The Role of Corneal Hysteresis in Ophthalmology

Abstract

Corneal hysteresis is the difference in the inward and outward pressure values obtained during the dynamic bidirectional applanation process employed in the ocular response analyzer, as a result of viscous damping in the cornea. The factors affecting corneal hysteresis can be classified into anatomical factors which include the central corneal thickness and physiological factors which include corneal viscoelasticity, intraocular pressure and age. The ocular response analyzer is an instrument used to assess the corneal hysteresis and intraocular pressure. It is based on dynamic bidirectional applanation. It also provides a basis for two additional new parameters: corneal-compensated intraocular pressure and corneal resistance factor. Some clinical conditions such as Fuchs’ corneal dystrophy, keratoconus and high myopia may induce lowering of corneal biomechanical properties and hence corneal hysteresis. However diabetes mellitus induces increase in corneal hysteresis by increasing collagen cross linking. Measurement of corneal hysteresis assists in predicting which patients stand the greatest risk for postoperative ectasia. It helps, in conjunction with corneal topography and pachymetry, in patient selection. It also helps in refractive surgery procedure selection, by specific knowledge of the biomechanical response of the cornea to different procedures.