Biology concepts – situs inversus, situs ambiguous,
dextrocardia, dextroposition, isomerism, canalization
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Catherine O’Hara has situs inversus, but she doesn’t
have any strange stories to tell about it as do Dr. No
and Donny Osmond. On the other hand, a weird
movie like Beetlejuice deserves a star with a
strange anatomical quirk.
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Dr. No of the James Bond stories was the quintessential bad
guy, but he still had a congenial relationship with 007. During a discussion
over tea, or perhaps as Bond hung over a pool of sharks, Dr. No explained to
James that he once survived an assassination attempt when a his would be
murderer tried to stab him in the heart.
Little did the assassin realize that Dr. No had situs
inversus. Even though he was stabbed on the left side, right where his heart
should have been, Dr. No laughed it off and then cut up the poor guy with a
laser. It helps to have an edge that people don’t know about.
Then again, sometimes situs inversus can be a pain. Donny Osmond, of the Osmond Brothers and then later of Donny and Marie, had situs
inversus. As a child he had a bout of lower left abdominal pain. Nothing
special is located in the lower left abdomen, so his family ignored it - so did
the doctors. When they arrived in England, he had to have emergency surgery
because of appendicitis. His situs inversus meant that his appendix was on the
opposite side.
In the cases mentioned above, the condition was situs
inversus totalis, but it isn’t always totalis. This is rare, but it happens,
and most often it involves the heart. Even when it comes to location, the heart
wants what the heart wants.
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We’re talking about looking for horses instead of zebras
and we’re talking about hearts, so why not zebrafish
hearts? It turns out that zebra fish can regenerate their
heart after injury, as can many animals. A new paper is
showing how the cells can change into the cell type they
need and then proliferate.
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But zebras do exist, and you have to know that they exist
and what they look like. One zebra is called isolated levocardia (levo
= left, and cardio = heart). If situs
inversus is rare (1in about 10,000 live births), then isolated levocardia is
very rare (in about 25,000 live births). Levocardia means that the apex (the
bottom of the heart comes to a blunt point) is directed to the left – this is
normal. But in isolated levocardia, the rest of the organs are situs inversus, only the heart is in the normal
orientation.
Remember that the lungs are designed to fit around the heart. The left lung has a cardiac notch for the apex of the organ. If the lungs
are switched, then the cardiac notch is on the right lung; this is fine if the
heart is mirrored as well. But in isolated levocardia, the right lung has room for
the heart apex, but the heart is point to the left.
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The top left image is the normal arrangement. B shows
something called dextroposition. This happens with
congenital malformations of the chest or sometimes in
injury. The heart points left but is moved to the right side
of the chest. C is dextrocardia without situs inversus,
there
are often congenital heart defects in this condition. D is
situs
inversus totalis. The heart works fine because
everything is switched.
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The prognosis of patients with isolated levocardia is not
good, only 5-13% live beyond the age of five years. This is usually due to
severe cardiac defects. The problems with the vessels may be a part of it, but these
patients often have defects of the heart organ itself. The chambers may be
transposed or the septa between the chambers could have holes or other defects.
Situs inversus with a normal heart position is one
exception, but how about the opposite exception? Can all the organs be in the
normal orientation except the heart?
Yes – it’s called dextrocardia. If
you want to be picky, dextrocardia refers to any situation where the apex of
the heart points to the right instead of to the left, but the vast majority of
these cases occur in situs inversus. What we are talking about is an isolated dextrocardia, although the
terms in dextrocardia get somewhat muddled. I prefer dextrocardia without situs inversus or dextrocardia with situs solitus (situs
solitus is the normal organ arrangement).
Dextrocardia without situs inversus does occur on its own (sporadic), but it seems that it occurs more rarely in mammals as compared to
lower forms (fish, amphibians). This may be an incidence of canalization, the tendency for a
conserved trait to become more resistant to mutation or environmentally induced
perturbation over evolutionary time.
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The normal heart s on the left. The aorta is connected to
the
left ventricle (the connection is hidden behind the
pulmonary artery. The right image transposition of the
arteries. The aorta empties the right ventricle and the
pulmonary artery empties the left ventricle. This sets up two
different circulatory systems: heart-body-heart,
and heart-lungs-heart.
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One of the most common congenital heart defects seen in dextrocardia with situs solitus is called transposition
of the great arteries. There are five great vessels that carry blood to and
from the heart. The superior (1) and inferior (2) vena cava bring blood from
the body back to the heart. The pulmonary artery (3) is an exception, being one
of the few arteries that carries deoxygenated blood, takes the blood from the
right ventricle to the lungs to pick up oxygen. Soon after the pulmonary artery
(PA) leaves the heart, it splits in two, one for each lung.
The pulmonary veins (4) bring blood back to the heart from the
lungs. Yet another exception – it’s a vein but it carries oxygenated blood.
There are actually four pulmonary veins, two from each lung, but they all join
together as the get to the left atrium of the heart.
Finally there is the aorta (5). This vessel carries oxygenated blood
from the left ventricle to the entire body. This is why the left ventricle has
to be so thick and strong. It has a long way to pump the blood. Of course, our
descriptions above are of the way the vessels are supposed to fit together.
In transposition of the great arteries (TGA), the aorta and the
pulmonary artery have switched connections. The aorta is attached to the right
ventricle instead of the left, and the reverse is true for the pulmonary
artery. Do you see the problem? In TGA, the aorta receives deoxygenated blood from the SVC and IVC via the right atrium and
pumps it right back out to the body. The pulmonary artery receives oxygenated blood from the PVs via the
left atrium and pumps it right back to the lungs!
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These are the holes in the heart. The left image is the
patent
ductus arteriosus from the aorta to the pulmonary artery.
The middle image is the atrial septal defect with a hole
between the two atria, while the right image shows the hole
between the two ventricles in the ventricular septal defect.
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Usually the ductus arteriosus closes on its own a couple days after
birth. In the rare case, the ductus will remain open (patent). These are some
of the babies you hear about with a “hole in their heart” (the hole could also
be from a patent septal defect). The PDA is the only thing that keeps TGA
babies alive – some oxygenated blood from the heart – lung – heart system can
mix with the deoxygenated blood of the body – heart – body system.
Patent ductus arteriosus is more common in babies with other
congenital heart defects, but doctors often give the infants prostaglandins to
ensure that the ductus will stay open until they can get in there and switch
the vessel connections surgically. The key is to find out there is a problem
before the baby is born.
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The two middle images show situs ambiguous. The heart is
in the center and has defects, the liver is all across the
abdomen, and the spleen is absent in one and too
numerous in the other.
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As with all sciences, terminology is important, and there is a
discussion now as to whether isomerism or even heterotaxy are appropriate terms
to uses for situs ambiguous (reviewed here and here). Isomerism (iso = same and mer =
part) means that parts are duplicated, like two right halves of the heart, but
it isn’t all the organs or even the whole heart, just the atria. So is this an
appropriate term?
Right
isomerism usually comes with more heart defects than left isomerism, and there
is no spleen in right isomerism as opposed to several spleens in left
isomerism. So if I had to have one or the other, I would pick left isomerism
for my heterotaxy. There are also malrotation problems in the intestines and
the stomach is sometimes on the wrong side. If you’re ever given the choice,
just take situs solitus; it’s boring, but it’s safe.
Next week
- how similar are twins? Sometimes they are so similar because they didn't completely separate.
For more information or
classroom activities, see:
Situs inversus –
situs ambiguous –
transposition of the great
vessel
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