In polycythemia vera (PV), thrombosis is a major cause of morbidity and mortality. However, It is easily overlooked at an early stage because of occult clinical symptoms. The pathophysiology of thrombosis in this disorder remains unclear (1). We report a case with PV that showed interesting brain magnetic resonance imaging (MRI) findings as a clue for diagnosing PV.

A 76-year-old female presented with difficulty walking for one week. A neurologist detected Barre’s sign in the right upper and lower limbs. Brain MRI revealed subacute infarction of the left corona radiata and left occipital lobe. Mild petechial hemorrhagic transformation coexisted in the left occipital lobe infarction. A T1-weighted image showed diffusely inhomogeneous skull bone marrow, while a diffusion-weighted image (DWI) revealed diffuse hyperintensities in the skull bone (Figure 1).

Figure 1: Axial T1-weighted and diffusion-weighted images of the brain.

1. An axial T1-weighted image shows inhomogeneous bone marrow in the skull. (arrowheads)
2. An axial diffusion-weighted image shows diffuse hyperintensities, indicating the disappearance of fat marrow in the skull. (arrowheads)

A T2*-weighted image (T2*WI) and susceptibility-weighted image (SWI) showed prominent cortical and deep medullary veins in the brain. Cerebral blood flow (CBF), evaluated by the arterial spin-labeling (ASL) method with MRI, decreased diffusely in the bilateral cerebral cortices (Figure 2). Magnetic resonance angiography showed no evidence of significant stenoses of the main trunk of the cerebral arteries. A hematological examination revealed pancytosis as follows: white blood cell, 13.7×10³/μL; red blood cell, 8.4×10⁶/μL; hemoglobin, 20.8 g/dL; platelet, 497×10³/μL; hematocrit, 66%. The patient was diagnosed with PV including the presence of a Janus kinase 2 (JAK2) mutation.

Figure 2: Cerebral blood flow by the arterial spin-labeling method and susceptibility-weighted image of the brain.

1. A cerebral blood flow image by the arterial spin-labeling method shows the diffuse hypoperfusion of the bilateral cerebral cortices.
2. A susceptibility-weighted image shows prominent cortical and deep medullary veins in both cerebral hemispheres.

PV is a rare hematological neoplasm that is characterized by the clonal proliferation of multipotent bone marrow progenitors due to acquired mutations in JAK2. Polycythemia may cause thrombotic diseases, particularly cerebral infarction. This risk has been attributed in part to increased blood viscosity due to an elevated hematocrit (2). Epidemiological evidence indicates that patients with a venous hematocrit at the upper limits of the normal range are prone to vascular occlusive disease (3). However, the pathophysiology of thrombosis in patients with PV is complex. Although multiple factors, such as hematological parameters, clinical factors, and molecular risk factors, are likely to contribute to the pathogenesis of thrombosis, it still remains unclear. (2).

 CBF is reduced in patients with PV and corrected by venesection (3). In our patient, CBF, evaluated by the ASL method, diffusely decreased in cerebral cortices other than the infarcted areas. Cortical and deep medullary veins were prominent on T2*WI and SWI. The hypointense signal within cortical and deep medullary veins on T2*WI and SWI in patients with acute infarction is widely assumed to be caused by uncoupling between oxygen supply and demand in hypoperfused tissue, resulting in a relative increase in deoxyhemoglobin and relative decrease in oxyhemoglobin (4, 5). Therefore, prominent cortical and deep medullary veins on T2*WI and SWI are interpreted as an increased oxygen extraction fraction, and a relative increase in deoxyhemoglobin and relative decrease in oxyhemoglobin in the brain of our PV patient.

 In our patient, DWI showed diffuse hyperintensity in the skull bone. This result can be interpreted as the cellular replacement of bone marrow from fatty marrow in patients with PV (6). Therefore, brain MRI can provide information for diagnosing PV based on decreased CBF, increased deoxyhemoglobin in the cerebral veins, and the cellular replacement of bone marrow. Moreover, brain MRI may provide information related to the risk of developing cerebral infarction in patients with PV.

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