There were two typographical errors in the Letter to the Editor in the October issue of the Journal of Glaucoma. The publisher regrets this error and reprints the letter here correctly in its entirety.
Pressure Dependence of Uveoscleral Outflow
To the Editor:
With the current widespread use of prostaglandin-like drugs to lower IOP in patients with glaucoma, a complete understanding of the mechanisms that these drugs affect is all-important. The impressive pharmacological activities of these drugs are believed to be exerted through increased uveoscleral outflow. Based on a few papers, many colleagues describe uveoscleral outflow as pressure independent. We have difficulty understanding how uveoscleral outflow can be pressure independent.
Conventional outflow facility is measured in units of μL/min/mm Hg. If uveoscleral outflow is not pressure dependent, its units would be μL/min. Essentially, μL/min are the units for secretion. If the units for uveoscleral outflow are μL/min, how could an increase in uveoscleral outflow lower IOP? One would have to consider some kind of secretory process removing aqueous humor from the eye or a decreased secretion of aqueous humor in response to the prostaglandin-like drugs.
We believe that uveoscleral outflow is pressure dependent, albeit with a high resistance pathway compared to the trabecular meshwork. The apparent driving force for uveoscleral outflow may be close to: IOP − 0 mm Hg. Uveoscleral outflow therefore contains a facility term, like trabecular outflow, and the proper units must be μL/min/mm Hg. We suggest modifying the Goldmann equation:EQUATIONso that EQUATIONwhere F = flow; Ct = trabecular outflow facility; Cu = uveoscleral outflow facility; Ctot = total outflow facility; Pev = episcleral venous pressure; and Peo = extraocular pressure, which is the pressure in the recipient space for uveoscleral outflow and may approach 0 mm Hg.
We would be interested in the thoughts of others in this field on the pressure dependence of uveoscleral outflow.
Bernard Becker, MD
Arthur H. Neufeld, PhD
Department of Ophthalmology and Visual Sciences
Washington University School of Medicine
St. Louis, Missouri
REFERENCEBecker B, Neufeld AH. Pressure dependence of uveoscleral outflow. J Glaucoma 2002; 11:464.
In the October issue of the Journal of Glaucoma, in the Editorial, “Translating the OHTS results into clinical practice” by George A. Cioffi, MD, and Jeffrey M. Liebmann, MD, Dr. Liebmann's affiliation was incorrect. The correct affiliation is:
Jeffrey M. Liebmann, MD, New York, New York
The publisher regrets this error.
REFERENCECioffi GA, Liebmann JM. Translating the OHTS results into clinical practice. J Glaucoma 2002; 11:375–7.© 2002 Lippincott Williams & Wilkins, Inc.