Abstract
Purpose: :
While finite element modeling studies and monkey histomorphometry studies suggest that the peripapillary sclera plays an important role in the pathophysiology of glaucomatous optic neuropathy, few studies have examined biomechanical characteristics of the anterior sclera in glaucoma. By measuring the degree of anterior scleral indentation obtained with a Schiotz tonometer, this study is designed to explore whether anterior scleral rigidity (compliance) differs between human subjects with and without glaucoma.
Methods: :
Consecutive Albuquerque VA Medical Center eye clinic patients diagnosed primary open-angle glaucoma (POAG), ocular hypertension (OH), glaucoma suspect due to optic nerve appearance (GS), and normal ocular health (NML) were recruited and informed consent was obtained. After bidirectional air-jet tonometry (Ocular Response Analyzer®), dynamic contour tonometry (Pascal® tonometry), Goldmann applanation tonometry, and central corneal thickness (ultrasound) were measured, Schiotz tonometry was performed using 7.5 and 10.0 gram weights with the edge of the footplate located 2mm from the infero-temporal limbus. Two measurements were taken with each weight and then averaged for statistical analysis. One eye was randomly included from each subject, unless only one eye met criteria for glaucoma diagnosis in which case that eye was included.
Results: :
101 POAG, 66 OH, 43 GS, and 42 NML eyes were included. Using ANOVA, scleral Schiotz (SS) measures were significantly lower in the glaucoma group versus the OH, GS, and NML groups. Weak but significant correlations were found between SS measures and age, intraocular pressure, ocular pulse amplitude, and corneal hysteresis (CH). In multiple regression analysis, SS (7.5g) and CH were the only variables independently associated with glaucoma diagnosis (both p<0.001).
Conclusions: :
Using Schiotz tonometry to obtain a relatively direct measure of scleral rigidity, we found that the anterior sclera of glaucomatous eyes is stiffer compared to eyes without glaucoma, and this difference is independent of age, IOP and axial length. This finding may help explain results from prior studies in which increased ocular rigidity was associated with glaucoma. This study’s results also support additional investigation of anterior scleral biomechanical properties, including how increased scleral rigidity is related to lower corneal hysteresis and how both are related to the glaucomatous process.
Keywords: sclera • cornea: clinical science • anterior segment