Essential Thrombocytosis

KEY FEATURES

Essentials of Diagnosis

• Elevated platelet count in absence of other causes

• Normal red blood cell (RBC) mass

• Absence of bcr/abl gene (Philadelphia chromosome)

General Considerations

• An uncommon myeloproliferative disorder

• Characterized by marked proliferation of the megakaryocytes in the bone marrow, leading to elevation of the platelet count

• High frequency of pathogenic variants of JAK2 and other genes (see also Polycythemia Vera)

Demographics

• The median age at presentation is 50–60 years

• There is a slightly increased incidence in women

CLINICAL FINDINGS

Symptoms and Signs

• Elevated platelet count

• Thrombosis

  • Most common clinical problem

  • Risk rises with age

  • Venous thromboses may occur in unusual sites, such as the mesenteric, hepatic, or portal vein

• Erythromelalgia: characterized by painful burning of the hands accompanied by erythema

• Bleeding

  • Typically mucosal

  • Uncommon occurrence

  • Related to a concomitant qualitative platelet defect

• Splenomegaly is present in at least 25% of patients

Differential Diagnosis

• Must distinguish from reactive causes of thrombocytosis (in which platelet count seldom is > 1,000,000/mcL [1000 × 10⁹/L])

  • Inflammatory disorders, such as rheumatoid arthritis and ulcerative colitis

  • Iron deficiency

• Other myeloproliferative disorders

  • Polycythemia vera

  • Myelofibrosis

  • Chronic myeloid leukemia (CML)

• Pathogenic variants of MPL and CALR genes frequently occur in patients with JAK2-negative essential thrombocytosis

DIAGNOSIS

Laboratory Findings

• Elevated platelet count (may be > 2,000,000/mcL [2000 × 10⁹/L]) (Table 15–14)

• Mildly elevated white blood cell count (usually not above 30,000/mcL [30 × 10⁹/L]), but with some immature myeloid forms

• Hematocrit is normal

• Peripheral blood smear

  • Reveals large platelets

  • Giant degranulated forms seen in myelofibrosis are not observed

• RBC morphology is normal

• Bone marrow shows increased numbers of megakaryocytes but no other morphologic abnormalities

• Peripheral blood should be tested for the bcr/abl fusion gene (Philadelphia chromosome) because it can differentiate between CML, where it is present, and essential thrombocytosis, where it is absent

TREATMENT

• Strict control of coexistent cardiovascular risk factors is mandatory for all patients

• Control of the platelet count, which should be kept at < 500,000/mcL (500 × 10⁹...