Abstract
Purpose: :
Geometric morphometrics (GM) is a statistical framework used to analyze variation in shape and covariation with other variables. Using GM, we hypothesize that ppRPE/BM layer mirrors the shape of the load bearing structures of/around the optic disc and may provide insight into the biomechanics of glaucomatous optic neuropathy. This study attempts to determine if the shape ppRPE in glaucoma (Glc) differs from normals(Nl) and how shape might be affected by age, IOP, or central corneal thickness (CCT).
Methods: :
We compared two groups : (i) 52 eyes (38 patients) with treated Glc (ii) 25 Nl eyes (17 patients). Data was analyzed using standard GM techniques including a Generalized Least Squares Procrustes Superimposition, Principal Component (PC) Analysis, thin-plate spline and permutation statistical analysis to evaluate differences in shape.
Results: :
Both Nl and Glc eyes have a slight V shape that gently steepens as it approaches the neural canal opening (NCO). Compared to Nl, Glc patients showed a slight posterior displacement of the ppRPE and widening of the NCO. Principal component analysis showed that the first 2 PCs account for approximately 80% of the shape variation. The shape implied by PC 1 showed variation in the size of the NCO and PC 2 showed variation in the anterior posterior position of the ppRPE. The two groups of patients were clearly segregated along PC2 . The difference using permutation statistics (n=10,000) was statistically significant (p=0.02%). Regression analysis failed to show any statistically significant dependence of shape variables on treated IOP, age or CCT as independant variables.
Conclusions: :
Glc patients showed slight posterior displacement of the ppRPE with widening of the NCO compared to Nl. There was no significant association between shape and IOP, CCT, or age. These findings in human are consistent with previous animal studies on the effects of increased IOP on the load bearing structures of the optic nerve.
Keywords: intraocular pressure • anterior chamber • lamina cribrosa