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Aortic dissection is a tear in
the wall of the aorta (the largest artery of the body).
This tear causes blood to flow between the layers of the
wall of the aorta and forces the layers apart. Aortic
dissection is a medical emergency and can quickly lead
to death, even with optimal treatment. If the dissection
tears the aorta completely open (through all three
layers) massive and rapid blood loss occurs. Aortic
dissections resulting in rupture have a near-100%
mortality rate even if intervention is timely.
Overview
As with all other arteries, the aorta
is made up of three layers. The layer that is in direct
contact with the flow of blood is the tunica intima,
commonly called the intima. This layer is made up
of mainly endothelial cells. Just deep to this layer is
the tunica media, known as the media. This
"middle layer" is made up of smooth muscle cells and
elastic tissue. The outermost layer (furthest from the
flow of blood) is known as the tunica adventitia
or the adventitia. This layer is composed of
connective tissue.
In an aortic dissection, blood
penetrates the intima and enters the media
layer. The high pressure rips the tissue of the media
apart, allowing more blood to enter. This can propagate
along the length of the aorta for a variable distance,
dissecting either towards or away from the heart or
both. The initial tear is usually within 10 cm of the
aortic valve.
The risk in aortic dissection is that
the aorta may rupture, leading to massive blood loss
resulting in death.
Classification systems
Several different classification
systems have been used to describe aortic dissections.
The systems commonly in use are either based on the
anatomy of the dissection or the duration of onset of
symptoms prior to presentation.
DeBakey
classification system
The DeBakey system is an anatomical
description of the aortic dissection. It categorizes the
dissection based on where the original intimal tear is
located and the extent of the dissection (localized to
either the ascending aorta or descending aorta, or
involves both the ascending and descending aorta.
-
Type I - Originates in
ascending aorta, propagates at least to the aortic
arch and often beyond it distally.
-
Type II – Originates in
and is confined to the ascending aorta.
-
Type III – Originates in
descending aorta, rarely extends proximally.
Stanford
classification system
Divided into 2 groups; A and B
depending on whether the ascending aorta is involved.
Pathophysiology
The initiating event in an aortic
dissection is a tear in the intimal lining of the aorta.
Due to the high pressures in the aorta, blood enters the
media at the point of the tear. The force of the blood
entering the media causes the tear to extend. It may
extend proximally (closer to the heart) or distally
(away from the heart) or both. The blood will travel
through the media, creating a false lumen (the
true lumen is the normal conduit of blood in the
aorta). Separating the false lumen from the true lumen
is a layer of intimal tissue. This tissue is known as
the intimal flap.
The vast majority of aortic
dissections originate with an intimal tear in either the
ascending aorta (65%), the aortic arch (10%), or just
distal to the ligamentum arteriosum in the descending
thoracic aorta (20%).
As blood flows down the false lumen,
it may cause secondary tears in the intima. Through
these secondary tears, the blood can re-enter the true
lumen.
While it is not always clear why an
intimal tear may occur, quite often it involves
degeneration of the collagen and elastin that make up
the media. This is known as cystic medial necrosis
and is most commonly associated with Marfan syndrome and
is also associated with Ehlers-Danlos syndrome.
In about 13% of aortic dissections,
there is no evidence of an intimal tear. It is believed
that in these cases the inciting event is an intramural
hematoma (caused by hemorrhage within the media). Since
there is no direct connection between the true lumen and
the false lumen in these cases, it is difficult to
diagnose an aortic dissection by aortography if the
etiology is an intramural hematoma. An aortic dissection
secondary to an intramural hematoma should be treated
the same as one caused by an intimal tear.
Etiology
Aortic dissection is associated with
hypertension (high blood pressure) and many connective
tissue disorders. Vasculitis (inflammation of an artery)
is rarely associated with aortic dissection. It can also
be the result of chest trauma. 72 to 80% of individuals
who present with an aortic dissection have a previous
history of hypertension.
The highest incidence of aortic
dissection is in individuals who are 50 to 70 years old.
The incidence is twice as high in males as in females
(male-to-female ratio is 2:1). Half of dissections in
females before age 40 occur during pregnancy (typically
in the 3rd trimester or early postpartum period).
A bicuspid aortic valve (a type of
congenital heart disease involving the aortic valve) is
found in 7-14% of individuals who have an aortic
dissection. These individuals are prone to dissection in
the ascending aorta. The risk of dissection in
individuals with bicuspid aortic valve is not associated
with the degree of stenosis of the valve.
Marfan syndrome is noted in 5-9% of
individuals who suffer from aortic dissection. In this
subset, there is an increased incidence in young
individuals. Individuals with Marfan syndrome tend to
have aneurysms of the aorta and are more prone to
proximal dissections of the aorta.
Chest trauma leading to aortic
dissection can be divided into two groups based on
etiology: blunt chest trauma (commonly seen in car
accidents) and iatrogenic. Iatrogenic causes include
trauma during cardiac catheterization or due to an
intra-aortic balloon pump.
Aortic dissection may be a late
sequalae of cardiac surgery. 18% of individuals who
present with an acute aortic dissection have a history
of open heart surgery. Individuals who have undergone
aortic valve replacement for aortic insufficiency are at
particularly high risk. This is because aortic
insufficiency causes increased blood flow in the
ascending aorta. This can cause dilatation and weakening
of the walls of the ascending aorta.
Signs and symptoms
About 96% of individuals with aortic
dissection present with severe pain that had a sudden
onset. It may be described as tearing in nature, or
stabbing or sharp in character. 17% of individuals will
feel the pain migrate as the dissection extends down the
aorta. The location of pain is associated with the
location of the dissection. Anterior chest pain is
associated with dissections involving the ascending
aorta, while intrascapular (back) pain is associated
with descending aortic dissections. If the pain is
pleuritic in nature, it may suggest acute pericarditis
due to hemorrhage into the pericardial sac.
While the pain may be confused with
the pain of a myocardial infarction (heart attack),
aortic dissection is usually not associated with the
other signs that suggest myocardial infarction,
including heart failure, and ECG changes. Also,
individuals suffering from an aortic dissection usually
do not present with diaphoresis (profuse sweating).
Individuals with aortic dissection
who do not present with pain have chronic dissection.
Less common symptoms that may be seen
in the setting of aortic dissection include congestive
heart failure (7%), syncope (9%), cerebrovascular
accident (3-6%), ischemic peripheral neuropathy,
paraplegia, cardiac arrest, and sudden death. If the
individual had a syncopal episode, about half the time
it is due to hemorrhage into the pericardium leading to
pericardial tamponade.
Neurologic complications of aortic
dissection (i.e., cerebrovascular accident (CVA) and
paralysis) are due to involvement of one or more
arteries supplying portions of the central nervous
system.
If the aortic dissection involves the
abdominal aorta, compromise of the branches of the
abdominal aorta are possible. In abdominal aortic
dissections, compromise of one or both renal arteries
occurs in 5-8% of cases, while mesenteric ischemia
(ischemia of the large intestines) occurs 3-5% of the
time.
Blood pressure changes
While many patients with an aortic
dissection have a history of hypertension, the blood
pressure is quite variable at presentation with acute
aortic dissection, and tends to be higher in individuals
with a distal dissection. In individuals with a proximal
aortic dissection, 36% present with hypertension, while
25% present with hypotension. In those that present with
distal aortic dissections, 70% present with hypertension
while 4% present with hypotension.
Severe hypotension at presentation is
a grave prognostic indicator. It is usually associated
with pericardial tamponade, severe aortic insufficiency,
or rupture of the aorta. Accurate measurement of the
blood pressure is important. Pseudohypotension
(falsely low blood pressure measurement) may occur due
to involvement of the brachiocephalic artery (supplying
the right arm) or the left subclavian artery (supplying
the left arm).
Aortic insufficiency
Aortic insufficiency (AI) occurs in
1/2 to 2/3 of ascending aortic dissections, and the
murmur of aortic insufficiency is audible in about 32%
of proximal dissections. The intensity (loudness) of the
murmur is dependent on the blood pressure and may be
inaudible in the event of hypotension.
There are multiple etiologies for AI
in the setting of ascending aortic dissection. The
dissection may dilate the annulus of the aortic valve,
so that the leaflets of the valve cannot coapt. Another
mechanism is that the dissection may extend into the
aortic root and detach the aortic valve leaflets. The
third mechanism is that if there was an extensive
intimal tear, the intimal flap may prolapse into the LV
outflow tract, causing intimal intussusception into the
aortic valve preventing proper valve closure.
Myocardial infarction
Myocardial infarction (heart attack)
occurs in 1-2% of aortic dissections. The etiology of
the infarction is involvement of the coronary arteries
(the arteries that supply the heart) in the dissection.
The right coronary artery is involved more commonly than
the left coronary artery. If the myocardial infarction
is treated with thrombolytic therapy, the mortality
increases to over 70%, mostly due to hemorrhage into the
pericardial sac causing pericardial tamponade.
Because aortic dissection may present
to the emergency room physician similar to a myocardial
infarction, the physician must be careful to make the
proper diagnosis prior to initiating treatment for
myocardial infarction, since the treatment regimen for
myocardial infarction can be lethal to an individual
presenting with aortic dissection.
Pleural effusion
A pleural effusion (fluid collection
in the space between the lungs and the chest wall or
diaphragm) can be due to either blood from a transient
rupture of the aorta or fluid due to an inflammatory
reaction around the aorta. If a pleural effusion were to
develop due to aortic dissection, it is more commonly in
the left hemithorax rather than the right hemithorax.
Diagnosis
Because of the varying symptoms and
signs of aortic dissection depending on the initial
intimal tear and the extent of the dissection, the
proper diagnosis is sometimes difficult to make.
In an individual with chest pain
radiating to the back, the differentials to consider
include:
While taking a good history from the
individual may be strongly suggestive of an aortic
dissection, the diagnosis cannot always be made by
history and physical signs alone. Often the diagnosis is
made by visualization of the intimal flap on a
diagnositic imaging test. Common tests used to diagnose
an aortic dissection include a CAT scan of the chest
with iodinated contrast material and a trans-esophageal
echocardiogram. Other tests that may be used include an
aortogram or magnetic resonance angiogram (MRA) of the
aorta. Each of these test have varying pros and cons and
they do not have equal sensitivities and specificities
in the diagnosis of aortic dissection.
In general, the imaging technique
chosen is based on the pre-test likelihood of the
diagnosis, availability of the testing modality, patient
stability, and the sensitivity and specificity of the
test.
Chest X-ray
Widening of the mediastinum on an
x-ray of the chest has moderate sensitivity (67%) in the
setting of an ascending aortic dissection. However, it
has low specificity, as many other conditions can cause
a widening of the mediastinum on chest x-ray.
The calcium sign is a finding
on chest x-ray that suggests aortic dissection. It is
the separation of the intimal calcification from the
outer aortic soft tissue border by 1 cm.
Pleural effusions may be seen on
chest x-ray. They are more commonly seen in descending
aortic dissections. If seen, they are typically in the
left hemithorax.
Another findings are obliteration of
the aortic knob, depression of the left mainstem
bronchus, loss of the paratracheal stripe, and tracheal
deviation.
About 12%-20% of individuals
presenting with an aortic dissection have a "normal"
chest x-ray.
ECG
There are no specific
electrocardiographic findings associated with aortic
dissection. About 1/3 of the time, the ECG will show
signs of left ventricular hypertrophy, which is due to
the long-standing hypertension seen in these
individuals. Another 1/3 of the time the ECG would be
considered "normal". If the ECG suggests cardiac
ischemia in the setting of aortic dissection,
involvement of the coronary arteries should be
suspected.
Biochemical markers
While there are currently no blood
tests that can accurately diagnose aortic dissection,
research has been performed into the serial measurement
of monoclonal antibodies to smooth muscle myosin heavy
chains that appears to be both sensitive and specific
for aortic dissection. The sensitivity of this test is
about 90% and the specificity is 97% within the first 12
hours of the beginning of the dissection, and this assay
can accurately differentiate myocardial infarction from
aortic dissection. This test is not currently available
for the diagnosis of aortic dissection in the clinical
setting.
Transesophageal echocardiography
The transesophageal echocardiogram
(TEE) is a relatively good test in the diagnosis of
aortic dissection, with a sensitivity of up to 98% and a
specificity of up to 97%. It is a relatively
non-invasive test, requiring the individual to swallow
the echocardiography probe. It is especially good in the
evaluation of AI in the setting of ascending aortic
dissection, and to determine whether the ostia (origins)
of the coronary arteries are involved. While many
institutions give sedation during transesophageal
echocardiography for added patient-comfort, it can be
performed in cooperative individuals without the use of
sedation. Disadvantages of the TEE include the inability
to visualize the distal ascending aorta (the beginning
of the aortic arch), and the descending abdominal aorta
that lies bellow the stomach. A TEE may be technically
difficult to perform in individuals with esophageal
strictures or varicies.
Aortogram
An aortogram involves placement of a
catheter in the aorta and injection of contrast material
while taking x-rays of the aorta. The diagnosis of
aortic dissection can be made by visualization of the
intimal flap and flow of contrast material in both the
true lumen and the false lumen.
The aortogram was previously
considered the gold standard test for the diagnosis of
aortic dissection, with a sensitivity of up to 88% and a
specificity of about 94%. It is especially poor in the
diagnosis of cases where the dissection is due to
hemorrhage within the media without any initiating
intimal tear.
The advantage of the aortogram in the
diagnosis of aortic dissection is that it can delineate
the extent of involvement of the aorta and branch
vessels and can diagnose aortic insufficiency.
The disadvantages of the aortogram
are that it is an invasive procedure and it requires the
use of iodinated contrast material.
Spiral CT with contrast
The spiral CT scan with contrast is a
fast non-invasive test that will give an accurate
three-dimensional view of the aorta. It is performed by
taking rapid-cut radiographs of the chest and combining
them in the computer to create cross-sectional slices of
the chest. In order to delineate the aorta to the
accuracy necessary to make the proper diagnosis, an
iodinated contrast material is injected into a
peripheral vein at a properly timed moment so that it
will enter the aorta at the time that the aorta is being
imaged.
It has a sensitivity of 96 - 100% and
a specificity of 96 to 100%. Disadvantages include the
need for iodinated contrast material and the inability
to diagnose the site of the intimal tear.
MRI
Magnetic resonance imaging (MRI) is
currently the gold standard test for the detection and
assessment of aortic dissection, with a sensitivity of
98% and a specificity of 98%. An MRI examination of the
aorta will produce a three-dimensional reconstruction of
the aorta, allowing the physician to determine the
location of the intimal tear, the involvement of branch
vessels, and locate any secondary tears. It is a
non-invasive test, does not require the use of iodinated
contrast material, and can detect and quantitate the
degree of aortic insufficiency.
The disadvantage of the MRI scan in
the face of aortic dissection is that it has limited
availability and is often located only in the larger
hospitals, and the scan is relatively time consuming.
Due to the high intensity of the magnetic waves used
during MRI, an MRI scan is contraindicated in
individuals with metallic implants. In addition, many
individuals succumb to claustrophobia while in the MRI
scanning tube.
Treatment
The risk of death due to aortic
dissection is highest in the first few hours after the
dissection begins, and decreases afterwards. Because of
this, the therapeutic strategies differ for treatment of
an acute dissection compared to a chronic dissection. An
acute dissection is one in which the individual presents
within the first two weeks. If the individual has
managed to survive this window period, his prognosis is
improved. About 66% of all dissections present in the
acute phase.
In all individuals with aortic
dissections, medication should be used to control high
blood pressure, if present.
In the case of an acute dissection,
once diagnosis has been confirmed, the choice of
treatment depends on the location of the dissection. For
ascending aortic dissection, surgical management is
superior to medical management. On the other hand, in
the case of an uncomplicated distal aortic dissections
(including abdominal aortic dissections), medical
management is preferred over surgical treatment.
Individuals who present two weeks
after the onset of the dissection are said to have
chronic aortic dissections. These individuals have been
self-selected as survivors of the acute episode, and can
be treated with medical therapy as long as they are
stable.
Medical management is appropriate in
individuals with an uncomplicated distal dissection, a
stable dissection isolated to the aortic arch, and
stable chronic dissections. Patient selection for
medical management is very important. Stable individuals
who present with an acute distal dissection (typically
treated with medical management) still have an 8 percent
30 day mortality.
Medical
management
The prime consideration in the
medical management of aortic dissection is strict blood
pressure control. The target blood pressure should be a
mean arterial pressure (MAP) of 60 to 75 mmHg. Another
factor is to reduce the shear-force dP/dt (force of
ejection of blood from the left ventricle).
To reduce the shear stress, a
vasodilator such as sodium nitroprusside should be used
with a beta blocker, such as esmolol, propranolol, or
labetalol. The alpha-blocking properties of labetalol
make it especially attractive in this situation.
Calcium channel blockers can be used
in the treatment of aortic dissection, particularly if
there is a contraindication to the use of beta blockers.
The calcium channel blockers typically used are
verapamil and diltiazem, because of their combined
vasodilator and negative inotropic effects.
If the individual has refractory
hypertension (persistent hypertension on the maximum
doses of three different classes of antihypertensive
agents), involvement of the renal arteries in the aortic
dissection plane should be considered.
Surgical
management
Indications for the surgical
treatment of aortic dissection include an acute proximal
aortic dissection and an acute distal aortic dissection
with one or more complications. Complications include
compromise of a vital organ, rupture or impending
rupture of the aorta, retrograde dissection into the
ascending aorta, and a history of Marfan's syndrome.
A number of comorbid conditions
increase the surgical risk of repair of an aortic
dissection. These include:
-
Prolonged pre-op evaluation
(increased length of time prior to surgery)
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Advanced age
-
Comorbid disease states (ie:
coronary artery disease)
-
Aneurysm leakage
-
Cardiac tamponade
-
Shock
-
Past history of myocardial
infarction or CVA
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History of renal failure (either
acute or chronic renal failure)
The objective in the surgical
management of aortic dissection is to resect (remove)
the most severely damaged segments of the aorta, and to
obliterate the entry of blood into the false lumen (both
at the initial intimal tear and any secondary tears
along the vessel). While excision of the intimal tear
may be performed, it does not significantly change
mortality.
Long term
follow-up
The long term follow-up in
individuals who survive aortic dissection involves
strict blood pressure control. The relative risk of late
rupture of an aortic aneurysm is 10 times higher in
individuals who have uncontrolled hypertension, compared
to individuals with a systolic pressure below 130 mmHg.
The risk of death is highest in the
first two years after the acute event, and individuals
should be followed closely during this time period. 29%
of late deaths following surgery are due to rupture of
either the dissecting aneurysm or another aneurysm. In
additions, there is a 17 to 25% incidence of new
aneurysm formation. This is typically due to dilatation
of the residual false lumen. These new aneurysms are
more likely to rupture, due to their thinner walls.
Serial imaging of the aorta is suggested, with MRI
being the preferred imaging technique.
From
Wikipedia, the free encyclopedia
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