1. The atrial septum normally develops between the 4th and 8th weeks of gestation through the formation of the septum primum and secundum. 2. Abnormal development can result in several types of atrial septal defects (ASDs), the most common being a secundum ASD located in the area of the fossa ovalis. 3. Rarer types include sinus venosus defects (above or below the oval fossa), coronary sinus defects, and ostium primum defects located at the atrioventricular junction. Each has distinct developmental features and associations.

1. ATRIAL SEPTUM DEVELOPMENT
and
COMMON ABNORMALITIES
K.NAGENDRA PRASAD

5. • Cephalic portion bends to
ventrally,caudally and right.
• Caudal portion bends
dorsocranially and to left.
• Starts on day 23 completed by
day 28.

6. 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.

7. As it grows into the atrial cavity, it extends down towards the endocardial
cushions that are developing concomitantly within the atrioventricular
canal.
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 endocardial
cushions, 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

8. • After the fusion between the primary septum and
the endocardial cushions of the atrioventricular
canal, 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 muscularised
antero-inferior rim, forms the true septum that
separates the cavities of the atrial chambers.

9. 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.

10. • 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.

14. • 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.

15. COMPOSITION OF ATRIAL SEPTUM
• 1.INTER ATRIAL portion -
• 2.ATRIOVENTRICULAR portion
a. muscular
b. membranous

17. 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 endocardial
cushion defect anomalies.

20. 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

21. • 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.

23. 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.

24. • 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.

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

27. • 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.

28. • 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 trabeculated
part of the right atrium and the smooth-walled
part (sinus venarum), which
originates from the right sinus horn.

29. • concomitant with development of the lungs in the body wall behind
the heart, a venous channel, the primary pulmonary vein canalises
within 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”.

30. • 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.

31. • 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 atrioventricular
cushions, 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.

32. • 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.

33. • 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 infolded
adjacent 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.

34. • 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.

35. 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.

37. • 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

38. • 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.

39. 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.

40. • 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 interatrial
defect necessitates breakdown not only of the wall of the discrete
venous channel, but also partial dissolution of the wall of the
adjacent left atrium.

41. • 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.

43. The Ostium Primum Defect
• Another lesion producing an interatrial communication is the “ostium
primum” 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.

45. • 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 atrioventricular
septal defects, even though shunting across the defect is exclusively
at atrial level.
• These hearts do not have separate atrioventricular junctions as in the
normal heart.

46. • The most important characteristic is that the COMMON
ATRIOVENTRICULAR VALVE has five leaflets. Only two leaflets are
exclusively within the right ventricle; and one is solely within the left
ventricle.
• The remaining two leaflets, the superior and inferior bridging leaflets, are
shared 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.

48. 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

49. 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 cavae
and coronary sinus.

51. • 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.

52. 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.

53. Thank you