Author reply

Abstract

We thank Dr Jonas for his encouraging comments and pertinent questions regarding our article. We acknowledge his recent work demonstrating an elevated ophthalmodynamometric force (ODF) required to induce retinal vein pulsation in glaucoma patients.1Jonas J.B. Central retinal artery and vein collapse pressure in eyes with chronic open angle glaucoma.Br J Ophthalmol. 2003; 87: 949-951Crossref PubMed Scopus (65) Google Scholar As part of our study, we calculated the neuroretinal rim area by subtracting the difference between disc area and cup area. The latter 2 parameters were calculated following optic disc photograph measurements of the horizontal and vertical discs and cup diameters using Littmann’s correction.2Barr D.B. An appraisal of the accuracy of Littmann’s method of determining the real dimension of a retinal object.Acta Ophthalmol Scand Suppl. 1995; 216: 1-12PubMed Google Scholar, 3Jonas J.B. Montgomery D.M. Determination of the neuroretinal rim area using the horizontal and vertical disc and cup diameters.Graefes Arch Clin Exp Ophthalmol. 1995; 233: 690-693Crossref PubMed Scopus (11) Google Scholar We used the ODF data from the right eye of subjects where measureable; otherwise, ODF data from the left eye were used. This was the same data set used in our Figures 2 and 3. Using the Spearman rank correlation method, we could find no significant relationship between optic disc area and ODF (r = −0.01, n = 90, P>0.05) in glaucoma suspects and glaucoma patients. Using data from glaucoma patients only, again we could find no significant correlation between ODF and disc area (r = −0.03, n = 52, P>0.05). Hence, we agree with Dr Jonas that the venous trans-lamina cribrosa outflow resistance may be independent of optic disc size. We cannot answer the question regarding a possible relationship between optic cup depth and venous outflow resistance because we do not have cup depth data. We do have cup area data, and there may be a relationship between cup depth and area in glaucoma patients. Using the same data set and the Spearman rank correlation method, we could find no relation between cup area and ODF in glaucoma patients (r = 0.143, n = 52, P>0.05) or glaucoma and suspects (r = 0.172, n = 90, P>0.05). The position of the vein in relation to the central retinal artery and how it passes into the neuroretinal tissue seem to affect the visibility or occurrence of spontaneous venous pulsation.4Hedges Jr, T.R. Baron E.M. Hedges III, T.R. Sinclair S.H. The retinal venous pulse its relation to optic disc characteristics and choroidal pulse.Ophthalmology. 1994; 101: 542-547Abstract Full Text PDF PubMed Scopus (36) Google Scholar This was the reason we excluded data from subjects where the vein entry into the optic disc was obscured by arterial or neuroretinal tissue. We did not measure the position of central retinal vein entry into the lamina cribrosa in our subjects, so we cannot further clarify this question. There may be several reasons why the central retinal veins may narrow in glaucoma. As Dr Jonas implies, lamina cribrosa condensation may cause compression of the central retinal vein and a raised flow resistance. Alternatively, the same condensation and thinning will lead to a greater translaminar pressure gradient because the same pressure difference is distributed over a shorter distance. This may increase the pressure gradient down the central retinal vein, leading to hemodynamic changes, such as elevated shear stress acting on venous endothelium and consequent luminal narrowing. We do not have data to separate these factors out, but these questions certainly remain fascinating and of potential importance regarding glaucoma etiology.