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Biomechanical response of optic nerve head to IOP stress differ in POAG, PACG eyes

Tristan Manalac
12 Apr 2018

The biomechanical response of the optic nerve head (ONH) appears to be related to disease severity in eyes with primary open-angle glaucoma (POAG), but not primary angle-closure glaucoma (PACG), a recent Singapore study has found.

After the first elevation in intraocular pressure (IOP), a greater change in the lamina cribrosa depth (LCD) significantly correlated with visual field loss, as represented by impaired visual field index (VFI; R, –0.57; 95 percent CI, –0.94 to –0.19; p=0.005) and mean deviation (MD; R, –0.64; –0.97 to –0.31; p=0.001). [JAMA Ophthalmol 2018;136:184-192]

A higher percentage decrease in the minimum rim width (MRW) of POAG eyes was likewise associated with worse VFI and MD both at the first (R, –0.57; –0.94 to –0.20; p=0.004 and R, –0.48; –0.86 to –0.09; p=0.02, respectively) and second (R, –0.60; –1.03 to –0.17; p=0.008 and R, –0.56; –0.98 to –0.13; p=0.01, respectively) IOP elevations.

In contrast, none of the associations of LCD displacements or MRW declines with visual field loss were observed in PACG eyes.

“[F]ollowing an acute elevation of IOP, we observed that LCs with more VF loss displaced anteriorly, whereas LCs with less VF loss displaced posteriorly in POAG eyes,” said researchers, noting that the direction of LC movement in response to IOP changes may have a potential role in patient stratification.

Moreover, LCD changes following IOP elevations were observed only in POAG eyes, “suggesting the possible existence of inherently different mechanisms involved, or similar mechanisms but with different extents of vulnerability of the ONH,” researchers added.

Patients included 91 Chinese adults (mean age 65.48±7.23 years; 42.9 percent female) with phakic eyes and who had no history of intraocular surgery from the Singapore National Eye Centre. Of the participants, 23 had POAG, 45 had PACG and 23 had no glaucoma.

Optical coherence tomography scans of the ONH were obtained at baseline and after two IOP elevations, which were induced by applying force to the anterior sclera with an ophthalmodynamometer. Standard automated perimetry was used to evaluate visual field.

“Our data suggest that the biomechanical response of the ONH to acute transient IOP elevations is different between POAG and PACG and may be of importance to improve our understanding of glaucoma pathogenesis,” said researchers.

However, several study limitations are worth noting, including the nine participants who were on losartan medication, which may have influenced scleral rigidity and consequently the findings of the study, according to researchers. This factor was adjusted for in statistical analyses, and the findings remained robust.

In addition, the study sample consisted purely of patients with Chinese ethnicity. While they comprise the majority of the Singapore population, variations in ONH biomechanics may exist among different ethnic groups and therefore negatively affect the generalizability of the present findings, researchers said.

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