ATRIAL SEPTUM DEVELOPMENT

and COMMON ABNORMALITIES

K.NAGENDRA PRASAD

• Cephalic portion bends to ventrally,caudally and right.

• Caudal portion bends dorsocranially and to left.

• Starts on day 23 completed by day 28.

SEPTUM PRIMUM

• The development of the normal atrial septum occurs following the initial looping of the heart that is after 28 days of gestation.

• The initial step in septation, a ridge of tissue develops from the superior aspect of the primary atrial component of the heart tube. This ridge is the primary septum (septum primum), and its the leading edge is covered by cushion-like mesenchymal tissue that is continuous over the dorsal mesocardium.

As it grows into the atrial cavity, it extends down towards the endocardialcushions that are developing concomitantly within the atrioventricularcanal.

Normal septal development also involves incorporation of another mass of tissue derived from the dorsal mesocardium. This is known as the vestibular spine (spina vestibuli), and it also carries on its leading edge a mesenchymal cap.

As the primary septum approaches the atrioventricular endocardialcushions, the various mesenchymal structures fuse together. The mass derived from the vestibular spine then muscularises, eventually forming the prominent infero-anterior border of the oval foramen

• After the fusion between the primary septum and the endocardial cushions of the atrioventricularcanal, the upper part of the primary septum disintegrates to form the ‘ostium secundum’.

• Development of the ostium secundum occurs during the fifth and sixth wks of embryologic life.

• The remaining part of the primary septum becomes the flap valve of the oval fossa.

• This flap valve, along with the muscularisedantero-inferior rim, forms the true septum that separates the cavities of the atrial chambers.

SEPTUM SECUNDUM

• After integration of the pulmonary veins into the left atrium the superior walls of the two atriums ‘infold’, creating the “septum secundum” in the superior portion of the atriums.

• Septum secundum is also concave in shape, with the concavity directed more posteriorly toward the opening of the sinus venosus of the primitive heart.

• It has 2limbs- Superior and inferior.

• Inferior limb fuses with the lowest portion of the atrial septum.

• The flap valve overlaps, but is not completely adherent to, the rims of this superior atrial fold, also known as Waterston's or Sonderggard's groove, providing a passage during fetal life for blood to pass from the right to the left atrium .

• The opening in the septum secundum is called the foramen ovale.

• Closure after birth –physiologically immediately

anatomically – 72 hours to 2 weeks .

• In about20% of cases, fusion of the septum primum and septum secundum is incomplete,and a narrow oblique cleft remains between the two atria.

• This condition is called probe patency of the oval foramen; it does not allow intracardiac shunting of blood.

COMPOSITION OF ATRIAL SEPTUM

• 1.INTER ATRIAL portion -

• 2.ATRIOVENTRICULAR portion

a. muscular

b. membranous

Types of interatrial communications

• 1. Abnormal development of the septa that normally devides atrial portion of heart into right and left.

• 2.Maldevelopment of partitioning of the AV CANAL and endocardialcushion defect anomalies.

SECUNDUM ASD

constitutes 70 – 80 % of ASD(most common)

Defect at the site of fossa ovalis.

The defect results from –

• Shortening of valve of foramen ovale

• Excessive resorption of septum primum

• Deficient growth of septum secundum

• Isolated secundum ASD accounts for 7% of congenital cardiac defects.

• Common in females.

• Here the defect present in the area borderd by the limbus of the fossa ovalis.

• Some times atrial septum may be fenestrated leading to multiple defects.

• Isolated defects of moderate and large size do not cause major symptoms in most cases during infancy and childhood.

• Left to right shunting increase with age in many patients.

Sinus venosus

• The tissue that seperates the right pulmonary veins from the svc and the posterior and inferior aspects of the right atrial free wall is termed as sinus venosus.

• Initially the systemic venous tributaries join with the primary atrium from both sides of the embryo. These channels are the horns of the systemic venous sinus, the “sinus venosus”, or the ducts of Cuvier.

• At this early stage, there are no discrete anatomical landmarks that mark boundaries between the venous horns and the primary atrium.

• At this stage, the lung buds are only just starting to develop, and the pulmonary vein has yet to appear.

• In the middle of the fourth week, the sinus venosus receives venous blood from the right and left sinus horns . Each horn receives blood from three important veins:

(a) the vitelline or omphalomesenteric vein

(b) the umbilical vein

(c) the common cardinal vein.

• With obliteration of the right umbilical vein and the left vitelline vein during the fifth week, the left sinus horn rapidly loses its importance .

• The right horn, which now forms the only communication between the original sinus venosus and the atrium, is incorporated into the right atrium to form the smooth-walled part of the right atrium .

• The left sinus horn diminishes in size through and beyond this period, becoming more fully incorporated into the left atrioventricularjunction as the coronary sinus.

• Throughout its development, the coronary sinus, and its precursor, the left sinus horn, possesses its own discrete walls.

• The sinuatrial orifice, is flanked on each side by a valvular fold, the right and left venous valves .

• Dorsocranially the valves fuse,forming a ridge known as the septum spurium .

• Initially the valves are large, but when the right sinus horn is incorporated into the wall of the atrium, the left venous valve and the septum spurium fuse with the developing atrial septum.

• RIGHT VENOUS VALVE-superior portion disappears completely

The inferior portion develops into two parts:

(a) the valve ofthe inferior vena cava,

(b) the valve of the coronary sinus

The crista terminalis forms the dividing line between the original trabeculatedpart of the right atrium and the smooth-walled part (sinus venarum), which originates from the right sinus horn.

• concomitant with development of the lungs in the body wall behind the heart, a venous channel, the primary pulmonary vein canaliseswithin the dorsal mesocardium .

• Canalisation of this channel brings the pulmonary venous plexuses into continuity with the cavity of the developing left atrium.

• Initially, a solitary pulmonary venous channel enters the left atrial part of the primary atrial component inferiorly and posteriorly, the entrance being bounded by two ridges which demarcate the site of the persisting dorsal mesocardium.

• The right of these two ridges becomes particularly prominent . This structure is called as the “spina vestibuli”.

• In the fully developed heart, the original embryonic left atrium is represented by little more than the trabeculated atrial appendage while the smooth-walled part originates from the pulmonary veins.

• the original embryonic right atrium becomes the trabeculated right atrial appendage containing the pectinate muscles, and the smooth walled sinus venarum originates from the right horn of the sinus venosus.

• The musculature of the atrioventricular canal itself becomes incorporated into the atrial chambers as the vestibules of the atrioventricular valves.

• Forward growth of the vestibular spine, binding the base of the primary septum to the upper surface of the fused atrioventricularcushions, has also carries the inferior ends of the valves of the systemic venous sinus.

• The expanded vestibular spine itself then becomes muscularised to form a bulbous structure that reinforces the base of the primary atrial septum.

• At this stage, the pulmonary vein continues to drain inferiorly to the left atrium, and divided into its right and left branches.

• After the eighth week of development only the initially solitary pulmonary vein begin fully to become incorporated into the body of the primary atrium, forms large part of the developing left atrium.

• By the 12th week of development, the superior right sided pulmonary vein has become a separate tributary of the left atrium.

• Concomitant with this change, the atrial roof has infoldedadjacent to the mouth of the superior caval vein to form the antero-superior margin of the oval foramen.

• This process of infolding, when complete, provides the buttress against which the flap valve can close in postnatal life.

• After completion of septation, the definitive atriums each possess a part of the body of the primary atrium, an appendage, a vestibule, and a venous component. They remain in continuity with each other through the oval foramen.

• The newly muscularised antero-inferior margin of the oval foramen, derived from the vestibular spine, is anchored to the fibrous skeleton, itself formed from the atrioventricular cushions.

Superior and Inferior sinus venosus defects

• These defects are the defects where the atriums communicate through a channel outside the boundaries of the true atrial septum.

• These defects develop secondary to abnormal attachment of the right pulmonary veins to either the SVC or IVC.

• An interatrial communication develops when the wall between the vena caval and pulmonary veins is resorbed.

• In cases of superior sinus venosus defect, the hole is located superiorly to the oval fossa, which can itself either be intact or deficient.

• here the superior caval vein usually overrides the crest of the defect.

• The caval channel then has biatrial connections, opening into the right and left atriums.

• The phenotypic feature of the lesion is the presence of the defect outside the confines of the normally formed oval fossa

• Inferior sinus venosus defects are uncommon, and occur at the mouth of the inferior caval vein, near the opening of the coronary sinus, but the oval fossa retains its discrete muscular border.

• These are often difficult to diagnose echocardiographically, since they can be mistaken for large defects of the oval fossa which extend back into the atrium so that the inferior caval vein overrides the entrance of the oval fossa, thus producing a biatrial communication in the setting of an oval fossa defect.

Coronary sinus defects

• The coronary sinus is a venous channel that is located within the left atrioventricular groove, above the annulus of the mitral valve.

• Distally located tributaries join to convey the deoxygenated blood back to the right atrium.

• Defects within the wall of the coronary sinus are a rarity.

• They are usually found associated with anomalous connection of the left superior caval vein to the roof of the left atrium, which enters between the right pulmonary veins and the left atrial appendage.

• In the normal heart, the muscular wall of the coronary sinus is separated by extracardiac tissue from the inferior wall of the left atrium.

• Developmentally, therefore, the existence of this type of interatrialdefect necessitates breakdown not only of the wall of the discrete venous channel, but also partial dissolution of the wall of the adjacent left atrium.

• The degree of disintegration of the two walls can vary widely, from small distinct fenestrations, to complete ‘unroofing’ of both walls, producing complete mixing of the venous deoxygenated blood and the oxygenated blood within the left atrium.

• The key phenotypic feature is again the presence of the defect outside the confines of the oval fossa.

The Ostium Primum Defect

• Another lesion producing an interatrial communication is the “ostiumprimum” variant of atrioventricular septal defect.

• The defect lie at the lowest part of the atrial septum at the level of the tricuspid and mitral valves and variable in size depending on the amount of atrial septal tissue that is deficient in addition to AV SEPTUM.

• Isolated ostium primum ASD characteristically aasociated with a cleft in anterior leaflet of mitral valve.

• The phenotypic feature in these hearts is the presence of a common atrioventricular junction, with separate valvar orifices into the right and left ventricles

• These anomalies, therefore, should be categorised as atrioventricularseptal defects, even though shunting across the defect is exclusively at atrial level.

• These hearts do not have separate atrioventricular junctions as in thenormal heart.

• The most important characteristic is that the COMMONATRIOVENTRICULAR VALVE has five leaflets. Only two leaflets areexclusively within the right ventricle; and one is solely within the leftventricle.

• The remaining two leaflets, the superior and inferior bridging leaflets, areshared between both ventricles, straddling the ventricular septum

• The distinguishing feature of the “primum” defect is that the two bridging leaflets are joined to each other by a tongue of fibrous tissue positioned directly on top of the crest of the ventricular septum, dividing the junction into discrete and separate left and right valvar inlets to the ventricles.

• Almost always, the bridging leaflets and the tongue are also fused to the ventricular septal crest.

• It is this feature which confines shunting across the septal defect at atrial level,with much of the shunting being below the level of the atrioventricular junction.

ASSOCIATIONS

• OSTIUM SECUNDUM –

holt oram syndrome

TAR syndrome

trisomies – edward’s syndrome.patau’s syndrome

NKX 2.5 mutations – along with conduction system disease

NKX 2.5 - master gene for heart development.

TBX mutations - plays important role in septation.

axenfeld reiger anomaly

• OSTIUM PRIMUM – Down syndrome

COMMON ATRIUM

• Common atrium is a rare variety of interatrial communication characterized by absence or virtual absence of the atrial septum, vestigial remnants of which may remain as diaphanous strands of tissue.

• The right-sided portion of the common chamber has features of a morphologic right atrium (crista terminalis, pectinate muscles, right atrial appendage) and receives the superior and inferior vena cavaeand coronary sinus.

• The left-sided portion of the common chamber has features of a morphologic left atrium (smooth nontrabeculated walls, a left atrial appendage) and receives the pulmonary veins.

• Common atrium is therefore a cyanotic malformation with increased pulmonary arterial blood flow.

• Associated with Ellis van crevald syndrome.

PATENT FORAMEN OVALE

• 1. VALVE COMPETENT PFO - 30 % of normal adult hearts have a probe patent , valve competent foramen ovale.

• 2. VALVE INCOMPETENT PFO - these are smallest ASD s are due to incompetent foramen ovale.

They may be congenital or may be acquired by stretching of the right / left atria in conditions in which these chambers are enlarged.

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