IOP Exposure Determines Scleral Shell Strain Changes in Nonhuman Primate (NHP) Experimental Glaucoma

Abstract

 

Purpose

 

To determine the time course of alterations in scleral shell mechanical behavior induced by exposure to chronic IOP elevations.

 

Methods

 

Posterior scleral shells from 5 bilaterally normal and 10 unilateral experimental glaucoma NHPs and were pressurized from 5 to 45 mmHg and the resulting full-field, three-dimensional, scleral surface deformations were measured using laser speckle interferometry. Scleral strain (local tissue deformation) was calculated directly from displacements. Maximum principal strain was averaged for a ~15 degree-wide-band adjacent to the ONH, and the relative difference in mean strain was calculated between contralateral eyes and compared with differential cumulative IOP exposure during the study period. The relationship between the difference in scleral strain and the difference in cumulative IOP exposure in contralateral eyes was assessed using statistical models, and the Akaike Information Criterion was used to determine whether a linear or nonlinear functional form provided the best fit of the hypothesized relationship, if present.

 

Results

 

Relative differential scleral tensile strain was significantly associated with differential cumulative IOP exposure in contralateral eyes. The linear model showed that relative differential mean posterior scleral tensile strain decreases with increasing differential cumulative IOP exposure (p=0.035). However, the quadratic statistical model fit significantly better than the linear model, and revealed a more complex relationship (AIC, p=0.0196; Figure).

 

Conclusions

 

These results show that scleral remodeling occurs in response to chronic IOP elevation, and the resulting mechanical behavior is dependent on the differential cumulative IOP exposure over time (a dose effect). As differential cumulative IOP exposure increases, the eye with the higher IOP exposure softens initially, resulting in a temporarily higher scleral tensile strains, but then stiffens significantly thereafter to exhibit lower mean tensile strains than the contralateral eye with lower IOP exposure (p<0.0001).

 

 

Figure. Linear and nonlinear statistical models showing the significant relationship between relative differential mean posterior scleral tensile strain and differential cumulative IOP exposure in NHPs. N, bilaterally normal NHPs; Gl, NHPs in which one eye was exposed to chronically elevated IOP.

 

Keywords: 568 intraocular pressure