Measurement of Episcleral Venous Pressure
The episcleral veins are the ultimate destination for aqueous humor draining through the trabecular pathway. Thus, the pressure in these veins, the episcleral venous pressure (EVP), represents the lowest possible IOP in an intact eye with normal aqueous humor production. It is a dynamic parameter that varies with alterations in body position and systemic blood pressure. EVP is often increased in syndromes with facial hemangiomas (eg, Sturge-Weber), carotid-cavernous sinus fistulas, and cavernous sinus thrombosis; and it may be partially responsible for the elevated IOP seen in thyroid eye disease. EVP may also be altered by some medications, including topical Rho kinase inhibitors.
EVP can be measured noninvasively by venomanometry, in which a transparent flexible membrane is placed against the sclera and the pressure is slowly increased. The pressure that begins to collapse an episcleral vein corresponds to EVP; however, visualizing this endpoint can be difficult. The use of automated pressure measurements, video recording of the vein collapse, and image analysis software can help to precisely identify the endpoint and associated EVP. The usual range of values is 6–9 mm Hg, but higher values have been reported when different endpoints and measurement techniques are used. According to the Goldmann equation, IOP acutely rises approximately 1 mm Hg for every 1 mm Hg increase in EVP. However, elevation of EVP can cause reflux of blood into the Schlemm canal, altering the normal outflow of aqueous humor. As a result, the change in IOP may be greater or less than that predicted by the Goldmann equation.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.