Polycythemias or
erythrocytoses in childhood and adolescence are very rare. Systematic data on the clinical presentation and laboratory evaluations as well as on treatment regimens are sparse. The diagnostic program in absolute
erythrocytosis includes extensive clinical, hematological, biochemical, and molecular
biological examinations which should be applied following a stepwise algorithm. Absolute
erythrocytoses are usually subdivided into primary and secondary forms. Primary
erythrocytosis is a condition in which the erythropoietic compartment is expanding independently of extrinsic influences or by responding inadequately to them. Primary
erythrocytoses include
primary familial and congenital polycythemia (PFCP) due to mutations of the
erythropoietin (Epo) receptor gene and the
myeloproliferative disorder polycythemia vera. Secondary
erythrocytoses are driven by hormonal factors (predominantly by Epo) extrinsic to the erythroid compartment. The increased Epo secretion may represent either a physiologic response to tissue
hypoxia, an abnormal autonomous Epo production, or a dysregulation of the
oxygen-dependent Epo synthesis. Congenital secondary
erythrocytoses are caused, e.g., by
hemoglobin variants with increased
oxygen affinity, by
2,3-bisphosphoglycerate deficiency, or by mutations in the von Hippel-Lindau gene associated with a disturbed
oxygen-dependent regulation of Epo synthesis.