IMAGING
Echocardiography,
cardiac magnetic resonance imaging,
and computed tomographic angiography are important imaging modalities used to identify and diagnose aortic arch variants and anomalies.
These noninvasive imaging modalities have largely replaced catheter angiography and barium esophagrams in the diagnosis of suspected vascular rings.
In the pediatric setting in particular,
transthoracic echocardiography is often the initial imaging modality used.
Ultrasonography poses minimal risk to the patient,
as it is noninvasive,
does not require use of intravenous contrast medium,
does not expose the patient to radiation,
and is readily available at the bedside.
However,
cross-sectional imaging with CT angiography or MR angiography is necessary in the setting of more complex anomalies and in older children and adults with inadequate echocardiographic visualization of the mediastinum.
CT angiography enables rapid and high-spatial-resolution evaluation of vascular anomalies but poses some disadvantages especially in a paediatric setting,
radiation exposure and a relatively high rate of intravenous contrast medium injection.
MRI allows detailed evaluation of the aortic arch without exposure to ionizing radiation.
However,
given the longer acquisition times,
sedation may be required in the pediatric setting.
Black-blood sequences such as breath-held,
cardiac-gated,
and double inversion fast (turbo) spin-echo sequences are performed for assessment of anatomy and to assess the relationships of vessels to the airway and esophagus.
EMBRYOLOGY AND ANATOMY
Aortic development begins during the 3rd week of gestation.
It is a complex process,
which lends itself to a variety of congenital variants and pathologic anomalies.
Each primitive aorta consists of a ventral and a dorsal segment.
The two ventral aortae fuse to form the aortic sac,
and the two dorsal aortae fuse to form the midline descending aorta.
Six paired primitive,
or pharyngeal,
aortic arches develop between the ventral and dorsal aortae.
The dorsal aortae also give rise to several intersegmental arteries.
The primitive arches appear and regress one after another in a cranial-to-caudal order.
The mature aortic arch system is formed as some of the primitive arches regress,
whereas others persist and develop.
- First and second primitive arches disappear early; however,
a remnant of the first arch forms portions of the maxillary artery,
and a remnant of the second arch forms portions of the hyoid and stapedial arteries.
- Primitive third aortic arch,
termed as carotid arch,
forms the common carotid and proximal portion of the internal carotid artery.
- Primitive fourth embryologic arches contribute to the definitive adult aortic arch.
- Pifth primitive arches typically do not form,
or they form incompletely and then regress
- Primitive sixth arches contribute to the ductus arteriosi and central pulmonary arteries.
ARCH VARIANTS AND ANOMALIES
Congenital anomalies of the aortic arch include left-sided,
right-sided,
and double aortic arches,
with various branching patterns of the great vessels.
Aortic arch sidedness refers to which bronchus is crossed by the arch.
The trachea normally deviates slightly to the side opposite the arch.
Normal Left Aortic Arch and Variants
The normal left aortic arch results from regression of the distal right fourth arch between the right subclavian artery and the descending aorta.
The first branch arising from the normal left arch is the right brachiocephalic artery,
followed by the left common carotid and left subclavian arteries.This typical arch branching pattern occurs in 70%–80% of patients.
The most common arch variant branching pattern occurs when the left common carotid artery has a common origin with,
or,
less commonly,
arises directly from,
the right brachiocephalic artery called the bovine-type arch variant.
Another relatively common arch variant is the left vertebral artery arising directly from the aortic arch proximal to the left subclavian artery(5%–6% prevalence)
Left Aortic Arch with Diverticulum of Kommerell
A loose vascular ring may be formed in the setting of a left arch when the aberrant right subclavian artery arises from a diverticulum of Kommerell,
which occurs in approximately 15%–30% of cases.
The Kommerell diverticulum is a remnant of the dorsal aortic arch.
Sidedness of the Ligamentum Arteriosum or Ductus Arteriosus
Left Circumflex Aorta
Another vascular anomaly in which an aortic retroesophageal diverticulum can exist is a left aortic arch with right descending aorta and a right ductus arteriosus or ligamentum arteriosum,
also referred to as a circumflex aorta.
In this anomaly the arch crosses the midline posterior to the esophagus.
It is extremely rare and results from regression of the right fourth arch between the right common carotid and right subclavian arteries.
Right Aortic Arch with Aberrant Left Subclavian Artery Arising from a Retroesophageal Diverticulum
Right aortic arch with aberrant left subclavian artery is the most common variation of a right aortic arch resulting from regression of the left fourth arch between the left common carotid and left subclavian arteries.
A portion of the left dorsal aorta typically persists as a retroesophageal diverticulum,
giving rise to the aberrant subclavian artery.
Right Aortic Arch with Aberrant Left Subclavian Artery without a Retroesophageal Diverticulum
A right aortic arch can be accompanied by an aberrant left subclavian artery in the absence of a retroesophageal diverticulum resulting from regression of the left fourth and left sixth arches.
The ductus arteriosus is absent or is right sided and therefore does not form a vascular ring.
Right Aortic Arch with Aberrant Brachiocephalic Artery
This very rare anomaly results from regression of the left fourth arch between the ascending aorta and the left common carotid artery with persistence of the left dorsal aortic root and the left ductus arteriosus.
The first branch to arise from the arch is the right common carotid artery,
followed by the right subclavian artery and left brachiocephalic artery originates as the last branch.
The ductus arteriosus persists on the left and connects the brachiocephalic artery to the left pulmonary artery,
creating a vascular ring.
Right Aortic Arch with Mirror Image Branching
This anomaly results from regression of the left dorsal aorta distal to the origin of the seventh intersegmental artery,
so that the left fourth arch becomes the proximal subclavian artery rather than the definitive aortic arch.
Right Arch with Isolation of the Left Subclavian Artery
This anomaly results from regression of the left arch at two segments,
between the left common carotid and left subclavian arteries,
and also distal to the left ductus arteriosus and left subclavian artery.
The left subclavian artery is not attached to either the arch or the left common carotid artery.
Right Circumflex Aorta
A rare anomaly with left descending aorta and left ductus arteriosus or ligamentum arteriosum.This anomaly results from regression of the left dorsal aorta between the left carotid and left subclavian arteries or regression of the left fourth arch between the left subclavian artery and the left ductus arteriosus or ligamentum arteriosum.
Double Aortic Arch
Results from persistence of both the right and left arches,
with each arch giving rise to ipsilateral separate carotid and subclavian arteries.
It is the most common cause of a symptomatic vascular ring,
as the trachea and esophagus are completely encircled and may be compressed by the two arches.
Clinical manifestations in infants and children include wheezing and stridor exacerbated by crying,
tachypnea or cyanosis,
and dysphagia.
Cervical Aortic Arch
Cervical aortic arch is characterized by an aortic arch extending abnormally high in the neck,
occurring more commonly on the right.
Occurs due to:
(a)development of the cervical arch from the second or third primitive arches with regression of the fourth arch
(b) failure of normal caudal descent of an otherwise normal fourth aortic arch
Persistent Fifth Aortic Arch
These are rudimentary vessels that quickly degenerate in approximately half of embryos,
whereas in others these arteries do not develop at all.
However,
rarely,
a primitive fifth arch may persist,
located below the fourth arch.
It can connect the ascending aorta to the descending aorta or the ascending aorta to a derivative of the sixth arch,
usually the left pulmonary artery.
Interrupted Aortic Arch
The clinical presentation of IAA is similar to that of aortic coarctation (typical shock or severe heart failure in the first 2 weeks of life) although the developmental mechanisms are different.
IAA is classified into three types on the basis of the site of aortic interruption in relation to the arch vessels.
Of the three types,
the right subclavian artery may arise normally (subtype 1) or abnormally (subtypes 2 and 3).
The most common abnormal sites for the right subclavian artery to arise are distal to the left subclavian artery (aberrant right subclavian artery,
subtype 2) and from a right ductus arteriosus (isolated right subclavian artery,
subtype 3).
Type A IAA
The arch interruption occurs distal to the origin of the left subclavian artery resulting from abnormal regression of the left fourth arch segment late in development,
after the left subclavian artery has ascended to its normal position.
Type A is the second commonest type of IAA(30%–40%)
Type B IAA
The arch interruption occurs between the origins of the left common carotid and left subclavian arteries resulting from abnormal regression of the left fourth arch segment early in development,
before cephalad migration of the left subclavian artery.
Type B is the most common form of IAA(50%–60%).
Type C IAA
In type C IAA,
the arch interruption occurs between the origins of the brachiocephalic and left common carotid arteries resulting from abnormal regression of the ventral portion of the left third and fourth arches.
Type C is by far the least common(less than 5%)
HYPOPLASIA,
COARCTATION,
AND PSEUDOCOARCTATION
Aortic Arch Hypoplasia
Gradual tapering of the distal aortic arch is a normal finding in the first 3 months of life; however,
persistence later in life is pathologic.
Arch hypoplasia has been defined as an external diameter of the proximal or distal aortic arch measuring <60% or <50% of the ascending aortic diameter,
respectively.
The corresponding limit for hypoplasia of the isthmus is an external diameter <40% of that of the ascending aorta.
However,
this approach assumes a normal diameter of the ascending aorta.
In children,
normal reference ranges for aortic diameters have been established using both CT and MR imaging data sets.
Coarctation
Aortic coarctation is defined as a focal discrete constricted aortic segment,
usually in a juxtaductal location.
A relatively common anomaly that accounts for 4%–8% of congenital heart defects,
with a prevalence of approximately four per 10 000 live births.
Coarctation may occur as an isolated defect or in association with other lesions.
However,
coarctation is only very rarely associated with right aortic arches.
Pseudocoarctation
Pseudocoarctation is a rare anomaly characterized by kinking or buckling of the descending aorta at the level of the ligamentum arteriosum.
One proposed embryologic cause is failure of compression and fusion of the third through 10th segments of the dorsal aortic roots and of the left fourth arch segment when the cephalic shift of the seventh dorsal intersegmental artery occurs.
Pseudocoarctation is usually asymptomatic and generally considered to be a benign entity that does not require surgical intervention.
However,
treatment may be required in symptomatic patients and those with associated aneurysm formation.