Potential Overestimation of Uveoscleral Outflow from Goldmann Tonometry Bias.

Abstract

In living humans, uveoscleral outflow (Fu) is calculated, not measured. Except in rare tracer studies it is the residual of the modified Goldmann equation, Fu = F - C(IOP - Pev), which treats Fu as pressure-independent and assigns all pressure-dependent outflow to facility (C). Goldmann applanation tonometry (GAT) underestimates manometric IOP by about 4-5 mmHg. Because F, C, and Pev are measured independently of GAT, that low IOP input understates C(IOP - Pev) and overstates the residual reported as pressure-independent uveoscleral outflow. With representative values, GAT 15.5 mmHg yields Fu of about 50% of flow; manometric 20 mmHg yields about 9%, in line with human tracer data. A pressure-dependent uveoscleral component, if present, already sits inside C and cannot be separated by this method. The same bias has likely overstated the pressure-independent effect of prostaglandin analogs.