A recent study indicates that episcleral venous pressure (EVP) is not the primary cause of high intraocular pressure (IOP) in patients with open-angle glaucoma (OAG). Results did show that EVP in patients with OAG was elevated slightly more than in normal subjects and so could contribute in small part to high IOP.
"Previous studies have produced contradictory results," says Arthur J. Sit, M.D., with the Department of Ophthalmology at Mayo Clinic in Rochester, Minn., who led the research team. "EVP's contribution to the elevation of intraocular pressure in patients with open-angle glaucoma is not yet fully understood. We want a clearer understanding of that relationship."
Venomanometer allows advanced measurement
The research team measured EVP using a computer-controlled venomanometer (pressure-chamber method) in one eye each of 101 subjects with untreated OAG and 191 eyes of 100 healthy volunteers. In the OAG group, ages ranged from 24 to 83 years and averaged 64 years, while in the control group ages ranged from 19 to 81 years and averaged 48 years.
"Measurement of EVP in humans is necessarily noninvasive," says Dr. Sit. "Currently, the only way to measure EVP noninvasively is the pressure-chamber method, which involves placing a clear inflatable balloon against the surface of the eye and increasing the pressure until the episcleral vein of interest is noted to collapse. The difficulty is in identifying the beginning of collapse, which is best correlated with true EVP, but cannot be visually identified at the time of measurement. Our computer-controlled system allows us to record video of the episcleral vein collapse and the synchronized pressure readings. Using video image analysis techniques, the pressure at the very beginning of collapse can be identified, corresponding to the EVP."
The study also evaluated relationships between EVP and other ocular and systemic variables in patients with OAG. The team measured intraocular pressure, axial length, central corneal thickness (CCT), systolic blood pressure, diastolic blood pressure, height and weight, and calculated body mass index (BMI). As with IOP, there were no significant correlations between EVP and any of the physiological variables assessed in patients with OAG.
"We found that EVP in patients with open-angle glaucoma was elevated by a small amount compared with normal subjects," says Dr. Sit. "Although the increase could contribute in a small part to the elevation of intraocular pressure, it was not a primary cause of high IOP in these patients. We also found that EVP is not related to age, central corneal thickness, axial length, body mass index or blood pressure."
Although the EVP elevation in glaucoma was not large, it does represent a potential target for treatment. "Even a small decrease in EVP could represent an important therapeutic effect in glaucoma treatment," says Dr. Sit. "It is currently not known if existing therapies affect EVP, since previous techniques were sufficiently precise to detect small changes. We plan to evaluate the effects of existing and novel therapies on EVP."