Keywords
essential thrombocythemia - myeloproliferative neoplasm - Von Willebrand syndrome
Introduction
Essential thrombocythemia (ET) is a chronic myeloproliferative neoplasm (MPN) with
clonal megakaryocyte proliferation. It is an extremely rare cause of thrombocytosis
in childhood and adolescence with an estimated annual incidence of 0.09 per million
under 15 year olds.[1] Especially in the young symptoms of ET are mild or even absent and the disease is
often diagnosed only accidentally. Therefore, real incidence of ET is likely to be
underestimated in these patients. In contrast to ET in adults thromboembolic events
or bleeding complications are uncommon in children[2] and somatic mutations are rarely present in childhood ET.[3] Therefore, diagnosis of childhood ET is challenging and World Health Organization
(WHO) diagnostic criteria for adult MPN cannot be simply adopted.[2]
[4] In general, ET is associated with a good prognosis,[5] but life expectancy is limited when thromboembolic complications or progression
to post-ET-myelofibrosis or acute myeloid leukaemia (AML) occurs. Here, we report
on a case of a 14-year-old girl with an impressively fulminant clinical presentation
of ET. Symptoms were highly acute and comprised headache, visual impairment and painful
paresthesia of the upper and lower extremities. Extraordinarily high platelet counts
were found and urgent intervention was required.
Case Report
A 14-year-old female patient consulted her general practitioner with recurrent stabbing
pain and palmoplantar erythema since 2 weeks. Blood count performed to exclude underlying
infection revealed massive thrombocytosis above the diagnostic range of the analyser
used and the girl was referred to the emergency department of our university hospital.
Two weeks before, when the patient consulted her general practitioner because of first
mild pain episodes, blood count had been performed for the same reason and showed
platelets of 1310 × 109/L. Retrospectively, elevated platelet counts of more than 600 × 109/L for the past 3 years and 996 × 109/L 12 months before diagnosis had been documented, measured in blood counts examined
for suspicion of iron- and vitamin D deficiency and fever. These results had not led
to further investigations.
Further history revealed paresthesia of both arms and intermittent headache with impaired
vision. Vital signs and clinical findings, including a detailed neurological examination,
were normal. In particular, there were no signs of secondary causes for thrombocytosis,
i.e. infection, trauma or recent surgery.
Apart from vitamin D substitution there was no other medication nor any drug abuse.
The adolescent had so far been healthy. The patient's parents were not consanguineous
and had immigrated from Afghanistan when the patient was 9 years old. Apart from multiple
miscarriages on the father's side of the family, family history was unremarkable with
no thromboembolic events or bleeding complications. Platelet counts of all first-degree
relatives were below 300 × 109/L.
At our institution, the complete blood count was repeated and confirmed thrombocytosis
of 2373 × 109/L determined by a Sysmex XN 9000 platelet counter. Haemoglobin and a differential
leukocyte count were within age-specific range.
Laboratory testing showed a level of C-reactive protein <3 mg/L. Serologic testing
for Epstein–Barr virus, cytomegalovirus, hepatitis A, B, C and human immunodeficiency
virus was negative. Further diagnostics showed a prolonged partial thromboplastin
time of 45 seconds (normal range: 25–36 seconds). Because of the known relationship
of very high platelet counts (>1000 × 109/L) and acquired von Willebrand syndrome, corresponding laboratory analysis was performed
as an explanation for the prolonged partial thromboplastin time. Other reasons for
the prolonged partial thromboplastin time were not found. No use of oral anticoagulants
or heparin was reported. Factor VIII plasma activity (95%) was measured later in the
course. Lupus anticoagulant and anti-cardiolipin antibody titre were not determined.
Both von Willebrand factor (VWF) activity (27%) and VWF antigen (48%) were reduced
with a pathological VWF activity/antigen ratio of 0.56 (normal ≥ 0.7). Multimer analysis
of VWF revealed a loss of the largest and relative reduction of large VWF multimers
as well as accentuated satellite bands ([Fig. 1], lane 1).
Fig. 1 Von Willebrand factor (VWF) multimer analyses. Separation of von Willebrand factor multimers in a medium resolution (1.7% agarose)
gel. Lanes N: normal human plasma pool; patient's samples (lanes 1–4): lane 1: at
diagnosis (platelet count: 2505 × 109/L; VWF ratio: 0.45; the loss of the large VWF multimers is clearly visible, satellite
bands are slightly more prominent than in normal human plasma pool); lane 2: 2 months
later (platelet count: 1243 × 109/L; VWF ratio: 0.65); lane 3: 10 months after diagnosis (platelet count: 688 × 109/L; VWF ratio: 0.79); lane 4: 11 months after diagnosis (platelet count: 267 × 109/L; VWF ratio: 0.88); lane 5: plasma of a patient with genetically confirmed Von Willebrand
disease 2A (IIA) (VWF ratio: 0.25) characterized by loss of large and medium sized
VWF multimers and clearly accentuated satellite bands.
Ultrasound showed slight splenomegaly. Bone marrow histology demonstrated increased
numbers of mature megakaryocytes exhibiting “staghorn' nuclei. In addition, mild fibrosis
without typical features of primary myelofibrosis (increased reticulin or collagen
fibres, hyperchromatic or ‘cloud-like’ nuclei) was seen ([Fig. 2]).
Fig. 2 Bone marrow histology. May–Grünwald–Giemsa (×400 magnification). Initial bone marrow histology showed mature
megakaryocytes with a lobulated nucleus also known as ‘staghorn’ nuclei typically
seen in essential thrombocythemia. No evidence of increased reticulin or collagen
fibres, hyperchromatic or ‘cloud-like’ nuclei. The remaining cell lines were normal.
Based on these classical findings, we diagnosed ET. However, histologically, there
is no clear distinction between a congenital thrombocytosis and ET and also a ‘prefibrotic’
primary myelofibrosis could not be entirely excluded.
Screening for the most common somatic mutations known to be associated with MPN using
clinical routine assays was negative for JAK2V617F, MPL W515L/K and deletions in calreticulin, respectively. Translocation of BCR-ABL1 was also ruled out. In addition, we analysed the mutational spectrum in our patient
using a targeted next-generation sequencing panel comprising all exons and the flanking
regions of JAK2, MPL and TPO and more than 60 cancer-related genes. However, even this approach did not detect
any clonal aberration.
Because of massive thrombocytosis with associated neurological symptoms, treatment
with hydration (2–3 L daily) and oxygen supply was initiated and oral acetylsalicylic
acid (100 mg increased to 200 mg divided in two daily doses) was started. Cerebral
ischaemia was excluded by an emergency magnetic resonance imaging (MRI) scan.
A therapy with oral 20 mg/kg hydroxycarbamide (Litalir) daily was started and gradually
increased to 40 mg/kg daily. Due to persisting symptomatic thrombocytosis 2 weeks
after initiation of treatment, platelet apheresis was performed. Since the patient's
initial ferritin of 34 µg (range: 7–140 µg) was always in a low normal range with
borderline haemoglobin levels of 120 g/L (range: 121–154 g/L), we decided to repeatedly
apply iron sucrose to minimize any possible additional reactive thrombocytosis due
to effective iron deficiency. However, we observed no effect on platelet counts.
Because of neutropenia, hydroxycarbamide had to be reduced twice and stopped three
times for several days, respectively. As a consequence, platelet counts repeatedly
increased significantly at around 3 weeks after stopping hydroxycarbamide ([Fig. 3]).
Fig. 3 Clinical symptoms, laboratory findings and therapeutic course. Measured along the horizontal axis is a follow-up of 30 months as days from diagnosis.
The vertical scale represents platelet count. Observations showed platelet count above
700 × 109/L caused bleeding and neurological symptoms, stabbing pain and erythema at the soles
and fingertips. Two weeks after diagnosis, platelet apheresis was performed and showed
transient decrease in platelet count. Intravenous iron sucrose showed no impact on
platelet count. The dose-dependent effect of treatment with hydroxycarbamide (20 mg/kg
increased to 40 mg/kg) on platelet counts is also illustrated.
Due to these fluctuations of cell counts and the convincing experience with pegylated
interferon in adult patients, we replaced hydroxycarbamide 16 months after the first
presentation and switched to treatment with peginterferon alfa-2a (Pegasys) as a subcutaneous
injection starting with 90 mcg/week and continuous increase of the dose up to 180
mcg/week over 3 months.
Under this treatment, the patient remained asymptomatic, platelet counts appeared
stabilized at around 800 × 109/L and no side effects were observed.
Persistent reduction of the platelet count also led to normalization of the VWF ratio.
Notably, there were three episodes with increased bleeding tendency in the further
course, two with epistaxis and one with significantly prolonged menstrual bleeding.
All of these episodes were associated with an increase in platelet counts ([Fig. 3]).
Follow-up bone marrow biopsy taken 1 year after diagnosis showed stable histologic
features.
[Fig. 3] illustrates the clinical course and platelet count over time with respect to different
therapeutic interventions.
Discussion
The most common cause of elevated platelet counts in adolescence is reactive, mostly
infection-associated, thrombocytosis. These cases are self-limiting and remain asymptomatic.
Complications like bleeding or thrombosis are hardly seen in reactive thrombocytosis.[6]
[7]
Other reasons for reactive thrombocytosis are surgery, anaemia in particular due to
iron deficiency, drug side effects, allergy,[7]
[8] chronic inflammatory disease and asplenia.[9]
One further but rare differential diagnosis is congenital thrombocytosis, an inherited
autosomal-dominant, autosomal-recessive or X-linked condition. However, in our case
all family members of the first generation had platelet counts below 300 × 109/L and previous blood samples from our patient taken by the family physician showed
lower platelet values gradually increasing over months. These findings and on the
absence of mutations in TPO and MPL as shown in our diagnostic screening rendered the diagnosis of congenital thrombocytosis
unlikely and we diagnosed ET.
ET is an extremely rare cause of thrombocytosis in adolescence. According to the current
WHO criteria, diagnosis of ET is made when the following criteria[4] are fulfilled: (1) prolonged platelet count over 450 × 109/L, (2) bone marrow aspiration showing an elevated number of mature megakaryocytes,
(3) not matching one of the WHO criteria for other chronic Philadelphia-negative myeloid
diseases and (4) confirmation of one acquired somatic mutation or clonal marker. If
the last criterion is not met, the diagnosis of ET can be made after 1 year without
evidence of reactive thrombocytosis. In our patient, no somatic mutation or clonal
marker was identified while all other criteria were present. Gene mutations typically
found in adult MPN are often lacking in children and adolescents suggesting a different
disease pathology.[10]
Only very few paediatric patients with ET present with microcirculatory symptoms,
thromboembolic events, haemorrhagic diathesis or splenomegaly. Symptoms in our patient
were uncommonly severe and together with extremely high platelet numbers prompted
immediate initiation of treatment.
Patients with ET have an elevated risk of thrombotic events but also may show a bleeding
tendency. The latter is explained by the quantitative and qualitative platelet alterations
resulting in thrombocytopathy which is, as in our case, often aggravated by an acquired
Von Willebrand syndrome, the result of adsorption of large VWF multimers on platelets.[11]
[12]
[13] Platelets with increased degradation might explain the overproportional reduction
of VWF activity with a decreased activity/antigen ratio. These mechanisms have likely
caused the episodes of increased bleeding tendency with epistaxis and prolonged menstrual
bleeding observed in our patient. It cannot be ruled out that the medication with
aspirin augmented this bleeding tendency. The initial therapeutic decision to apply
aspirin, however, was based on the significantly increased risk of thromboembolic
complications associated with the extremely high platelet counts found in our patient
with ET. The two bleeding episodes observed in our patient were clinically not severe.
After these minor bleeding episodes, we intermittently halved the aspirin dosage.
Since bleeding symptoms occurred in phases of increased platelet counts >1200 × 109/L and subsided when platelet counts dropped, it can be assumed that they were mainly
caused by the secondary Von Willebrand syndrome.
Regarding the low numbers of treated children with ET and the long-term implications
in this age group, the therapeutic approach of childhood and adolescence MPN remains
challenging.
We have chosen to use hydroxycarbamide as primary cytoreductive therapy in our patient
because of our experience with this medication in sickle cell anaemia patients where
hydroxycarbamide is used for the induction of foetal haemoglobin and acts as an endothelial
cell adhesion modulator.[14] However, an aggravated leukaemogenic potential in adult ET patients after long-term
use is still controversially debated. In addition, due to its potential fetotoxic
and embryotoxic effects, hydroxycarbamide will have to be replaced for pregnancy.
Alternative therapeutic options are anagrelide and pegylated interferon (Pegasys).
Both substances are established in the treatment of adult MPN.[15]
Like polycythemia vera, ET can transform to myelofibrosis or AML. No prognostic markers
are known to predict transformation. Due to the mild bone marrow fibrosis observed
in our patient however, we could not entirely exclude ‘prefibrotic’ primary myelofibrosis
which is associated with a less optimal prognosis.
In summary, diagnosis of ET is very rare in children and adolescents and should be
considered as a differential diagnosis in persistent thrombocytosis only after exclusion
of secondary causes. Childhood ET appears to have a different genetic background than
adult ET and often mutational status cannot serve as a diagnostic criterion. Paediatric
patients with ET are mostly asymptomatic and only a very small minority present with
microcirculatory symptoms, thromboembolic events, haemorrhagic diathesis or splenomegaly.
Lacking a standard therapy in children and adolescents, management and especially
long-term treatment of ET in these age groups can be challenging.
