Thrombotic thrombocytopenic purpura Review articles J Thachil

Review articles
© The Intensive Care Society 2011
Thrombotic thrombocytopenic purpura
J Thachil
Thrombocytopenia is the most common coagulation problem in intensive care units with an incidence of up to 60% in
some studies. It is often associated with multiple organ failure and high mortality. One of the life-threatening causes of
thrombocytopenia in intensive care is thrombotic thrombocytopenic purpura (TTP). It is important that an early
diagnosis of this condition is made in critically ill patients since early initiation of treatment can translate into a
successful outcome. Patients who are diagnosed with TTP outside critical care units may very often require intensive
care management due to the high rate of complications associated with this condition. This review summarises recent
advances and outlines a practical approach to patients who are suspected to have TTP.
Keywords: thrombocytopenia; thrombotic thrombocytopenic purpura; microangiopathy; ADAMTS-13
Introduction
Thrombocytopenia is a well-known complication in intensive
care unit (ICU) patients and is associated with increased
mortality.1 A low platelet count develops, secondary to various
factors including sepsis, drugs, disseminated intravascular
coagulation and, rarely, the potentially life-threatening
condition of thrombotic thrombocytopenic purpura (TTP).
TTP is categorised as a microangiopathic haemolytic anaemia,
which is a group of disorders characterised by haemolytic
anaemia, thrombocytopenia and small vessel damage
(microangiopathy).2 It is not easily distinguishable from
conditions causing thrombotic microangiopathy, where
haemolysis is not common (see Table 1). There has been
tremendous progress in understanding of the pathogenesis and
treatment of TTP in the last two decades, with its
transformation from a universally fatal condition (greater than
90% mortality) to a treatable disease with less than 10%
mortality with early treatment.3
Epidemiology
Thrombotic thrombocytopenic purpura is a rare disease, with
an incidence of 4 cases per million.4 However, an enhanced
awareness of the condition and an increase in predisposing
conditions such as bone marrow transplant, has led to more
cases being diagnosed in recent years. TTP is more common in
women in their third and fourth decades, reflecting its autoimmune nature and the triggering nature of pregnancy for this
condition.4 It is also more common among black people.4 TTP
occurs less frequently in children. However, TTP can be
inherited or acquired and the inherited form of TTP usually
presents in the neonatal period, with jaundice secondary to
haemolytic anaemia, or in infancy where infections often
precipitate its onset.5 Some cases of congenital TTP may be
first diagnosed in adulthood (eg in pregnant females).6 In
1960, Schulman and colleagues described an 8-year-old girl
who had repeated episodes of bleeding from the neonatal
period with chronic thrombocytopenia and microangiopathic
JICS Volume 12, Number 3, July 2011
Microangiopathic haemolytic anaemias
•
Thrombotic thrombocytopenic purpura
•
Haemolytic uremic syndrome
•
Disseminated intravascular coagulation
•
Disseminated cancer
•
Haemolysis elevated liver enzymes and low platelet (HELLP)
syndrome
Thrombotic microangiopathy (without overt haemolysis)
•
Catastrophic antiphospholipid syndrome
•
Macrophage activation syndrome
•
Malignant hypertension
•
Pre-eclampsia
•
HIV infection
•
Antibodies to vascular endothelial growth factor (VEGF)
treatment
•
Allogeneic bone marrow transplant recipients
•
Renal allograft rejection
Table 1 Causes of thrombotic microangiopathy.
haemolytic anaemia.7 Two decades later, Upshaw described a
29-year-old woman who, since childhood, had had multiple
episodes of thrombocytopenia and microangiopathic
haemolytic anaemia, similar to Schulman’s case.8 These
descriptions have led to recognition of inherited TTP, which is
also termed Upshaw-Schulman syndrome.
Clinical features
The original description of TTP in 1924 by Moschowitz
detailed the pentad of signs and symptoms:9
• anaemia
• thrombocytopenia
• fever
215
Review articles
common cause of death especially if there has been a delay in
initiation of plasma exchange. Pancreatitis can precipitate TTP
and may be responsible for non-specific abdominal pain in the
absence of intestinal ischaemia.14
In this context, it is useful to bear in mind that the presence
of large ecchymotic patches, early lung involvement and large
vessel thrombosis are all features leading away from the
diagnosis of TTP.
Pathogenesis
Figure 1 Pathogenesis of thrombotic thrombocytopenic purpura
(TTP). In the top figure, the normal physiological state is
explained. Von Willebrand factor (red circles) is normally coiled in
the subendothelium (yellow rectangles), but unfolds in ultra-large
high molecular weight multimer forms with endothelial injury.
These are normally cleaved by the protease, ADAMTS-13
(scissors), preventing platelets (purple circles) aggregating on to
these multimers. TTP occurs when there is deficiency of this
enzyme (congenitally or through antibody formation) allowing
persistence of these multimers which cause platelet aggregation
and thrombotic consequences.
• neurological disturbance
• renal impairment.
However, these five classical features are often not evident in
the majority of patients. The absence of the pentad should
not discourage physicians from making the diagnosis of
TTP, which requires only the presence of microangiopathic
haemolysis and thrombocytopenia that cannot be explained
otherwise.10
The most common presenting symptoms of TTP are
nonspecific and can include abdominal pain, vomiting, and
generalised weakness.2 Many patients may have these
symptoms for many weeks before full-blown TTP develops.11
Fever is rare unless a septic illness triggers the onset of the
condition, but a high temperature should raise the suspicion of
a non-TTP cause of thrombotic microangiopathy. In a recent
study, Escherichia coli infections predominantly, but also
bacteria other than Enterobacteriaceae, were associated with
thrombotic microangiopathy in intensive care patients,
especially those patients who are immunocompromised (eg
with human immunodeficiency virus infection or systemic
lupus erythematosis).12
Neurological symptoms are seen in about half of the
patients and can vary from minor symptoms like headache and
transient confusion to hemiparesis and coma.2 It is important
to remember that the neurological symptoms can be fluctuant
in their presentation and severity and can develop during
therapy. Renal impairment is unusual with very few patients
having elevated serum creatinine although microscopic
haematuria and proteinuria are common.11 Cardiac
involvement with biochemical evidence of myocardial injury is
frequent in TTP and an elevated troponin has been suggested
to be a useful biomarker in some studies.13 This is also a very
216
The pathological basis of TTP is the widespread formation of
platelet thrombi in the microcirculation of several organs,
leading to thrombocytopenia (Figure 1). The accelerated
platelet aggregation is secondary to the binding to ultra-large,
high molecular weight multimers of von Willebrand factor
(VWF) in the circulation.13 VWF unfolds under conditions of
high shear stress, allowing proteolysis by a protease enzyme,
ADAMTS-13 (a disintegrin and metalloprotease with
thrombospondin repeats), into smaller fragments of VWF.15 In
physiological states, ultra-large VWF multimers released by the
endothelium induce platelet aggregation in areas of high shear
stress, but this only occurs briefly as VWF is rapidly broken
down by ADAMTS-13.15 However, in congenital deficiency or
where there is failure of ADAMTS-13 to act because of an
inhibitory antibody, there is excessive platelet aggregation. In
acquired TTP, antibodies develop against this enzyme, while in
the rare congenital type, enzyme deficiency occurs due to
mutation in the ADAMTS-13 gene.16,17 The resulting ADAMTS13 deficiency allows the unusually large multimers of VWF to
remain in the circulation, resulting in multiple platelet
aggregates.18 It has also been suggested that activated
endothelial cells can secrete large amounts of the ultra-large
VWF, which can overwhelm the enzyme activity of the
protease, leading to TTP in the absence of antibodies.14,19
Haemolysis in TTP is the result of platelet microthrombi in
the circulation which shear the red cells. This results in
schistocytes, which are fragmented red cells, easily observed on
a peripheral blood smear examination. Detection of
schistocytes is essential in the diagnosis of TTP because it
means that thrombocytopenia is caused by thrombotic
microangiopathy. This is especially important in intensive care
patients where there are multiple causes for a low platelet
count. Certain studies have suggested the possibility that the
red cell damage is responsible for the development of platelet
aggregates, especially in selected cases where the ADAMTS-13
enzyme is not severely depleted.20 Haemoglobin released from
haemolysed red cells mops up the platelet anti-aggregatory
agent nitric oxide, which also has vasodilatory properties.14,20
Uninhibited platelet aggregation and vasoconstriction can lead
to the thrombotic complications of TTP. In addition, the release
of adenosine diphosphate from inside the red cells can act as a
strong platelet aggregating agent.14
Diagnosis
The diagnosis of TTP is a clinical one, in the presence of
haemolysis and thrombocytopenia with no alternate
explanation (Figure 2).10 As it occurs frequently in previously
healthy individuals and the acute presentation can be dramatic,
Volume 12, Number 3, July 2011 JICS
Review articles
Involve
haematologist
lab
lgy
o
t
o
em
Ha
Suspect
TTP
Coagulation lab
n
io
us
sf
an
Tr
Bioc
hem
istr
y la
Vir
b
olo
gy
lab
Request blood film,
reticulocyte count
Request clotting
screen &
ADAMTS-13 work-up
Request LDH &
haptoglobin
b
la
Request hepatitis B,
C & HIV status
Ask for Coombs test
Arrange for urgent
plasma exchange
Figure 2 Diagnosis of thrombotic thrombocytopenic purpura.
the diagnosis should be suspected and treatment initiated on
an urgent basis. The laboratory abnormalities associated with
TTP include:
• Full blood count – the platelet count is often very low in the
acute phase, with a median value of 16 x109/L in the
Oklahoma registry data.11 Severe bleeding secondary to
thrombocytopenia is uncommon, as the basic pathogenic
mechanism for TTP is platelet aggregation. Anaemia
develops secondary to haemolysis, with haemoglobin about
8 g/dL.11
• Blood film – examination of the peripheral blood smear is
critical in the diagnosis of TTP. The presence of schistocytes
or red cell fragments in the peripheral blood smears should
be confirmed or excluded by an experienced morphologist.
TTP cannot be diagnosed in the absence of schistocytes.
However, there may be a time lag of up to 24 hours before
these fragments are noted under the microscope and hence
serial examination by an experienced observer is necessary.10
• Signs of haemolysis – anaemia, increased reticulocyte count,
elevated lactate dehydrogenase (LDH) and decreased
haptoglobin with negative direct Coomb’s or antiglobin
tests. LDH in TTP has been suggested to be also secondary
to tissue ischaemia in addition to haemolysis and, as such,
the normalisation of this parameter can take longer than the
other haemolytic markers.21
• The coagulation screen is usually normal and can be helpful
in differentiating TTP from other causes of microangiopathy,
such as disseminated intravascular coagulation.
• Abnormal renal function tests (abnormal in only a few
patients with TTP) can be helpful in pointing towards a
diagnosis of haemolytic uraemic syndrome.
• Abnormal liver function tests can suggest HELLP syndrome
JICS Volume 12, Number 3, July 2011
in pregnant women.
• HIV tests –TTP can be triggered by the virus in some cases.
The largest series reported so far included 30 episodes of
HIV-associated TTP noted in 24 patients over 10 years.22 In
this paper, a new diagnosis of HIV corresponded with the
first presentation of TTP in eight patients. Strict adherence
to once-daily highly active anti-retroviral therapy in
combination with plasma exchange was associated with a
good outcome in most cases with immunosuppression with
rituximab needed in approximately 10% of cases.
ADAMTS-13 assays
The identification of the ADAMTS-13 enzyme has been a
major advance in the understanding of TTP. Undetectable or
very low plasma levels of enzymatic activity (less than 10%)
establish a diagnosis of inherited or acquired TTP
unequivocally. However, patients with normal levels of
ADAMTS-13 activity may have the characteristic features of
TTP, while low enzyme levels (10-40%) may also occur in
disorders other than TTP, especially those secondary to other
conditions such as HIV infection and allogeneic bone marrow
transplantation.23 In congenital TTP, ADAMTS-13 assay is
beneficial in diagnosing the deficiency, while in acquired TTP,
inhibitory antibodies to the enzyme should be measured in
specialised laboratories. Currently, the usefulness of measuring
ADAMTS-13 is not in the initial diagnosis of TTP, which
remains a clinical one, but in predicting future relapses.24 Both
small-scale prospective and retrospective studies have shown
that patients with severe ADAMTS-13 deficiency and
antibodies to ADAMTS-13 protease had a much higher
likelihood of relapse.25 The assay may also be helpful in cases
where the diagnosis of TTP is not straightforward. In these
cases, plasma exchange is started and ADAMTS-13 levels are
awaited to confirm or refute the diagnosis.
Differential diagnosis
There are several other disorders that should be considered
(Table 2):
1. Haemolytic uraemic syndrome (HUS) – HUS is classified
into two categories, depending on whether it is associated
with Shiga-like toxin. Toxin-associated HUS (D+HUS) is the
classic or typical form seen in young children and is
associated with diarrhoea and more severe and sometimes
persistent renal impairment. The atypical form of HUS (DHUS) is much more difficult to differentiate from TTP,
although neurological symptoms would suggest the
diagnosis of TTP and moderate to severe renal failure,
HUS.26 ADAMTS-13 deficiency is unusual with typical HUS
although non-severe (>10%) deficiency may be noted in the
atypical form. Atypical HUS is a disease of complement
dysregulation, where genetic mutations of both complement
regulators and activators, and autoantibodies against the
complement regulator factor H occur. Identification of the
underlying molecular abnormality can help in planning
future management of these patients.27
2. Disseminated intravascular coagulation (DIC) – the main
differentiating factors are abnormal coagulation screen, low
fibrinogen and high D-dimers. Serial testing of these
217
Review articles
TTP
DIC
CAPS
MAPS
Thrombocytopenia
Sudden
Gradual
Sudden
Gradual
Coagulation abnormalities
Rare
Common
Rare
Rare
Haemotology
Anaemia
Reticulocytosis
Anaemia rare
Cytopenias
Cytopenias
Biochemistry
High LDH
Low haptoglobin
No change
May show renal or
liver abnormalities
May show renal or
liver abnormalities
Table 2 Differential diagnosis of thrombotic thrombocytopenic purpura.
Key: TTP: thrombotic thrombocytopenic purpura DIC: disseminated intravascular coagulation CAPS: catastrophic antiphospholipid syndrome
MAS: macrophage activation syndrome LDH: lactate dehydrogenase.
Begin
plasma exchange
Patient responding
Patient not responding
Add anti-platelets and
start thromboprophylaxis
once platelet count >50
Double plasma exchange
Consider rituximab
Platelet count & LDH
normal with haemoglobin
normal or near normal
for 2 days
Other immunosuppressives
or experimental therapies
Taper plasma exchange
Figure 3 Treatment algorithm for thrombotic thrombocytopenic
purpura.
markers may be necessary to confirm DIC.
3. The HELLP syndrome (Haemolysis, Elevated Liver enzymes
and Low Platelets) and pre-eclampsia – in pregnancy, both
these diagnoses can mimic TTP, although in HELLP, there
are abnormal liver tests and in both HELLP and preeclampsia, hypertension and proteinuria co-exist.
4. Catastrophic antiphospholipid syndrome (CAPS) and
macrophage activation syndrome (MAPS) – in both these
conditions, there may be a history of autoimmune disorders
such as systemic lupus erythematosis or juvenile
rheumatoid arthritis. CAPS (also known as Asherson’s
syndrome) presents in a dramatic fashion, with multiorgan
failure developing over a very short period of time,
associated with histopathological evidence of multiple vessel
occlusions.28 It is often preceded by a precipitating event,
mainly infectious, and is associated with high mortality.
Laboratory tests such as anti-cardiolipin antibodies and
lupus anticoagulant tests will be positive in CAPS especially,
in very high titres.29 MAPS is another potentially fatal
condition associated with excessive activation of
218
macrophages and T-cells leading to an overwhelming
inflammatory reaction.30 Patients with MAPS can have
pancytopenia and hepatosplenomgaly. Ferritin is increased
in these cases, along with triglycerides, which can aid the
diagnosis.31 Clinically, both CAPS and MAPS affect major
venous and arterial vessels rather than the microvasculature,
as in the case of TTP.
Management
Plasma exchange is the most important component of therapy
for all patients with suspected TTP. (Figure 3) In all cases of
suspected TTP, plasma exchange should be initiated as a matter
of urgency.10 Before the empirical use of plasma exchange
therapy, TTP was associated with a fatal outcome in almost all
patients. Following reports from trials of plasma exchange,
survival of 80-90% is reported.3 This treatment modality may
work by removing ADAMTS-13 antibodies, and/or replacing
the deficient protease. However, it has been reported to be
efficacious even in cases with normal plasma levels of
ADAMTS-13 and no detectable autoantibody.11 The
effectiveness of the plasma exchange procedure may also be
that it ‘cleans up’ the haemolytic products, which in turn mop
up beneficial nitric oxide.20 It is important to note that in
hospitals with no access to plasma exchange that infusion of
fresh frozen plasma (30 mL/kg) may be tried to ‘buy time,’
although volume overload can be a problem. The British
Committee for Standards in Haematology suggest that singlevolume daily plasma exchange should be commenced ideally
within 24 hours of TTP presentation, although reports of early
deaths due to cardiac involvement suggest that the sooner the
exchange is commenced, the better the outcome.10 Solvent
detergent-treated plasma is preferred, to minimise the risk of
viral transmission through blood products. Hepatitis B
vaccination should be considered in patients who are negative
for hepatitis B antibodies, to prevent transfusion-transmitted
infection.10 Plasma exchange, once commenced, is continued
until two days after remission is confirmed with the platelet
count greater than 150 x109/L, normal LDH and schistocytes
no longer present on blood films.10,11 Anaemia can worsen
during therapy and renal dysfunction, if present, may not
recover in all patients.11
The presence of antibodies to ADAMTS-13 in acquired TTP
has led to the use of immunosuppressive therapies. Most
commonly, steroids are prescribed at 1-2 mg/kg/day.10 In very ill
Volume 12, Number 3, July 2011 JICS
Review articles
Plasma exchange
Second and third
trimester
Steroids +
plasma exchange
TTP
No
tb
et
te
r
First trimester
Aim to
Deliver
Be
tte
r
HELLP
Pre-eclampsia
Postpartum
Treat as TTP
Figure 4 Management of thrombotic microangiopathy in
pregnancy. The main distinction for a suspected case of TTP in
pregnancy is from HELLP and PET. In the first trimester, delivery
of a viable baby is not possible. So continuation of pregnancy is
advisable since TTP is more likely and plasma exchange is
administered. In the second and third trimesters, since both
HELLP and PET are diagnostic considerations, glucocorticoids in
combination with plasma exchange is a relevant option with the
hope that the delivery of the fetus can be undertaken at a a safe
period. If the delivery of the fetus makes the clinical and
laboratory parameters better, PET or HELLP can be confirmed
and TTP can be excluded with the discontinuation of plasma
exchange. In the post-partum period, the female is treated as
having TTP since PET and HELLP are less likely.
patients, methyl prednisolone 1g IV for three days is
recommended.11 In the past, vincristine, cyclophosphamide,
azathioprine, intravenous immunoglobulins and splenectomy
have been tried. All patients should receive folate
supplementation for haemolytic anaemia.
Adjunctive treatment for patients with TTP includes an
antiplatelet agent, like aspirin, and an antithrombotic agent,
like low-molecular-weight heparin, to retard the platelet
aggregatory process and prevent the high incidence of
thrombosis in these patients. A randomised trial reported a
non-significant reduction in mortality when antiplatelet agents
were added to plasma exchange, although it may be prudent to
wait for the platelet count to be over 50 x109/L before staring
aspirin.11,32 A study of 68 patients with TTP demonstrated
seven cases of venous thromboembolism, with the authors
recommending thromboprophylaxis with low-molecularweight-heparin once the platelet count is adequate.33 Platelet
transfusions are considered to be relatively contraindicated in
TTP as they may add ‘fuel to the fire’ by enhancing further
platelet aggregation. However, if there is a need for central
venous access for plasma exchange or urgent surgery, platelet
transfusions may be considered. A recent systematic review did
not show added risk from platelet infusions.34
Refractory and relapsed cases
Despite the considerable success of plasma exchange,
approximately 10-20% of patients are refractory or relapse
early.35 In refractory cases of TTP, plasma exchange may be
intensified to twice daily regimen.9 Recently, there has been
JICS Volume 12, Number 3, July 2011
interest in the monoclonal antibody to CD20, rituximab, at
doses of 375 mg/m2 weekly for four cycles, with good response
noted in about three weeks.36,37 Over 100 TTP cases have been
treated with rituximab, with about 88% of refractory cases and
almost all relapsed cases achieving remission.38 Front-line
therapy with rituximab has not been extensively studied in
TTP, although there is interest in this regimen in combination
with plasma exchange due to the immune-mediated nature of
acquired TTP. While using rituximab and plasma exchange
concurrently, an interval of 24-36 h after rituximab should be
provided to prevent washout of the drug. Drugs on the horizon
include recombinant ADAMTS-13 concentrates and an aptamer
with the function of inhibiting the binding between platelets
and ultra-large VWF multimers. A recent publication also
noted N-acetyl cysteine to play a role in reducing the size and
activity of VWF in vitro and animal experiments.39
TTP in pregnancy
TTP in pregnant females can be a very difficult challenge. The
main differential diagnosis includes other causes of thrombotic
microangiopathy such as pre-eclampsia and HELLP syndrome.
It is important to distinguish these conditions since both
HELLP and preeclampsia are managed by delivery of the fetus,
which is not effective for TTP.10 Although both HELLP and preeclampsia usually present in the third trimester and have
classical clinical accompaniments, eg elevated liver enzymes
(with HELLP) and proteinuria/hypertension (pre-eclampsia),
the distinction can be difficult (Figure 4 for a practical
algorithm).40 In these situations, multidisciplinary involvement
with experienced obstetricians, haematologists and intensivists
is essential. Once TTP has been diagnosed, the treatment is
similar to that for a non-pregnant patient with TTP, with
plasma exchange.10 This procedure has been successfully tried
in many cases of TTP, although increased plasma requirement
may be noted during gestation.10 Monitoring of ADAMTS-13
activity can guide the adequacy and frequency of plasma
exchange.41 TTP can relapse in future pregnancies; those at risk
are patients with idiopathic TTP compared to those who
presented with TTP in association with pregnancy (25% vs
12%).41 Severe deficiency of ADAMTS-13 activity during
pregnancy predicts a high risk of relapse.42 These patients may
benefit from prophylactic therapy. Rituximab treatment is NOT
safe during pregnancy. A recent literature review identified that
out of 153 pregnancies with known outcomes, there were 90
live births, 22 premature births, one neonatal death, eleven
neonates with haematological abnormalities and two with
congenital malformations.43
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Jecko Thachil Consultant in Haemostasis and Thrombosis,
Roald Dahl Haemostasis and Thrombosis Centre, Royal Liverpool
University Hospital
jeckothachil@yahoo.co.uk
Volume 12, Number 3, July 2011 JICS