An excessive increase in viscosity of blood, as may occur secondary to erythrocytosis in patients with cyanotic congenital heart disease. Hyperviscosity symptoms include: headache; impaired alertness, depressed mentation or a sense of distance; visual disturbances (blurred vision, double vision, amaurosis fugax); paraesthesiae of fingers, toes or lips; tinnitus; fatigue, lassitude; myalgias (including chest, abdominal muscles),and muscle weakness. (Perloff JK at al. Adults with cyanotic congenital heart disease: hematologic management. Ann Intern Med 1988;109:406-413.) Restless legs or a sensation of cold legs may reflect hyperviscosity (observation of Dr. E. Oechslin). As the symptoms are non-specific, their relation to hyperviscosity is supported if they are alleviated by phlebotomy. Iron deficiency and dehydration worsen hyperviscosity and must be avoided, or treated if present.

Excessive blood thickness.