The atrial septum is a fibromuscular wall that separates the right and left atrial chambers.
Structurally it is characterized by two overlapping embryological structures, the left-sided partially fibrous septum primum, and the right-sided muscular septum secundum1.
The mean thickness of the septum primum covering the fossa ovalis was 1.8±0.7 mm. The septum primum was significantly thicker in patients with significant with disease of the atrioventricular valve (2.0±0.6 mm) compared to those without significant disease of the atrioventricular valve (1.6±0.7 mm; p<0.01).
Mean IAS thickness was 2.56 (SD 0.92) mm at anterior region (AR), 1.89 (SD 0.75) mm at fossa ovalis (FO), and 2.95 (SD 0.97) mm at posterior region (PR).(ref)
Functionally in fetal life, it serves to enable flow of blood rich in nutrients from the maternal circulation via placenta and IVC to cross from the right sided circulation to the left so that the brain can be supplied with this blood. Postnatally it serves to uncouple the mixing of deoxygenated and oxygenated blood between the two atrial chambers.
Diseases of the atrial septum are mostly congenital. Patent formaen ovale is seen in 30% of adults and can be the source of systemic emboli from the right sided circulation. Atrial septal defect (ASD) is the most common disorder involving the interatrial septum and is also the most common congenital cardiac abnormality diagnosed in adult life .
Diagnosis of atrial septal disease should be suspected clinically in adults presenting with symptoms of dyspnea, fatigue and palpitations, an flow type ejection murmur, loud P2, and a fixed wide splitting of the second heart sound on auscultation. In ASD secundum the EKG shows a RSR prime pattern suggesting RV overload. The CXR may show an enlarged right ventricle and pulmonary arteries and increased flow. Transthoracic echocardiography allows direct visualization of the defect and can be supplemented by bubble study, Transesophageal echo or cardiac catheterization as needed .
Medical therapy, minimally invasive techniques and surgical options are available depending on the disease process in question.
The nomenclature of the atrial septum is confusing since there are a variety of names that describe the same structures. Sometimes the embryological terms are used and sometimes the anatomical terms are used
Diagnostic elements relate to these diseases and include the clinical evaluation for an atrial septal defect (systolic murmur, split fixed P2, right ventricular enlargment, and pulmonary hypertension later in life) EKG showing enlarged RV, and echocardiography that can document defects in the fossa ovalis.
Treatment is usually surgical.
The atrial septum has an upper middle and lower portions, divided as such for both anatomical, embryological reasons.
The upper portion arises from sinus venosus tissue while the middle portion arises from he muscle itself (mesoderm), while the lower portion arises from the endocardial tissue.
Septum Primum
Fossa Ovalis
Imaging the Septum Primum
Fetal Ultrasound
The fetal ultrasound shows the right atrium in blue, left atrium in red, limbic band in green and the septum primum white, which is open and lies on the left atrial side.06428c03 heart cardiac atrial septum RA septum secundum superior limbic band inferior limbic band endocardial cushions ventricular septum fossa ovalis fetal echocardiogram LaA left atrium normal anatomy Courtesy Ashley DAvidoff MD copyright 2008
Normal Appearance
Mean IAS thickness was 2.56 (SD 0.92) mm at anterior region (AR), 1.89 (SD 0.75) mm at fossa ovalis (FO), and 2.95 (SD 0.97) mm at posterior region (PR).(ref)
ASD secundum
An atrial septal defect of the secundum type is a defect of the septum primum as shown below.
Surgical Repair of an ASD Secundum
References
Anderson H , Webb S , Brown NA. Clinical anatomy of the Atrial septum with references to its developmental components. Clinical Anatomy 1999;12:362-74.
Anderson H , Brown NA, Webb S. Development and structure of the atrial septum. Heart 2002;88:104-10.
Lazarova D GjorgovNJ. Dimensions of the triangle of Koch. Bratisl Lek Listy 2006;107 (4):107-9.