Polycythemia vera is a myeloproliferative disorder of unknown cause, characterized by a marked increase in RBC mass and in blood volume and often by an increase in WBCs and platelets. The condition must be distinguished from the many secondary or symptomatic forms of polycythemia (erythrocytosis), in which the platelets and white cells remain normal. A high proportion of patients with polycythemia vera will have mutations in JAK2. The slightly increased incidence of thrombosis in primary polycythemia is attributed to the high blood viscosity, engorgement of vessels, and reduced rate of blood flow. The majority of patients with cerebrovascular manifestations have TIAs and small strokes, but we have seen one case of sagittal sinus thrombosis. With very high hematocrit, sludging of red cells may be seen in the retinal vessels. The cause of cerebral hemorrhage in this disease is less clear, although a number of abnormalities of platelet function and of coagulation have been described (see Davies-Jones et al).
Instances of platelet counts above 800,000/mm3 are considered to be a form of myeloproliferative disease allied with polycythemia vera. In some patients, there is an enlarged spleen, polycythemia, chronic myelogenous leukemia, or myelosclerosis. In several of our patients, no explanation of the thrombocytosis was found. They presented with recurrent cerebral and systemic thrombotic episodes, often of minor degree and transient. Cytapheresis, to reduce the platelets, and antiplatelet drugs (hydroxyurea anagrelide) to suppress megakaryocyte formation, are helpful in ameliorating the neurologic symptoms. In one of the cases we followed, several small lesions, presumably infarctions, were situated in the white matter and simulated multiple sclerosis. Another patient with essential thrombocytosis developed dramatic new migraine with aura when her platelet counts exceeded 1,000,000/mm3, a phenomenon commented on in the literature.
A wide variety of bleeding disorders—such as leukemia, aplastic anemia, thrombocytopenic purpura, and hemophilia—may also give rise to cerebral hemorrhage. Many rare forms of bleeding disease may be complicated by hemorrhagic manifestations; these are reviewed by Davies-Jones and colleagues.
This inherited disease is related to the presence of the abnormal hemoglobin S in the red corpuscles. Clinical abnormalities occur mainly in patients with sickle cell disease—that is, with the homozygous state, and not in those with the sickle cell trait, which represents the heterozygous state. We have seen neurologic symptoms in patients with a heterozygous mixed hemoglobinopathy, such as sickle-thalassemia, sickle-S, and sickle-D, but all are less severe and less frequent than in sickle cell anemia. The disease, which is practically limited to persons of central African and certain Mediterranean origins, begins early in life and is characterized by “crises” of infection (particularly pneumococcal meningitis), pain in the limbs and abdomen, chronic leg ulcers, and infarctions of bones and visceral organs. Ischemic lesions of the brain, both large and small, are the most common neurologic complications, but cerebral, subarachnoid, and subdural hemorrhage may also occur, and the vascular occlusions may be either arterial or venous. Patients with sickle cell anemia may develop progressive stenosis of the supraclinoid intracranial carotid artery with consequent collateral formation, producing a syndrome akin to moyamoya disease described earlier in the chapter. These fragile collateral vessels may rupture causing intracranial hemorrhage.