The imaging showed extensive cerebral venous sinus thrombosis, a rare form of stroke in which the veins that drain blood from the brain become occluded by clot. CVST accounts for only about 0.5 to 3 percent of all cerebrovascular events, but it disproportionately strikes younger people, particularly women. In this patient, the thrombosis was not confined to one vessel. The superior sagittal sinus, both transverse sinuses, both sigmoid sinuses, and the right internal jugular vein were all involved, effectively shutting down the supratentorial venous drainage system. Even more ominously, the deep venous system, including the internal cerebral veins and the vein of Galen, was thrombosed as well.

Deep venous involvement matters because those vessels drain structures that have almost no collateral blood supply: the thalami, basal ganglia, hypothalamus, and periventricular white matter. When congestion builds in these territories, venous infarction can develop rapidly and carries high morbidity and mortality. Magnetic resonance imaging in this patient confirmed exactly that pattern. T2-weighted and FLAIR sequences showed bilateral thalamic hyperintensity with periventricular edema, and diffusion-weighted imaging demonstrated restricted diffusion in both thalami, the radiological signature of acute venous infarction. Magnetic resonance venography showed absent or markedly reduced flow through the major sinuses, with prominent cortical collaterals suggesting the brain was desperately rerouting its venous outflow.

Complicating the picture further was obstructive hydrocephalus, a rarely reported consequence of CVST. The noncontrast CT scan showed mild dilatation of the lateral ventricles and prominence of the temporal horns, along with a hyperdense cord sign along the superior sagittal sinus, a classic marker of acute sinus thrombosis. Two mechanisms can drive ventricular enlargement in this setting. Elevated venous sinus pressure can impair cerebrospinal fluid resorption across the arachnoid granulations by altering the pressure gradient, while venous congestion and edema in the thalami and adjacent periventricular structures can physically obstruct cerebrospinal fluid pathways. The authors of the report note that their imaging could not pinpoint a single anatomical obstruction site, so the hydrocephalus was considered likely multifactorial rather than attributable to one definitively demonstrated blockage.

The initial misdiagnosis is perhaps the most instructive element of the case. Idiopathic intracranial hypertension and CVST share strikingly similar presentations: progressive headache, vomiting, visual disturbance, and raised opening pressure on lumbar puncture. The key clinical discriminator is the response to treatment. Acetazolamide typically improves idiopathic intracranial hypertension, whereas untreated CVST worsens over time as the thrombus propagates, potentially leading to venous infarction, hydrocephalus, and neurological deterioration. Plain CT can miss venous sinus thrombosis in up to a third of cases, which is why recent recommendations urge that patients with suspected idiopathic intracranial hypertension be screened with MRI and magnetic resonance venography as part of the diagnostic workup. In this patient, progressive symptoms despite acetazolamide therapy were the crucial clue that prompted advanced imaging and the correct diagnosis.

Once the thrombosis was identified, the team turned to the question of why it had happened. Her complete blood count revealed persistent, marked thrombocytosis, with platelet counts repeatedly measured at 1140 × 10^9 per liter, roughly four times the upper limit of normal. A peripheral blood smear showed large platelets, and a bone marrow biopsy demonstrated hypercellularity with megakaryocytic hyperplasia. Molecular testing identified the JAK2 V617F mutation with a variant allele frequency of 14 percent, while CALR and MPL variants and BCR::ABL1 transcripts were absent. The diagnosis was essential thrombocythemia, a myeloproliferative neoplasm in which the bone marrow produces excessive, functionally abnormal platelets.

The JAK2 V617F mutation, present in roughly half to sixty percent of essential thrombocythemia patients, activates the JAK-STAT signaling pathway, driving platelet overproduction and producing platelets that are prone to inappropriate activation. Beyond thrombocytosis, the mutation contributes to endothelial dysfunction, leukocyte activation, and chronic inflammation, all of which amplify thrombotic risk. The result is a hypercoagulable state capable of producing clots at unusual arterial and venous sites. Venous thromboembolism is a recognized complication of essential thrombocythemia, but cerebral venous sinus thrombosis remains a rare manifestation, and the combination of extensive CVST, obstructive hydrocephalus, and JAK2-positive essential thrombocythemia has seldom been described together in a single patient. Current guidelines therefore recommend evaluating for underlying myeloproliferative neoplasms, including JAK2 mutation testing, in any patient with CVST who has no obvious provoking factor.

Treatment proceeded on two fronts. Immediate anticoagulation with low-molecular-weight heparin, enoxaparin 80 milligrams, was initiated on 21 April 2026, and later transitioned to the oral factor Xa inhibitor apixaban at 5 milligrams twice daily in May 2026. Given the extent of the thrombosis and the persistent prothrombotic risk posed by the underlying blood disorder, long-term anticoagulation was deemed appropriate. Cytoreductive therapy with pegylated interferon alfa was also started to bring the platelet count under control, a choice the report highlights as particularly suitable for women of childbearing age because of its disease-modifying potential. A multidisciplinary team spanning neurology, hematology, radiology, and ophthalmology managed the case, with ophthalmological assessment confirming bilateral papilledema and continuous intracranial pressure monitoring instituted. Notably, the hydrocephalus was managed without neurosurgical cerebrospinal fluid diversion; the patient improved with treatment directed at the thrombosis and the hematological disease, underscoring that management must be individualized to neurological status and imaging progression, while recognizing that deterioration may still demand urgent surgery.

The outcome was striking. By a follow-up visit on 2 June 2026, the patient had improved clinically and her platelet count had fallen to 355 × 10^9 per liter, with hemoglobin of 124 grams per liter, hematocrit of 0.41 liters per liter, and a white cell count of 5.6 × 10^9 per liter. Follow-up magnetic resonance venography demonstrated partial recanalization of the right transverse sinus and the posterior portion of the superior sagittal sinus, though recanalization remained incomplete. Susceptibility-weighted imaging showed blooming in the left basal ganglia, suggestive of residual blood products, but no clinically significant bleeding complication occurred during anticoagulation, and no new thrombotic events were observed on subsequent follow-up. The patient remains under combined neurological and hematological care.

The authors are careful to note the limitations inherent in a single-patient report: the temporal relationship and biological plausibility linking JAK2-positive essential thrombocythemia to this catastrophic venous thrombosis are strong, but causality cannot be proven, and generalizability is limited. Still, the clinical lessons are clear. Worsening symptoms in a patient labeled with idiopathic intracranial hypertension should trigger urgent magnetic resonance venography, because delayed diagnosis of CVST allows thrombus propagation with potentially fatal consequences. Once CVST is confirmed, a thorough workup for hidden prothrombotic disorders, including myeloproliferative neoplasms, is essential, since identifying an underlying condition fundamentally changes long-term management. The case also adds to the sparse literature on hydrocephalus as a complication of cerebral venous thrombosis and stands as a reminder that a young brain, given early diagnosis and coordinated multidisciplinary care, can recover even from near-complete occlusion of its venous drainage system.

Subject of Research: Cerebral venous sinus thrombosis with obstructive hydrocephalus caused by undiagnosed JAK2 V617F-positive essential thrombocythemia

Article Title: Extensive Cerebral Venous Sinus Thrombosis With Obstructive Hydrocephalus in a Young Female With Underlying Essential Thrombocythemia: A Case Report

Article References: Nafea, S., Farooq, T., Taher, M. A., Barros, A., Soe, K. N., Ali, F., & Thomas, K. (2026). Extensive Cerebral Venous Sinus Thrombosis With Obstructive Hydrocephalus in a Young Female With Underlying Essential Thrombocythemia: A Case Report. Clinical Case Reports, 14(10), Article e73669. https://doi.org/10.1002/ccr3.73669

Image Credits: AI Generated

DOI: 10.1002/ccr3.73669

Keywords: cerebral venous sinus thrombosis, essential thrombocythemia, JAK2 V617F, obstructive hydrocephalus, myeloproliferative neoplasm, idiopathic intracranial hypertension, venous infarction, anticoagulation, pegylated interferon alfa, magnetic resonance venography, thalamic infarction, thrombocytosis

The study, conducted by Özge Sönmez and Ahmet Emre Eşkazan of the Division of Hematology at Cerrahpaşa Faculty of Medicine, Istanbul University-Cerrahpaşa, used a web-based questionnaire to probe how hematologists manage extreme thrombocytosis, the technical term for platelet counts that far exceed the normal range of roughly 150,000 to 450,000 per microliter. Fifty-six hematologists participated in the survey, which was supported by the Turkish Society of Hematology and approved by the local clinical studies ethics committee at Istanbul University-Cerrahpaşa. The authors set out to map real-world clinical behavior in a domain where evidence is thin, expert opinion varies widely, and the consequences of over-treatment or under-treatment can be serious for patients with essential thrombocythemia, one of the classical Philadelphia-negative myeloproliferative neoplasms.

The central finding concerns the platelet threshold at which physicians choose to intensify therapy. Cytoreductive treatment, which uses medications such as hydroxyurea to suppress platelet production in the marrow, was favored by 41.1 percent of respondents once platelet counts reached or exceeded 1000 × 10⁹ per liter, the celebrated million-platelet mark. But when the threshold was raised to 1500 × 10⁹ per liter, the proportion of hematologists opting for cytoreduction jumped to 83.9 percent. In other words, the million mark alone is not a universal trigger for action; it is a point of debate. Only when counts approach one and a half million does a strong majority of specialists converge on drug-based platelet lowering, a pattern that mirrors the cautious, risk-adapted philosophy embedded in contemporary myeloproliferative neoplasm guidelines.

Why does the million mark inspire such hesitation? The answer lies in the biology of extreme thrombocytosis. Very high platelet counts do not simply increase the risk of blood clots; paradoxically, they also raise the risk of bleeding. The mechanism is acquired von Willebrand disease, a condition in which the sheer excess of circulating platelets adsorbs and proteolytically cleaves von Willebrand factor, the adhesive protein that platelets and clotting depend on. As platelets multiply, they effectively devour their own hemostatic infrastructure, and the largest von Willebrand multimers disappear first. A patient with a platelet count of a million or more may therefore present with gastrointestinal bleeding or bruising rather than thrombosis, and giving aspirin to such a patient in the belief that high platelets mean high clot risk can precipitate dangerous hemorrhage.

This biological nuance explains the second pillar of the survey: laboratory assessment. A striking 78.6 percent of hematologists reported testing for von Willebrand factor ristocetin cofactor activity, abbreviated vWF:RCo, at the time of diagnosis of extreme thrombocytosis. This assay measures the functional capacity of von Willebrand factor to support platelet adhesion under shear stress, and it is the workhorse test for detecting the acquired form of the deficiency. The near-universal uptake of this test among survey respondents suggests that the field has internalized the lesson that platelet number alone is an unreliable guide to bleeding risk, and that documenting acquired von Willebrand disease is now considered a standard part of the initial evaluation when counts soar into the extreme range.

Aspirin decisions revealed an equally disciplined logic. In the presence of cardiovascular risk factors and in the absence of acquired von Willebrand disease, 91.1 percent of hematologists favored aspirin therapy for patients with platelet counts of at least 1000 × 10⁹ per liter, and 78.6 percent favored it at counts of at least 1500 × 10⁹ per liter. The slight decline at the higher threshold is telling: the very patients with the most extreme counts are also the most likely to harbor acquired von Willebrand disease, so the antiplatelet drug that protects one patient’s arteries becomes a hemorrhagic hazard for another. The survey thus captures, in numerical form, the delicate balancing act that defines extreme thrombocytosis management, where the same count can argue for and against the same drug depending on a single laboratory result.

When asked why they initiate cytoreductive treatment at all, respondents cited three goals with roughly equal frequency: prevention of thrombosis, prevention of bleeding, and control of symptoms such as headaches, erythromelalgia, and visual disturbances. This even distribution is itself informative. It shows that hematologists do not view cytoreduction solely as an anti-thrombotic strategy. Lowering the platelet count can restore von Willebrand factor levels and relieve bleeding, and it can also ameliorate the microvascular symptoms that plague many patients with essential thrombocythemia regardless of their thrombotic risk score. The therapeutic rationale, in practice, is tripartite, and the survey suggests that experienced clinicians weigh all three aims rather than fixating on any single one.

The most novel element of the report is its comparison with artificial intelligence. The authors posed the same clinical scenarios to AI programs and recorded the recommendations the models produced. The AI-generated advice supported von Willebrand testing, endorsed cytoreductive treatment particularly at platelet counts of 1500 × 10⁹ per liter or above, and generally recommended aspirin only for patients with counts of at least 1000 × 10⁹ per liter who did not have acquired von Willebrand disease. Across several clinical scenarios, the differences between the human specialists and the machine recommendations were few. The authors are careful to frame this comparison as exploratory, and they stop short of claiming that AI can substitute for clinical judgment, but the convergence is noteworthy at a time when large language models are being proposed as clinical decision aids.

The significance of this convergence deserves careful interpretation. On one hand, it suggests that the consensus positions in hematology, such as testing for acquired von Willebrand disease and reserving aggressive cytoreduction for the most extreme counts, are encoded deeply enough in the published literature that AI systems trained on that literature reproduce them faithfully. On the other hand, it raises the question of whether AI is genuinely reasoning or simply echoing the dominant patterns of its training data, including their blind spots. Where expert opinion is divided, as it clearly is at the million-platelet threshold where only about four in ten specialists opt for cytoreduction, an AI that confidently recommends a single course of action could flatten legitimate clinical nuance into false certainty. The survey’s exploratory design is a prudent acknowledgment of that risk.

For patients with essential thrombocythemia, the practical message of the study is reassuring in one respect and cautionary in another. Reassuring, because the management of extreme thrombocytosis appears to follow a coherent, biologically informed playbook: check von Willebrand function, tailor aspirin to its result, and reserve cytoreductive drugs chiefly for the most extreme counts or for patients with thrombosis, bleeding, or disabling symptoms. Cautionary, because the survey also exposes genuine variability at the margins, with more than half of specialists declining to treat at exactly one million platelets while others intervene. As Sönmez and Eşkazan conclude, extreme thrombocytosis itself remains an important driver of treatment decisions, particularly above 1500 × 10⁹ per liter, and the near-agreement between experienced hematologists and AI recommendations offers a provocative glimpse of a future in which algorithms may help standardize care, provided they are validated rigorously and deployed with the same humility that the survey’s authors bring to their own exploratory analysis.

Subject of Research: Clinical management of extreme thrombocytosis in essential thrombocythemia, comparing hematologist survey responses with artificial intelligence recommendations

Article Title: When platelets reach a million: insights from hematologists

Article References: Sönmez, Ö., & Eşkazan, A. E. (2026). When platelets reach a million: insights from hematologists. Annals of Hematology. https://doi.org/10.1007/s00277-026-07183-x

Image Credits: AI Generated

DOI: 10.1007/s00277-026-07183-x

Keywords: essential thrombocythemia, extreme thrombocytosis, platelets, cytoreductive therapy, acquired von Willebrand disease, aspirin, von Willebrand factor, myeloproliferative neoplasms, hematology, artificial intelligence, clinical survey, thrombosis