9/05/2017

Author: Danilo Andrade de Jesús, Wroclaw University of Technology

Intraocular pressure (IOP) is routinely assessed by noncontact applanation tonometry and its normal level ranges from about 10 to 21 mmHg. The accuracy of applanation tonometry is influenced by corneal stiffness that varies with a number of parameters, such as thickness, curvature and age. 

Cornea is found to increase its stiffness as the characteristics of viscoelastic behaviour decrease with age. Such changes in corneal micro-structure contribute to a variation of corneal biomechanical variables which may be independent of central corneal thickness (CCT) or IOP. However, the assessment of the micro-structure effect in corneal tissue stiffness was limited to few studies due to the lack of means to measure it in vivo. Recent studies have suggested that the measured IOP should be corrected by the CCT and an age-dependent correction factor, pointing that an increase of corneal thickness in a young person has a lower influence on the measured IOP than the same increase of thickness in an older subject. Despite the assumption made by these studies, ageing is not necessarily a linear process and may differently affect each subject. Parameters such as medical history (i.e., diabetes or topical drugs), ocular anatomy, sun exposure, alimentation or ethnicity may all play a role in that process. Moreover, the actual correction does not apply to other factors such corneal swelling, wound healing, and diseases such as keratoconus that all have possible effect on corneal micro-structure and hence, subsequently, on the IOP measurements. Therefore, new techniques are needed to measure in vivo the contribution of the corneal micro-structure on the IOP measurement.

In a manuscript published this month in PloS One, we show, for the first time, that corneal micro-structure plays an important role in IOP measurements obtained from noncontact tonometry. The proposed technique of statistically modelling OCT speckle introduces a new approach to provide complementary information to better understand the influence of alterations of the collagen framework on ocular tonometry.

Article Source: Influence of eye biometrics and corneal micro-structure on noncontact tonometry 
Jesus DA, Majewska M, Krzyżanowska-Berkowska P, Iskander DR (2017) Influence of eye biometrics and corneal micro-structure on noncontact tonometry. PLOS ONE 12(5): e0177180. https://doi.org/10.1371/journal.pone.0177180