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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.
- A = Type I and II DeBakey
- B = Type III Debakey
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:
- Aortic dissection
- Myocardial infarction
- Acute aortic insufficiency
- Non-dissecting aortic aneurysm
- Pericarditis
- Musculoskeletal pain
- Mediastinal tumors
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)
- Advanced age
- Comorbid disease states
(ie: coronary artery
disease)
- Aneurysm leakage
- Cardiac tamponade
- Shock
- Past history of
myocardial infarction or CVA
- 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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