Cor triatriatum sinister (CTS) is a rare pathology (0.4% congenital heart disease) and 80% of the cases is associated with other heart diseases. The embryonic base of the CTS isn't well established. The hypotesis mostly accepted is inadequate implantation of the pulmonary veins on the left atrium which therefore creates a fibromuscular intraatrial accesory membrane. Clinically, depending on the degree of partition of the left atrium and the size of the orifice, patients may remain asymptomatic. 

Methods:
A case review

Results:
A 4-month-old male infant, admitted to the pediatric ICU with a RSV+ bronchiolitis, inititally requires NIV BIPAP with initial major clinical improvement being transferred to the Ward.  After 7 days upon admission he presents with hypotension, skin pallor, difficulty of breathing, inspiratory/expiratory weething,hepatomegaly. Echocardiogram show a partially collapsed left ventricle (FE Teich 74%) but not hypertrofic. Dilated right ventricle with slightly depressed systolic function and severe tricuspid insufficiency with a 105mmHg gradient suggesting severe pulmonary hypertension. In the presence of severe pulmonary hypertension with cardiogenic shock, the patient requires intensive pulmonary vasodilator treatment and ECMO. A follow-up echocardiogram shows the left atrium divided by an accesory membrane with the diagnosis of Cor Triatriatum Sinister (CTS).The patient underwent a surgical resection of the intraatrial membrane with favorable evolution.

Conclusions:
If the intra-atrial foramen is small, the obstruction can create a pressure gradient that mimics semiology of mitral stenosis. It can be associated with atrial arrhythmias that facilitate cardiac decompensation. In symptomatic patients with high pressure gradient, treatment of choice is surgical resection

A 4 month old patient presenting with clinically deteriorating bronchiolitis. Should we think about something else?