Essential Thrombocytopenia

Overview & Pathophysiology

Definition

• Essential Thrombocythemia (ET) = a chronic Philadelphia-negative myeloproliferative neoplasm (MPN) defined by sustained thrombocytosis (platelets >450 × 10⁹/L) from clonal megakaryocyte proliferation.
• A JAK-STAT–driven MPN in the same family as Polycythemia Vera and Primary Myelofibrosis — and the most indolent of the three.

Epidemiology

• Incidence: ~1–2 per 100,000/year.
• Age: bimodal — main peak 50–60 yrs, second peak in young women (~30 yrs).
• Sex: slight female predominance.

Top 3 driver mutations (mutually exclusive in ~90%)

• ~10–15% are "triple-negative" (no identifiable driver).
• CALR pearl: CALR-mutated ET = better prognosis and lower thrombosis risk than JAK2.

Pathophysiology

• Driver mutation → constitutive JAK-STAT activation → megakaryocytes proliferate independent of thrombopoietin.
• Massive platelet release → hyperviscous, prothrombotic blood.
• Paradox: at very high counts (>1,000–1,500 × 10⁹/L), platelets adsorb large vWF multimers → acquired von Willebrand disease → bleeding.

The shared molecular lesion is constitutive JAK-STAT signalling, illustrated in the JAK2 mutation pathway diagram below.

To separate ET from the other clonal myeloid neoplasms, see the four chronic myeloproliferative disorders for the diagnostic features and mutations that distinguish ET from CML, PV, and PMF.

Clinical Presentation

Most patients are asymptomatic at diagnosis (incidental CBC). Symptomatic disease follows a classic triad.

1. Vasomotor / microvascular symptoms

• Erythromelalgia — burning, red, painful hands/feet, dramatically relieved by aspirin (the classic clue).
• Headache, dizziness, transient visual disturbances, paresthesias, tinnitus.

2. Thrombosis (arterial > venous)

• Arterial: stroke / TIA, MI, peripheral arterial occlusion.
• Venous: DVT, PE, and splanchnic vein thrombosis (Budd-Chiari, portal vein).

Splanchnic clots in a young patient are a red flag — refer to the Budd-Chiari syndrome workup for the hepatic-vein outflow obstruction picture and its JAK2 testing.

ملاحظة سريرية – Clinical Note
أي مريض شاب لديه خثار في الوريد البابي أو الكبدي، يجب فحص طفرة JAK2 لاستبعاد كثرة الصفيحات الأساسية.	ملاحظة

3. Paradoxical bleeding (platelets >1,000 × 10⁹/L)

• Mucocutaneous bleeding from acquired von Willebrand disease (platelets adsorb large vWF multimers).
• Use aspirin cautiously at very high counts.

فخ امتحاني – Exam Trap
ارتفاع الصفائح الشديد (>١٠٠٠) يسبب نزيفاً وليس تخثراً بسبب استهلاك عامل فون ويلبراند.	ملاحظة

Exam findings

• Mild splenomegaly in ~25–50%.
• No significant hepatomegaly and no lymphadenopathy.

Diagnostic Approach

Workup is step-wise: confirm true thrombocytosis → exclude reactive causes → molecular & marrow studies.

Step 1 — Confirm true thrombocytosis

• CBC: platelets persistently >450 × 10⁹/L (≥2 occasions).
• Peripheral smear: increased, often giant/abnormal platelets; WBC and RBC usually normal.

Step 2 — Exclude reactive (secondary) thrombocytosis

• Iron studies (iron deficiency) and CRP/ESR (infection, inflammation).
• History: recent bleeding, surgery, splenectomy, or malignancy.
• Reactive thrombocytosis is the single most common cause of a high platelet count (IL-6 → hepatic thrombopoietin) — exclude it first.

Step 3 — Molecular & bone marrow studies

• Driver panel: JAK2 V617F → CALR → MPL.
• BCR-ABL must be negative (excludes CML).
• BM biopsy: hypercellular with large, mature, hyperlobulated ("stag-horn") megakaryocytes; no significant reticulin fibrosis.

WHO criteria (all 4 major required)

1. Platelets >450 × 10⁹/L.
2. BM megakaryocyte proliferation with large, mature forms.
3. Does not meet criteria for CML, PV, PMF, MDS, or other myeloid neoplasm.
4. JAK2, CALR, or MPL mutation (a clonal marker / absence of reactive cause serves as the minor criterion when criterion 4 is absent).

Centerpiece: ET vs. reactive thrombocytosis

To separate ET from CML and the other clonal myeloid neoplasms, see the four chronic myeloproliferative disorders for the mutation-based differential.

Management

Treatment is risk-stratified; ET is not curable (except by transplant) but very treatable. Goals: prevent thrombosis, control bleeding, relieve symptoms.

Risk stratification (IPSET, simplified)

• High risk: age >60 OR prior thrombosis (± JAK2-positive with cardiovascular risk factors).
• Low risk: none of the above.

نقطة قرار علاجي – Treatment Decision
لا نبدأ العلاج الخلوي (Hydroxyurea) إلا للمرضى ذوي الخطورة العالية (عمر >٦٠ أو جلطة سابقة).	ملاحظة

Treatment by risk

Cytoreductive agents (rule of 3)

• Hydroxyurea 500 mg–1 g PO daily — first-line for most high-risk patients.
• Pegylated interferon-α — preferred in pregnancy and young patients (non-teratogenic, non-leukemogenic).
• Anagrelide — second-line; inhibits megakaryocyte maturation (side effects: palpitations, headache, fluid retention).

Note – ملاحظة
Pregnancy & ET: increased risk of miscarriage and placental thrombosis. Use low-dose aspirin throughout pregnancy; add LMWH postpartum. If cytoreduction is needed → interferon-α (hydroxyurea is teratogenic).	ملاحظة

Complications & Prognosis

Complications (rule of 3)

• Thrombosis — leading cause of morbidity (stroke, MI, DVT, splanchnic vein thrombosis).
• Bleeding — when platelets >1,000–1,500 × 10⁹/L (acquired von Willebrand disease).
• Transformation (late, uncommon): post-ET myelofibrosis (~5–10% at 15 yrs) or AML (~1–5%, higher with alkylating agents).

Transformation to post-ET myelofibrosis produces the same fibrotic marrow seen in primary myelofibrosis:

Prognosis

• Most indolent MPN; median survival approaches that of the general population if well-managed.
• CALR-mutated patients fare better than JAK2-mutated patients.
• Additional risk: pregnancy loss from placental thrombosis.