Cor triatriatum is a rare congenital heart defect in which a thin membrane divides one of the heart’s upper chambers (an atrium) into two separate compartments, effectively creating a heart with three atria instead of two. It accounts for roughly 0.1% of all congenital heart diseases. Most cases involve the left atrium, though a rarer version affects the right side. Despite sounding alarming, surgical repair carries excellent long-term outcomes when the condition is caught and treated.
How the Extra Chamber Forms
During normal heart development around the fifth week of embryonic life, the pulmonary veins merge into the left atrium through a shared opening. In cor triatriatum, that process goes wrong. The shared opening fails to fully incorporate into the atrial wall, leaving behind a fibrous, muscular membrane that stretches across the left atrium like a partial wall. This divides the chamber into an upper compartment, where blood arrives from the lungs, and a lower compartment, which connects to the mitral valve and the rest of the heart.
One leading explanation is that tissue from the right side of the developing heart essentially traps the pulmonary vein opening during formation, preventing normal absorption. Another theory holds that the membrane is simply an abnormal overgrowth of one of the heart’s early dividing walls (the septum primum). Either way, the result is the same: a physical barrier that can restrict blood flow from the lungs into the functioning part of the left atrium.
Left-Sided vs. Right-Sided Types
The left-sided form, called cor triatriatum sinistrum, is the more common and more clinically significant version. The membrane sits above the mitral valve and below the pulmonary veins, creating a bottleneck for oxygenated blood returning from the lungs. How much obstruction it causes depends entirely on the size of any openings in the membrane.
The right-sided form, cor triatriatum dextrum, is even rarer. It happens when a structure called the right valve of the sinus venosus, which normally shrinks and disappears between the 9th and 15th weeks of gestation, fails to regress properly. Instead of dissolving into the small, harmless flaps that guide blood flow in a normal right atrium, it persists as a membrane that partially or completely divides the right atrium into two sections. In mild cases this remnant may appear as a web-like network (called a Chiari network) that causes no symptoms at all.
Classification by Membrane Opening Size
The severity of cor triatriatum sinistrum depends on how much blood can pass through the dividing membrane. A widely used classification system breaks this into three groups:
- Group 1: No opening connects the two chambers. Blood from the pulmonary veins has no direct route into the lower atrium. Some pulmonary veins may drain abnormally into other structures, or the upper chamber may connect to the right atrium instead.
- Group 2: One or a few small openings (fenestrations) exist in the membrane, allowing limited blood flow through. This creates significant obstruction, similar in effect to a narrowed mitral valve.
- Group 3: A single large opening allows blood to pass freely between the upper and lower chambers. This causes little to no obstruction and may produce no symptoms at all.
A case described in clinical literature showed a restrictive opening of just 2 millimeters, small enough to cause severe obstruction and significant backup of pressure into the lungs.
Symptoms in Infants and Adults
When the membrane opening is small or absent, cor triatriatum behaves much like mitral stenosis, the condition where the mitral valve itself is narrowed. Blood backs up behind the obstruction, raising pressure in the pulmonary veins and eventually in the lungs. In infants, this can cause difficulty breathing, poor feeding, failure to gain weight, and signs of pulmonary hypertension (high blood pressure in the lung arteries). Severe cases may present in the first months of life as a cardiac emergency.
When the membrane has a larger opening, symptoms may not appear until adulthood, sometimes not until the 30s, 40s, or later. Adults typically develop shortness of breath with exertion, fatigue, or heart rhythm disturbances like atrial fibrillation. Some people are diagnosed only incidentally, when imaging is done for an unrelated reason. Because the symptoms so closely mimic mitral valve disease, cor triatriatum can be misdiagnosed for years.
How It’s Diagnosed
Echocardiography (ultrasound of the heart) is the primary tool for identifying cor triatriatum and is considered superior to older techniques like cardiac catheterization for this particular defect. The characteristic finding is a membrane visible inside the atrium on multiple imaging angles. The membrane curves forward and downward, inserting into the atrial wall some distance away from the mitral valve ring, with the left atrial appendage (a small pouch on the atrium) sitting below the membrane rather than above it.
During the relaxation phase of the heartbeat, the membrane moves forward toward the mitral valve, giving it an arching appearance that helps distinguish cor triatriatum from other conditions that can look similar on imaging, such as abnormal pulmonary vein drainage. Transesophageal echocardiography, where the ultrasound probe is positioned in the esophagus behind the heart, can provide especially clear views of the membrane and measure the size of any openings.
Why It Mimics Mitral Stenosis
Cor triatriatum sinistrum and mitral stenosis produce nearly identical symptoms because both conditions obstruct blood flow at roughly the same point: between the pulmonary veins and the left ventricle. Both cause elevated lung pressures, shortness of breath, and potential fluid buildup. The key difference is location. In mitral stenosis, the valve itself is narrowed or stiff. In cor triatriatum, the valve is normal but a membrane upstream creates the blockage. On echocardiography, the membrane’s position above the left atrial appendage and away from the mitral valve ring is the distinguishing feature. A supravalvular mitral ring, another rare anomaly, sits even closer to the valve and can also cause confusion, but it attaches directly at the valve ring rather than higher in the atrium.
Surgical Repair and Outcomes
The definitive treatment is surgical removal of the membrane. The procedure is performed on cardiopulmonary bypass (a heart-lung machine), and the surgeon excises the obstructing membrane to restore a single, open atrial chamber. In a 50-year experience at the Mayo Clinic, 80% of patients required additional cardiac procedures at the same time, reflecting the fact that cor triatriatum frequently coexists with other congenital heart abnormalities such as atrial septal defects (holes between the upper chambers).
Long-term outcomes are excellent. In one series of 32 patients followed for a median of about six years, overall survival was 96.9% at 15 years after repair. The single death occurred in a high-risk patient with a severely underdeveloped left heart. Among survivors, none required reoperation, and nearly all (96.9%) returned to full, unrestricted activity levels. Follow-up echocardiograms showed no recurrence of the membrane in all but one patient, who had residual mild-to-moderate pulmonary hypertension.
Cases involving isolated cor triatriatum with no other heart defects tend to have the best prognosis. When the condition occurs alongside complex congenital anomalies, outcomes depend more on those associated defects than on the membrane itself.