Effects of biological stimuli on the viscosity of the vitreous.

Changes in the physical properties of the vitreous induced by various biological stimuli were investigated with Cannon-Fenske kinematic and cone-plate rotary viscometers. Heating greatly decreased the viscosity of the human vitreous, the critical temperature being between 50 degrees C and 60 degrees C. Freezing also affected the viscosity, but less so than heating. Viscosity of hyaluronic acid was also decreased by heating to approximately 60 degrees C. Preliminary incubation with hyaluronidase made the viscosity of both the vitreous and hyaluronic acid heat-stable. Lens extraction decreased vitreous viscosity markedly, suggesting a decrease of hyaluronic acid concentration and/or a lowering of hyaluronic acid molecular weight. Vitreous haemorrhage resulted in decreased vitreous viscosity. Cryoretinopexy at the level employed in this study induced little change in vitreous viscosity, while retinal diathermy caused a moderate decrease. In the course of vitreous haemorrhage and retinal diathermy, a return towards normal of the viscosity was observed, suggesting the presence of local hyaluronic acid production.