Essential Thrombocythaemia

Overview

ET (essential thrombocytosis/primary thrombocythaemia) = clonal myeloproliferative disorder, markedly ↑platelets, NO recognisable stimulus. Same family as CML, polycythaemia vera — shared stem-cell-clone origin.

ET vs secondary/reactive thrombocytosis

ET: clonal, no stimulus, no definitive clonal marker (diagnosis largely by exclusion). Secondary: known stimulus — chronic infection, haemorrhage, postoperative, chronic iron deficiency, malignancy, RA, post-splenectomy.

Pathophysiology

Loss of normal thrombopoietin control over megakaryocyte endomitosis → uncontrolled megakaryocyte+platelet proliferation. Familial pattern reported in some cases — hereditary component possible.

Clinical features

Insidious, older patients. Paradox: BOTH bleeding AND thrombotic events occur (platelets numerous but functionally abnormal):

• Arterial/venous thrombosis
• Easy bruising
• Spontaneous bleeding
• TIA/stroke (platelet aggregation in CNS microvasculature)

Lab findings

Platelet count >400,000/µl sustained. Smear: large platelets, megakaryocyte fragments, hypogranular forms. Consistently abnormal platelet function (esp. aggregation) — the functional basis for bleeding despite high count. Marrow: numerous hyperdiploid megakaryocytes, variable fibrosis.

Treatment/prognosis

Benign course, often no therapy needed. Treatment only if platelets >1 million. Complications: acquired von Willebrand disease (large vWF multimers adsorbed onto excess platelets, proteolysed → ↓functional plasma vWF), bleeding. Notably: thrombosis incidence NOT higher than matched controls, despite intuition.

Clinical relevance

Bleeding+thrombosis coexisting in same disease = key concept — explained by qualitatively defective platelet function, not just platelet number. Corrects the common “more platelets = more clotting” assumption. Acquired vWD complication shows how excess of one haemostatic component (platelets) can paradoxically DEPLETE another (functional vWF) via adsorption/proteolysis. Distinguishing ET from reactive thrombocytosis is a practical everyday exercise — most high platelet counts are reactive; ET diagnosis requires excluding these first.

Overview

Essential thrombocythaemia (ET) — also called essential thrombocytosis or primary (idiopathic) thrombocythaemia — is a clonal myeloproliferative disorder with markedly elevated platelet count in the absence of any recognisable stimulus. It belongs to the same family of chronic myeloproliferative neoplasms as chronic myeloid leukaemia and polycythaemia vera (see Chronic Myeloid Leukaemia, Polycythaemia), sharing their origin from a single transformed stem cell clone.

ET vs Secondary (Reactive) Thrombocytosis

This distinction matters clinically far more than the platelet count alone:

• ET — clonal overproduction of platelets from megakaryocyte colonies, with no added stimulus. No specific clonal marker distinguishes it definitively from secondary thrombocytosis, so diagnosis is substantially one of exclusion.
• Secondary/reactive thrombocytosis — occurs in response to a known stimulus: chronic infection, haemorrhage, the postoperative state, chronic iron deficiency, malignancy, rheumatoid arthritis, or post-splenectomy.

Pathophysiology

The underlying mechanism is loss of normal thrombopoietin control over endomitosis in megakaryocytes — the process by which megakaryocytes replicate their DNA without cell division to build up the ploidy needed for platelet production. Without this control, both megakaryocytes and platelets proliferate uncontrolled. A familial pattern has been reported in some ET families, suggesting a hereditary component in at least some cases.

Clinical Features

Insidious onset, more frequent in older individuals. Despite the platelet excess, both haemorrhagic and thrombotic events occur — a seeming paradox explained by the platelets being functionally abnormal despite being numerous:

• Arterial or venous thrombosis.
• Easy bruising after minor trauma.
• Spontaneous bleeding.
• Transient ischaemic attacks or stroke, from platelet aggregation within the CNS microvasculature.

Laboratory Findings

• Sustained platelet elevation above 400,000/µl.
• Peripheral smear: large platelets, megakaryocyte fragments, hypogranular forms.
• Consistently abnormal platelet function, particularly defective aggregation — the functional basis for the haemorrhagic tendency despite high platelet numbers.
• Bone marrow: numerous hyperdiploid megakaryocytes, with variable increased fibrosis.

Treatment and Prognosis

ET runs a benign course and often requires no therapy at all; treatment is generally reserved for platelet counts above one million/µl. Notable complications include acquired von Willebrand disease (large vWF multimers get adsorbed onto the excess platelet mass and are proteolysed, depleting functional plasma vWF) and bleeding — but, notably, the incidence of thrombosis in ET is not higher than in matched controls, despite the intuitive expectation that “more platelets” should mean “more clotting.”

Clinical Relevance

• The coexistence of both bleeding and thrombotic tendencies in the same disease is the single most conceptually important feature of ET — it is explained by qualitatively defective platelet function rather than platelet number alone determining clinical risk, a useful corrective to the common assumption that thrombocytosis simply equals hypercoagulability.
• ET’s acquired von Willebrand disease complication is a genuinely instructive example of how an excess of one haemostatic component (platelets) can paradoxically deplete another (functional vWF), via adsorption and proteolysis of large vWF multimers onto the greatly expanded platelet mass.
• Distinguishing ET from reactive/secondary thrombocytosis is a practical, everyday diagnostic exercise — most markedly elevated platelet counts encountered clinically are reactive (infection, iron deficiency, malignancy, post-splenectomy), and recognising ET requires first excluding these more common explanations rather than assuming a clonal disorder from platelet count alone.

No Rendered Diagram for This Topic

Essential thrombocythaemia is a comparison-and-classification topic (ET vs reactive thrombocytosis; the bleeding-and-thrombosis paradox) rather than a multi-step mechanism — a rendered diagram would not add clarity beyond the comparison already laid out in notes.md.

Hand-Draw Suggestion

Draw a simple two-column table by hand — ET vs Secondary Thrombocytosis — with rows for Clonality, Stimulus, and Typical Setting, as a quick recall aid for the distinction that matters most in this topic.