Movement of the inter-atrial septum: everything has meaning
The inter-atrial septum (IAS) is a thin wall between the right and left atria. Normally, its motion (i.e., whether it sways towards the left or right atrium) is influenced by the pressure gradient between the atria. Here is a normal apical 4 chamber view and a table showing normal cardiac pressures.
When performing focused cardiac ultrasound (FoCUS), just observing the anatomy on greyscale clips can provide valuable information. For example, take a look at this apical 4 chamber view obtained from a patient with severe pulmonary hypertension. The IAS is bowing to the left with each beat indicative of high right sided pressures. As expected, the IVC is plethoric.
Apical 4 chamber 
Zoomed image 
IVC long axis
Below is a different yet related case. Images were obtained from a CKD patient seen in the nephrology clinic.
Apical 4 chamber view 
IVC transverse view (vessel on the left) 
Subcostal view. Additionally, note the left ventricular hypertrophy and a small pericardial effusion
This IAS movement is weird – does not seem to be moving with each beat. Instead, it is protruding into the left atrium once in a few beats (with inspiration). Moreover, IVC is not plethoric and nicely collapsing with inspiration (= no elevated right atrial pressure). This is a case of IAS aneurysm. It is a localized deformity of the IAS, generally at the level of fossa ovalis which protrudes into right or left atrium or both (LA in our case). By definition, this protrusion should be at least 10 mm (15 mm per some authors). The movement is more apparent during inspiration because of the increased venous return into the RA.
IAS aneurysm may be encountered as an isolated abnormality or in association with other structural cardiac abnormalities such as mitral valve prolapse or atrial septal defects/ patent foramen ovale, Marfan’s syndrome etc. Interestingly, it may also be associated with autosomal dominant polycystic kidney disease (ADPKD) [our patient did not have ADPKD]. Uncomplicated and isolated IAS aneurysm requires no specific treatment, but these patients should be evaluated for the presence of thrombus in aneurysm. Detection of above-mentioned associated abnormalities may require a transesophageal echocardiogram and/or other imaging and hence cardiology consultation should be sought.
Below image was obtained from a young patient with no known heart disease. There was no pulmonary hypertension; IVC was small and collapsible. However, the IAS seems to be bowing to the left. Likely explanation for this phenomenon is the sudden drop in LA pressure (relative to RA) as the LV strongly sucks blood during diastole in a hyperdynamic heart.
