Author reply

Abstract

Morgan et al raise important considerations to keep in mind while interpreting the results of our recent study. We acknowledge that using retrospective data, regardless of how carefully selected, imposes limitations on the interpretation of our results. Assessing “normal” mean intracranial pressure (ICP) is difficult and in this case “normal” is usually expressed as a range of ICP values. Although both the primary open-angle glaucoma group and the control group had a mean ICP within the “normal” range, the control group had a mean ICP toward the upper end of normal. This demonstrates the importance of establishing a comparable control group, and avoiding the use of historical controls. The overarching concern is that our control group may not be representative of the “normal population.” One important consideration may be age. Although age did not seem to be an independent predictor of cerebrospinal fluid (CSF) pressure in our data, most historically “normal” values for ICP have been ascertained in young, presumably healthy volunteers. These “normal” data may not be comparable to our more aged glaucoma population. However, we do not have CSF pressure data on normal or young subjects at the Mayo Clinic who were outside of our control group. We agree that examining this information would be helpful in determining whether bias was introduced by our approach in selecting a control group.Morgan et al raise an excellent point regarding postural position and reference planes when measuring pressures within the body. Although ICP measured by lumbar puncture may not represent the true retrolaminar CSF pressure, it is the most accessible measurement available, and the only surrogate available for our retrospective study. However, we agree that the optimal method to determine the translaminar pressure would be to obtain simultaneous IOP and ICP measurements in a standard position. Future studies will need to address these and other important issues regarding the methods used to elucidate the role of ICP in the pathogenesis of glaucoma. Morgan et al raise important considerations to keep in mind while interpreting the results of our recent study. We acknowledge that using retrospective data, regardless of how carefully selected, imposes limitations on the interpretation of our results. Assessing “normal” mean intracranial pressure (ICP) is difficult and in this case “normal” is usually expressed as a range of ICP values. Although both the primary open-angle glaucoma group and the control group had a mean ICP within the “normal” range, the control group had a mean ICP toward the upper end of normal. This demonstrates the importance of establishing a comparable control group, and avoiding the use of historical controls. The overarching concern is that our control group may not be representative of the “normal population.” One important consideration may be age. Although age did not seem to be an independent predictor of cerebrospinal fluid (CSF) pressure in our data, most historically “normal” values for ICP have been ascertained in young, presumably healthy volunteers. These “normal” data may not be comparable to our more aged glaucoma population. However, we do not have CSF pressure data on normal or young subjects at the Mayo Clinic who were outside of our control group. We agree that examining this information would be helpful in determining whether bias was introduced by our approach in selecting a control group. Morgan et al raise an excellent point regarding postural position and reference planes when measuring pressures within the body. Although ICP measured by lumbar puncture may not represent the true retrolaminar CSF pressure, it is the most accessible measurement available, and the only surrogate available for our retrospective study. However, we agree that the optimal method to determine the translaminar pressure would be to obtain simultaneous IOP and ICP measurements in a standard position. Future studies will need to address these and other important issues regarding the methods used to elucidate the role of ICP in the pathogenesis of glaucoma. Glaucoma and Cerebrospinal Fluid PressureOphthalmologyVol. 115Issue 12PreviewRegarding the paper by Berdahl et al, who used retrospective cerebrospinal fluid (CSF) pressure data to assess differences in CSF pressure between glaucoma patients and nonglaucoma control patients,1 we have a question regarding the control data and would like to comment on the use of different reference planes for pressure measurement. The mean CSF pressure in their glaucoma patients was 12.4 cm H2O versus 17.7 cm H2O for the controls. The only other study measuring CSF pressure in glaucoma patients found a higher CSF pressure range, from 11 to 17 cm H2O in 7 subjects, but did not report a mean or use a normal control group. Full-Text PDF