| Interatrial septum | |
|---|---|
 Interior of right side of heart. (labeled at left of image, as 'atrial septum') | |
 Interior of dorsal half of heart of human embryo of about thirty-five days. | |
| Details | |
| Identifiers | |
| Latin | septum interatriale |
| MeSH | D054087 |
| TA98 | A12.1.00.019 |
| TA2 | 3964 |
| FMA | 7108 |
| Anatomical terminology | |
Theinteratrial septum is the wall of tissue that separates the right and leftatria of theheart.
The interatrial septum is aseptum that lies between theleft atrium andright atrium of thehuman heart. The interatrial septum lies at angle of 65 degrees from right posterior to left anterior because right atrium is located at the right side of the body while left atrium is located at the left side of the body.[1] The interatrial septum represents the posterior wall of the right atrium.[2]
The interatrial septum forms during the first and second months offetal development. Formation of the septum occurs in several stages. The first is the development of theseptum primum, a crescent-shaped piece of tissue forming the initial divider between the right and left atria. Because of its crescent shape, the septum primum does not fully occlude the space between the left and right atria; the opening that remains is called theostium primum. During fetal development, this opening allows blood to be shunted from the right atrium to the left.
As the septum primum grows, the ostium primum progressively narrows. Before the ostium primum is completely occluded, a second opening called theostium secundum begins to form in theseptum primum. Theostium secundum allows continued shunting of blood from the right atrium to the left.
To the right of the septum primum, theseptum secundum begins to form. This thick, muscular structure initially takes on the same crescent shape as the septum primum, except that it originates anteriorly, whereas the septum primum originates posteriorly. As the septum secundum grows, it leaves a small opening called theforamen ovale. The foramen ovale is continuous with the ostium secundum, again providing for continued shunting of blood.
The ostium secundum progressively enlarges and the size of the septum primum diminishes. Eventually, the septum primum is nothing more than a small flap that covers the foramen ovale on its left side. This flap of tissue is called the valve of the foramen ovale. It opens and closes in response topressure gradients between the left and right atria. When the pressure is greater in the right atrium, the valve opens; when the pressure is greater in the left atrium, the valve closes. Because thelungs are nonfunctional in fetal life, pressure in thepulmonary circulation is greater than that of thesystemic circulation. Consequently, the right atrium is generally under higher pressures than the left atrium, and the valve of the foramen ovale is normally open.
At birth, there is a reversal in the pressure gradient between the atria, resulting in functional closure of the valve of the foramen ovale. Permanent anatomical closure of the foramen ovale occurs with time in normal infants. Inappropriate failure of closure of the foramen ovale results inpatent foramen ovale.
AnAtrial septal defect is a relatively common heart malformation that occurs when the interatrial septum fails to develop properly.
Persistence of the ostium secundum is the most commonatrial septal defect.[3] Additionally, in a subset of the population, the foramen ovale is not overtly patent but the two septa have not fused. In normal physiologic circumstances, the septum primum acts as a one-way valve preventing blood flow as described above; but, if pathologic conditions cause right atrial pressure to exceed left atrial pressure, blood may flow through the foramen ovale from right to left.
Failure of the septum primum to fuse with the endocardial cushion can lead to an ostium primumatrial septal defect.[4] This is the second most common type of atrial septal defect[3] and is commonly seen inDown syndrome. Typically, this defect will cause a shunt to occur from the left atrium to the right atrium. Children born with this defect may be asymptomatic, however, over timepulmonary hypertension and the resultinghypertrophy of the right side of the heart will lead to a reversal of this shunt. This reversal is calledEisenmenger syndrome.
Lipomatous atrial septal hypertrophy (LASH) is the fat deposition in the infoldings of the interatrial septum adjacent to the true atrial septum. It is shaped like a “dumbbell” because the deposition is located at the above and the below the fossa ovalis, sparing the fossa itself. The incidence of LASH increases with older age and obesity. On CT scan, it shows homogeneous, dumbbell mass which is non-enhancing at the interatrial septum.[5] MRI shows extension of the mass into interventricular septum and ventricular wall.[6]
