Why Low Outflow Facility and Low Aqueous Production May Be Risk Factors in Endoscopic Cyclophotocoagulation: A Goldmann-Based Perspective

Endoscopic cyclophotocoagulation (ECP) lowers intraocular pressure (IOP) by reducing aqueous humor production, yet the magnitude of its effect varies considerably among eyes and may occasionally lead to hypotony. Based on the Goldmann equation, in which IOP is expressed as IOP = episcleral venous pressure (EVP) + (aqueous humor production (F) - uveoscleral outflow (U)) / outflow facility (C), the pressure reduction induced by aqueous suppression can be approximated as ΔIOP (change in IOP) = dF (change in F) / C, when EVP and U are assumed to be constant. This relationship indicates that the IOP response to ECP depends not only on the degree of aqueous suppression but also strongly on the conventional outflow facility. A conceptual model assuming a fixed preoperative IOP demonstrates that eyes with low outflow facility exhibit a steeper pressure response to aqueous suppression and that eyes with low baseline aqueous production are mathematically linked to such low outflow states. These findings suggest that eyes characterized by both reduced aqueous production and impaired outflow may be particularly susceptible to excessive IOP reduction after ECP. Clinically, this includes conditions such as neovascular glaucoma with extensive angle closure, uveitic glaucoma, and glaucoma in elderly patients. The model presented is based on simplified assumptions, including constant episcleral venous pressure and uveoscleral outflow, and may not fully account for alterations in the pressure-flow relationship under pathological conditions; therefore, it should be interpreted with caution as a conceptual framework with limited physiological generalizability. However, the present conceptual framework provides a theoretical explanation for variability in ECP outcomes and suggests that securing a certain level of outflow facility prior to aqueous-suppressive procedures may be a reasonable strategy in selected cases.