Thoracic Imaging 2003 - Society of Thoracic Radiology

swelling of the entire leg, calf swelling with the symptomatic
leg measuring more than 3 cm greater than asymptomatic leg
(10 cm below the tibial tuberosity), pitting edema, and collateral
(nonvaricose) superficial veins. 7
Relationship between PE and DVT
Although a causal relationship between DVT and PE is
accepted, the temporal relationship between these entities is less
well established. Depending on patient population and method
of investigation, widely variable percentages of patients with
proven PE have DVT. For this reason, evaluation of the venous
system is not an appropriate first-line test for patients who present
with clinical syndromes suggesting PE rather than DVT.
For example, in 89 patients with suspected PE and no risk factors
or symptoms of DVT, Sheiman et al found no DVT. 8
However if patients who are suspected of having PE also have
risk factors for or symptoms of DVT, an assessment of the deep
venous system may allow for a diagnosis of VTE; these same
authors found a 14 to 25% rate of DVT in patients with risk factors
for and/or symptoms of venous disease. Additionally, an
increased risk of DVT at ultrasound has been shown in one
study in patients with larger perfusion defects on lung scintigraphy,
so lower extremity investigation may be warranted in
patients without lower extremity symptoms but with demonstrated
large embolic burdens.
DVT by location
DVT is frequently isolated to one of two areas within the
lower extremity deep venous system: it may be iliofemoral or
calf-popliteal. Isolated superficial vein thrombosis occurs with
varying frequency, but has been seen in as large a percentage as
22.3 % of patients.Within the lower extremities, calf vein
thrombosis is associated with a relatively low risk of PE.
However, calf vein thrombosis may extend and involve the
popliteal and femoral veins, and the need for imaging of the calf
veins therefore remains controversial.
The risk of isolated pelvic vein thrombus is an even more
important area of inquiry, since pelvic vein thrombus is clearly
associated with a risk of PE. Isolated pelvic vein DVT has been
historically thought to be rare, approximately 2% of total DVT. 9
Some recent MR literature has reported much higher rates of
isolated pelvic DVT, with some CTV studies showing intermediate
results. As with so many other factors in VTE, the prevalence
of this particular manifestation probably depends on
patient risk factors.
Upper extremity DVT is a small minority of DVT, estimated
to represent 1-4% of total DVT. The overall incidence may be
increasing as a consequence of increased use of central venous
catheters, a major risk factor for the development of upper
extremity DVT. 5 The relative risk of PE in these patients compared
with the risk in patients with lower extremity DVT is not
known, but high-probability scans were found, in one series, in
five of 19 asymptomatic patients with upper extremity DVT.
Hypercoagulable States
Of the traditional risk factors for VTE, hypercoagulable
states represent an expanding category. Three inherited conditions,
antithrombin III, protein C, and protein S deficiencies,
were discovered early but account for a only a small percentage
of cases of VTE. These are seen in from 0.1 to 0.5 % of the
population and from 2 to 5 % of patients with VTE. However,
several more recently discovered conditions are associated with
a much larger percentage of VTE cases. These include Factor V
Leiden mutation, prothrombin G-A gene variant, elevated Factor
XI and Factor VIII, and hyperhomocystinemia. 6 Including all of
these conditions, hypercoagulable states now may be identified
in more than 25% of patients with VTE. It may be that, as more
of these conditions are recognized, it will be possible to find a
predisposing hypercoagulable state in the majority of patients
with VTE, which may allow for prophylaxis tailored to patients
who are truly at very high risk for disease.
Mortality of PE
PE was a significant cause of mortality in the Olmsted
County study, and even excluding patients initially diagnosed
with PE at autopsy, only 71.1% of patients with PE survived 7
days. PE was an independent predictor of reduced survival for
up to 3 months after onset. Other series show better survival in
treated patients (92%), and it is generally accepted that the
majority of deaths due to PE occur in patients who are not treated
because the diagnosis is not made. 6 However, some authors
have questioned whether PE is always the highly lethal event
that these data suggest. 10
In autopsy series, it is difficult to ascertain the contribution
of PE to patient death, and the possibility that PE is sometimes
clinically unimportant is reinforced by the fact that some studies
have shown that PE may be asymptomatic, even relatively frequently.
For example, studies of patients with DVT with no respiratory
symptoms show a high, 40-60%, rate of PE. Perfusion
scans may be used to quantitate the extent of physiologic abnormalities
secondary to PE. One study showed that the severity of
clinical signs increased with increasing perfusion abnormalities,
so many of these asymptomatic PE may have smaller physiological
consequences. The idea that some PE is clinically insignificant
is reinforced by the use of declotting procedures for dialysis
access grafts. In these procedures, thrombus is often pushed
into the lungs, with very rare negative consequences in patients
of whom many have very poor cardiopulmonary reserve. 10
Nevertheless, it is still widely accepted that PE causes significant
mortality in patients in whom a pre-mortem diagnosis is
not made, and that treatment reduces this mortality. It has been
estimated that less than 10% of PE deaths occur in patients in
whom treatment is initiated, and that while a minority of these
patients will die too soon to be treated, the missed diagnoses
remain a major opportunity for improvement in patient outcomes.
6
Natural History of VTE
One of the most significant risk factors for VTE is a history
of prior VTE, with particularly high rates of recurrence, compared
with other studies, documented in the Olmsted County,
Minnesota, study. 5 There is no well established protocol for
imaging followup of patients with a history of VTE, although
some authors have suggested that attention should be given to
establishing the degree of lysis that occurs following an episode
of PE, to help prevent misdiagnosis of chronic clot as recurrent
acute clot, and to facilitate identification of patients at risk for
chronic thromboembolic pulmonary hypertension.
The natural history of PE is for the native thrombolytic system
to dissolve clots over time. In some patients, however, this
does not completely occur, with chronic PE as a result. 11 Some
of these patients will develop the syndrome of chronic thromboembolic
pulmonary hypertension. This condition is thought
to occur in 0.1 to 0.5 % of patients who survive after acute PE.
Approximately 10% of patients with this disorder have anticardiolipin
antibodies, however the remainder have no discernible
underlying predisposition. Many present late in the course of the
disease without any history of diagnosed acute PE. 12
CONCLUSIONS
Unlike the other major acute cardiovascular events, heart
attack and stroke, the difficulties of diagnosis in VTE have led
to persistent uncertainty about its prevalence and natural history.
This uncertainty adds to the difficulty of making educated judgments
about which patients are at risk for this disease.
Nevertheless, an awareness of at least the limited information
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