Can hyperviscosity reduce cerebral perfusion and cause neurologic symptoms?

Hyperviscosity syndrome is a clinically important, potentially reversible cause of impaired cerebral circulation and neurologic dysfunction, most clearly documented in polycythemia and monoclonal gammopathies such as Waldenström macroglobulinemia.

Cerebral perfusion and neurologic effects

• In 39 people with relative polycythemia, cerebral blood flow was substantially lower than in controls and inversely correlated with hematocrit and measured whole-blood viscosity. Venesection reduced viscosity and increased cerebral blood flow, supporting a reversible viscosity-related flow limitation.
• Earlier ^133Xe clearance studies in polycythemia vera likewise found low cerebral blood flow that rose after hematocrit reduction. Cerebral arterial velocities also improve after treatment/phlebotomy in newly diagnosed polycythemia vera.
• Clinically, hyperviscosity can cause headache, dizziness, poor concentration, “slow mentation,” confusion, altered mental status, somnolence, seizures, focal deficits, and coma. Confusion and cognitive slowing are particularly recognized in Waldenström-associated hyperviscosity.

Mechanisms and associated manifestations

• Increased blood or plasma viscosity raises resistance to microvascular flow, compromising cerebral perfusion and oxygen delivery. Changes in hematocrit may additionally affect oxygen content and cerebrovascular autoregulation.
• In Waldenström macroglobulinemia, excess intravascular pentameric IgM increases plasma/serum viscosity. Visual disturbance or hyperviscosity retinopathy and mucosal bleeding are other classic manifestations, alongside neurologic symptoms.
• Therapeutic plasma exchange is first-line acute treatment for symptomatic monoclonal-gammopathy hyperviscosity and generally lowers viscosity by approximately 20–30% per session.

Clinical implications

• New confusion or cognitive decline in a compatible hematologic setting warrants urgent assessment, including neurologic and retinal examination, serum viscosity, and paraprotein testing. Treatment should be guided by the clinical syndrome rather than a viscosity cutoff alone.
• Bottom line: Hyperviscosity is a well-supported cause of reduced cerebral microvascular perfusion and neurologic symptoms, including confusion and cognitive slowing; symptomatic presentations require urgent evaluation and, when appropriate, prompt plasma exchange.