Now, why should we care?
What is the practical significance of this have to do with anything?
Essentially, you wanna remember that these blood vessels
are not developing in isolation.
We have nerves, and bones, and viscera,
different organs all developing at the same time.
And one thing that this explains
is why the recurrent laryngeal nerve takes such a bizarre course
on both sides of the body.
Initially, the recurrent laryngeal nerve
which innervates structures from the 6th pharyngeal arch
is gonna be going from the vagus nerve
into the developing muscles of the larynx,
and it's looping underneath the 6th aortic arch
in order to reach the 6th pharyngeal arch.
As the body elongates that nerve gets stretched out
and were left with the vagus nerve coming down into the thorax
giving off other branches
but having it's recurrent branch loop back up to our larynx
and have a very extended U, as it does so.
But on the right, we find it looping around our subclavian artery
and on the left, looping around the aortic arch.
The reason for that is on the right side,
the 6th aortic arch loses its connection to the dorsal aorta
and as the body grows, that nerve gets pulled further up
and the next thing it hits is the 4th aortic arch.
And that remains as part of the subclavian artery
which is why we have a different course on the right than on the left
for the recurrent laryngeal nerve
because on the left it winds up going around the arch of the aorta
and gets tethered there.
And in particular, it's gonna be found just beside the ductus arteriosus
which after birth becomes fibrous
and is known as ligamentum arteriosum.
So that's why we have two different courses
for the same nerve on the right side versus the left side.
It's reflecting the lack of symmetry
between the right and left vascular development.
Now, other things that can go wrong are gonna be called vascular slings,
where the vasculature actually constricts another organ.
In this case, we can have the esophagus and trachea
get compressed by the vessels.
The most common form of this is gonna be either
a double aortic arch or a subclavian artery
that wraps around the back of the esophagus to get to the right upper limb.
What happen in this case
is we have the heart pumping blood to the aortic arches
and we have the two dorsal aortae,
in this picture, making a nicely mimetic heart shaped loop
as they go to the dorsal aorta.
Normally on the right side, the segment of the dorsal aorta
between the 7th intersegmental artery
and the fused dorsal aorta would rescind
and the subclavian artery on the right would detach,
move up, and be connected to the upper limb only.
If that portion of the aorta fails to rescind, we wind up with a tether.
And that tether, as the body elongates is gonna move up, up, up, up, up
and constrict the esophagus and the trachea.
I'm gonna back up just a moment,
and you can see that if the esophagus and the trachea
are passing posterior to the heart but anterior to the dorsal aorta,
as that dorsal aorta shortens,
it's gonna wind up effectively zipping up
and then constricting those organs causing difficulty swallowing
and possibly difficulty breathing as those two organs get constricted.
Another form of this is called the subclavian sling
and it's due to the right dorsal aorta rescinding
but doing it in the wrong place.
Instead of rescinding between the 7th intersegmental artery
and the fused dorsal aorta,
it will occasionally rescind above that point.
And will have the right dorsal aorta disappear
but superior to the 7th intersegmental artery.
Now, I want you to remember,
is that the 7th intersegmental artery is tethered to the upper limb.
It can't just move around.
It can't just detach and find another more efficient route
to where it wants to go.
It's stuck right where it is.
So as the body elongates, and that vasculature gets more and more tight,
we wind up with that passing
posterior to the esophagus and the trachea.
So in this case, we have a right subclavian artery
coming off most distally from the arch of the aorta
because it has no other choice.
It came from much further down and had to take that posterior course
simply to get where it was going.
Again, this can cause difficulty swallowing
and even constriction of the airway if it's very pronounced.
Last and certainly not least, there are gonna be coarctations of the aorta.
As development happens, normally, the aorta should stretch,
lengthen and stay open along its entire length.
But occasionally, there can be constrictions
and these are called coarctations.
The most common one is called a post-ductal coarctation.
And that simply denotes that the ductus arteriosus
leaves the pulmonary trunk for the aortic arch
and the coarctation happens after that.
Now our coarctations is gonna narrow this massive artery,
meaning there's gonna be a lot of blood pressure above that constriction
but much less blood pressure below.
And on our lower limbs, organs are gonna be deprived of blood
if there's no way for the blood to reroute pass that blockage.
So in these cases, the ductus arteriosus is going to close
because there's a lot of pressure in the aorta,
there's a lot of oxygen that signals it to close,
and we line up with the ductus arteriosus closing.
Yet, we have impeded blood flow.
What happens in this case is that
arteries that bypass the aorta will enlarge tremendously
to get blood to the organs in the lower limbs.
Some blood will back up into the subclavian arteries,
go through the internal thoracic arteries on either side of our sternum
and then back fill the intercostal arteries into the thoracic aorta.
And those intercostal arteries will enlarge
to the point that actually leave pretty pronounced grooves
on the underside of the ribs.
And then the blood will fill in to the aorta,
move down to the organs and the lower limb.
One way to diagnose this at least early,
is to look for differences in the pulse,
in the upper limb and the lower limb.
A very strong bounding pulse in the two upper limbs
but absent or very, very less pronounced pulses in the lower limb
can be an early sign of coarctation of the aorta.
Now sometimes these are subtle and aren't diagnosed until adulthood.
So, I want you to note that it's not always gonna be obvious.
These are not constrictions that are always
so severe that they result in immediate distress
but can be brought on by exercise
and they're sometimes associated with the bicuspid aortic valve
rather than the normal tricuspid set of semilunar valves leaving the aorta.
Now much less common,
maybe between 2 to 5% of coarctations occur proximal to the ductus arteriosus,
and these, not surprisingly are called preductal coarctations of the aorta.
In this case, we've got a constriction of the aorta
but distal to that, we have the ductus arteriosus.
Because the pressure in that distal portion of the aorta is so low,
the ductus arteriosus stays open,
and it's gonna take poorly oxygenated blood,
from the pulmonary circuit that's trying to get to the lungs
and spill into the descending thoracic aorta.
Because of this, there may be
cyanosis or blue tinting of structures in the lower limb,
but not in the upper limb.
That's going to be, out getting fresh blood from the aortic arch.
Patients of the preductal coarctation of the aorta
are gonna show cyanosis in any vessel that is distal to the coarctation.
And that is important because the coarctation doesn't always leave
all the subclavian and carotid arteries unaffected.
If I have a coarctation that occurs between
my left common carotid and left subclavian artery,
I'm gonna have good reddish perfusion of my head and neck,
but I'll have cyanosis on my left arm but not my right.
If that coarctation happens between
the right brachiocephalic trunk and the left common carotid artery,
I may have one sided cyanosis of the head and neck,
cyanosis of the left upper limb but unaffected right upper limb
and relatively decent reddish or pinkish appearance
of the mucus membrane on my right side.
So remember that the position of the coarctation
can determine which vessels are getting cyanotic blood
and which ones are not.
So it's typically evident, right after birth, if these are present,
that cyanosis will be noted right away
as opposed to a postductal coarctation,
which may only become notable
as the patient gets older and becomes more active.
All right, thank you very much for your attention and I'll see you on our next talk.