Asymmetry in intraocular pressure and retinal nerve fiber layer thickness in normal-tension glaucoma

The etiology of glaucoma is most probably multifactorial. This study intended to investigate the asymmetry in intraocular pressure (IOP) and that in retinal nerve fiber layer (RNFL) thickness in normal-tension glaucoma patients. Two diurnal tension curves, obtained within 3 months and counting at least five IOP readings each, including an early morning IOP measurement upon awaking, were obtained in 15 normal-tension glaucoma patients. None of the patients received IOP-lowering therapy. IOP asymmetry was present in at least three readings and was always in the same direction. The optic nerve was imaged in both eyes in each patient by means of confocal scanning laser ophthalmoscopy (Heidelberg Retina Tomograph). The interocular difference in RNFL thickness and the RNFL cross-sectional area were correlated with the interocular difference in IOP by means of Spearman's rank correlation factor. Nine female and 6 male normal-tension glaucoma patients (mean +/- SD age was 62. 4 +/- 16.9 years) were included in this study. Interocular IOP asymmetry varied between 0.30 and 4 mm Hg. Strong negative correlations were found between interocular asymmetry in IOP and interocular asymmetry in RNFL thickness asymmetry (R = -0.652, p = 0. 0083) and interocular asymmetry in RNFL cross-sectional area (R = -0. 702, p = 0.0034). The present results demonstrate for the first time a more marked thinning of the neuroretinal nerve fiber layer in the eye with the higher IOP in normal-tension glaucoma patients.