy of Fallot, atrioventricular canal, and double outlet right ventricle. Associated bicuspid pulmonary valve, aortic valve atresia, and heterotaxy have also been described. 
In classic cor triatriatum, the pulmonary venous chamber receives all pulmonary veins and it drains into the left atrium through a variable-sized orifice. The natural history of this defect depends on the size of the communicating orifice between the upper and lower atrial chambers. If the communicating orifice is small, the patient presents critically ill and may succumb at a young age (usually during infancy) to congestive heart failure and pulmonary edema. If the connection is larger, patients may present in childhood or young adulthood with a clinical picture similar to that of mitral stenosis. Cor triatriatum may also be an incidental finding when it is nonobstructive.
Other anatomical variants of cor triatriatum include a pulmonary venous chamber that receives part of the pulmonary veins and communicates with the left atrium, and a pulmonary venous chamber receiving all or part of the pulmonary veins that does not connect with the left atrium.
Congenital pulmonary vein stenosis is a very rare association with cor triatriatum.  Variable types of subtotal cor triatriatum may be noted, with only the right or left pulmonary veins draining into the upper chamber. 
Initially, the fetal lungs and pulmonary veins are connected to the systemic venous circulation. Subsequently, a dorsal outgrowth from the common atria, also referred as common pulmonary vein, evaginates and joins the pulmonary veins, whereas the connection to the systemic circulation disappears. As the fetal heart grows, the common pulmonary vein is completely absorbed. Failure of this dorsal outgrowth to join the pulmonary veins results in total anomalous pulmonary venous drainage (TAPVD). An abnormal connection between the common pulmonary vein and the atria results in any of the variants of cor triatriatum.
The critical anatomic feature of cor triatriatum is a diaphragm that divides the left atrium into 2 chambers (see the videos below).
It consists of fibromuscular tissue, and the proximal chamber that is created represents a vestigial common pulmonary vein. The pulmonary veins thus drain into the proximal chamber.
Outcome depends on the size of the communication to the distal chamber, which communicates with the mitral valve. If present, a patent fossa ovalis or secundum atrial septal defect permits decompression of the proximal chamber into the right atrium.
The presentation of cor triatriatum is one of decreased cardiac output and pulmonary venous hypertension. If a connection between the common pulmonary venous chamber and the right atrium is present, pulmonary overcirculation may result in significant right ventricular enlargement.
This is a very rare malformation. Incidence is approximately 0.1-0.4% of all infants with congenital cardiac disease.
Approximately 75% of patients die in infancy (generally from pulmonary hypertension) if the defect is unrepaired. If the communication between the proximal and distal chambers is not restrictive or if an atrial septal defect allows decompression of the hypertensive left atrium, the prognosis is significantly improved.
The only treatment is surgical correction. Most postoperative deaths occur in the first 30 days. The early mortality rate in each large series was consistently 15-20%. Early deaths had a higher rate of associated severe cardiac anomalies.
Long-term results are excellent, with long term survival of 80-90% in patients surviving surgery. Survivors have excellent functional results without residual sequelae and a life expectancy that approaches that of the general population. This is particularly true when repair is performed in infancy.
If the condition is unrecognized and untreated, problems include pulmonary edema, right heart failure, and death.
Late complications include recurrence of the obstructive membrane with recurrence of symptoms.  Similar symptoms may occur later on in life if the membrane is incompletely excised.
Ischemic stroke as a late complication from undiagnosed cor triatriatum (sinister) in an adult has been described. 
No race predilection is reported.
A slight male predilection is observed, with a male-to-female ratio of 1.4:1.
Diagnosis is primarily made in infancy. Later presentation does occur but is usually in childhood or early adulthood; in this patient population, 85% are younger than 40 years. Rarely, patients presenting in their eighth or ninth decade of life have been reported.
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