Perinatal Polycythemia and Hyperviscosity Syndrome By Andrew W Walter, MS, MD, Sidney Kimmel Medical College at Thomas Jefferson University Reviewed / Revised Dec 2022 View Patient Education Etiology Symptoms and Signs Diagnosis Treatment Key Points Polycythemia is an abnormal increase in red blood cell mass, defined in neonates as a venous hematocrit ≥ 65%; this increase can lead to hyperviscosity with sludging of blood within vessels and sometimes thrombosis.