Introduction

Essential thrombocythemia (ET) is an uncommon blood disorder in which the bone marrow produces an excessive number of platelets. This overproduction is usually driven by genetic mutations involving JAK2, CALR, or MPL.^1,2 Many patients live for years without major complications, but the disorder carries a heightened risk of blood clots.

Among the possible consequences, stroke is one of the most devastating. In some cases, stroke can even be the very first manifestation of ET, occurring before the condition has been formally diagnosed.³ Notably, strokes may appear even when platelet counts are only moderately raised. Certain factors, including older age, prior clotting events, traditional vascular risks such as high blood pressure, and the presence of the JAK2 mutation, are known to amplify this risk.²

Recognising the connection between ET and stroke is therefore crucial. For clinicians, this knowledge shapes decisions about preventive strategies, such as the use of low-dose aspirin or platelet-lowering treatments. For patients, it underscores the importance of regular medical follow-up and controlling other modifiable risks. When both doctors and patients remain vigilant, neurological complications can be reduced, and long-term outcomes improved.

What is essential thrombocythemia?

Definition and pathophysiology

Essential thrombocythemia is a chronic myeloproliferative neoplasm, a disorder arising from clonal expansion of bone marrow stem cells. The hallmark is a persistently elevated platelet count, generally above 450 × 10⁹/L.¹ The underlying driver in most cases is activation of the JAK–STAT signalling pathway, triggered by acquired mutations. Around 64% of patients carry the JAK2 V617F mutation, approximately 23% have CALR mutations, and about 4% show MPL mutations.²

These genetic changes disrupt normal regulation of blood cell production, causing excessive platelet formation. In addition to their sheer number, platelets in ET often display abnormal function, contributing both to clot formation and, paradoxically, to bleeding tendencies.¹

Symptoms and clinical presentation

A significant proportion of individuals with ET are asymptomatic, with the disorder discovered incidentally during routine blood tests.² When symptoms occur, they often result from abnormal microvascular circulation or bleeding. Common complaints include:

• Headaches and dizziness
• Visual disturbances such as blurred vision or flashing lights
• Tingling, burning pain, or redness in the hands and feet (erythromelalgia)
• Easy bruising, recurrent nosebleeds, or excessive bleeding during minor procedures

In rare cases, patients may experience severe bleeding episodes if platelet counts rise to extreme levels, particularly above 1,500 × 10⁹/L.¹

Risk factors for thrombosis in ET

Several factors are recognised as increasing the risk of clotting events in ET:²

• Age over 60 years: older patients face a substantially higher risk of thrombosis.
• Genetic background: JAK2-positive patients tend to have more arterial events than those with CALR mutations, the latter often linked with lower clotting risk.
• History of thrombosis: prior blood clots strongly predict future events.
• Traditional vascular risks: hypertension, diabetes, smoking, and high cholesterol act synergistically with ET to drive clot formation.

How does ET increase the risk of stroke?

Excess platelets and clot formation

In ET, the bone marrow’s unchecked platelet production leads to both excessive numbers and dysfunctional platelet behaviour. These abnormalities increase the likelihood of thrombosis in the arterial and venous systems.¹

Mechanisms: arterial vs. venous thrombosis

• Arterial thrombosis: Clotting within arteries, especially cerebral arteries, is a direct cause of ischemic stroke and transient ischemic attacks (TIAs). These events may occur even without significant narrowing of large arteries, often manifesting as small-vessel blockages or watershed infarctions⁴
• Venous thrombosis: While venous clots such as deep vein thrombosis or cerebral venous sinus thrombosis are less common, they remain part of the disease’s thrombotic spectrum¹

Types of stroke linked to ET

• Ischemic stroke: The most frequent type, often presenting as the first clinical sign of ET. In some series, up to 60% of affected patients had their ET diagnosis made after a stroke³
• Transient ischemic attacks (TIAs): Brief, reversible neurological deficits are common in ET and may herald a full stroke⁴
• Hemorrhagic stroke: Less common but possible, particularly in patients with very high platelet counts or in those undergoing complex treatment regimens²

In one review of 63 ET-related stroke cases, 55 were ischemic and 8 were hemorrhagic. Remarkably, nearly two-thirds of these patients were diagnosed with ET only after the cerebrovascular event³

Beyond platelet-driven mechanisms, standard vascular risks such as hypertension, diabetes, and smoking further elevate the risk of stroke, particularly in patients carrying the JAK2 V617F mutation^2,3

Recognising stroke in patients with ET

Warning signs and neurological symptoms

Stroke in ET presents the same classic symptoms seen in the general population: sudden facial droop, weakness in one arm, and slurred or lost speech. However, ET patients may also report more subtle or transient symptoms in the lead-up, including headaches, dizziness, visual disturbances, numbness, or tingling. These can correspond to TIAs, which frequently precede a major event.⁴

Practical memory aids such as FAST (Face, Arms, Speech, Time) and BE-FAST (which adds Balance and Eyes) remain invaluable tools for rapid recognition.

Importance of early recognition and emergency action

Stroke treatment is highly time-dependent. Interventions such as intravenous thrombolysis are most effective when given within 4.5 hours of symptom onset, while mechanical thrombectomy can be considered in select patients beyond that window.⁵ For ET patients, rapid hospital assessment is even more critical: studies suggest they experience higher short-term mortality and worse outcomes following stroke compared with the general population.⁵

For clinicians, any sudden neurological deficit in an ET patient should be treated as a medical emergency. Standard stroke pathways, urgent brain imaging, thrombolysis evaluation, and thrombectomy assessment must be activated without delay^[4].

Differences in presentation compared with general stroke patients

Strokes associated with ET differ in several ways:

• They may occur in younger adults, sometimes being the first clue to an underlying myeloproliferative disorder³
• Infarcts are often multifocal or atypical, involving multiple vascular territories or showing small-vessel and watershed patterns rather than large-artery blockages⁴
• JAK2 mutation carriers demonstrate a higher propensity for arterial events, influencing both risk assessment and prevention strategies²
• ET patients face a dual risk of clotting and bleeding, complicating decisions around antiplatelet or anticoagulant therapies¹
• Outcomes tend to be worse, with studies reporting higher in-hospital mortality rates⁵

Prevention and management strategies

Standard treatment options

The guiding principle in ET management is preventing blood clots while minimising bleeding complications.

• Low-dose aspirin is widely used in patients without contraindications. It reduces the risk of arterial thrombosis, including stroke, and is supported by both clinical trials and international guidelines¹
• Cytoreductive therapy is recommended for high-risk patients those over 60 years of age or with a history of clotting. Hydroxyurea remains the first-line cytoreductive agent and has demonstrated superiority over anagrelide when combined with aspirin in reducing vascular events²
• Interferon-α (especially pegylated forms) is an effective alternative, particularly for younger patients and women of childbearing age, as it avoids the long-term risks of hydroxyurea and is considered safer during pregnancy²

Therapy must always be individualised. For example, low-risk patients with certain CALR mutations may not benefit from aspirin and could face a greater bleeding risk.³

Monitoring and lifestyle modifications

Effective care relies on regular follow-up. This includes:

• Periodic blood counts to track platelet levels and white blood cells
• Monitoring for symptoms such as headaches, visual changes, or bleeding
• Adjustment of treatment based on established response criteria

Lifestyle factors are equally important. Patients are encouraged to stop smoking, maintain a healthy weight, exercise regularly, stay hydrated, and manage blood pressure, cholesterol, and diabetes. Primary care providers play a central role in supporting these preventive efforts.^1,2

Collaboration between specialties

Managing ET patients at risk of stroke requires coordinated care:

• Haematologists oversee diagnosis, genetic testing, and disease-specific treatments¹
• Neurologists and stroke specialists provide rapid assessment and acute management of cerebrovascular events, as well as guidance on secondary prevention^4,5
• Primary care and cardiovascular teams address modifiable risk factors to reduce overall vascular burden²

This multidisciplinary approach ensures that treatment strategies are balanced, evidence-based, and responsive to individual patient needs.

Conclusion

Essential thrombocythemia is more than just a rare blood disorder; it is a condition with potentially life-changing neurological consequences. By driving excessive platelet production and platelet dysfunction, ET significantly increases the risk of ischemic stroke and related events.

Awareness of this link is vital. For clinicians, it directs preventive strategies, acute management, and long-term monitoring. For patients, it emphasises the importance of adherence to therapy, lifestyle modification, and vigilance for early neurological symptoms.

Effective prevention and management involve a combination of low-dose aspirin where appropriate, cytoreductive therapy for higher-risk patients, regular monitoring, and aggressive control of cardiovascular risk factors. Importantly, collaboration between haematologists, neurologists, and primary care teams ensures that patients receive holistic and timely care.

Looking forward, continued research is needed to refine risk stratification, clarify the role of newer therapies, and explore patient-centred approaches to education and engagement.^2,3 With early recognition, proactive management, and shared decision-making, the burden of stroke in ET can be significantly reduced, helping patients lead longer, healthier lives.^4,5

Frequently asked questions (FAQs)

Can stroke be the first sign of essential thrombocythemia?

Yes. In many cases, a stroke or a transient ischemic attack (TIA) is the first clinical event that leads to an ET diagnosis. Studies show that a significant proportion of ET patients are only diagnosed after experiencing a cerebrovascular event.

How does ET increase the risk of stroke?

In ET, both the high platelet count and abnormal platelet function promote clot formation. Strokes may occur through arterial thrombosis (causing ischemic stroke or transient ischemic attacks) or, less commonly, haemorrhage due to extreme platelet counts or treatment effects. Risk is especially high in older patients, JAK2 mutation carriers, and those with traditional vascular risk factors.

What are the warning signs of stroke in ET patients?

ET-related strokes often present with the same classic symptoms: sudden facial droop, weakness in an arm, and slurred speech. Some patients may first notice headaches, dizziness, vision changes, or tingling, which can signal transient ischemic attacks (TIAs). Early recognition and urgent hospital care are critical.

How are strokes in ET patients managed differently from strokes in the general population?

While standard emergency treatments like thrombolysis and thrombectomy still apply, ET patients pose unique challenges because they face a dual risk of clotting and bleeding. Long-term management often combines antiplatelet therapy (aspirin), cytoreductive drugs, and strict control of cardiovascular risk factors, tailored to each patient’s genetic background and overall risk profile.